STUDIES ON SOME HETEROCYCLICS AS CORROSION INHIBITORS

ABSTRACT

THESIS SUBMITTED FOR THE DEGREE OF

Soctor of $I|ilos;opIip IN

APPLIED CHEMISTRY

• • y^\- I'T ^ •

SUPERVISOR f ^ : t , , BY

DR. M. A. QURAISHI M. A. WAJID ! HA 1

DEPARTMENT OF APPLIED CHEMISTRY FACULTY OF ENGINEERING & TECHNOLOGY

ALIGARH MUSLIM UNIVERSITY ALIGARH (INDIA)

1994

The „c.. embodied in the p„sent thesis deais with the

stud. Of so^e nitrogen and sulphur containing heterocyclic

compounds as corrosion inhibitors for .ild steel i„ , , „ , ,

- d IK H^so^. .he performance of these compounds as corrosi­

on inhibitors has been investigated using the Weight Loss

Studies .nd Potentiostatic Polarisation technique.

Electron Spectroscopy and Scanning Electron Microscopy have

also been used to examine the inhibitive performance of some

selected compounds.

The compounds examined in the present investigations are

listed in Table (1-4). Their inhibiting action has been

discussed in the following four series.

1. 2-aminobenzothiazole and its substituted analogues.

2. 2-salicylideneaminobenzothiazole and its substituted

analogues.

3. 2-amino-4-phenylthiazole and its anil derivatives.

4. Azathiones.

The results of the present investigations reveal the

fact that aminobenzothiazole and its derivatives inhibit the

corrosion of mild steel effectively in both the acid

solutions at all the studied concentrations (100-500 ppm).

Maximum value of inhibition efficiency is achieved at a

concentration of 500 ppm. The order of inhibition efficiency

has been found as follows :

ACLBT > ABT > AMEOBT > AMEBT

The higher inhibition efficiency of ACLBT may be

attributed to its high dipole moment. The better performance

of methoxy derivative as compared to methyl derivative has

1

T a b l e i

SI .No

1

2

3

U

N A M E

2 - AMINO BENZOTHIAZOLE

2 - A M I N O - 6 - CHLORO-

B E N Z O T H I A Z O L E

2 - A M I N O - 6 - M E T H Y L -

B E N Z O T H I A Z O L E

2 - A M I N O - 6 - M E T H O X Y -

B E N Z O T H I A Z O L E

S T R U C T U R E

j f iCV-H - ^ ^ ^ - ^

c i - ^ ^ ^ ^ s ^

ABBREVIAriON USED

ABT

HiC

^ NH2

H3CO

^ NH:

A C L 8 T

AMEBT

AMEOBT

T a b l e Z

SI.No. N A M E S T R U C T U R E ABBREVIATION

USED

OH

2- SALICYLIDENE AMINO

BENZOTHIAZOLE

f^^^^""

2-SALICYLIDENE AMINO-

6-CHLORO- BENZOTHIAZOLE

2-SALICYLlDENE AMINO-

6-METHYL-BENZOTHIAZOLE

2-SALI CYLIOENE AMINO-

S-METHOXY- BENZOTHIAZOLE

0.>"'" W "''

ci-

OH

H3C

V N ^ C H - T ^ \ SACLBT

OH

^ | | "^N^CHY/ \ SAMEBT

OH

^ \ yN:CH-(V^\sAMEOBT

Tables

SI No. N A M C S T R U C T U R E ABBREVIATION

USED

2-AMINO - U - PHENYL

THIAZOLE

10

n

2-CINNAMALIDENE AMINO i,- PHENYL- THIAZOLE

2-VANILLIOENE AMINO­

S-PHENYL- THIAZOLE

A PT

S ^ N = CH-CH:CH

CA PI

12 2-SALlCYLlDENE AMINO

^-PHENYL - THIAZOLE

CA«.c„/\ 0CH3

/7-A VAPT OH

OH

S ^ N t C H ^ r \ SAPT

Tabled-

SI. No. N A M E S T R U C T U R E ABBREVIATION

USED

13 CYCLO PENTYL -

TETRAHYDRO-AZA-THIONE

11*

IS

DIMETHYL -TETRAHYDRO-AZA-THIONE

ETHYL - METHYL-

TETRAHYORO - A Z A - T H I O N E

HN NH

HN , ^ ^ N H

S

C P T A T

H 3 C ^ ^ C H 3

HN ' •"^NH I I

HN ^ , ^ N H

S

H5C2 CH3

H N ^ ^ N H I 1

HN NH

V S

O M T A T

EMTAT

been explained on the basis of Pearson's HSAB principle.

The inhibition of corrosion by 2-aininobenzothia2ole and

its derivatives may be explained on the basis of adsorption

of these compounds on the metal surface in terms of the

following interactions, a) lone pair of electrons of N and

S atoms of the benzothiazole ring can interact with metal

surface; b)Tf-electrons of benzothiazole ring can interact

with positively charged metal surface; c) protonated amino-

benzothiazoles can also interact with negatively charged

metal surface.

All the anils derived by the condensation of salicylal-

dehyde and cuninobenzothiazoles are found to give better per­

formance than the corresponding amines. Their enhanced inh­

ibition efficiency may be attributed to the presence of an

additional7r-bond of the azomethine group, 7T-electrons of

the benzene ring and an electron releasing -OH group.

The inhibition efficiency values of 2-amino-4- phenylth-

iazole and its condensation products, anils in acidic

solutions follow the order :

CAPT > VAPT > SAPT > APT

The higher value of inhibition efficiency of SAPT than

APT can be explained due to the presence of an additional

azomethine double bond, benzene ring and an electron relea­

sing -OH group. VAPT gives better inhibition efficiency than

SAPT because it contains an additional -OCH^ group. The

highest inhibition value obtained by CAPT can be explained

due to the presence of an additional /T-bond in its molecules

which is absent in the case of VAPT, hence it gives the best

performance among the investigated anils.

The inhibition efficiency of azathiones follow the order:

CPTAT > EMTAT > DMTAT

The highest inhibition efficiency achieved by CPTAT

among azathianes has been explained on the basis of its

larger molecular area.

The order of performance of various classes of organic

compounds exajnined in the present investigations is :

Anils > Amines > Azathiones.

The lower inhibition efficiency of azathiones can be

explained due to the absence of aromatic character in

these compounds.

All the compounds investigated have shown good

inhibition efficiency at the studied temperatures ranging

from 40 to 60 C. The inhibition efficiency of the

investigated compounds decreased at lower concentrations

except ACLBT and all the anils which showed nearly 90%

inhibition efficiency even at 60 C. All the inhibitors

are found to obey Temkin's adsorption isotherm.

The Potentiostatic Polarization studies were carried out

at 35±2 C. The polarization behaviour of different series of

compounds in both hydrochloric and sulphuric acids were

studied. All the compounds are found to be mixed inhibitors

except aminobenzothiazoles which are predominantly cathodic

and SABT predominantly anodic in hydrochloric acid.

The interesting feature of the investigation is that the

inhibition efficiency of all the amino compounds (ABT and

APTs) enhanced significantly on the addition of Potassium

Iodide (KI). This has been explained on the basis of the

synergistic model given below : H

/^

/

4

; * ) I N < ^

0 ©/" I Q-

0 0^

H H

All the investigated compounds are found to reduce the

permeation of hydrogen through steel surface effectively in

both the acid solutions.

The decrease in double layer capacitance values as

evident from AC impedance study in presence of SAMEBT

supports the adsorption of SAMEBT inhibitor on the steel

surface. The results of Auger Electron Spectroscopy studies

show that the adsorption of heterocyclic compounds on the

metal surface occurs through N and S atoms. The better

appearance of mild steel surface in inhibited acid solutions

than in plain acid solutions as evident from Scanning

Electron Microscopic (SEM) studies, further supports the

fact that inhibitor molecules are adsorbed over the steel

surface and prevent the attack of corrosive solution on the

surface.

CONCLUSIONS

1. All the compounds studied perform well as inhibitors in

hydrochloric acid and in sulphuric acid. They are found

to be more effective in hydrochloric acid than in

sulphuric acid.

2. All the compounds examined are found to be mixed

inhibitors in both the acids but aminobenzothiazoles and

its derivatives show predominantly cathodic behaviour

in hydrochloric acid where as the salicylideneamino-

benzothiazole shows predominantly anodic behaviour.

3. A good correlation has been observed in the values of

inhibition efficiency among the different techniques

adopted for the investigations.

4. A fairly good agreement is observed between the

corrosion inhibition by the compound and the reduction

in hydrogen permeation.

5. Anils of aminophenylthiazole have shown nearly 90%

inhibition efficiency at the concentrations even as low

as 25 ppm.

6. All the Amino compounds are found to exhibit synergistic

action in presence of potassium Iodide.

7. 2-Amino-6-chlorobenzothiazole and anils of 2-Ainino-4-

phenylthiazole are found to show good inhibition effici­

ency even at a temperature of 60 C.

STUDIES ON SOME HETEROCYCLICS AS CORROSION INHIBITORS

THESIS SUBMITTED FOR THE DEGREE OF

JBottor of $I|ilo£[opt)p IN

APPLIED CHEMISTIIY

SUPERVISOR

DR. M. A. QURAISHI BY

M. A. WAJID KHAN

DEPARTMENT OF APPLIED CHEMISTRY FACULTY OF ENGINEERING Qc TECHNOLOGY

ALIGARH MUSLIM UNIVERSITY ALIGARH {INDIA)

1994

T4547

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T4547

Betricatetr to

consifuclien J^usiice

raiher ikan desTfucllen

PEACE UNDERSTANDING

LOVE

K^ei'ld f-tee ffom jntpfooentent

aftns, beundafles, in

disease, peeei*Tif, halloed mofal lvalues

FACULTY OF ENGINEERING & TECHNOLOCy Aligirh Muslim University, Aligarh-202002 (INDIA)

Vv. i rn . %. (Purai5hi READER

Data:

CERTIFICATE

This is to certify that, the work incorporated

in this thesis entitled, "Studies on Some

Heterocyclics as Corrosion Inhibitors", is the

original contribution of Mr. M.A. Wajid Khan,

carried out under my supervision and guidance. The

work submitted in this thesis has not been

submitted elsewhere for any degree and is suitable

for submission to the award of Ph.D. degree.

(Dr. M.A. Quraishi)

Supervisor

AcknowZ^dgQ,me,nt6

I exp^s-i^ mij.Qn.ati.tadQ- to mg ^upe.n.vt^on., VJI. M.A. Q^iViai^hl ^OH. hi-h active and ^cholan.lLf gutdance; and i-on. pfLOvtdlng i-tnanclal a^^l^tanco. a-4 a S>2.'{hL0h. Re.^e.axc.h ¥e.llo[io, thn.ough CSJR, Meu; V2.lht, ^on. the. n.Q.^Q.ah.c.k \noH.k.

I thank ?h.o^. K.M. Sham^Liddtn, Chatn^man, VQ.pan.tme.nt o^ AppliQ.d ChQ.mi^tn.L[, ^on. hl^ CLOopQ.n.atton.

I am gn.atQ.iuL2. to Vn.. S.U.K. lijQ.n. and V>i. S,. MuxaLidhan-an, R.T. Lab, CECRJ, kan.aiku.dl ion. thQ.vi hoAp and immo-n^o. aoopoxatton in tho. n.Q.^Q.an.ch ujon.k.

Thanks an.Q. duo. to my collQ.aguQ.-6, ln.lQ.nd^ and ujQU-u^l^hcn.^, ion. tho. coopoxatlon they n.Q.ndQJiQ.d.

I am pn.Qioundlg gn.at2.iul to my pan.Q.nt-6, ^i^tQ.n.-s, bn.othe.n. Vn.. Majld, hn.otheJi.6-in-law and othen. n.elative-6 ion. thein. encoun.agement and ^uppon.t.

-- r/

(M.A. {\)ajid Khan)

CONTENTS

Preface . . . 1

Chapter - I

Introduction ... 1

Chapter - II

Experimental ... -72

Chapter - III

Ke3ult5 and Diecussion ... 91

Section - I

Am\nobenzoth\azo\ee ae

Acid Corrosion inhiipitore

Section - II

Anile of Aminobenzothiazoles

ae Acid Corroeion Inliihitore

Section - III

Amino piienylthiazoJe and ite

Anile ae Acid Corroeion Inhibitore

Section - IV

Azathionee ae Acid Corroeion

inhibitore ... 171

91

124

150

Sunmary 1 1 1

PREFACE

Corrosion is a phenomenon of universal interest. It

affects the economy of a country and causes hazards to the

health of human lives, as well. The seriousness of the prob­

lem has made the scientists all over the world very much co­

rrosion conscious and devising ways of preventing corrosion

has become a part of their struggle against corrosion. Nume­

rous methods of prevention have been suggested and among

them corrosion control through the application of inhibitors

has received the attention of the scientists to a very great

extent, due to simplicity of the method. Being specific to a

system this needs thorough consideration. In the present

study, use of some heterocyclic compounds as inhibitors of

corrosion of mild steel in acidic environment has been

investigated.

The thesis begins with an introduction highlighting the

economic and technological significance of the corrosion

problem. The forms and theory of corrosion have been descri­

bed to explain the mechanism of corrosion prevention of

corrosion using organic inhibitors in acid medium has been

described, with greater emphasis on mode of action of inhi­

bitors towards prevention of corrosion. A brief description

of different techniques employed for investigation of corro­

sion inhibitors is also given.

The literature on heterocyclic compounds as acid corros­

ion inhibitors has been surveyed.

The description of synthesis of inhibitors, materials

used and methods adopted such as Weight Loss method Potenti-

ostatic Polarization, AC Impedance, Hydrogen Permeation

techniques and surface examinations using Auger Electron

Spectroscopy (AES) and Scanning Electron Microscopy (SEM) is

given in the chapter, Experimental.

The results,obtained from potentiostatic and weight loss

measurements have been discussed in various sections in

terms of various corrosion parameters such as inhibition ef­

ficiency, corrosion rate, corrosion current, corrosion pote­

ntial. Selected compounds have been examined by Hydrogen

permeation technique to investigate their effectiveness in

preventing hydrogen through steel. To get more information

about the mechanism of inhibition one of the compounds

SAMEBT has been evaluated by AC Impedance techniques. The

influence of concentration, temperature, molecular structure

of heterocyclic compounds on corrosion inhibition has also

been discussed. SEM and AES techniques have also been used

to study the adsorption of organic compounds on the metal

surface.

A plausible mechanism of corrosion inhibition has been

proposed. An attempt has also been made to correlate the

structure and efficiency of the inhibitor molecules.

Corrosion is the loss of useful properties of a

metallic material as a result of chemical or electrochemical

reaction with its environment. Metals such as gold and

platinijjn remain unaffected for centuries while others like

copper, iron, aluminium, etc.are likely to change into their

compounds when kept exposed to aggressive environment. The

latter metals tend to revert back to the combined state

forming oxides, sulphides, carbonates, etc. Deterioration by

physical causes is not called corrosion but is described as

Erosion, Galling or Wear. When the chemical or electrochemi­

cal and physical deterioration takes place simultaneously

the damage proceed is frequently far greater than the damage

caused when they produced one at a time. Corrosion-erosion,

stress corrosion cracking, corrosion fatigue etc. are some

examples of this type of cojoint action. Rusting of iron and

iron based alloys is a very common example of corrosion.Non-

ferrous metals corrode but do not rust. Corrosion processes

form an interesting area for scientific studies which are

frequently under taken by chemists, particularly electroche-

mists, metallurgists and chemical engineers.

1.1 COST OF CORROSION

Losses due to corrosion are so high that it has

assumed great economic importance throughout the world. It

is expected that 1/4 part of the total production of metals

and alloys go waste due to corrosion. Technological and

economic consequences of the wastage of metals and alloys by

corrosion can not be ignored. A nujnber of reports have

appeared from time to time about the enormous financial

losses in India and other countries of the world.

According to NACE (International) bulletin [1] the

annual losses due to corrosion in USA were estimated to be

more than $ 200 Billions. In India the annual losses due to

the impact of corrosion has increased to more than Rs.1500/-

crores [2].

From national economic point of view, it is

necessary for scientists and engineers to adopt various

ways and means to reduce the loss due to corrosion. With

technological and industrial growth, the use of metals and

their alloys is increasing very rapidly and any step in the

direction of understanding the nature of corrosion, its

mechanism and the way to control it, would be of great help

to nation's economy.

1.2 CLASSIFICATION OF CORROSION

Corrosion has been classified in many different ways

as low temperature and high temperatu r e corrosion, direct

oxidation and electrochemical corrosion, etc. The preferred

classification is i. dry or chemical corrosion, ii. wet or

electrochemical corrosion.

Dry corrosion occurs in the absence of a liquid phase

or above the dew point of the environment. Vapours and gases

are usually the corrodents. It is often associated with high

temeratures. An example is attack on steel by furnace gases.

Wet corrosion occurs when a liquid is present which

involves aqueous solutions or electrolytes. A common example

is corrosion of steel by water .

A general scheme for the classification of corrosion

processes is presented separately in the form of a chart-1.

In the present work we are concerned with mainly

" Hydrogen evolution type " immersed corrosion. Since the

mechanism of under water corrosion is generally electrochem­

ical in nature, we have to consider the corrosion process as

being made up of anodic and cathodic components.

A simple voltaic cell ideally represents the

"Hydrogen evolution type" of corrosion in which corrosion is

a function of the amount of hydrogen evolved ; the cathodic

reaction may be represented as:

ZH" + 2e~ > 2H > H^

Hydrogen evolution corrosion is normally associated with the

more acid electrolytes (eg.acid industrial waters).

1.3 FACTORS INFLUENCING CORROSION

Except for the noble metals, such as gold, metals

occur in the earth's crust as certain stable compounds,usua­

lly oxides, hydrated oxides or sulphides, some times basic

sulphates, basic chloride or carbonates, etc. In reducing

the "ore" to metallic state, energy must be expanded to over

come the affinity between the metal and non-metal. The metal

thus produced, represents an energy rich state and if, as

usually happens in service, it is exposed to oxygen and / or

water, or to sulphur compounds,etc.,they return to the lower

energy state in which they originally occured in the earth

through the reactions involving drop of free energy i.e. an

operation which will occur spontaneously.

Factors like structural features of the metal, nature

of the environment and the type of reaction that occurs at

CO CO 0) r-(0 •p

o z

CO c 0

• ^

•p

u (0 « L.

cn O ct (r o o

I 0 L. •P

o 9 0 (0

w C 0

"D -t-- 3 O

Q- (0 •^ 9 r- I. I

r" ("" fl 0 +3 +> 0 >^

C 0

•p CO

• o 1- O) X C 0 1 -r p 0 o •«-0 C L L f- aj

o P

L 0 X a 0 0 E P <

c 3 C 0 0 I . 1-0) 0 I- 0 0 L tJ U c 0 D 0

c "0 0 0 t-0 0 I. 0 0 u i 0 H 0

0 0 a > 1-

i. 0 0 c •r-_I

«f-0

u T"

" 0 D 0 U 0 Q.

E p —

T-

4-

1 x: p •r" U 0 (7> 0 -1

1 E L 0 <«-

0 - E

C 0 T-

P 0

0 • I. p 0 E > 0 <

0 X 0

TJ 0 L. 0 +3

0 X CO

"O 0 0 0 a X Ul

0 T"

X 0 i. 0 <

0 u 0 0 c <

c 0 0) >. X o c 0 0) 0 L T3 ><

c 0 •r-0 0

i. 0 u

c •r-0 I.

0 p

0 a > p

u t -

X

1 u 0 0 X 0

1 <p 3 » 0 >

0 a > p

c 0 0 1- a +j >>

a .p 0 a >» •p

c 0

3 0

0 r- >, m 0 p

O T-•<- TJ

oB p - 1 -•r- E

0 L. 3 > O X

- 0 X <

1 0 1-

metal-environement interface, must be considered for the

understanding of corrosion phenomenon.

The important factors which may influence the

corrosion process are i. nature of the metal, ii. nature of

the environment, iii. electrode potential, iv.temperature v.

solution concentration, vi. aeration, vii. agitation, viii.

pH of the solution, ix. nature of corrosion products and x.

Hydrogen overpotential.

Generally high temperatures (more than 100 C) produce

more intense corrosion.

1.4 VARIOUS FORMS OF CORROSION

Corrosion can manifest itself in the following main

forms [3]. All of them are interrelated to varying degrees.

Except the general or uniform corrosion, all the other forms

are insidious in nature and they are considerably more

difficult to predict.

1.4.1 GENERAL CORROSION OR UNIFORM ATTACK

This is the most common type in which the corrosion

is uniform over the entire exposed surface.

Eg.: Water tanks subjected to atmospheric exposure.

1.4.2 PITTING OR LOCALIZED ATTACK

It is one of the most destructive and insidious form.

It is an example of highly localised corrosion, the attack

being limited to the extremely small areas. Examples are the

corrosion of stainless steel and chromium nickel alloys

in presence of ferric chloride and aluminium in presence of

neutral solutions.

1.4.3 GALVANIC CORROSION

It is an accelerated electrochemical action due to the

two different metals being in electrical contact and exposed

to an electrolyte. Heat exchanger failure in which aluminium

tubes are supported by a perforated steel sheet provides an

example for this type of corrosion.

1.4.4 CREVICE CORROSION

This type of corrosion takes place where only one

metal is in constant with different concentrations in the

environment. Rectangular metal containers and rivetted lap

joints offers the possibility of this type of corrosion.

1.4.5 STRESS CORROSION

It is the spontaneous cracking resulting from the

combined effect of prolonged stress and corrosive attack.

Caustic embrittlement of boilers provides an example for

this type of corrosion.

1.4.6 EROSION-CORROSION

It is the acceleration in rate of attack on a me t al

because of relative movement netween a corrosive fluid and

the metal surface e.g. heat exchanger tube handling water.

1.4.7 FRETTING CORROSION

It is a case of deterioration resulting from repeti­

tive rubbing at the interface between two surfaces in a

a corrosive environment. It is found in many aircraft engine

parts.

1.4.8 FILIFORM CORROSION

It is a special type of rusting which occurs on

certain metals under protective films like paints and is

characterised by a thread like growth.

Filiform corrosion may be found on tools coated with

oil films, refregerator doors, etc.

1.5 ELECTROCHEMICAL THEORY OF CORROSION

Though corrosion problem is as old as man's knowledge

about the uses of metals but its mechanism was not known

uptill eighteenth century. Wollaston [4]produced first paper

paper in 1801 regarding the mechnaism of corrosion. The most

acceptable electrochemical theory of corrosion was given by

Whitney [5] in 1903. Various other theories namely acid

theory [6,7] direct chemical attack theory [8], colloidal

theory [9] have also been put forwarded but they are mainly

restricted to specific systems only. The electrochemical

theory of corrosion is the only theory which is universally

accepted and is applicable to most of the corrosion

processes.

Most of the corrosion reactions can be separated into

two or more partial reactions which can be further divided

into two classes, oxidation and reduction. An oxidation

reaction is indicated by production of electrons as given

below :

M " M^ + e~ .. • (1)

This.reaction constitutes the basis of corrosion of

metals. In a similar fashion, a reduction r e action is

indicated by the consumption of electrons. For every

oxidation reaction there must be a corresponding reduction

reaction. In aqueous solutions, various reduction reactions

are possible depending upon the system. Some examples of

reduction reactions are :

Hydrogen evolution: 2H + 2e > H„ (in acidic system)

...(2)

Oxygen reduction: 0„ + 4H + 4e >2H_0 (in acid solution)

...(3)

0 + 2H„0 + 4e~ >40H~(in neutral and alkaline solution)

Metal ion reduction: M + e > M ^ ...(4)

Metal deposition : M + ne > M ...(5)

Oxidation reactions are known as anodic reactions

while reduction reactions as cathodic. During the corrosion

more than one anodic and more than one cathodic reactions

may occur. Oxidation-reduction (redox) reactions can be

understood by the example of corrosion of mild steel in

sulphuric contaminated by ferric ions. Anodic reaction will

occur as below:

M " M"" + ne~ ...(5a)

All the component element of mild steel (e.g. Fe,Mn,

etc.) go into the solution as their respective ions. The

electrons produced by these anodic (oxidation) reactions

will be consumed by the cathodic (reduction) reactions. In

this case, reaction (4) can be represented as below:

3+ - 2+

Fe + e » Fe ...(6)

Removing one of the availble cathodic reactions (e.g.

reaction (6) by removal of the Fe ions) will reduce the

corrosion rate.

When a metal or alloy is immersed in a corrosive

environment (conductive) different potential zones are deve-

loped on the surface of metal or alloy due to the presence of

different metallic phases, grain boundaries, segregates,

crystalline imperfections, impurities, etc. This difference

in potential leads to the formation of anodic and

cathodic areas on the metallic surface where oxidation and

reduction reactions occur, respectively. These areas result

in the formation of local action cell on the metallic

surface. Local action cell can also form where there are

variations in the environment or in temperature. The

electrode potential is calculated from the Nernst equation :

E = E + m - ^ ...(7) o zF (red) ^ '

where,

E = Standard electrode potential

R = Gas constant (1.98 cal/gm equivalent)

F = Faraday constant(96,500 coulombs/gm equivalent)

T = Absolute temperature

z = Number of the electrons transferred in the reaction

(ox) = Concentration of oxidized species (mol/1)

(red) - Concentration of reduced species (mol/1)

The potential of a reaction is related to its free energy

(AG) by :

AG = zFE ...(8)

A negative value for the free energy corresponds to a

spontaneous reaction, whereas a positive value of AG

indicates that the reaction has no tendency to proceed. The

change in free energy accompanying an electrochemical or

corrosion reaction can be calculated from a knowledge of the

cell potential of the reaction.

10

It is the redox potential by which one can predict

whether a metal will corrode in a given environment or not.

1.5.1 POTENTIAL-pH DIAGRAM

The use of oxidation reduction potentials can be

further extended ploting these potentials as a function of

solution pH. Such diagrams, often called Pourbaix diagram,

[10] are constructed using electrochemical calculations

based on solubility data, equilibrium const a nts and the

Nernst equation. The potential -pH diagram for iron exposed

to water has been shown in Fig.(1.0). It is necessary to

consider the following equilibria before drawing the potent­

ial diagram for iron :

2 + - • •

Fe ^ Fe + 2e Corrosion reaction ...9 3+ - . . .

Fe ^ Fe + e Oxidation reaction ..10

2Fe + 3H_0 ^ Fe_0,X + 6H + 6e Precipitation reaction

Fe "*" + H-0 -—^ Fe(OH) "*"+ H"*" Hydrolysis reaction.. 12

2Fe + 3H_0 -—^ ^ O'-'T "•" 6H + 6e Corrosion reaction ..13 Fe + 2H-0 -—^ HFeO + 3H + 2e Corrosion reaction ..14

(Hypo-ferrite)

HFeO" + HO -—^ Fe(0H)_J + e Precipitation reaction '^ ^ _ ^ . .15

Fe + 20H — ^ Fe(0H)_^ Precipitation reaction " ..16

Reactions (9), and (10) and (15) are independent of pH and

will be represented by straight horizontal lines; while reac­

tions (11), (13) and (14) are dependent upon pH and potential

and will be represented on the E /pH plots by sloping lines.

Reaction (12) and (16) which only depend on pH will be repre­

sented by vertical lines. Oxygen is evolved above but not

below line (cd) in accord with the reaction :

11

UJ

^ ( Q

z 0.0 UJ t-o a

•0.4

- 0 . 8

-1.2

•1.6

--il^^9en,„^

(d:

Fe F e ( 0 H ) 3

-''-^'°-'-i..

Fe

6 B

PH

10 12 K 16

FIG.1.0 SIMPLIFIED POTENTIAL-pH DIAGRAM FOR THE Fc-HjO SYSTEM

12

H2O > ^ 0^ + 2H"*" + 2e" ..17

Hydrogen is evolved below but not above (ab) in accord with

the reaction :

H" > I H^ - e~ . .18

As can be seen in Fig.2, the redox potential of the hydrogen

electrode (line "ab") lies above immunity region along all

the pH scale. This means that Fe may be dissovled with

evoluation of hydrogen in aqueous solutions of all pH values.

In the pH interval (9.4 -12.5), however, a passivating layer

of Fe(OH)„ is formed (reaction (16)).At lower (reaction (9))

At higher pH values soluble hypoferrite can form with in a

restricted active potential range. At a higher redox

potential in the corroding medium, the passivating layer

consist of Fe(OH) or Fe_0 ,nH-0 or Fe 0- cr Fe 0 in differ-

-2 ents situations.Soluble ferrate (FeO ) can form in alkaline

solutions at a very noble potential, but the stable field is

not well defined.

Though the potential-pH diagram is quite u seful in

showing at a glance specific conditions of potentia 1 and pH

underwhich the metal will corrode,there are serveral limita­

tions regarding their use in practical corrosion problems.

Since the data in potential-pH diagram are thermodynamic,

they convey no information abo ut the rate of reactions.

The major uses of such diagram which can be constru­

cted for all metals are :

i. predicting whether or not corrosion will occcur,

ii.estimating the composition of corrosion products formed,

and

13

iii.predicting environmental changes which will prevent or

reduce corrosive attack.

1.6. METHODS OF CORROSION CONTROL

The methods of corrosion control are many and varied.

Details of these various methods may be found in the extens­

ive literature on corrosion control [11-13]. The general

classification may be given as follows.

Corrosion Control method

^ ^ _ ^

Modification Modification Change of metal of of Metal of environment environment potential

1 1 1 By alloying or By the use of Cathodic and anodic surface modification, inhibitors. protection.

1.6.1 MODIFICATION OF METAL

Mostly corrosion protection involves bulk alloying or

surface coatings. Surface modification is far more economic

than bulk alloying. Surface coatings may pose porblems

related to adhesion, thermal expansion combatibility, etc.

Surface processing of metals has been improved by ion impla­

ntation technique [14] and Laser treatment [15,16] which

result in a homogeneous often single phase surface layer.

Recently Electron beam surface area glazing has been

found to increase the wear life of iron base tool

materials [17].

1.6.2 CATHODIC PROTECTION

Both cathodic [18] and anodic protection are important

among the electrochemical methods when an external d.c.

power source and an auxiliary are used to pass a net

14

cathodic current, corrosion rate is reduced. It is thus

possible to bring down the corrosion rate to the desired

level by the application of a suitable cathodic current.

With large structures like pipe lines, ships, etc., this

method becomes expensive. In such cases "sacrificial anodes"

are used to protect the metal cathodically. For e.g. in the

protection of the hulls of ships and boats from corrosion,an

active metal, generally zinc is used as a sacrificial anode

in contact with corrodable metal. The two metals in contact

form a cell, the terminals of which have been shorted. Zinc

slowly dissolves while the other metal is cathodically

protected.

The other type of anodes used for cathodic protection

are called "impressed current anodes". In this system the

anodes are not used as the source of electrical energy.

Instead, an external source of direct current is connected

within the structure to be protected and the anode. The

positive terminal of the power source is always connected to

the anodes and the negative terminal is connected to the

structure to be protected.

Examples of impressed current anodes are graphites,scrap

iron and platinum and lead-silver alloys. Rectifiers are

used as power sources in this system.

The main advantage of impressed current system over

Galvanic system which has a definite maximum current input,

is that the protecting current can be increased, if

necessary by increasing the applied voltage.

Although the impressed current system is more

15

versatile the main difficulty is that anode leads must be

insulated well and water proof and that continuous electrical

power must be available.

1.6.3 ANODIC PROTECTION

Here the structure to be protected is made the anode,

and the potential is set in the passive region. The nature

of auxiliary electrode is of little importance in this case

since it is the cathode and is cathodically protected. The

current density in the passive region is extremely small and

so that electric power needed for anodic protection is much

smaller than that required for cathodic protection. For this

reason it is preferred to cathodic protection.

The fact that passivation does not occur on iron and

ferrous alloys in the presence of CI ions limits the

application of this method. Anodic protection [19] is applied

to steel containers for storage of sulphuric and phosphoric

acid.

1.6.4 OTHER MEASURES

1 DESIGN

Many premature and unnecessary corrosion failures of

plant equipments, buildings and transportation units have

been traced to improper design. The design should consider

mechanical and service requirements together with an allow­

ance for corrosion,

ii. ELECTROPLATING

Electroplating with tin,chromium,nickel,etc., improve

to surface finishing and control the corrosion of metals.

16

iii. METALLIC COATINGS

Metallic coatings is a valuable and well tried method

for the protection of relatively corrodible metals such as

iron and steel. It can be divided into two classes,viz.,

noble or cathodic and sacrificial or anodic. In noble

coating, more electropositive metals is coated on the base

metal whereas in sacrificial coating, a metal wit h more

electronegative potential than the base metal is employed on

the surface,

iv. ORGANIC COATINGS

The organic coatings are more widely used than the

inorganic coatings. Generally, these have two functions,

i. they provide a protective film on the surface and ii. the

second function is decorative which gives pleasant effect to

the eye. The organic coatings are generally c 1 assified as

paints, enamel,lacquers and varnishes. The main constituents

of paints and enamels are a. pigment, b. binding medium c.

solvent and d. drier. The pigments are insoluble solid

particles which give a dried film, its characteristic

properties of colour and capacity.The binding medium is the

continuous phase in which the pigment is dispersed and driers

are substances which when incorporated in relatively small

proportions in drying oils or in paints, bring about

appreciable reduction of drying time at ordinary temperatures,

The solvents are volatile liquids added to paints and

varnises to facilitate application and to aid penetration by

lowering the viscosity. Besides paints and enamel,lacquers

and varnishes are also employed for protective coatings. The

17

term lacquers is more commonly used for the combinations of

cellulose derivatives,like cellulose nitrate or acetate, in

a volatile solvent.

1.7 INHIBITORS

One of the major methods of corrosion control

particularly in closed systems is the use of corrosion

supression reagents called inhibitors. An inhibitor is a

chemical substance which when added in small concentration

to the corrosive environment causes a substantial reduction

in the rate of corrosion of metal either by reducing the

probability of its occurance (deterrent) or by reducing the

rate of attack (ratardent) or by both. An inhibitor useful

for a particular corrosion system may be harmful to another

under certain situations.

The definition of an inhibitor favoured by the NACE

is "A substance which retards corrosion when added to an

environment in small concentrations [20] and the recent ISO

definition of an inhibitor is "chemical substance which

decreases the corrosion rate when present in the corrosion

system at a suitable concentration without significantly

changing the concentration of any other corrosive agent"[21].

Inhibitors may also be defined on electrochemical basis as

substances that reduce the rates of either or both of

partial anodic oxidation/or cathode reduction reaction.

From 19th century onwards vegetable wastes, plant

extracts [22,23] were used as inhibitors. Putilovaet al [24]

have reviewed metallic corrosion inhibitors. Reviews on

organic inhibitors [25-2 7 ] and organic sulphur compounds

18

[28] have been published. Several books have been published

on this subject [29,30]. Besides, the university of Ferrara,

Italy, conducts a symposium on corrossion inhibition once in

five years [31]. All the international Seminars on corrosion

discuss the developments and application of corrosion inhi­

bitors [32,33]. Various books on corrosion, review the subj­

ect in a precise manner [34,35].These show that the informa­

tion on this subject is extensively available and also shows

the importance of this topic.

1.7.1 CIASSIFICATION OF INHIBITORS

Inhibitors are classified in different ways.Depending

on the environment, they are called acid inhibitors, neutral

and alkaline inhibitors and vapour phase inhibitor. Depend­

ing on the mechanism of inhibition they are classified as

cathodic, anodic and mixed inhibitors.

According to Putilova [24] inhibitors are of three

types:

Type A forms a protective film on the metal surface.

Type B reduces the aggressiveness of corrosive media.

Type AB forms a protective film and also reduces the

aggressiveness of the corrosive agent.

1.7.2 ACID INHIBITORS

This may be further classified into Inorganic and

Organic inhibitors,

i. INORGANIC INHIBITORS

In strong acid solutions, Br , I have been found to

be effective inhibitors [36]. The oxides like As_0-, Sb-0_

have been reported as inhibitors in acid media. These subst-

19

ances get deposited [35] in the form of metal on iron and

increase the hydrogen over voltage and subsequently reducing

the corrosion. Recently addition of heavy metal ions like

2+ ''+ 2+ Pb , Mn" Cd is found to inhibit corrosion of iron in

acids. This effect is explained as due to under potential

deposition of metal ions leading to complete coverage on the

iron surface [37].

ii. ORGANIC INHIBITORS

A large number of organic compounds have been studied

as inhibitors. These compounds include alcohols, amines,

aldehydes,mercaptans,alkaloids,anisidines, sulphur compounds

such as thiourea,etc. They have an active functional group

containing N,S or oxygen atom.

The effectiveness of a large number of organic

inhibitors has been correlated [38-41] to different factors

like chemical structure, substituent effect, steric effect,

Hammet constant, molecular weight, carbon chain length, bas­

icity (pka), dipole moment, magnetic susceptibility and NMR

shift, etc.

The study of a variety of organic compounds in relat­

ion to their different aspects of inhibition in diffe r ent

environments has been excellently reviewed by Sanyal [42].

An organic corrosion inhibitor can be anodic,

cathodic or both depending on its reaction at the metal

surface and how the potential of the metal is affected [43].

Generally cathodic inhibitors increase cathodic polarization

and shift the corrosion potential to more negative values,

and anodic inhibitors enhance anodic polarization and shift

20

the corrosion potential to more positive values.

The effectiveness of an organic inhibitor depends

mainly on i.size ii.carbon chain length iii.bonding strength

to metal surface iv, aromaticity and / or conjugated bonding

and v. nature and number of bonding atoms.

1.7.3 ALKALINE AND NEUTRAL INHIBITORS

These inhibitors include cathodic inhibitors, those

which increase cathode polarisation, anodic inhibitors (pas-

sivating inhibitors) which enhance the anodic polarization

and mixed or general inhibitors which act as both cathodic

and anodic areas.

Anodic inhibitors form an oxide or some other insol­

uble film. Insufficient concentration of anodic inhibitors

will lead to severe pitting.

Sodium chromate is one of the most widely used and

efficient inhibitors. Sodium silicate is generally used in

hot water systems. The other compounds used in neutral and

alkaline media are borates, molybdates and salts of organic

acids, like benzoates and salicylates.

1.7.4 VAPOUR PHASE INHIBITORS

These are also called volatile corrosion inhibitors.

These are used in boilers,to prevent corrosion in condenser

tubes by neutralizing the acidic CO-.They are transported to

the site of corrosion in a closed system by voltalisation

from a source. Compounds of this type inhibit corrosion by

making the environment alkaline.

Volatile solids such as nitrate and benzoate salts of

dicyclohexyl amine and cyclohexyl amine are used in closed

21

vapour spaces like shipping containers.

The inhibitor vapour condenses on contacting a metal

surface and is hydrolysed by moisture present to liberate

nitrite and benzoate ions which in presence of available

oxygen are capable of passivating steel as they do in

aqueous solution.

1.7.5 ANODIC INHIBITORS

Those substances which reduce the anode area by

acting on the anodic sites and polarise the anodic reaction

are called anodic inhibitors. In the presence of anodic

inhibitor, displacement in corrosion potential (E ) takes

place in positive direction, supress corrosion current

(I ) and reduce the corrosion rate. In aqueous acid ^ corr'

media, the corrosion of metals takes place at the anodic

area through metal dissolution. The cathodic reaction

generally involve the reduction of hydrogen ions or by

oxygen reduction to hydroxyl ions. These reactions may be

represented schematically shown in Fig.1.1a.The curve E A ^ J ^ corr

represents the anodic reaction while E C represents the '^ corr '

cathodic reaction and the point B where both anodic and

cathodic reactions intersect corresponds to corrosion

potential (E ) and corrosion current(I ). The ^ ^ corr' ^ corr'

substances which retard the anodic reaction lead to the

enhancement of anodic polarisation. In this situation,

anodic curve becomes E^^^^ A (Fig.1.1a) and the current

I^^„__ corresponding to 0 is less than I (corrosion corr corr ^

corrosion current in the absence of the inhibitors) and the

rate of corrosion is decreased. Anodic inhibitors which

22

iriirr 'corr CURRENT

( Q )

•z- t^corr c a, c "o '^corr a.

'corr 'corr

CURRENT

(b )

O ^corr

a> '^corr o 0.

'corr 'corr CURRENT

( c )

FIG l . lMECHANiSM OF ACTION OF CORROSION INHIBITORS BASED ON POLARIZATION EFFECTS

23

causes a large shift in the corrosion potential are called

passivating inhibitors, if used in insufficient concentrat­

ions, they cause pitting and sometimes an increase in corro­

sion rate.With careful dosage control, anodic inhibitors are

frequently used because they are very effective in sufficie­

nt quantities.Anodic inhibitors cause passivity by speeding

up the corrosion reaction to the extent that the anodes are

polarized to a passive potential. If corrosion of a metal or

alloy is controlled by the anodic reaction (anodic control),

it is abvious that decrease in overall corrosion should be

proportional to the portion of the anodic areas being

polarized. On the other hand, if corrosion is controlled by

the cathodic reactions(cathodic control), the corrosion

current and, therefore, the total amount of corrosion is not

affected by decreasing the anodic areas. In this case, the

same amount of corrosion must be distributed over a smaller

anodic area,resulting in intensified localised attack (pitt­

ing type of corrosion).

The inhibition mechanism of the anodic corrosion

inhibitors has been a matter of long dispute and there have

been two points of view advanced to explain their action.

One supports the formation of protective insoluble film on

metals in the presence of the inhibitors while the other can

be understood in such a way that the inhibitors get

adsorbed by specific force interaction or through

chemisorption on the surface of metals,

i. PROTECTIVE FILM MECHANISM

It has been observed that most of the potassium and

24

sodium salts containing anions act as anodic inhibitors by

forming a sparingly soluble salt with the metal. Hoar and

Evans [44] have shown that chromates react with ferrous ions

and precipitate an adhe r ent protective film of hydrated

ferric and chromic oxides on the anodic areas. It has been

shown that during the inhibition of corrosion by sodium

hydroxide [45], orthophosphate [46], nitrite [47], chromate

[48] and other anodic inhibitors like sodium carbonate,

acetate, benzoate and molybdate [49] in aerated solutions,

there occurs the formation of an invisible protective thira

film by y-Fe 0 .

ii. ADSORPTION MECHANISM

According to Uhlig [50], for inhibition oxide film

formation is not necessary ; primary inhibition by chromates

and other oxidizing inhibitors is due to physical and

activated adsorption-chemisorption, through which valence

forces of the surface metal atoms are satisfied. These

views were confirmed from the measurements of electrode

potential as well as the measurement of residual activity of

an iron sample immersed in radioactive chromate solution

and subsequently washed thoroughly with distilled water

51-53]. Later on, It was found that anions adsorbed at

the oxide solution interface were responsible for inhibition

rather than the formation of metal oxide film [53].

1.7.6 CATHODIC INHIBITORS

Those substances which reduce the cathode area by

acting on the cathodic sites and polarise the cathodic

reactions are called cathodic inhibitors. They displace the

25

. . . . c corrosion potential m the negative direction (E ) and ^ ^ ^ corr'

reduce corrosion current, thereby retard cathodic reaction

and supress the corrosion rate (Fig.1.1b) . In this situati­

on, the point of intersection is at O and corresponding

c current I will be lower than that without inhibitor corr

^ corr'

The cathodic inhibitors, with a fev? exceptions [54] do

not lead to intensified or localised attack, since, cathode

areas are not attacked during corrosion. If corrosion is

controlled by cathodic reactions, the added cathodic

inhibitor would decrease cathodic area and hence over all

corrosion rate. On the other hand, if the corrosion is

controlled by anodic reaction, decrease in cathodic area

would increase cathodic current density but will have no

effect on the nature of corrosion. The increase in cathodic

current density may cause the reduction of substances present

which would not otherwise be reduced. Mann et al [55-57] and

other investigators [58-61], working on numerous organic

inhibitors in acid media have proposed that inhibitors exist

in onium structure and get adsorbed on the cathodic areas of

the surface by force of physical adsorption, and

chemisorption. In contrast, Bockris and Conway [62] have

claimed that the action of cathodic inhibitors is due to an

increase of the hydrogen over voltage rather than that by an

adsorbed inhibitor film on the metal surface. The cathodic

inhibition due to the general adsorption of the inhibitors

on the metal surface remains, however, the most accepted

theory [63,64].

26

Like anodic inhibitors, cathodic inhibitors are not

dangerous but safe, when present in solution in insufficient

quantities and involve no additional risk of pitting attack,

attack.

1.7.7 MIXED INHIBITORS

There are a number of chemicals which inhibit the

metallic corrosion by interfering with both the anodic and

cathodic reactions are called mixed inhibitors. This tyoe of

inhibition can be represented by Fig.(1.1.c).The anodic and

cathodic reactions are represented by E A and E C • -' corr corr

respectively and corrosion current I in presence of such ^ ^ corr ^

type of inhibitors is considerably less than that in their

absence. Glue Gelatin and other high molecular weight

substances fall in this category. It is believed that the

action of such type of inhibitors at the metal-liquid

interface is due to their concentration or coagulation

providing a shield to the metal surface.Machu [65,66] claims

that their action is mainly due to fonnation of porous

layer which increases the electrical resistance of the surf­

ace layer.

1 . 7 . 8 RECENT CLASSIFICATION OF INHIBITORS

Many authors have classified inhibitors as organic

and inorganic solution and vapour phase anodic and

cathodic.These classification systems do not cover the full

range of inhibitors which are in current use. Selection of a

corrosion inhibitor to combat a specific corrosion problem

requires a detailed knowledge of the mechanism of corrosion

process.

27

A recent classification [67] based on the mechanism of

inhibitive action, has however divided the corrosion inhibi­

tors into four major categories :

i. Barrier layer formers

ii. Neutralizers

iii. Scavengers and

iv. Miscellaneous,

i. BARRIER LAYER FORMERS

These materials form barrier layers on the corroding

metal surface and reduce the corrosion rate. This type is

the most important and forms the largest category of

corrosion inhibitors. The barrier formers have been further

classified into oxidiers, adsorbed layer formers and

conversion layer formers. In general, these inhibitors are

effective in reducing both the cathodic and anodic reaction

rates except for the oxidizing inhibitors which shift the

corrosion potential of the metal to more positive value at

which a stable oxide or hydroxide is formed and protects

the metal surface,

ii. NEUTRALIZERS

The neutralizing inhibitors remove the hydrogen ions

from the corrosive environment thereby reducing the corrosi-

vity of the environment and hence the concentration of the

cathodic reactant. These inhibitors are used in the treatment

of boiler waters, oil field applications and also in ethylene

glycol cooling systems,

iii. SCAVENGERS

These are similar to neutralizers but used to remove

28

corrosive species other than hydrogen ions. Typical example

of scavenger system is the use of hydrazine in boiler

systems to remove the traces of oxygen which is a cathodic

reactant.

iv. MISCELLANEOUS

These inhibitors include material such as scale

inhibitors and biological growth inhibitors which reduce

corrosion by interfering with other processes.

The above classification of inhibitor types by

function appears to give a fairly simple and concise

approach, although it has limitations in cases where the

mechanism is not known.In general the use of neutralizing and

scavenging type inhibitors seems to be the best suited to

closed systems where such chemicals are not lost in the

systems. In open systems however the use of inhibitors is

difficult to justify.

A summary of the classification system is presented

in the following table 1.0.

29

TABLE 1.0

Type Example

I Barrier layer formers

A. Adsorbed layer formers

1. Cathodic inhibitors

2. Anodic inhibitors

3. Mixed inhibitors

B. Oxidizing inhibitors-Passivators

C Conversion layer formers

1. Insoluble corrosion products

2. Cathodic deposits

II Neutralizing inhibitors

1. Volatile

2. Non-volatile

III Scavengers

1. Oxygen scavengers

2. Decomposition inhibitors

Acetylenic alcohol in HCl

NaNO_ in water

Phosphate

CaCO^

Cyclohexylamine in boilers Amines, brine

Na_SO in boilers

Dioxane in CH C CI

IV Miscellaneous

1. Biological growth inihibitors

2. Scale inhibitors

Quaternary amines

Phosphate

3. Other H^O in NH

30

1.8 MECHANISM OF INHIBITION IN ACIDS

The inhibitive action of organic compounds occur on

the metallic surface due to interaction between the

inhibitor and the metal surface by adsorption phenomenon. In

this process [25], the molecules are held on to the surface

of the adsorbent by valence forces i.e., variation in the

charge of the adsorbed substance and a t r ansfer of charge

from one phase to the other. Therefore, the molecular struc­

ture of the inhibitors assumes special significance [68].The

electron density at atoms of functional group constituting a

reaction centre affects the strength of the adsorption bond

[67] it also depends on the properties of the metal, as well

as on the polarizability of the functional group [69,70].

Inhibition by adsorption can be explained by LFER corre­

lation [71,72].

1.8.1 FACTORS AFFECTING ADSORPTION MECHANISMS

i. SURFACE CHARGE ON THE METAL

The magnitude and sign of the surface charge of the

metal play a very important role for the establishment of

the adsorption bond. The eff e cts excercised by organic

inhibitors on the electrode reactions must be connected with

the modifications induced in the structure of the electroch­

emical double layer because of their adsorption. In solution

the charge on a metal can be expressed by its potential with

respect to the zero charge potential. This potential, often

referred to as the 4> potential, is more important than the

potential on a hydrogen scale and sign of these two

potentials are different [73]. As the potential becomes more

31

positive, the adsorption of anions is favoured and as the

potential becomes more negative , the adsorption of cations

is favoured.

ii. REACTION OF ADSORBED INHIBITORS

In some cases, the adsorbed corrosion inhibitor s may

react to form a product by electrochemical reduction, which

may also be inhibitive in nature. Inhibition due to the

added susbstances has been termed as primary inhibition and

that due to the reaction product, secondary inhibition [74].

In such cases, the inhibitive efficiency may incre a se or

decrease with time according to whether the secondary

inhibition is more or less effective than the primary

inhibition [75].

iii. INTERACTION OF ADSORBED INHIBITOR SPECIES

Lateral interactions between adsorbed inhibitor

species becomes significant with increase of surface

coverage of the adsorbed species. This lateral interaction

may be either attractive or repulsive. Attractive

interaction occurs between molecules containing large

hydrocarbon components. Repulsive interactions occur between

ions or molecules containing dipoles and lead to weaker

adsorption at high coverage [76].

iv. INTERACTION OF THE INHIBITOR WITH WATER MOLECULES

The surfaces of metals in aqeuous solution are covered

with adsorbed water molecules. Adsorption of inhibitors

takes place by the displacement of adsorbed water molecules

from the surface, which involves free energy for adsorption.

It is found to increase with the energy of solvation of the

32

adsorbing species [77],

V. STRUCTURE OF INHIBITORS AND THEIR ADSORPTION

Inhibitors can bond to metal surfaces by electron

transfer to the metal to form adsoption bond. Generally the

inhibitors are the electron donor and the metal is the

electron acceptor. The strength of this bond depends on the

characteristics of both the adsorbate and adsorbent.

Electron transfer from the adsorbed species is favoured by

the presence of relatively loosely bound electrons, such as

may be found in anions and neutral organic molecules

containing lone pair electrons or n electron systems

associated with multiple, especially triple bonds or

aromatic rings.

Most organic compounds have atleast one polar atom,

i.e. nitrogen, sulphur, oxygen and in some cases selenium

and phosphorous. In general, the polar atom is regarded as

the reaction center for the establishment of the chemisorp-

tion process [69]. In such cases, the adsorption bond

strength is determined by the electron density of the atom

acting as the reaction centre and by the polarizability of

the polar atoms. The effectiveness of the polar atoms with

respect to the adsorption process varies in the following

sequences [71] :

Selenium > Sulphur > Nitrogen > Oxygen.

The importance of electron density in chemisorption of

organic substances in relation to inhibition phenomena has

been evaluated by Donahue [74]. The idea of electron density

acquires particular importance in aromatic or heterocyclic

33

inhibitos whose structure may be affected by the

introduction of susbtituents in different positions of the

rings [73]. The availability of electron pairs for the

formation of chemisorption bonds can thus be altered by

regular and systematic variations of the molecular

structure.

1.8.2 INFLUENCE OF INHIBITOR ON CORROSION REACTION

An inhibitor may decrease the rate of anodic

process,the cathodic process or both processes.The change in

corrosion potential on addition of the inhibitor is the

indication of a retarded process [78].Shift of the corrosion

potential in the positive direction indicated mainly

retardation of the anodic process (anodic control ) whereas

shift in the negative direction indicates the retardation of

the cathodic process (cathodic control). Little change in

the corrosion potential suggests that both anodic and

cathodic processes are retarded.

In the presence of an inhibitor, a shift of polariza­

tion curves without a change in the Tafel slope indicates

that the adsorbed inhibitors acts by blocking active sites

so that reaction can not occur rather than affecting the

mechanism of the reaction [79]. A change in the Tafel slope

is the indication of affecting the mechanism of the reaction.

Inhibitors in acid solutions affect the corrosion

reactions of metals in the following ways :

i. FORMATION OF A DIFFUSION BARRIER

The adsorbed inhibitor which forms a surface film on

the metal surface, can act as a physical barrier to restrict

34

the diffusion of ions or molecules to or from the metal

surface and so retarded the corrosion reactions. This type

of behaviour occurs in inhibitor containing large molecules

[80].

ii. BLOCKING OF REACTION SITES

The inhibitors may adsorb on the metal surface to

prevent the surface metal atoms from participating in either

the anodic or cathodic reactions of corrosion. This blocking

process reduces the surface metal atoms at which these

reactions can occur, and hence the rates of these reactions.

The mechanism of the reactions are not affected and the

Tafel slopes of the polarization curves remain unchanged.

Adsorption of inhibitors at low surface coverage tends to

occur at anodic sites, causing retardation of the anodic

reaction. At high surface coverage,adsorption occurs on both

anodic and cathodic sites, and both reactions are inhibited,

iii. PARTICIPATION IN THE ELECTRODE REACTIONS

The electrode reactions involve the formation of

adsorbed intermediate species with surface metal atoms. The

presence of adsorbed inhibitors will i n terfere with the

adsorbed intermediate but the electrode processes may then

proceed by alternative paths through intermediates

containing the inhibitor. In these processess, the inhibitor

affects the reaction and the inhibitor remain unchanged

with a change in the Tafel slope [81]. Inhibitors may retard

the rate of hydrogen evolution on metals by affecting the

mechanism of the reaction with the increase in Tafel slopes

of cathodic polarization curves. This effect has been

35

observed on iron in the presence of inhibitors such as

phenylthioureas [82].

iv. ALTERATION OF THE ELECTRICAL DOUBLE LAYER

The adsorption of ions or species which can form ions

on metal surfaces will change the electrical double layer at

the metal solution interface, and this in turn will affect

the rates of the electrochemical reactions.

V. ADSORPTION ISOTHERMS

An adsorption isotherm gives the relationship between

the coverage of an interface with an adsorbed species ( the

amount adsorbed) and the concentration of the species in so­

lution [83]. Various adsorption isotherms have been formula­

ted. Table 1.1 gives the list of isotherms and their corresp­

onding equations [84].

Interpretation of the inhibition characteristics of

organic molecules can be made by fitting the data to one of

the adsorption isotherms.

36

TABLE 1.1

Adsorption Isotherms

S.No. Isotherms Equations

1. Freundlisch

2. Langmuir

3. Frumkin

ftC = e

ftC = e 1-0

^C = lyl^y exp (-2ae)

Temkin ^ ^ Exp(ae)-1 ' 1-exp.[-a(l-0)]

5. Blomgren-Bockris / C = jIg exp.(Pe^/^-q0^)

6. Parsons Q 9—0

ftC = Tj- ^ exp. 2 exp(-2a0) 1-e (l-S)'

7. Bockris,Devanathan logC ± log j ^ = 0 + 3 0 ^ and Muller.

where,

~G /RT I? = e ads' = adsorption constant

G = free energy adsorption

6 = surface coverage

c = concentration

a = interaction parameter

a > 0 = > attraction and a < 0 = > repulsion

p and q = constants expressed in terms of dipole moments

37

1.9 SYNERGISM IN INHIBITION

"Synergism" is the term applied to the marked rein­

forcement of the inhibiting action of one inhibitor by the

addition of small amounts of a second inhibitor, eventhough

the second inhibitor is less effective when used alone.

It was shown by Foley [85] that tetraisoamyl ammonium

sulphate has little influence on the dissolution of iron in

4N sulphuric acid . However when 0.005N KI was added, the

organic cation is adsorbed reducing the double layer capacity

and the dissolution of iron is very much decreased.

The inhibition efficiency of acetylenic compounds has

been greatly improved when combined with amines or thio com­

pounds [86] .

1.10 EXPRESSIONS FOR CORROSION RATE

Corrosion rates have been expressed in a variety of

ways in the literature.The following are the major systems of

corrosion units.

i. ipy ... inches per year

ii. mpy ... mils per year

(1 mil = 0.001 inch = 25.4 t^m = 0.0254 mm)

iii.mmy ... milli mils per year

iv. ipmo ... inches per month

v. mdd ... milligrams per square

decimeter per day

Usually corrosion rates are expressed in two basic

units mpy and mdd.

The corrosion rates in mpy and mdd scales can be

directly calculated from the following expressions :

38 W X 534 1) mpy = -

where

W = the weight loss in grains

a = the area of the specimen in square inches

3 d = the density of the specimen in gram/cm

and t = the time in hours

[If the area is calculated in square centimeters then the

>.75 atd

. • 82.75 W , expression for mpy is mpy = T-T ] ...20

and ii.

, , 53.5 X W „, mdd = z— ...21

a X t

where

W = the weight loss in grams

t = the time in hours

and a = the area in square centimeters.

All the other expressions can be calculated from

these two basic units for corrosion rates, using the

conversion table [3]. 1 44

mpy = 1000 X ipy = 12100 x impo = —^— x mdd ...(22)

It can be mentioned here that in general a corrosion

rate of less than 5 mpy indicates satisfactory service beha­

viour 5-50 mpy, moderate to fair corrosion resistance and

corrosion rates above 50 mpy would be unsatisfactory for

service.

1.11 CORROSION AND INHIBITION MONITORING TECHNIQUES

The various techniques employed for corrosion

monitoring have been classified as

a. Non electrochemical methods and

b. Electrochemical methods

39

The measurement of corrosion rates in the presence

of corrosion inhibitors by weight loss and electrochemical

methods have been reviewed by Mercer [87].

1.11.1 NON ELECTROCHEMICAL METHODS

These include techniques like weight-loss measurement

and Gasometric methods.The main disadvantage of these

methods is that these require relatively long exposure times

of the corroding systems. Also the Chemical methods are in

general restricted to systems which do not form adherent

layers of corrosion products,

i. WEIGHT LOSS MEASUREMENTS

This method is the most reliable method. The

electrochemical measurement results are usually compared

with weight loss data. Here the change in weight of the

specimen is determined by immersing the specimen in the

corrosive medium for a fixed time. The weight loss is

expressed in mils per year (mpy) or milligrams per square

decimeter per day (mdd) which can be converted to current

using Faraday's laws. The conversion factor [35] for iron is

-7 2 1 mdd - 4.0 X 10 amp/cm . This method is also used to

evaluate the inhibitors.

ii. GASOMETRIC METHODS

This method yields reliable and accurate results with

a high degree of reproductivity. In this method the volume

of hydrogen gas (in acid corrosion) evolved during a

corrosion reaction is directly measured at a constant

temperature. The correpsonding metal loss can be calculated.

This technique has been used for the inhibitor studies by

40

Nathan [88] and Hackerman [89]. Mathur [90] et al have

designed a gasometric unit with which corrosion rates could

be monitored under controlled conditions of temperature and

pressure without any aqueous tension correction. Also this

technique has been successfully applied for the determination

of corrosion kinetic parameters by them [90].

However this technique has certain limitations such

as it can not be applied to a strong oxidising medium like

nitric acid, to systems where the inhibitor used undergoes

reduction with the hydrogen gas evolved, etc.

1.11.2 ELECTROCHEMICAL METHODS

The electrochemical methods are most widely used for

the study of inhibitors,

i. POLARIZATION METHODS

In this method the behaviour of inhibitor is

understood by drawing a Tafel plot Fig.i .2- in absence and

presence of inhibitor.

The percentage inhibition is calculated from the

folmula I — I __o _ o corr corr

IEJ-S — =

o corr

I = Corrosion current density (corrosion rate) in absence of inhibitor.

I = Corrosion current density (corrosion rate) in presence of inhibitor.

The corrosion rate is determined from the polarizat­

ion data in two ways.

a. Tafel extrapolation method

b. Linear polarization method

41

( • ) Nobl»

i>

^VH,

1

E ( V )

'

EcorrW)

^M^M

' \ ^ ^ \ , ^ ^ v

«. - ,-;;;;| || '')k ' TheOfeticQl curves

^J^^^'^^^x Experimental curves

1 \ 1 \ ON 1 . \ 1 \

•cor r (M) \ \ y

Active ( - )

i H j ^ M )

1 f\n i a U W ^ 1 — •

FlGa.2 POLARIZATION CURVES FOR A CORRODING ELECTRODE Ecorr = Corrosion potential 'corr = Corrosion current

42

In Tafel extrapolation method the linear portion of

the Tafel curve is extrapolated. The point of intersection

is referred to as !„(-,__•

Linear polarization method, provides the value of

absolute corrosion rate from the following relation.

fta.Oc "corr 2.3(/?a +ftc\ X Rp

where fta. and pc are Tafel constants,

1/R = Ai/AE = polarization resistance,

ii IMPEDANCE METHOD

The impedance technique [91-96] has become a popular

tool for the measurement of corrosion rate, in recent years.

The main advantages of this method are :

i. applicable to low conductivity systems

ii. provides mechanistic information

iii. solution resistance is completely eliminated.

The electrical equivalent circuit for the corroding

system is given below :

— I I

_ ' s . _ 'dl

h. -M-R = Solution resistance, s

R. = Charge transfer resistance.

W = Warburg impedance.

C,-,= Double layer capacity.

The inhibition eifficiency of the inhibitor can be

determined form AC impedance method [97,98] by the following

43

formula :

1/ — 1/ to t

IE % = •• ,„ — X 100

^/\o

R, and R. are the charge transfer resistance with and

without inhibitor.

for determination of R , very small potential is

applied as a function of frequency (usually 60 KHz-lmHz).

The impedance of the corroding system for various

frequencies can be measured using lock-in-amplifier. A plot

of Z (real)/Vs Z"(imaginary) for various frequencies gives

a semicircle ( Nyquist ) plot which cuts the real axis

at higher and lower frequencies. At higher frequency it

corresponds to R and at lower frequency it corresponds to

(R + R ) . The difference between the two values gives R :

From R the corrosion current can be calculated using

stearn-Geary equation. fta.ftc 1 corr 2.3(/?a+/?c) R

The double layer capacitance can be determined from the

frequency at which Z" is maximum from the relation.

z"max 2nc ,, .R

1.12 OTHER METHODS

The methods such as Radio tracer technique, Spectro­

scopic methods, X-ray photo electron spectroscopy. Auger

electron spectroscopy, Ellipsometry,Hydrogenpermeation have

also been used for studying the inhibition phenomenon.

44

1.12.1 RADIO TRACER TECHNIQUE

A better knowledge of inhibition phenomena can be

obtained by Radio Tracer method [99] with labelled inhibitor.

It is possible to detect traces of substance adsorbed

even under extreme dilution.

The method consists of bringing about the adsorption

of the compound under examination on the metal(electrode) by

putting the electrode in the electrolyte containing the

radio active organic substance. The electrode is taken out

and washed. It is subjected to a count determination to

measure the activity. It is compared with a standard and

the amount of substance adsorbed is found. Also the

decrease in the concentration of the labelled additive in

the solution as a result of adsorption can be measured.

Bockris [100] developed a special type of cell which permits

the study of adsorption under definite conditions of applied

potential.

1.12.2 SPECTROSCOPIC TECHNIQUE

The results of I.R. and U.V. spectra of the adsorbed

products are very useful in the interpretation of

inhibition phenomena. I.R. studies help to predict the

functions of the adsorption bonds and on the arrangement of

the inhibitor molecules on the surface of the metal.

Schwabe [101] showed from I.R. studies that in the case of

corrosion inhibition with dibenzylsulfoxide, the product

adsorbed on the electrode was dibenzylsulphide.

U.V. spectroscopy has been used to determine the

amounts of inhibitor adsorbed on the electrode by evaluating

45

the decrease in concentration in solution under condition

of free corrosion [102],

Suetaka [103] developed a technique to determine

directly the amount of inhibitor adsorbed by spectra recorded

on metallic electrodes.

Riggs et al [104] obtained NMR spectra of anilines and

substituted anilines. They have observed a good correlation

between chemical shift and coefficient of inhibition of steel

corrosion.

X-ray, Electron diffraction and Ellipsometric techni­

ques [105] have been employed to study the films formed on

the metal surface by the inhibitors.

1.12.3 AUGER ELECTRON SPECTROSCOPY

In Auger Electron Spectroscopy [106,107],a specimen is

excited with an electron beam causing inner shell electrons

to be removed from the atoms present. Through a relaxation

mechanism, outer shell electrons fill the created vacancies

and so called "Auger Electrons" are ejected from the material.

An "Auger Spectrum" is obtained by plotting the derivative of

the electron energy distribution versus energy. The typical

depth analysis with AES is of the order of 10 A or less and

elemental concentrations as low as 0.1% of monolayer can be

detected and identified. Both qualitative and quantitative

information can be obtained for all elements above helium and

sensitivity varies less than an order of magnitude for all the

elements.

The inner core vacancy is created by electron

bombardnent of the surface with electrons having energies in

46

the range of 1-5 KeV. Auger electrons are generated having

energies in the range 0 to 2000 eV and only those electrons

coming from within few monolayers of the surface escape with

characteristic energy. The depth profiles of t he surface

films are obtained by sputtering the surface away slowly by

Ar ions. The absolute thickness of the surface film cannot

be determined but the information about the thickness

relative to the sputtering of Ta O . can be easily obtained.

1.12.4 POLAROGRAPHIC TECHNIQUE

The polarographic method is employed for the study of

corrosion and has the practical utility in the detection of

minute changes in the corrosive system. In this method, the

potential is gradually increased in the direction of

reducing the substance present in the aqeous solutions. At

the reduction potential of the substance, current suddenly

increases. The height of the peak in the current versus time

curve will indicate the concentration of the substance

present.

1.12.5 ELECTROCAPILLARY TECHNIQUE

The concept of the electro-capillarity has recently

been introduced for the study of corrosion inhibitors [108,

109]. It consists of measuring the interfacial tension of the

electrode-electrolyte interface as a function of applied pote­

ntial. The shift of electro-capillary curve, i.e. potential

and surface tension curve, in the negative region after the

addition of the inhibitor shows that adsorbed species are

anionic in nature. In the presence of cationic type of inhibi­

tors, the curve shifts towards anodic potential.

47

1.12.6 NUCLEAR MAGNETIC RESONANCE

This method has been applied to study the electronic

structure of organic compounds. Using this method, it has

been verified that the electron density on the nitrogen of

anilines determines the ability of these compounds as

inhibitor of corrosion for steel in acids [110].

1.12.7 ULTRASONIC TECHNIQUE

For the inspection of the corrosion damage, ultraso­

nic technique [111] is successfully used. This is specially

important for plants where the testing needs to be done

without unnecessary shut down for inspection or testing.

Generally, two methods of ultrasonic inspection are

employed: reflection and resonance. By the reflection

method, the pin points of the position of many different

types of internal flows, e.g. stress corrosion crackings,

pits and fatigue cracks etc. are searched. The resonance

method measures the flows which are perpendicular to the

direction of ultrasonic beam.

1.12.8 HYDROGEN PERMEATION TECHNIQUE

When metals are in contact with acids, atomic

hydrogen is produced. Before these combine to produce

hydrogen molecules, a fraction may diffuse into the raetal.

Inside the metal, the hydrogen atoms may combine to fom

molecular hydrogen. Thus a very high internal pressure is

built up. This leads to heavy damage of the metal. This is

known as "Hydrogen embrittlement".

This phenomenon of hydrogen entr ' into the metals can

occur in industrial processes like pickling, plating,

48

phosphating, etc. Hydrogen permeation depends on the nature

of the acid used and it has been shown [112] that hydrogen

permeation decreases in the order HCl > H SO > H^PO^ > HCIO

for concentration greater than 0.5N.

A typical cell for permeation studies was introduced

by Devanathan [113] et al in which the hydrogen entered, is

ionised and recorded as permeation current.

The effect of inhibitors on the permeation of

hydrogen has assumed remarkable importance in the pickling

processes. Thus not only the loss of metal must be protected

but also the entry of hydrogen into the metal must be

restricted by the application of inhibitor.

An inhibitor can be considered as completely effect­

ive only if it simultaneously inhibits metal dissolution and

hydrogen penetration into the metal.

Bockris et al [114] showed that naphthalene increases

the rate of hydrogen penetration into iron. Also it has been

shown [115,116] that thiourea and derivatives act as good

inhibitors for iron and steel but stimulates hydrogen

penetration. This has been interpreted as due to the

formation of hydrogen sulphide. Antropov et al [117] studied

extensively the effects of numerous inhibitors on the

corrosion of iron and on the diffusion of hydrogen through

the metal. They showed that pyridine derivatives practically

eliminates the diffusion of hydrogen through iron membranes.

The behaviour of the inhibitors with regard to

hydrogen permeation can be understood by measuring the

permeation current with and without inhibitors. Those

49

inhibitors which reduce the permeation current are good at

inhibiting the entry of hydrogen into the metal concerned.

Other methods of evaluating hydrogen penetration [118]

consist in the determination of brittleness of the metal

previously subjected to acid attack in inhibited solution or

charged cathodically with hydrogen in acid solutions contain­

ing the inhibitors under study.

The percentage inhibition of hydrogen penetration is

given by N. , .,— N

ini o

. . . (23)

where N. ., the fracture data determined on the metal speci-

men in the absence of adsorbed hydrogen.

N , the after cathodic charging with hydrogen in the

absence of inhibitor and

N. ^•u.r the fracture data after cathodic charging in

the presence of the inhibitor.

1.13 INHIBITION OF CORROSION OF STEEL IN ACIDS

Inhibitors play an important role in controlling the

corrosion of steels [119,120]. The major use of inhibitors in

acid solutions is in pickling processess [121,122],for removal

of rust, scale and corrosion products. The chief requirements

of the inhibitors are that it should neither decompose

during the life of the pickle, nor increase hydrogen adsorp­

tion [123] by the metal. It should also not lead to the form­

ation of surface films with electrically insulating properties

that might interfere with subsequent electroplating or other

surface treatments. Pickling inhibitors require a favourable

50

polar groups by which the molecule can attach itself to the

metal surface. These include organic N,amine,S and OH groups

groups. The size, orientation and shape of the molecule play

a part in the effectiveness of inhibition [124]. The surface

charge of the metal and its constituents effect the relative

strength of the adsorbed bond and corrosion inhibition [123].

Granese and Resales [125] elucidated the mechanism of corro­

sion inhibition of iron and steel in HCl media. They observed

reduced corrosion by N-containing organic coumpounds like

acridine hexamethylene, quarternary ammonium sulphate etc.

at 85°C.

The anion of the pickling acid may also take part in

the adsorbed film accounting for differing efficiencies of

inhibition for the same compound in HCl as compared to

H^SO .This was supported by Hanna et al [126] for the use of

ethoxyiated unsaturated fatty acid. Pickling inhibitors may

act as a good inhibitor for iron but not for other metals or

vice versa due to specific electronic interactions of polar

groups with the metal. Sometimes temperature plays a

significant role in affecting the inhibition efficiency [127,

128] e.g., 0-tolylthiourea in 5% H SO acts as a good inhibi­

tor at elevated temperatures than at room temperature due to

increased adsorption.

It the USSR acid inhibitors are made by the use of

industrial byproducts. Katapan A which is alkylbenzyl pyridi­

ne chloride [129] and its analouges are efficient in prevent­

ing the corrosion of high C steel.

Compounds containing N or S have shown vast applica-

51

tions as corrosion inhibitors [130,131]. Machu [130] has

shown the use of S-containing compounds for H _ S 0 and

N-containing compounds for HCl solutions. Hackerman [132]

gave the idea that higher percentage of n orbitals of the

free electrons on the N atom leads to inhibitive action.

N-containing compounds used as acid inhibitors include

heterocyclic bases such as pyridine, quinoline and various

amines [133,134] S-containing compounds like thiourea and

its derivatives, raercaptans and sulphides in concentrations

0.003-0.01% give 90 % protection [135,136]. According to

every and Riggs [137], a mixture of N and S compounds was

better than either type alone.Highly substituted N atoms may

increase the inhibition efficiency due to increase of

electron density. Alkyl susbtitution on N atom or p-position

of aromatic nucleus improve inhibition efficiency in contrast

to meta derivatives. Effects of anions such as I and SH, in

the promotion of pronounced inhibiting action by organic

cations in acid solutions are well known [138,139].

52

1.14 HETEROCYCLICS AS ACID CORROSION INHIBITORS FOR MILD STEEL

The inhibiting action of pyrrole and its derivatives

was investigated in 5N HCl and 5N H SO at 20°C by weight

loss and polarization methods. The inhibition efficiency was

found to be dependent on the dipole moment and pK values

[140].

2-mercapto-benzimidazole (1) was studied by Balezin

et al. They found it to be effective for mild steel in IN

H2S0^ upto 70°C [141] .

r * * ^

(1)

Lee [142] evaluated the corrosion inhibiting effect

of 7-nitroso-8-hydroxyquinoline (2) against mild steel in

hydrochloric acid solution. It exhibited greater than 90%

efficiency.

(2)

The use of thiomorpholine (3), phenothiazine derivatives

(4) and vinylpyridine polymers (5) as potential pickling

inhibitors for ferrous metals has been reported in US

patents [143 - 145] .

53

H I

CH-CH^-

N-

(3) (4) (5)

Singh and his co-workers [146] reported about 97%

efficiency of 2-mercapto-benzothiazole (6) at the concentra­

tion of 6 X 4 X 10~'*M in IN H SO at 40°C.

(6)

The inhibitive action of pyridine (7), pyrrole (8),

furan (9) and thiophene (10) was investigated by galvanost-

atic measurement for Fe-IN H SO system. Thiophene exhibited

maximum efficiency among the heterocyclics examined [147].

^ . / ^ N'

(7)

H

(8)

^0^

(9)

^ s ^

(10)

54 The corrosion of mild steel in 2.5N H SO contain-

-2 m g 1 X 10 M concentration of 3-substituted-4-ainino-5-

mercapto 1,2,4-triazolines (11) was studied [148].

HN-

I

(11)

The inhibition efficiency of these triazolines was

found to increase on introducing electron donor substituents

at position-3. p-methoxyphenyl substituted triazoline showed

maximum efficiency among the inhibitors studied, 4-amino-3-

hydrazino-5-thio-l,2,4-triazole and some of its derivatives

(12) were studied as corrosion inhibitors by Abd-el-Nabey et

al [149]. They also found that inhibitors' efficiency incre­

ase on increasing the electron density at active centres of

the inhibitors.

N

. ^ NHNHj

NrCHR

\L

55

such as porphyrins and pthalocyanines (13) as acid corrosion

inhibitors for steel was investigated by potentiostatic and

AC impedance methods. Pthalocyanine gave 82% efficiency at

25 C in acid chloride environment (pH = 2 ) .

(13)

1,10-phenanthroline (14) was examined as corrosion

inhibitor for mild steel by gasometric and gravimetric

methods [151]. The maximum efficiency was found to be 92.1%

-3 and 92.5% in IN H SO and IN HCl respectively at 3 x 10 M

concentration.

(14)

Anderson et al [152] examined the effectiveness of 4,

7-diphenyI-l, 10-phenanthroline (15) in acid medium. They

found that the inhibiting action is attributed to the

presence of H-electrons of the aromatic ring.

56

(15)

The inhibition efficiencies of 1,1'-alkylene-bispyri-

dinium compounds (16) have been studied for the corrosion 0-

of mild steel in IN H SO . The inhibitors namely 1,1'-

ethylene-3,3'-dimethyl-bis-pyridiniumiodide and 1,1-ethylene-

bispyridiniumiodide showed 87.8% and 86.7% efficiencies in

concentration ranges of 250 and 1500 ppm respectively at

30°C.

^-(CHj)n- 21

(16)

R. = R2 = H, CH , COOH etc.

n = 1,2,3, etc.

The authors have found that substitution in the pyri­

dine ring has a pronounced effect on the inhibition efficie­

ncy [153].

The influence of soine series of heterocyclic compounds

containing more than one nitrogen atoms in their molecules

on corrosion of carbon steel in IN HCl was investigated by

Trabanelli and co-workers [154] with a view to establish

57

correlations between molecular structure and the inhibition

efficiency of the various compounds.Among the examined subs­

tances 2,2'-Biquinoline (17), Quinoxaline (18), Quinozoline

(19), and 2-mercaptopyrimidine (20) show good inhibiting

efficiencies (80% - 90%) at temperatures from 25°-60°C.

(17; (18:

kAsH (19) (20)

The influence of some of the substances on hydrogen

penetration process into the steel was also studied by these

authors.

Sethumadhavan and his co-workers [155] studied 1,10-

phenanthroline as corrosion inhibitor for mild steel in pure

sulphuric acid and found 87% efficiency at room temperature

-2 m 1 X 10 M concentration.

Granese et al [156] studied the inhibition action of

some nitrogen containing heterocyclic compounds such as

58

pyridine, quinoline, acridine and their n-hexadecyl deriva­

tives in HCl media by electrochemical and surface analysis.

They reported that the efficiency of these compounds incre­

ases with number of aromatic systems and electrons availabi­

lity in the molecules.

Stupnisek et al [157,158] investigated the inhibiting

action of various substituted N-arylpyrroles (21,22) on cor-

rossion of iron in strong acid solution (5 mol. dm HCl)

using electrochemical methods, with a view to studying the

relationship existing between the molecular structure and

inhibition efficiencies of pyrroles are significantly influ­

enced by the type and the position of the functional groups.

Thus N-arylpyrrole bearing fluorine at ortho position gave

better performance than other pyrrole derivatives.

H3C y\ CH,

R ^

(21). R = halogen at ortho,

meta or para position

(22) R = Alkyl or halogen

Raicheva and co-workers [159] investigated several

diazoles (23,24) such as imidazole, benzimidazole and their

derivatives as acid corrosion inhibitors of iron and steel.

They have found a very good relationship between the struct­

ure of diazoies and inhibition efficiency.

59

R, \

23.a.

b.

c.

24.a.

b.

c.

d.

H

(23)

^1 " ^2 " "'

R — H, R_ = CH-j

R^ = CH^, R^ = CH^OH

^1 " ^2 " ^3 H

f. R^= R3 = H, R^ = CH^CgH^

g. R2 - R3 = R, = CH^C^H^

h, R^ = H, R, = R- = CH^C^H^ -J i Z Z o o

i. R, - R^ = H, R„ = C^H^N

^ = K3

R,

R.

H, R - CH3

H, R^ = NH^ k,

R^ = R3 = H, R^ = CgH^N

R^ = R3 ^ H, R3 = NO2

R = H, R = CH^OH 1 R, R2 = H, R = COOH

R^ = R3 = H, R^ = Cg H3

They have concluded that the annelation of the benzene

nucleus to the diazole ring increases the protective action

of the diazole significantly, thus the inhibition efficiency

of benzimidazole (IE 29%) increased to 96% in 1,2 dibenzyli-

midazole (25). ^ ^ \ ^

N A C H , / ^ ^ \

CH,

<^

X ^ (25)

Ajmal et al [160] studied the inhibitive action of

2-hydrazino-6-methyl-benzothiazole (26) on corrosion of mild

steel in acidic solutions. They found that this compound

a mixed inhibitor in IN H_SO 60

and behaves acts as

predominantly as cathodic inhibitor in IN HCl. They have

also found that inhibitor effectively inhibits permeation of

hydrogen through mild steel.

r ^ ^

N=:CH

(26)

The inhibitive action of 4-ainino-5-mercapto-3-methyl-

1,2,4-trizole (27) on corrosion of mild steel in IN H_SO

and IN HCl was investigated by potentiodynamic polarization,

AC impedance and hydrogen permeation methods [161]. The

results of the investigations indicated the improved perfor­

mance of this compound in H_SO.. The inhibitor was also

found to be very effective in bringing down the hydrogen

permeation current considerably in both the acid solutions.

^N^ 5H I NHj

I, R = CHj; 2, Rs Cj H,

(27)

These authors [162] have further synthesized a few

anils (28) by condensing 3-alkyl-4-amino-5-mercapto-l,2,4-

trizoles with salicylaldehyde with a view to investigate the

inhibitive action of these compounds on the corrosion of

mild steel in acidic medium. They have found that all these

6 1

compounds show b e t t e r pe r fo rmance t h a n t h e c o r r e s p o n d i n g

amines .

N^ 5H

LCHJ/ \

HO'

f = H ; 6 R = CHj ; 3,R= C, Hy

( 2 8 )

62

AIM OF THE PRESENT WORK

The aims of the present investigations were to synthesi­

ze benzothiazole derivatives from cheap, readily available

raw materials to study :

a. Their inhibitive action on corrosion of mild steel in

acid medium.

b. Their effectiveness in preventing hydrogen permeation

in to the steel in acid media.

The choice for inhibitor synthesis is based on following

considerations :

1. Aminobenzothiazole and its derivatives have been syn­

thesized to study the combined influence of the benz­

othiazole ring and substituent on corrosion inhibiti­

on.

2. The condensation products of aminobenzothiazoles and

salicylaldehydes (anils) have been synthesised to

study the influence of n-bon d of the azome t hine

group, n-electron of the benzene ring, which are

expected to induce greater adsorption of organic

compounds on the steel surface. The presence of -OH

group is expected to increase the solubility of the

compounds in the acidic solutions.

3. The condensation products of 2-amino-4-phenylthiazole

with substituted aromatic aldehydes have been synthe­

sised to study the influence of substituents attached

to the benzene ring on corrosion inhibition.

63

4. Azathiones have been synthesised to study the influe­

nce of four nitrogen and sulphur atoms on the corros­

ion inhibition.

64

REFERENCES

1. NACE, Inter. Bull., Houston (1992).

2. The Hindu, Aug. 24 (1994) 27.

3. M.G. Fontana and N.D. Greene, "Corrosion Engineering", Mc Graw Hill International Book Company, New York (1978); (1986).

4. W.H. Wollaston, phil. Mag., 11 (1801) 206.

5. W.R. Whitney, J. Chem. Soc, 25 (1903) 395.

6. F.C. Calvert, Chem. News, 23 (1871) 98.

7. G.T. Moddy, J. Chem. Soc, 89 (1906) 720.

8. W.D. Bengouch and J.M. Staurt, J. Inst. Metals, 28 (1922) 31

9. J.N. Friend, Trans. Am. Electrochem. Soc, 40 (1921) 63.

10. M. Pourbaix, "Atlas of Electrochemical Equilibria in Aqueous solution, Pergamon Press, New York (1966).

11. O.L. Riggs Jr. and C.E. Locks, "Anodic Protection Theory and Practice in the prevention of corrosion", plenum Press, New York (1981).

12. S.W. Dean, R. Derby and G.T. Vondem Bussche, "Inhibitors types". Paper 253, presented at corrosion/81. National Association of corrosion Engineers (1981).

13. O.L. Riggs, M. Hutchison and N.L. Conger, corrosion, 16 (1960) 58t.

14. I.L. Singer, C.A. Carosella and J.R. Reed, Nucl. Instriim. Methods 182/183 (1981) 923.

15. R. Arghode, S. Venkatakrishna Iyer and D. Mukherjee, Corros. Bui. 3,2 (1983) 35.

16. J.D. Ayers et al, corrosion, 37,1 (1981) 55.

17. Y.W. Kim, P.R. Strutt et al Mat. Trans. A., lOA (1979)

881.

18. L.N. Applegate, "Cathodic Protection", Mc Graw Hill Book

Co., Inc., New York (1960).

19. H.H. Uhlig, "Corrosion and Corrosion Control", John Wiley and Sons, New york (1964) 68.

20. NACE-Glassary of Corrosion Terms, Mat. Prot.,4 (1965) 79.

65

21. Corrosion Metals and Alloys-Terms and definitions ISO 8044-1986.

22. Pengkeru, Peiyulian, Huang Qiulong and Wangkingin, Proc. 10th Inter. Cong, on Metallic Corros., India, 111 (1987) 2709.

23. R.M. Saleh, A.A. Ismail and A.A. El-Hosary, Brit. Corros. J. 17 (1982) 131.

24. I.N. Putilova, S.A. Balezin and O.P. Barannik, "Metallic Corrosion Inhibitors", E. Bishop (Edt.) Pergamon Press (1960) 2.

25. G. Trabanelli and V. Carassiti, "Advances in Corrosion Science and Technology", G. Fontana and R.W. Stachle (Edt.), Plenum Press, New York 1 (1970) 147.

26. B. Sanyal, Progress in Organic Coatings, 2 (1981) 165.

27. P.N. Girijashankar, J. Balachandra and K.I. Vasu, J. Electrochem. Soc, India, 29 (1980) 195.

28. G. Trabanelli and F. Zucchi, "Reviews on Coatings and Corrosion", J. Penciner (Edt.) 1 (1973) 97(2).

29. J.J. Bregmann, "Corrosion Inhibitors", Mc Millan, New -York (1963).

30. C.C. Nathan, "Corrosion Inhibitors",NACE, Houston (1971).

31. Proc. of Eur. Symp. Corrosion Inhibitors, Ferrara, Italy (1960), (1965), (1970), (1975), (1980), (1985) and (1990),

32. Proc. Inter. Cong, on Metallic Corrosion, (1961), (1963), (1966), (1969), (1972), (1975), (1978), (1981), (1984), (1987), (1990) and (1993).

33. Proc. of NACE Conf. on Corrosion, Houston (1990).

34. J.G.N. Thomes, "Corrosion", L.L. Shrier(Edt.) Newnes and

Butterworths, London 2 (1976).

35. R.H. Hausler, "Corrosion Chemistry" (Edt.), G.R. Bruba-ker and P.B.P. Phipps, Acs. Symp. Series, 89 (1979) 262.

36. F. Mazza, N.D. Greena, 2nd Symp. Eur. Sur Les Inh. de Corr. Ferrara, Italy, 13, Ses 5, Suppl. 4(1) (1965) 401.

37. K. Juttner, Workstoffe Under Korrosion, 31 (1980) 358.

38. V.R. Evans, "The Corrosion and Oxidation of Metals", second Suppl. Edward Arnold Publ. London (1968) 104.

39. S.Z. Altsybeeva, S.Z. Levin and Dorokhov, 3rd Eur. Symp. on Corros. Inh. Ferrara, Italy (1970) 501.

40. A. Akiyama, K. Nobe, J. Electrochem. Soc,117 (1970) 999.

66

41. K. Aramaki, Boshoku Gijutsu, 26(6) (1977) 297,

42. B. Sanyal, Progress in Organic Coatings, 9(2) (1981) 165.

43. U.R-. Evans, "Metallic Corrosion, Passivity and Protection" Edward Arnold and Co., London (1948) 535.

44. T.P. Hoar and U.R. Evans, J. Chem. Soc, 2476 (1932) 76.

45. J.E.O. Mayane, J.W. Menter and M.J. pryor, J. Chen. Soc, (1950) 3229.

46. M.J. Pryor and M. Cohen, J. Electrochem. Soc, 98 (1933) 263.

47. R. Pyke and M. Cohen, J. Electrochem. Soc, 93 (1948) 63.

48 J.E.O. Mayane and M.J. Pryor, J. Electrochem. Soc, 95 (1949) 1831.

49. M.J. Pryor and M. Cohen, J. Electrochem. Soc, 100 (1953)

203.

50. H.H. Uhlig and A. Geary, J. Electrochem. Soc, 101 (1954) 215.

51. D. Brasher and E. Stove, Chem. Ind., 8 (1952) 171.

52. M. Simnad, J. Inst. Metals, London (1953) 23.

53. R. Powers and N. Hackerman, J. Electrochem. Soc, 100

(1953) 314.

54. R.S. Thornhill, Ind. Eng. Chem., 37 (1945) 706.

55. S. Chiao and C.A. Mann, Ind. Eng. Chem., 39 (1947) 910.

56. C.A. Mann, Trans. Electrochem. Soc, 69 (1936) 115.

57. C.A. Mann, B.E. Lauer and C.T. Hultin, Ind. Eng. Chem., 28 (1936) 159.

58. E. Jimeno, L. Grifoll and F.R. Morral, Trans. Electro­chem. Soc, 69 (1936) 105.

59. L. Eleze and H. Fischer, J. Electrochem. Soc, 99 (1952) 259.

60. G.A. Marsch and H.J. Mc Donald, Pittsburgh, Inter. Conf. on Surface Reactions, The Corrosion Publishing Co., Pittsburgh (1948) 1.

61. J.C. Warner, Trans. Electrochem. Soc, 55 (1927) 287.

62. J.CM. Bockris and B.E. Conway, J. Phys. and Colloid Chem. 53 (1949) 527.

67

63. N. Hackerman and A.C. Makrides, Ind. Eng. Chem.,46 (1954) 523.

64. H.C. Gatos, J. Electrochem. Soc, 101 (1954) 433.

65. W. Machu, Korr. Met., 10 (1934) 284.

66. W. Machu, Trans. Electrochem. Soc, 72 (1937) 333.

67. S.Q. Deans Jr. R. Derby and G.T. Von Dem Burrche, Mat.

Perform, 20, 12 (1981) 47.

68. N. Hackerman and F.H. Finley, J. Electrochem. Soc, 107(4) (1960) 259.

69. N. Hackerman, An Adsorption Theory of Corrosion Inhibi­tion by Organic Compounds, Comptes Reudus Symp. Eur. Sur les Inh. de Corros., Annali Univ. Ferrara, Italy, N.S. Sez, V. Suppl. n. 3 (1961) 99.

70. R.C. Ayers Jr. and N. Hackerman, J. Electrochem. Soc, 110 (1963) 507.

71. F.M. Donahue and K. Nobe, J. Electrochem. Soc, 114 (1967)

1012 and 112 (1965) 886.

72. P.R. Wells, Chem. Rev., 63 (1963) 171.

73. L.I. Antropov, Corros. Sci., 7 (1967) 607.

74. L. Horner, Werkstoffe U Korrosion, 23 (1972) 466.

75. G. Trabanelli, F. Zucchi, G. Gullini and V. Carassiti, Br. Corr. J., 4 (1969) 212.

76. T.P. Hoar and R.P. Khera, 1st European Symposium on Corrosion Inhibitors, Univ. of Ferrara Italy (1961) 73.

77. E. Blomgren, J.O.M. Bockris and C.J. Jesch, Phys. Chem., 65 (1961) 2000.

78. G.W. Poling, J. Electrochem. Soc, 114 (1967) 1209.

79. E.J. Kelly, J. Electrochem. Soc, 115 (1968) 1111.

80. W. Machu, 1st European Symposium on Corrosion Inhibitors, Ferrara, 1960, Univ. of Ferrara, Italy (1961) 183.

81. Z.A.Iofa, 2nd European Symposium on Corrosion Inhibitors, Ferrara, Italy (1966) 93.

82. L. Cavallaro, L. Felloni, F. Pulidori and G. Trabanelli, Corrosion, 18 (1962) 396t.

83. B.M.W. Trapnell, "Cheraisorption", Butterworths Scientific Publication, London (1955) 109.

68

84. B.B. Damaskin, O.A. Petrii and V.V. Batrakov, "Adsorption

of Organic Compounds on Electrodes", Plenum Press, New-York (1971) 86.

85. R.T. Foley, Corrosion, 26 (1970) 58.

86. Jean, Frasct Societe Franalite, Fr. I. 475, 895, 7 April (1967) .

87. A.D. Mercer, Brit. Corros. J. 2G (1985) 61.

88. C.C. Nathan, Corrosion, 9 (1959) 199.

89. N. Hackerman et al, J. Electrochem. Soc, 115, 16 (1968) 1006.

90. P.B. Mathur and T. Vasudevan, Corrosion, 38,3 (1982) 171.

91. I. Epelboin, M. Keddam, H.J. Takenouti, Appl. Electro­chem., 2 (1972) 71.

92. S. Haruyama, T. Tsuru and M. Anan, Boshoku Gijutsu, 27 (1978) 449.

93. N. Kirtivasan, T. Tsuru and S. Haruyama, Boshoku Gijutsu, 29 (1980) 275.

94. F. Taib Heakal and S. Haruyama, Corrosion Sci., 20 (1980) 887.

95. F. Mansfeld, Corrosion, 37 (1981) 301 and 38, (1982) 570.

96. K. Hladky, L.M. Callow and J.L. Dawson, Brit. Corros. J., 15 (1980) 21.

97. F. Bourelier, K. Vugang, Proc. 10th International Cong­ress on Metallic Corrosion (1987) 2813.

98. F. Mansfeld, M.V. Kendig, A.J. Allen and W.J. Lorenz, Proc. 9th International Congress on Metallic Corrosion (1984) 1368.

99. P. Lacombe, 2nd Symp. Eur. Sur les. Inhibiteurs de Corr­osion, Annali Univ., Ferrara, Italy, N.S. Sez V. Suppl. 4 (1966) 517.

100. J.O.M. Bockris, D.A.J, Swinkels, J. Electrochem. Soc,

111 (1964) 776.

101. K, Schwabe and W. Leonhardt, Chemie-Ingenieur Technik 38(1) (1966) 59,

102. L. Cavallaro, L. Felloni and G. Trabanelli, Symp. Eur. Sur les. Inhibiteurs de Corros. Annali Univ. Ferrara, Italy, N.S. Sez V., Suppl. 3 (1961) 11,

103. W, Suetaka, Bull, Chem, Soc, Japan, 38 (1965) 148.

69

104. P.F. Cox, R.L. Every and O.L. Riggs Jr. Corrosion, 20 (1964) 299t.

105. G. Mrowcezynski and Szklarska - Smialowska, J. Appl.

Electrochem. 9 (1979) 201.

106. D.T. Lorson, Corros. Sci., 19 (1979) 657.

107. A. Joshi, L.E. Davis and P.W. Palmerg, "Methods of Surface Analysis", A.W. Czanderna(Edt.),Vol. 1 of Method and Phenomena, Elsevicer, New York (1975) 159.

108. C.P. De, Nature, 180 (1957) 803.

109. Z. Ostrowski, Proc. of European Symp. on Corrosion Inhibitors, Univ. Ferrara, Italy (1960) 239.

110. P.F. Cox, R.L. Every and O.L. Riggs Jr., Corrosion, 20 (1964) 299t.

111. W. Suetaka, Bull. Chem, Soc, Japan, 38 (1965) 148.

112. G. Devarajan, Ph.D. Thesis, Madurai Kamaraj University (1982).

113. M.A.V. Devanathan and Z. Starchurski, Proc. Roy. Soc, 270A (1962) 90.

114. J.O.M. Bockris, J.Me. Breen and L. Nanis,J. Electrochem. Soc, 112 (1965) 1025.

115. G. Anderson, U. Tragardh and G. Wranglean, Current Corrosion Research in Scandinavia, Almquist and Wiksell, Stockholm (1965) 11.

116. L. Cavallaro, G.P. Bolognesi, L. Felloni, Werk Und Korr.,

10 (1959) 81.

117. L.I. Antropov, M.A. Gerasimenko, Yu.S. Gerasiraenko and Yu.A. Savgira, 3rd International Congress on Metallic Corrosion (Moscow) May (1966) 97.

118. M. Smialowski, Comptes 2eme Symp. Europ. Sur les. Inhibiteurs de Corrosion, Annali Univ., Ferrara, Italy, N.S. Sez. V. Suppl. 4 (1966) 203.

119. G.Schmitt and B. olbertz; 1 werkst Korros; 29 (1978) 451.

120. N.S. Rawat, G. Udayabhanu and R.K. Arora, Trans. SAEST, 20 (1985) 63.

121. A. Kozlowska, H. Kryszezynska, E. Radomska, B. Szeptycka, S. Wlodarczyk, Powloki onchr, 14 (1986) 8.

122. T. Das, Metalloberflache, 41 (1987) 465.

70

123. K. Ravindranath, V.P. Sastry, 10th International Cong, on Metallic Corrosion, Madras, India, 3 (1987) 2629.

124. B. Skorupska, M. Studnicki, J. Leskiewiez, Ochr. Prized, Korz, 29 (1986) 231.

125. S.L. Granese, B.M. Rosales, 10th International Cong, on Metallic Corrosion, Madras, India, 3 (1987) 2733.

126. F. Hanna, G.M. Sherbini, Y. Barakat, 10th International Cong., Madras, India, 3 (1987) 2771.

127. O.L. Riggs Jr. and R.M. Hurd, Corrosion, 23 (1967) 252.

128. N.V. Bagoyavlenskaya, Metallurgiya, Moscow, (1967) 14.

129. A.S. Afanasev, E.N. Chankova, S.G. Tyr and R.A. Eremeeva, Zasch Met., 9 (1973) 743.

130. W. Machu, 3rd European Symp. on Corrosion Inhibitors, Ferrara, Italy, (1970), (1971) 107.

131. J.A. Haslegrava, D.S. Sullivan, Ausz, Eur. Patentanweld 44, (3) (1987) 2562.

132. N. Hackerman and R.M, Hurd. 1st Inter. Cong, on

Metallic Corrosion, Butterworths, London (1962) 166.

133. B. Sathianandan, K. Balakrishnan and N. Subramanyan,

Brit. Corros. J., 5 (1970) 270.

134. P.N.G. Shankar, K.I. Vasu, J. Electrochera. Soc, India 32 (1983) 47.

135. K. Shekhter, N. Lokhonya, V. Kolloot and E. Talimets, Tr. Tallin, Politekh Inst., 542 (1983) 95.

136. A.G. Alshkel, M.M. Hefny, A.R. Ismail, M.A. El-Basionny, Corros. Prev. Control, 34 (1987) 155.

137. R.L. Every and O.L. Riggs, Mat. Prot, 3 (1964) 46.

138. Z.A. lofa, V.V. Batrakov and K. Ngok Ba, Protection of Metals, 1 (1965) 44

139. N. Pebere, M. Dupratt, F. Dabosi, A. Lattes and A. De

Savingnac; J. Appl. Electrochem., 18 (1988) 225.

140. V.P. Grigorev, V.V. Kuznetsov, Zashch Metal, 3, (1967) 178.

141. A. Balezin, S.M. Belenkii, V.T. Aronson and N.M.

Belenkaya, Zashch Metal,4 (1968) 385;CA 69 (1968) 109104.

71

142. K.S. Lee, Dachan Hwakak Hwoejee, 13 (1969) 1375; CA 72,

14978y.

143. N.T. Fred, US pat, 3, 414 (1968) 521; CA 70, 31157f.

144. M. Koloblelski, US pat, 3514411 (1970); CA 73, 28076k.

145. T.N. Muzychko, S. Share and J.A. Martin, US pat, 3, 505235 (1970); CA 72, 122536 m.

146. I. Singh, A.K. Lahiri and V.A. Attekar, Proc. 5th Inter. Cong. Metallic Corrosion, Tokyo, (1972) 570.

147. G. Singh and G. Kaur. Trans. SAEST 19 (1984) 106.

148. B.A. Abdel-Nabey and El-Toukhy, Surf. Coat. Tech. 27, (1986) 325.

149. A.B. Tadros and B.A. Abdel-Nabey, J. Electroanal. Chem.

246, (1988) 433.

150. S. Hettiarchchi, Y.W. Chan, R.B. Wilson and V.S. Agarwal; Corros. Sci., 45 (1989) 30.

151. S.N. Banerjee and S. Mishra 45 (1989) 780.

152. C.R. Anderson, G.M. Schmid, Corros. Sci., 24 (1984) 325.

153. G. Subramaniam, K. Balasubramaniam and P. Shridhar, Corros. Sci., 30 (1990) 1019.

154. F. Zucchi and G. Trabanelli, Proc. 7th European Symp­osium on Corrosion inhibitors, Ferrara, Italy, (1990) 339.

155. R. Sethumadhavan, V. Murugopandran, A. Muthuson, Trans. SAEST, 26 (1991) 4.

156. S.L. Granese, B.M. Resales, C. Oviedo and J.O. Zerbino, Corros. Sci., 33 (1992) 1439.

15 7. E. Stupnisek-Lisac, M. Metikos-Hukovic, D. Lencic, J. Vorkapic-Furac and K. Berkovic, Corrosion, 48 (1992) 924.

158. E. Stupnisek-Lisac, K. Berkovic, J. Vorkapic-Furac, Corros. Sci., 28, 12 (1988) 1189.

159. S.N. Raicheva, B.V. Aleksiev and E.I. Sokolov, Corros. Sci., 34 (1993) 343.

160. M. Ajmal, A.S. Mideen and M.A. Quraishi, Corros. Sci., 36 (1994) 79.

161. S. Muralidharan, M.A. Quraishi and S.V.K. Iyer, Portg. Electrochim. Acta, 11 (1993) 255.

162. S. Muralidharan, M.A. Quraishi and S.V.K. Iyer, Proc. 184th Meeting Electrochem. Soc, 93-2 (1993) 185.

72

2.0 MATERIALS

2.1 TEST SPECIMEN

The mild steel sheet used for the investigation had the

following composition:

C Mn Si S P Fe

0.14 0.35 0.17 0.025 0.03 rest

2.2 TEST SOLUTION

A.R. grade sulphuric and hydrochloric acids were used.

Double distilled water was used to prepare all the solutions

required for the experiments. 1-2% ethanol was used to

dissolve inhibitors.

2.3 INHIBITORS USED

1. 2-aminobenzothiazole(ABT)

2. 2-amino-6-chlorobenzothiazole(ACLBT)

3. 2-aitiino-6-methoxybenzothiazole(AME0BT)

4. 2-amino-6-methylbenzothiazole(AMEBT)

5. 2-salicylideneaminobenzothiazole(SABT)

6. 2-salicylideneamino-6-chlorobenzothiazole(SACLBT)

7. 2-salicylideneamino-6-methoxybenzothiazole(SAMEOBT)

8. 2-salicylideneamino-6-raethylbenzothiazole(SAMEBT)

9. 2-amino-4-phenylthiazole(APT)

10. 2-cinnamalideneainino-4-phenylthiazole (CAPT)

11. 2-vanillideneainino-4-phenylthiazole( VAPT)

12. 2-salicylideneaminG-4-phenylthiazole(SAPT)

13. Cyclopentyl-tetrahydro-aza-thione{CPTAT)

14. Diinethyl-tetrahydro-aza-thione(DMTAT)

15. Ethylmethyl-tetrahydro-aza-thione(EMTAT)

73

2.4 SYNTHESIS OF 6-SUBSTITUTED-2-AMINOBENZOTHIAZOLES

2.4.1 PREPARATION OF p-SUBSTITUTED ARYLTHIOUREA [1,2]

An appropriate p-substituted aniline (0.1 mol) was

dissolved in a mixture of concentrated HCl (9ml) and water

(25 ml) by warming on a water bath. The solution of amine

hydrochloride thus obtained was cooled and solid

ammonium thiocyanate (0.1 mol) added. After the addition,

the reaction mixture was heated on a wate r b ath for 5

hours. There after, the reaction mixture was cooled and the

precipitated crude product was filtered, washed with water,

dried and crystallized from aqueous ethanol. p-susbstituted

arylthioureas, thus prepared are phenylthiourea :m.p.l48 C,

p-tolylthiourea :m.p.l80 C, p-anisidylthiourea : ra.p. 209 C,

p-chlorothiourea : m.p.l74 C.

2.4.2 6-SUBSTITUTED-2-AMINOBENZOTHIAZOLES

GENERAL PROCEDURE [1,3,4] (Scheme-1)

To a suspension of an arylthiourea (0.1 mol) in chlor­

oform (100 ml);bromine (0.15 mol) in chloroform (100 ml) was

added. The mixture was heated under reflux for 15 minutes

which was accompanied by the evolution of dense white fumes

of hydrogen bromide. After the reaction was over, chloroform

was distilled off and the semi-solid product, thus obtained,

was treated with sulphurous acid till the brown colour was

discharged and the solid dissolved. The solution was then

filtered from undissolved matter ammonia. The solid which

separated out was filtered, washed with water, dried and

crystallized from ethanol. 2-arainobenzothiazole :m.p.l28 C,

2-amino-6-chlorobenzothiazole : m.p.l97 C, 2-amino-6-methoxy

74

O X o

m

X I I

cr

^ o I I

cr

m X o I I

Q:

X u o

II

cr

r - i OJi m i ^ 1

2 o

X

U

UJ

u in

75

benzothiazole : m.p. 159°C. 2-ainino-6-methylbenzothiazole :

m.p. 135°C.

2.5 SYNTHESIS OF 2-SALICYLIDENEAMlNOBENZOTHIAZOLE AND ITS 6-SUBSTITUTED ANALOGUES [5] (Scheine-2)

Salicylaldehyde (0,1 mole) was added to 0.1 mole of 2-

aminobenzothiazole disssolved in 100 ml of benzene. The

solution was refliaxed on a water bath for an hour. The water

produced during the reaction was removed in a Dean-Stark

trap connected with a reaction vessel. On cooling, the solid

was obtained.

It was crystallized from ethanol. The pure 2-salicylid-

eneaminobenzothiazole, m.p. 201°C. The spectrum of this anil

gave broad peaks at 2900-3000 cm~ due to hydroxy group

and at 1635,1580 cm due to salicylaldimine group.

Similary other anils were synthesized by the condensat­

ion of the salicylaldehyde with 6-substituted benzothiazole

derivatives.

2.6 SYNTHESIS OF 2-AMINO-4-PHENYLTHIAZOLE AND ITS ANILS

2.6.1 SYNTHESIS OF 2-AMINO-4-PHENYLTHIAZOLE [6](Scheme-3)

An equimolar mixture of phenacyl bromide and thiourea

was refluxed on water bath in 50 ml of anhydrous ethanol for

5 hours. On cooling,a white precipitate was obtained. It was

washed with dilute aqueous Na^CO^ solution and crystallized

from ethanol :m.p. 125 C.

2.6.2 SYNTHESIS OF ANILS [7] (Scheme-4)

An equimolar mixture (0.05 mol)of 2-amino-4-phenylthia-

zole and suitable aldehyde was dissolved in absolute alcohol,

76

X I I

cc

^ , o II

D:

m X o II

cr

ro X u o I I

Q:

U 0 | CDl GOl

r . ^ ^

o X

o X o

+

CM

X

o X

X (J

(N I

u

UJ X

u in

77

T^:" 1 /•

fN

(/) = 0

2

CD CM

O o o

(J}l

n I

u

78

O

o

I u II

o

cr.

m

o

cr

-f fN

= <

o

X fN

u

X

o r X

o

CO

r ^

It

c

-X I I

m cr

I I

(-M

I I

^ cr

0 | ^^ 1

o II

c

I II

m cc

-m

X u o 11

fN

cr -

X o II V —

cr T— 1

o II

c ,

X o

I I

m cr

-X

I I f N

cr II

— cr

^\

u

U I u in

79

two drops of piperidine were added and refluxed on a water

bath for 2-3 hours. The reaction mixture was poured in ice

cold water, the solid thus separated was filtered and air

dried. The anils thus prepared are 2-Cinnainalideneainino-4-ph

enylthiazole :in.p.l80 C, 2-Vanillideneamino-4-phenylthiazole

:160 C, 2-Salicylideneamino-4-phenylthiazole :170°C:

[2)l] SYNTHESIS OF AZATHIONES (Scheme-5)

2.7.1 PREPARATION OF THIOCARBOHYDRAZIDE [8]

To a vigorously stirred solution of 250 grains of 100%

hydrazine hydrate(5 mol) in 150 ml of water;76 grams (1 mol)

of carbon disulphide was added dropwise. The reaction

mixture was then heated at reflux for 30 minutes. Cooled in

ice bath for 30 minutes. The precipitated thiocarbahydrazide

was filtered off, washed with ethanol and ether and air

dried and crystallized from minimum amount of water

acidified with a few drops of concentrated HCl.

2.7.2 PREPARATION OF AZATHIONES [9]

Thiocarbohydrazide (0.1 mol) was dissolved in 50 ml of

water and was added to suitable ketones (0.1 mole) in 25 ml

of ethanol. On keeping the reaction mixture over night,white

precipitates were obtained, which upon crystallization with

aqueous ethanol gave the desired compounds. Azathiones thus

prepared are Cyclopentyl-tetrahydro-azathione : m.p.lSO C,

Ethylmethyl-tetrahydro-azathione :m.p.134°C,Dimethyl-tetra-

hydro-azathione:m.p.197 C.

2.8 TECHNIQUES EMPLOYED

The experimental work was carried out with the help of

the following techniques:

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81

1. Weight Loss Method

2. Potentiostatic Polarization Technique

3. AC Impedance Technique

4. Hydrogen Permeation Technique

5. Scanning Electron Microscopy (SEM)

6. Auger Electron Spectroscopy (AES)

2.8.1 WEIGHT LOSS METHOD

Specimens of size 5 x 2 x 0.025 cm. were cut from the

mild steel sheet and mechanically polished with 1/0 to 4/0

grades of emery papers. After polishing, the specimens were

washed with distilled water and finally with acetone. The

specimens were stored in a dessicator over silica gel. The

weight of the specimen was measured before exposing it to

corrodent on a single pan electrical balance. During weight

loss experiments,the specimen were fully immersed in 100 ml,

test solution using beaker of 250 ml capacity. After a

definite exposure time,the specimen was taken out and washed

with distilled water. If there is any corrosion product on

mild steel surface it was removed from the surface by

mechanical rubbing with a rubber cork. Specimens were then

dried and loss in weight was recorded.The thermostatic water

bath was used for carrying out the weight loss experiments

at higher temperatures. Thermostat was maintained within an

accuracy of ± 2 C. The percentage inhibition efficiency and

surface coverage (©) were calculated using the following

equations.

_Uninhibited corros. rate-inhibited corros. rate ^ '^^~ Uninhibited corros.rate

82

2.8.2 POTENTIOSTATIC POLARIZATION TECHNIQUE

For potentiostatic polarization studies, working elect­

rodes Icm X 1 cm with a tag of 4 cm were cut from the mild

steel sheet and polished with 0/0 to 4/0 grade of emery-

papers. The specimens were then thoroughly washed with dist­

illed water and finally with acetone, unwanted area of the

the electrode was coated with a lacquer to get a well defin­

ed working area. The polarization was carried out using a

a potentiostat ( E.G. & G PARC 173 ), Universal Programmer

(model : 175) and X-Y recorder (RE 0089).

All the potentials were measured against a saturated

calomel electrode.The inhibition efficiency were calculated

using the following equations:

IE% = ^°Corr~ ^Corr ^ ^^^

oCorr

I _ - Corrosion current density without inhibitor. oCorr ^

Ip . = Corrosion current density with inhibitor.

2.8.3 AC IMPEDANCE TECHNIQUE

The compound SAMEBT was studied by AC impedance method.

In this technique a three compartment cell was used in

which the mild steel working electrode was kept in the

middle compartment. Platinum auxiliary and reference elect­

rodes were kept in the side compartment. The electrode was

polished and degreased before each experiment and immersed

in the test solution taken in the cell. The reference and

auxiliary (pt) electrodes were assembled and connections

were made. The block diagram of the circuit is shown in the

83

Fig.(2.0).A time interval of 10-15 minutes was given for the

O.C.P.to read a steady value. The potentiostat was set at

O.C.P. using Electrochemical interface.

Impedance measurements were carried out using Solartron

Instrument 1250 FRA in combination with 1286 Electrochemical

Interface employing a frequency of 60 kHz to mHz range. The

real part (Z') and the imaginary part (Z") of the cell impe­

dance were measured for various frequencies (60 kHz to mHz).

Plots of Z' versus Z" were made from the impedance

diagrams ( Nyquist plot ) t he charge transfer resistance

values (R, ) were determined by taking the difference in

impedance values of high and low frequency intercepts of the

real axis. I values were calculated using stearn-Geary corr ^ ^

equation.

Impedance measurements were carried out for mild steel

in IN H_SO. and IN HCl both in the absence and presence of

inhibitors.

2.8.4 HYDROGEN PERMEATION TECHNIQUE

MATERIALS AND SOLUTIONS

Mild steel specimen of the same size and composition

used for gravimetric studies, were used for permeation stud-

dies also. The specimens were mechanically polished and deg-

reased with trichloroethylene. G.R.grade NaOH and BDH grade

palladium chloride were used. Conductivity water was used

for solution preparation.

PALLADIUI-1 PLATING OF THE STEEL MEMBRANE

The steel membrane was made to function as bipolar

84

x ( t )

F.R. A

Generator

Analyser

S x ( t )

F.R.A -

WE.RE2 -

SE -

REi -

E . I -

cell

Frequency Response Analyser

working electrodes

counter electrodes

Reference electrodes

Electrochemical Interface

F;o. 2 0 Block diagram of Impedance set up

85

electrode in the cell. The anodic side of the membrane was

electroplated with a thin layer of palladium (palladised) by

the following procedure.

Specimens were polished, degreased and one si d e of the

specimen was masked with lacquer. The specimen was cathodic-

ally cleaned at the ambient temperature (35± 2 C) in the

cleaning solution containing sodium hydroxide (35 gms/litre)

and sodium carbonate (25 gms/litre) at a current density of

- 2 . . • 150 mA.cm for 5 minutes with mild steel as anode.

Specimens were gently washed in tap water and then with

double distilled water. The membrane was electroplated with

a thin layer of palladium from the solution of the following

composition.

100 ml of double distilled water was heated to 80 C, "

this 1 gm of Pdcl„ was added followed by NaNO_ , until the

PdCl reacted completely to form a yello w solution of the

complex Na [Pd(NO„) ]. This complex salt solution was added

to 1000 cc of 0.2M NaOH solution prepared from double disti­

lled water and A.R.grade NaOH pellets. At low current densi-

-2

ties 100 tJk.cm this solution gives a bright coherent colo­

ured coating of palladium for 90 minutes duration using

platinum as the anode. The specimen was washed well with

distilled water and then the lacquer was removed.

MEASUREMENT OF PERMEATION CURRENT

The palladised steel membrane was inserted in the clamp

and screwed tightly without any leakage of the solution. The

two compartment were fitted. The compartment facing palladi­

um plated side was filled with 0.2N NaOH solution which was

86

pre-electrolysed for a period of 12 hours at a current of

2.5 mA. A Hg/HgO/0.2N NaOH reference electrode and platinum

counter electrode were introduced to complete the circuit.

The cell is connected to a potentiostat (Wenking model:

POS 73) and a constant of potential of -300 mV was applied

to the specimen. It has been reported by Srinivasan et al

[10] that -300 mV is the most suitable potential for ionisi-

ing the diffused hydrogen rapidly and efficiently at pd/0.2N

NaOH interface. Water was circulated through the double wall

of the cell and the temperature was maintained at 35 ± 2 C.

The potential was maintained steady and const a nt till a

steady background current was obtained. In all these studies

a residual current of very low value was obtained. After

reaching the study background current at the anode compartm­

ent, the test solution ( acid or inhibited solution ) was

introduced into the cathode compartment and and allowed to

corrode the steel membrane. The permeation current was simu­

ltaneously recorded using X-Y-T (Rikadenki) recorder. The

complete permeation setup is shown in Fig.(2.1).

2.8.5 SCANNING ELECTRON MICROSCOPY

To study the morphology of corroded surface of the

specimen and formation of film at various stages in presence

and absence of inhibitors,Scanning Electron Microscope(SEM),

model : JEOL (JSM-35 CF) was used.

The specimens were thoroughly washed with double

distilled water before putting on the slide. The photographs

have been taken from that portion of specimen from where

better information was obtained. They were photographed at

87

r-.i] COMPLETE PERMEATION CELL SET UP

1 Steel membrane 2 Teflon bushings

3 PVC coupling

5 Counter electrode

7 Water circulation

4 Reference electrode

6 Tap

8 Anode

88

appropriate magnifications.

To understand the morphology of the steel surface in

absence and presence of inhibitors, the following cases have

been examined:

i. Polished mild steel specimens

ii. Mild steel specimens dipped in IN HCl and IN H SO

iii. Mild steel specimens dipped in IN HCl and IN H SO

containing optimum concentrations of inhibitors.

2.8.6 AUGER ELECTRON SPECTROSCOPY (AES)

Auger Electron Spectroscopy is one of the most popular

and powerful techniques to analyse the chemical species

within the outermost surface layers of the specimen. The

presence of different element species can be distinguished

by their characteristic peaks in the Auger spectra.The Auger

analysis of the samples was accomplished by using Sc a nning

Auger Microprobe (SAM), model :PH 15 90 A. A primary electron

beam of 3 Kev, a target current of 2-3)UA and a beam diameter

7000°A were used.

Specimens were treated in surface pre-treatment and

inhibitors containing solutions and then stored in a

dessicator for 3-4 days. Specimen of dimension 1 cm x 1 cm

were fixed on a sample holder with the help of a silver

paste. Specimens were then introduced into the Fast Entry

Air Lock (FEAL) chamber which is maintained at a vacuxim level

of 10 Torr, After allowing for degassing, the specimens

were transferred into the preparation chamber (which is

~9 maintained at a base pressure of (2 x 10 Torr ) , with the

help of transfer mechanism consisting of a carosel. The

89

specimen were again kept for a long enough time to

facilitate further degassing.The specimen is then introduced

into the analyser chamber. The area of interest is then

selected with the help of SEM attached to the instrument and

excited with a 3 Kev electron beam. The area analysed is 200

t-iA and depth resolution is 5-15 A .

90

REFERENCES

1. Huggerchaff, Chem. Ber., 34 (1901) 3131.

2. J.L. Wood "Organic Reactions", A. Roger (Edt.), John Wiley and Sons, New york, 3 (1959) 240.

3. C.G. Stuckwash, J. Amer. Chem. Soc, 71 (1949) 3417.

4. N.P. Kaufman and W.C.A. Ortiring, Arch. Pharm., 226 (1928) 197.

5. V.K. Mishra and D.K. Saxena, Synth. React. Met-Org. Chem. 17 (1987) 987,

6. J.M. Sprague and A.H. Land,"Heterocyclic Compounds",R.C. Elderfield (Edt.), John Wiley and Sons, New York, 5 (1957) 496.

7. B.Dash, P.K.Mahapatra, J. Indian Chem. Soc, LXII (1985) 460.

8. R.Standstrom, Acta Chem. Scand., 14 (1960) 1037.

9. R.W. Lamon, J. Org. Chem., 34 (1969) 756.

10. K.N.Sr inivasan, R.Subramanian, V.Kapali and S.V.K. Iyer , Bul l .E lec t rochem. , 2 (1986) 548.

CHAPTER - Ul

RESULTS AMD DISCUSSION

SECnOM -1

AMIM03£M20THIAZOL£S AS ACID CORROSIOM IMHIBITORS

91

Heterocyclic compounds comprise a potential class of

corrosion inhibitors. There is a wide consideration in the

literature regarding corrosion inhibition studies by nitrog­

en containing heterocyclics [1-11]. On the contrary,investi­

gations on heterocyclics bearing N and S atoms in the same

ring have received a little attention. 2-mercaptobenzothia-

zole has been reported as an effective corrosion inhibitor

for copper and its alloys in different corrosive environments

[12-15]. Singh and co-workers have studied the effect and 2-

mercaptobenzothiazole on corrosion inhibition and hydrogen

absorption in acid medium [16-17]. A few hydrazinobenzothia-

zoles were synthesized and evaluated as acid corrosion

inhibitors [18,19]. Survey of literature reveals that 2-ami-

nobenzothiazole and its 6-substituted analogues have not

been studied as acid corrosion inhibitors.

This section deals with the influence of 2-aminobenzo-

thiazole and its derivatives on corrosion of mild-steel in

IN HCl and IN H_SO.. The molecular structures and other 2 4

details of the studied inhibitors are given in Table (3.1).

The weight loss measurements were conducted in both the

acids at different temperatures (40-60 C), using 100-500 ppm

concentrations for all the inhibitors. Polarization experim­

ents have been performed at (35 ± 2 C) to understand the

behaviour of these compounds as corrosion inhibitors.Hydrog­

en permeation experiments were also carried out to study

the effect of these compounds on the permeation of hydrogen

through the steel surface. Elemental analysis by Auger Elec­

tron Spectroscopy (AES) has been carried out for ABT+HCl and

92

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93

ACLBT+H^SO^ systems to investigate the adsorption of these

inhibitors on the mild steel surface.

3.1.1 WEIGHT LOSS STUDIES

Various corrosions parameters for mild steel in IN HCl

and IN H_SO in the presence of different concentratio ns

of aminobenzothiazole and its derivatives at 40°-60°C are

summarized in Tables (3.1.1-3.1.3). It is found from these

tables that, inhibition efficiency increases with increase

of inhibitor concentrations from 100 to 500 ppm. M a ximum

inhibition efficiency is achieved by these compounds at a

concentration of 500 ppm, further increase of concentration

does not cause any appreciable change in the performance of

these inhibitors. The inhibition efficiency (IE) values of

aminobenzothiazole and its derivatives in IN HCl at 40 C

follows the order:

ABT > ACLBT > AMEOBT > AMEBT

In the case of IN H_S0. the order of corrosion inhibition 2 4

caused by different compounds for a common concentration of

400 ppm is as follows :

ACLBT > ABT >AMEOBT > AMEBT

There is a slight discrepancy in the order of inhibition by

these compounds in both the acid solutions. However at elev­

ated temperatures the order of inhibition efficiency for

different compounds is same as above.

Various corrosion parameters at 50 and 60 C are given

in Tables (3.1.2-3.1.3). Fig. (3.1,3.1.1) shows variation of

inhibition efficiencies with temperature in INHCl and IN

H-SO respectively. It is evident from these tables and

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( N i r O ' f i n r H o o ' T v o T f r o o o O O < - '

( - - ' * f o O c o r - i o i c N L n i n o w c o o OvOOiO^OO f t N C M i - t ^ ( s j ^ ^

r - < N o » ' r o o 0 ' - ' [ ^ t ^ r v i i n i n r - v o r o r o m i n i n r o o o i n v o r . r n r - t ^ v o

v o M o o i n o i O M P v i ^ p o v D i n e ^ i n r - ( f » i r o i n v o ^ i H i n o i i n ^ i H r ^ f v j

r^ t rr •«• m p o T t r c i n

E-" E-i

O O O O O CQ O O O O O iJ in 'T ro fM t-t o

<

m 0 O O 0 0 O 0 0 0 O 0 U in ' 1 ' ro (N f-1 £

{H

O 0 O O O CQ O 0 O O O U in 'T ro (N rH 2

O O 0 O O o o o o o U1 ^ fO (M I-H

96

97

100

80

60

' AO

20

0 AO

j _

50^ Temp.'c

( . 1 )

60

I E .

100

80

60 h

AO

20

0 AO 50

Tetnp C 60

F i n . 3 .1 TIIF VARTATIOn OF I t l l l i n i T I O t l F .FFICI FtJCI TS WITH Tl^riP-FRATUI'.F IM It) n C l 1, 500ppm;2,-100 p[jm;3, 300 pt'm; 4, 200 pyn

a , A i r r , h, ACI.lVl'; c , nMF.OnT; d , AME^T

98

100

» 80

60 E.

AO

20

n

-

1 1 1

\

1 1

/.o

(a)

50 Temp. C

(H)

60

I.E.

00 r

80

60

AO

20

0 AO

A.

50 Temp. C

I

60

F i g - S - I . l THE VARIATION OF INHIBITION EFFICIENCIES WITH TEMP­ERATURE IN IN SULPHURIC ACID 1 ,500 ptJin; 2 , 4 0 0 yym; 3 , 3 0 0 ppm; 4 , 2 0 0 pum.

a , ABT; b , ACLBT; c , AMEOBT; d , AflEBT

99

figures that the inhibition efficiency of all aminobenzothi-

azoles except ACLBT decreases with rise in temperature. The

compound ACLBT maintains inhibition efficiency of nearly 97%

in both the acids indicating its stronger adsorption over

the metal surface.

The higher inhibition efficiency obtained for the chlo-

roderivative may be attributed to high dipole moment of the

molecule. Smialoska et al [20] have observed similar behavi­

our of chloro substituent in the case of thiophene derivati­

ves. Recently Gad-Allah et al [21] have also reported that

chlorosubstituted aminopyrazole gives better inhibition eff­

iciency than parent compound during acid corrosion of copper

in the presence of aminopyrazoles.

Lesser inhibition shown by methoxy and methyl derivati­

ves "as compared to the parent and its chloro derivative may

be attributed to the orientation of these groups in the ring,

which may be responsible for influencing the extent of inhi­

bition. These groups might prevent a flat orientation of

molecules resulting in lesser coverage on the metal surface

and lesser inhibition. Similar explanation has been given by

Rengamani et al [22], to explain the discrepancy in order

of inhibition efficiency for isomers of anisidines, while

studying the influence of anions on the performance of anis­

idines as inhibitors for the corrosion of mild steel in

acidic solutions.

The higher inhibition efficiency for methoxy derivative

than methyl derivative can be explained in the following way.

According to Pearson's [23,24 ] classification of "HSAB"

100

principle (Hard and Soft Acid Bases) class (a) acid prefer

to bind to "hard or non-polarizable bases" and class (b)

acids prefer to bind to "soft or polarizable bases". Since

class (a) are themselves "soft" a simple rule is proposed.

Hard acids bind strongly to hard bases and soft acids bind

strongly to soft bases. In methyl derivative the inductive

effect of methyl group makes the nitrogen centre a "softer

base". According to this classification, ferrous ions are

intermediate acids,ferric ions are hard acids. So both these

ions would prefer to form a weaker bond with a soft base,

methyl compound, (AMEBT). This may lead to lesser adsorption

with methyl derivative (AMEBT) and lesser inhibition of cor­

rosion of mild steel in acids. But methoxy derivative is a

harder base as compared to methyl derivative, so it forms a

strong bond with ferrous and ferric ions which leads to more

adsorption and more inhibition.

The inhibition of corrosion by ABT and its derivatives

may be due to the adsorption of inhibitor molecules on the

metal surface. The adsorption of inhibitor molecules on the

metal surface can be explained in terms of following inter­

actions:

i. Lone pairs of electrons of N and S atoms of the thiazole

ring and the metal surface,

ii.n electrons of benzene and thiazole ring and the metal

surface.

In addition to this, in acidic solution, aminobenzothia-

zole can exist as protonated species [25]

BZNH + H" - > BZNH^

101

These protonated species can interact electrostatically with

negatively charged metal surface.

A noteworthy feature of the investigation is that all

compounds under study show more inhibition efficiency in IN

HCl than in IN H SO . This can be explained by the fact that

specific adsorption of the anions depends on the polarisabil-

ity and the size of the ion. Heavily hydrated ions of small­

er size are less specifically adsorbed than larger ions

which are devoid of hydration sheaths. It appears that the

loss of hydration sheath at least in the direction of the

metal surface is a necessary condition for specific adsorp­

tion [26]. The number of primary water molecules surrounding

each ion(s) is given below [27].

No.of primary water molecule

'^ in}

I~ 0.3

Br" 1.0

Cl~ 1.6

F~ 3.9

S 0 ~ 11.0

Solvation number or primary hydration number is found

to decrease, with increase in ionic radius, and specific

adsorption increases with ionic size and polarisability of

the anion. In this respect chloride ions are more specific­

ally adsorbed than sulphate ions. Stronger adsorption of

chloride ions on the metal surface favours more adsorption of

organic molecules as cations on the metal surface,leading to

102

enhanced inhibition. Lesser specific adsorption of sulphate

ion leads to lesser adsorption of organic molecules and less

inhibition. It is reasonable to assume that acid anions spe­

cifically adsorb on the iron surface in competition with

oriented water molecules. Specifically adsorbed anions are

known to increase synergistically the efficiency of corrosion

inhibition in acid solutions.

According to Frumkin [28] and lofa [29], the increased

adsorption of organic cations thereby more inhibition can be

attributed to changes of surface charge on the metal in the

presence of halide ions. The metal at the open circuit

potential, becomes negatively charged and cations are

attracted by electrostatic forces to the metal surface. The

extent of specific adsorption is more in the case of

chloride ion, leading to more inhibition in this case. Acco-

ording to lofa and Tomashova [30], the enhanced corrosion

inhibition in HCl may be due to the fact that anion being

specifically adsorbed, creates an excess negative charge

towards the solution phase and favours the adsorption of

of cations. Murakawa and Hackerman [31] are of the view that

the strong adsorption of organic molecule is not always a

direct combination of organic molecule with the iron

surface, but in some cases adsorption occurs, through the

already adsorbed CI or SO ions. If there are already

adsorbed chloride or sulphate ions on the surfaces they

will interfere with the adsorption of organic molecules. The

lesser interference by SO. than by CI ion emphasises the

weaker adsorbability and thereby lesser inhibition in %SOi^

solution.

103

3.1.1.1 APPLICATION OF ADSORPTION ISOTHERM

The dependance of adsorption of organic compounds on

their concentration in the case of iron has been usually

characterized by Langnuir, Temkin and Freundlisch isotherms.

In the present study, values of surface coverage ( S) were

evaluated using values of inhibition efficiency in the abs­

ence and presence of inhibitors in both the acids. Langmuir

and Temkim isotherms were tested for all the compounds under

study. Langmuir isotherm was tested by plotting C/0 vs C for

all the compounds, but a straight line relationship could

not be obtained. So none of these compounds obey Langmuir

isotherm. Temkin isotherm was tested by plotting & vs log C

for all the compounds reported in this study. Fig.(3.1.2)

show the fact that a straight line was obtained in all the

cases, thereby clearly proving the fact that the adsorption

of these compounds on the surface of mild steel obeys Temkin

adsorption isotherm [32].

3.1.2 POTENTIOSTATIC POLARIZATION STUDIES

The cinodic and cathodic polarization behaviour of mild

steel in IN H_SO and IN HCl in absence and presence of dif­

ferent concentrations of benzothiazole and its derivatives

is shown in Figs.(3.1.3-3.1.6). Table (3.1.4) gives various

corrosion parameters obtained from the polarization curves

such as corrosion current (I ), corrosion potential

^ corr'' '^

(E ) and Inhibition efficiency (IE).

It is found from this table and figures that addition

of aminobenzothiazole and its derivatives both in IN H SO

and IN HCl do not show any definite trend in the shift of

, 104

0-38

t 0.9A

e

0.86

0.78 h

Fig .3.1.2, TEMKIN ISOTHERM PLOTS FROM IN HCl AND IN SULPHURIC-ACID AT 40°C.a, IN HCl; b, IN sulphuric acid IjABT; 2j ACLBT; 3rAME0BT; 47-AMEBT

105

-398

^ - 4 9 8 -

> > E

z

o a.

- 5 9 8 -

- 6 9 8

CURRENT DENSITY, j i A . Cm"^

Fig . ' 3 . 1 .3 POTENTIOSTATIC POLARIZATION CURVES OF MILUSTEEL ia) HCl CONTAINING 2-AMirJOBENZOTHIAZOLE AT DIFFERENT

ENTRATIONS.

IN IN CONC-

-«39

CURRENT DENSITY, jj.A. Cm"^

Fig-3.1.3POTENTIOSTATIC POLARIZATION CUP.VES OF MILDSTEEL IN lU () SULPHURIC ACID CONTAINING 2-AMinOBE:;ZOTHIAZOLE AT DIFF-SULPHURIC ACID CONTAINING

ERENT CONCENTRATIONS.

106

-A02

•502

> E

o Q.

-602 -

-702

1 2 3

IN IN IN

IICI IlCl 11 CI

+ + +

300 400 500

ppm ppm ppm

ACI.iri ACl.U'l ACLBl

10'

CURRENT DENSITY, jiA. Cm-^ Fig.3,.tA POTENTIOSTATIC POLARIZATION CURVES OF MILDSTKEL /cA HCl CONTAINING 2-AMINO-6-CHLOROBENZOTHIAZOLE AT Dl

CONCENTRATIONS.

IN I:; FFERENT

-437

u

> E

-537

t -637h o CL

1 IH H^SO^ + 300 ppm ACLBT

2 IN H^SO^j + 400 ppm ACLBT

3 IN H SO j + 500 ppm ACLBT

•737 10 10*

CURRENT DENSITY, oiA. Cm"^

Fig-3-14 POTEWTIOSTATIC POLARIZATIOH CURVES OF MILDSTEEL IN l:i SULPHURIC ACID CONTAINING 2-AI-1IN0-6-CHLG?.0BE:JZ0TH1A-

ZOLE AT DIFFERENT CONCENTRATIONS. (.)

107

^ -512

>

E

z

O Q.

CURRENT DENSITY, ju.A. Cm"2

F i g - 3 . 1 . 5 POTENTIOSTATIC POLARIZATION CURVES OF MILDSTEEL IN iN (CC) !IC1 CONTAINING 2-AMINO-6-METHOXYBENZOTHIAZOLE AT D I F F ­

ERENT CONCENTRATIONS.

•437

LJ U Ul

1/1

> > E

•537

z Ui •- - 6 3 7 o Q.

-737

1 IN H^SO, + 300 ppm A!-1E0BT 2 4

2 IN H_SO. + 400 ppm Al-IEOBT

3 IN H-SO^ + 500 ppm AI-IEOBT

10 10

CURRENT DENSITY, jiA- Cm"^

Fig.3.t^ POTENTIOSTATIC POLARIZATION CURVES OF MILUSTEEL IN IN SULPHURIC ACID CONTAINING 2-Ar-lINO-6-METHOXYBENZOTHIAZOLE AT DIFFERENT CONCENTRATIONS. (t)

- 4 2 4

108

- 5 2 4

1 IN HCl + 300 ppm AMEBT 2 lU HCl + 4 00 ppm AMEBT 3 IN HCl + 500 ppm AMEBT

CURRENT DENSITY, i i A . C m " ^

F i g . 3 . 1 . ^ POTENTIOSTATIC POLARIZATION CURVES OF MILUSTEEL ItJ Irj L^\ HCl COI-ITAINIUG 2-AMINO-6-METHYLBENZOTHIAZOLE AT DIFFERENT

CONCENTRATIONS.

-417

UJ o in

> > E

z UJ o a.

-517 -

-2 CURRENT DENSITY, aiA. Cm

Fig.3.1. POTNETIOSTATIC POLARIZATION CURVES OF SULPHURIC ACID CONTAINING 2-AHINO-6-HI AT DIFFERENT CONCENTRATIONS. (t)

MILUSTEEL I:.' Hi THYLBE[JZOTHi;-.ZOLE

109

Table 3.1.4 POTENTIOSTATIC POLARIZATION PARAMETERS IN IN HYDROCHLORIC ANC

IN SULPHURIC ACIDS AT 35 ± 2°C

IN HCl IN H SO

Inhibitor E I IE E I IE corr corr corr corr

(ppm) (mV) ( A.cm" ) (%) (mV) ( A.cm" ) (%)

BLANK -576 350 -581 320

ABT

500 -560 10 97.1 -603 10 96.8

400 -570 12 96.5 -601 10 96.8 300 -560 15 95.7 -599 13 95.9 ACLBT 500 400 300

AMEOBT

500 400 300

-562 -566 -566

-592 -590 -580

15 25 50

50 80

100

95.7 92.8 85.7

85.7 77.1 71.4

-603 -601 -601

-584 -591 -586

20 25 30

60 90

120

93.7 92. 1 90.6

81.2 71.8 62.5

AMEBT

500 400 300

592 596 579

80 110 130

77.1 68.5 62.8

-593 -593 -589

110 110 200

65.5 65.6 37.5

110

the corrosion potenial. In IN H„SO , parent and chloro deri­

vative, shift the Ef,Qj.j. to more negative values indicating

their predominant cathodic behaviour while methoxy and meth­

yl derivatives do not cause appreciable change in E

value. In IN HCl methoxy and methyl derivatives shift the

E to more negative values suggesting that they are predo­

minantly cathodic in behaviour,while parent (ABT) and chloro

derivative (ACLBT) do not cause any appreciable shift of

E , indicating that they are of mixed type.

The values of corrosion current obtained by the extra­

polation method for the corrosion of mild steel in IN HCl

and IN H_SO in the absence and presence of different conc­

entrations of aminobenzothiazoles are shown in Table (3.1.4).

It is seen from this table that a corrosion current of

-2 350 / A.cm is obtained for mild steel m IN HCl. A lesser

-2 value of corrosion current of 320 MA. cm is obtained for

mild steel in IN H„SO . These observations clearly bring out

the fact that mild steel corrodes to a greater extent in

IN HCl than in IN H SO . It is found that all the compounds

bring down I values in both the acids. All the compounds

decrease I to a maximum extent at a concentration of 500 corr

ppm. It is also found that a decrease in values of I in '^^ corr

the presence of these compounds is more in IN HCl, this

clearly shows that these compounds are more effective in

inhibitors in IN HCl.

Among the aminobenzothiazole and its derivatives, the

reduction in I in both the acids follows the order: corr

ABT > ACLBT > AMEOBT > AMEBT

Ill

The values of inhibition efficiency obtained by these compo­

unds also follows the same order.

It is evident from the tables that there is a fairly

good agreement between the values of inhibition efficiency

(IE) obtained by two different methods namely weight loss

method and polarization methods.

The effect of Potassium lodide(KI) on the polarization

of mild steel in IN HCl and IN H SO containing 100 ppm of

ABT and ACLBT; 300 ppm of AMEOBT and AMEBT in combination

with 0.25% KI is shown in Figs.(3.1.7-3.1.10). Various elec­

trochemical parameters obtained from polarization curves

with KI effect are summarized in Table (3.1.5).

There is an obvious decrease in I values when a corr

co-mbination of inhibitor and KI is used.The maximum decrease

in the values of I of AMEBT and ACLBT is observed. corr

In the case of AMEBT, the I value from 200AJA is corr

brought down to 40MA in and in the case of ACLBT, from 120AJA

to 24/JA in sulphuric acid and in hydrochloric acid, in the

case of ACLBT, the values of I are brought down to 55/JA

corr ^

from 200(UA.

This significant reduction in Corrosion Current values caused by KI can be explained on the basis of synergistic mechanism [33].The synergistic model can be shown as follows:

^ H H

112

- 4 1 2

ui <_) 1/1 (A

> > E

z UI

o Q.

- 5 1 2

-612

- 7 1 2

1 IN HCl 2 IN HC 3 IN HC

10

CURRENT DENSITY, jjiA. Cm"2

F i y . 3 4 . T POTENTIOSTATIC POLARIZATION CURVES OF MILD STEEL IN IN HCl CONTAINING 2-AMINOBENZOTHIAZOLE AND K I .

1

541

-641

-741

IN HoSO, Z -t

2 IN H2SC4 + 100 ppm ABT

3 IN H2SO4

10 10' 10'

CURRENT DENSITY , / i A- cni '

F i y . 3 . 1 . 7 FOTENTIOSTATIC POLARIZATION CURVES OF MILD STEEL IN

ib) SULPHURIC ACID CONTAINING 2-AMINOBENZOTHI.AZOLE AND

113

i;i HCl IM HCl + 100 ppm ACLBT IJJ HCl + 100 ppm ACLBT + 0

CURRENT DENSITY, MA.Cm ^

Fia.3.1.8 POTENTIOSTATIC POLARIZATION CURVES OF MILD STEEL rt) IN HCl CONTAINING 2-AMINO-fi-CHLOROnENZOTHI AZOLE ,

KI

-437

-537

-637 •

-131'-

IH H.sn, 2 4

2 IN H SO, + 100 pp

3 IN H SO. + 100 ppi

10 10 10' CURRENT DENSirr.AJ A. cm'

fig-S.t. } POTENTIOSTATIC POLARIZATION CURVES OF MILD STEEL IN (j IN SULPHURIC ACID CONTAINING 2-AMINO-6-CHLOROBENZO-

THIAZOLE AND KI.

114

-394

CURRENT Df NSITY, jxA. Cm~2

F i g . 3.-].'^ POTENTIOSTATIC POLARIZATION CURVES OF MILD STEEL IN (CK_] IN HCl CONTAINING 2-AMINO-6-METHOXYBENZOTHIAZOLE AND

K I .

-52A

S -624 UJ

o

1 IN H SG j

2 IN H SO, +300 ppm AHEOBT

3 IN H^SO, + 300 ppm AMEOBT + 0.25% KI 2 4

-724 10 »'

CURRENT DENSITY,/J A. cm

Fiy.3. t .c) POTENTIOSTATIC POLARIZATION CURVES IN MILD STEEL IN /^j IN SULPHURIC ACID CONTAINING 2-AMINO-6-METHOXYBENZO-

THIAZOLE AND K I .

115

hi (_)

t/i > > E

z UJ - 6 K -

CURRENT DENSITY, i i A . Cm~2

f i y - S - I I O P O T E N T I O S T A T I C POLARIZATION CURVES OF MILD STEEL IN jc^) IN HCl CONTAINING 2 - A M I N 0 - 6 - M E T H Y L B E : J Z 0 T H I A Z 0 L E AND

K I .

•'.161-

-516

1 IN lUSO, 2 4

2 IN H.SO. + 300 ppm AMEBT 3 IN H^SO, + 300 ppm AMEBT + 0.25% KI

2 4

5 - 6 1 6 111

S a

-716 10 10'

CUHRENT DENSITY,/J A cn i '

E"i9-3.1.10 POTENTIOSTATIC POLARIZATION CURVES OF MILD STEEL IN (j IN SULPHURIC ACID CONTAINING 2-AMinO-6-HETHYLBENZO-IN SULPHURIC ACID CONTAINING

THIAZOLE AND KI.

116

Table 3.1.5

POTENTIOSTATIC POLARIZATION PARAMETERS IN IN HYDROCHLORIC AND IN SULPHURIC ACIDS AT 35 ± 2°C

IN HCl KI=0.25%

IN H^SO, 2 4

Inhibitor (ppm)

BLANK (IN HCl) ABT

100 100+KI ACLBT

100 100+KI AMEOBT

300 300+KI AMEBT

300 300+KI

E corr

(mv)

-576

-5 6 5 -556

-566 -558

-581 -571

-579 -571

I corr

{lJk.cn )

350

100 45

200 55

100 70

130 100

IE

(%)

-

71.4 87.10

42.80 84.20

71.40 80.00

62.80 71.40

E corr

(mv)

-581

-589 -579

-597 -595

-585 -571

-593 -577

corr

(MA.cm

320

45 19

120 24

120 35

200 40

IE

" ^ (%)

-

85.90 94.00

62.50 92.50

62.50 89.00

37.50 87.50

117

3.1.3 HYDROGEN PERMEATION STUDIES

A study of hydrogen permeation has been under taken

with an objective of evaluating the inhibitors with regards

to their effectiveness on the reduction of hydrogen uptake.

The results have been discussed in terms of :

1. Effect of inhibitors on reduction of Hydrogen permeation.

2. Correlation between the permeation current and corrosion

inhibition.

3. Relation between molecular structure and the extent of

permeation current.

4. Effect of anions on the permeation current.

Table 3.1.6 PARAMETERS OF HYDROGEN PERMEATION CURVES AT A CONCENTRATION

OF 500 PPM AND TEMPERATURE 35± 2°C

Inhibitor

BLANK

ABT

ACLBT

AMEOBT

IN HCl

Permeation current(MA)

24.0

8.0

9.0

11.2

Percentage reduction

66.67

62.50

53.34

IN H :

Permeation current

12.5

6.6

4.5

9.0

(/ A)

SO4

Percentage reduction

47.2

64.0

28.0

It is found from this table that permeation current

for mild steel in IN HCl is almost twice than that for IN

H_S0.. For HCl the permeation current is found to be 24 f^A

and for IN H_S0 the permeation current is found to be 12.5

MA. This fact clearly shows that CI ions are more corrosive

2- . in nature than SO ions. Lamberto et al [34] have reported

that the hydrogen embrittlement produced in IN H_SO pickl­

ing bath is less than that in IN HCl. The influence of these

118

compounds in reducing the permeation current in both the

acids is discussed below.

The permeation current versus time curves for mild steel

in IN HCl and IN H„SO in the absence and presence of

2-aminobenzothiazole and its derivatives are shown in Figs.

(3.1.11,3.1.12). All the four compounds are found to reduce

the permeation current in both acids. It is also found that

the percentage reduction in the permeation current is more

from HCl than in H„SO^ and the order of re d uction in the 2 4

permeation current exactly correlates with that of weight

loss studies.

The reduction in hydrogen intake may be due to the

formation of surface compounds on the metal surface by

adsorption thereby hindering the hydrogen permeation. The

inhibitor may retard the discharge of hydrogen ions and

lower the coverage of the metal surface by hydrogen atoms,

thereby resulting in less hydrogen permeation. The formation

of hydrides by freely dissolving metals may also create a

barrier for the transfer of hydrogen ions in to the metal.

3.1.4 AUGER ELECTRON SPECTROSCOPY

Figs. (3.1.13, 3.1.14) show Auger Electron Spectra for

mild steel surface exposed to IN HCl and IN H_SO containing

500 ppm of 2-aminobenzothiazole and 500 ppm of 2-amino-6-

chlorobenzothiazole respectively. The appearance of peaks at

154 and 387 eV supports the adsorption of 2-aminobenzothia­

zole through S and N atoms and the presence of peaks at 156,

18 7 and 387 eV respectively proves the adsorption of 2-amino-

6-chlorobenzothiazole through S, CI and N atoms.

119

12 16 20

TIME (Min)

Fig-34.1t HYDROGEN PERMEATION CURVES AT A CONCENTRATION OF 500 ppm IN IN HCl At 35+2"C

120

2U

20

E u <

=1 16

1

2

3

4

IN H2S0^

IN H2S0^ + AMEOBT

IN H2S0^ + ABT

IN H2SO4 + ACLBT

- • - • • !

-• • • 3

-> • >

12 16

TIME (Min)

20 2A 28

Fig.3.1.12. HYDROGEN PERMEATION CURVES AT A CONCENTRATION OF 500 ppm IN IN SULPHURIC ACID At 35+2°C

121

<r>

O o t o

o o

o o l O

o o i n

O o -J

o o n

O o fNt

o o

> 01

>-o cr UJ

^ u z o (T 1— ( ) I I I _ i UJ

O M

2 Qj W w o

^ r

2

^ P-i

Q S ri U H 2 •^ O

u u. o <: s t D rt K E-i p U v-3

w o

H

O tH p i O E-i tsj U 2

w o 2

W 2

D 1

• a X

z rt CN r-H CO

* • CO

Z UJ • D - D

122

til T3

123

3.1.5 CONCLUSIONS

The main conclusions drawn from the studies are :

1. Aminobenzothiazole and its derivatives have been found to

perform well in both the acids, but a better performance

is noticed in the case of HCl.

2. In sulphuric acid, aminobenzothiazole and chloro derivat­

ive behave predominantly as cathodic; methoxy and methyl

as mixed. Whereas in hydrochloric acid methoxy and methyl

derivatives behave predominantly as cathodic; parent and

chloro derivative as mixed type.

3. All the compounds reduce the hydrogen permeation current

considerably in both the acids.

4. The adsorption of compounds on the mild steel surface

from both the acids obeys Temkin's adsorption isotherm.

5. It is found from AES that the inhibitor molecules are

adsorbed on the metal surface.

stcnoM - u

A\Mas OF AMIMOBEMZOTHIAZOIES AS ACID CORROSION IMHISITORS

124

A Perusal of literature [35,36] reveals the fact that

most of the effective coiiunercial inhibitor formulations

include aldehydes and amines as their essential ingredients.

Turbina et al [37] observed that condensation products of

carbonyls and amines which are known as anils or Schiff's

bases give higher inhibition effici e ncy than that of the

constituent carbonyls and amines. Desai et al [38] have

studied a few Schiff's bases as inhibitors for the corrosion

of mild steel in hydrochloric acid. They found that the inh­

ibition efficiency of the investigated Schiff's bases is

much greater than that of corresponding amines and aldehydes.

Recently in our laboratory a few anils have been synthesized

by condensing a few substituted aminotriazoles and Salicyl-

aldehyde and all the anils were found to be very effective

acid corrosion inhibitors for mild steel [39].The inhibition

efficiency for all the compounds was found to be greater

than that for the corresponding amines and salicylaldehyde.

In the present investigation, the influence o f a few anils

synthesized in the laboratory by condensing 2-aininobenzo-

thiazole and its substituted analogues with salicylaldehyde,

has been studied as inhibitors for the corrosion of mild

steel in IN HCl and IN H„SO.. The molecular structure and 2 4

other details of anils selected for the present study are

given in the Table(3.2).

Self-corrosion studies were conducted in IN HCl and IN

H SO at different temperatures ranging from 40 -60 C, using

100-500 ppm concentrations of all the anils. Polarization

experiments were carried out at 35 ±2°C. Hydrogen permeation

125

126

experiments were also conducted to study the effect of these

compounds on permeation of hydrogen through steel. Auger

Electron Spectroscopy (AES) has been employed to investigate

the adsorption of 2-salicylideneamino-6-chlorobenzothiazole

in IN H_SO. on mild steel surface. The compound namely 2-

salicylideneamino-6-methylbenzothiazole (SAMEBT) has been

studied using AC impedance technique to investigate the

mechanism of inhibition.

3.2.1 WEIGHT LOSS STUDIES

Various corrosion parameters such as percentage inhibi­

tion efficiency(IE) and corrosion rate of anils obtained by

weight loss method at different concentrations and temperat­

ures are given in Table (3.2.1-3.2.3). It is seen that all

the anils show good inhibition of the corrosion of mild

steel for all the concentrations under study. The percentage

inhibition efficiency for all the inhibitors increases with

increasing the concentrations. All the compounds (anils)

show their maximum inhibition efficiency at 500 ppm concent­

ration in both the acid solutions.

The various corrosion parameters at 50 and 60 C are

summarized in tables (3.2.2,3.2.3). Figs.(3.2,3.2.1)show the

variation of percentage inhibition efficiency for anils with

temperature in IN HCl and IN H SO.. It is seen from these

tables and figures that the percentage inhibition efficiency

decreases with increasing the temperature in all the cases

inboththe acids except parent anil (SABT), which is quite

stable in IN H_SO. at a concentration of 500 ppm even at 60

C, indicating its stronger adsorption over the metal surface

127

in

in

O C/5

•z < • J

CQ

<

in vo ^ r~ in O f^ CO O ^

O O iH m oi

Oi ^ O O f^

oi 01 r~ in "4* O l CTi O^ <T> 0 0

VO O l VD f ~ OO

r H Cri CT> ( N C7>

O O •* 00 in (N

o o o o m (NJ

O cTi r~ CO O •H in M n o\

r- l rH M m m

(N T rn in t

CD r~- l o Tj- o C^ O^ 0> C71 C71

r~ O IN O cTv oi n •-( iH CO

fN ^ lO (TV m

o o o o

(^ m CO I f I CO !Ti Ov ' J ' n >-l

00 CTv (N i n n i-H 1-1 r\j

•<t 00 r~ o) n

in m CT\ in (N 00 00 r^ r ' vD

vD rH o in O •-I CO vo O ^

>* lO m r-< (N (N <N m •<• l O

O O O O O CD O O O O O UJ m ^ m oi rH 2:

O w (N 00 rn <N (N O ITi in

O (N •«»• in I -

'f O (N O -I

n O r- 1" O CO CO r» r» vo

r- O ' I * n «!• f <-i r- O O

r^ r^ r* f \o r j ro r^ T}* o

O O O O O O O O O O in "* m (N ^

in •<}•

ID M

fM in

(M

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O O M 0

m ' f in r

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m fS

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n in

r-t

00 r-(

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n r-(N

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n

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r- vo in 'J- (M 0^ Ol C\ ^ Ol

in <«• O •-< t

f vo in w O Cl Ol 01 01 Ol

0 o O O '-' f>- in ^ r-l Ol 01 01 Ol Ol OQ

(N

lO 01

O CO 00 lO CO lo 10 in fN Ol

n VD rH 00 IM I-l rH (N (N "T

r-j p- • * r~ M rH lo (N r- r*

^ Ifl 00 r-l ^ r-t rH

0 \o '-' O "* 01 O 00 in CO

^ r- c^ in CO H i-l

00 M t^ 10 ON CO lo r» 10 M

in 00 T-t f^ »-i tH iH (N

X

2 E-rH CD

^ < O O O O O ffl O O O O O HJ in «}• M IN iH CJ

W

E-i m o o o o o o

O O O O O W in «!• m (N rH Z

W

O O 0 0 O 03 O O O O O W in 'J- n (N rH £

U)

88888 in ' f fo (N •-<

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u CO m O O •^ O 1"^ ( ^ CTl • *

O ' - ' U^ <D <N

rH a \ (N rH i n i n 00 rH r - 00

c\ rH o r~ rn f-H <N (N VD

CTi r-l (N » t lO

C7 CD ^ CO r n (J^ CJi O^ \ ^ i n

vo O O r~ fN)

(7^ r^ CO 0 \ O CD 00 ( ^ vo n

^ r- 00 >-< vo IN VD <N CO ( N

• * • * r -

r-l rH r~ i H 00 VO O r-( VD CO

i n (N « * t r f - i M m i n r~ r~

•<«• i n (N ^o o^ n (N CO O

CTV r- VO i-l (T\ •-* (N m in in

rg i-i 'J" n in

CO O O rn i n r- r^ lO ^ n

CO m rH (N m vo vD in ^ 00

n m ("- 01 00 i n t^ cTi n i n

H a <c

H O O O O O CQ O O O O O i J

col i n >* ro CM ^ U

<: t/1

en O O O O O O o o o o o u i n «t m f\) cH s

< w

H O O O O O ffl O O O O O W i n ' f m M >-i 2

< w

o o o o o o o o o o i n ^ M (N «H

128 •-H CO \o rn «f ^ r~ cj\ t j - vo

VO r^ r-( 00 TT

n n in ID r-

r\j r^ (M N '»•

O CO M i n 00 «5 i n ' I - tN rH

r-< o ^ O •^ O > CO (N 01

00 <-< o i ^ r O 01 o n 00 O

i-C i H i - l CM

Ci] — I

O ^ i n O ru vo cji CO CO in

0 0 <N 0 0 (N CO . H r-i ^ i n

1/) r - CO 00 CO

CO (N ^ (N H CO 00 r » " t N

r o O ro > CO m r~ i n ' i - O

vo cji i-t O in •* lo O (^ '-'

r-i ru M

CO in • t ^ •>r lo \o tj- lo in

i n r~

^ >-< r» PH f\)

<N r-t ^ Ol n t ^ o i 00 r^ vD

(N in TT r-i r> VO CO VO CN <N

O ID rH » t CO ro n ko 00 ^

O i n r- 00 iH rH i n 00 i n vD

r - r~ CN r - r^ >H i-< (N

in (71 00 in rH

O o i <N VJD f n CT> CO CO r~ l o

r» c i O 00 (--

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-"<

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c/) in •* n ni r-i u be; < 2 en «; HJ CQ

E-i ID

o o o o o o O O O O O U i n •<«• M r>J i-H 2

W

H o o o o o en O O O O O U i n I - M CM r-t S

C/3

O 0 O O O O 0 O 0 O i n V n (M r-i

O r- l o O o i r^

la lO rn •-t (N ro

f n O >o <-< "-<

o i • * f " i n i n 00 00 r~ vo i n

<-( vo i n iH i-( •H r r CM vo 01

ro ^j- O O O ' f »o <ji rr CO

u o o

< W UJ hH

Q D f-

cn w O in

U 1-1 w 3

S o OS

K

<

2 O

in o Di Qi o u

w

r;

0) e 3 —

C CL

in -H

4-1 - H c £: 0) G U -H c 0 u-l u o

O ^ <^ f n CO

CO ^f r-l (M U3 c^ CT CO ^o r»i

Lfl in

in CD n in in • * 00 CO ' H lO

CTi r~ 00 CO CO (N 00 r~- a^

2

o o o o ca O O O O KJ •^ m (N r-i u

•* (Nl m n (N

O in in O O 00 (^ l o T m

n • *

fNJ

o\

<T\ r-<

r~ r H

r H

r~ CT\

ro ^o .-(

r H

.—( tN 00 r j

f i 00

(T> (N ro

o o o o

n rH 00 (N in

CO r r- 00 O lo lo ^ n n

(7^ lO t~- m (N r~ "^ 10 O • ^

CT^ i n 10 IN CO ^ i n ' T CTi (N 1-H r-i (N r^ n

129

in in

in t~-

•-' in O 00 'T in n O rH tri

n O ro 'O O •-I n lO rH

n m rH o fo n in en 00 in

>*• m O • ^ M n ^ lO O (N

I - in n CO O vo r- ic T O

in r rH 00 (N in in a O IN

r- in in in M O r r f cr.

r- O n r- c^ 00 CTv rH rvi in

o o o o o o o o M- ro (M rH

«* rn (N ^ CT>

O CO in r- flo i n "T ro rs)

00 O rH r-- tJ\ ro ro fNi O 'J"

Tt Tf 10 ro O ro ^ O •^ ro (N fN ro ro I -

O O O O O O O O O O in •* ro rsj rH

QJ 4-1 n u

w

u c •H IT (1) e 3 --

e 4-1 a 0 Q

U £1 •J -H

c a 0) C U-H c U 0

CO

o

c^ r~ r in in O Ch O rH r^

i n t t O i n r^ rH (M ^r r - 00

rH O ' - ' 00 (M

ID r» ro O ff> 00 r» lO ro i-t

(N r-

• *

00 in

r- CO If) (N m r\j I t r- vo ^

rH ^ i n ^ rM 00 r o rH O t~.

rH fSI T ' t

i n Oi O i n CO lO o \ 00 r^ ( N

CT\ ro t-H ro •JT

rH ro i n ID CO

0 \ (^ ro r o ^

rH 00 (M rH (N CO lO i n ' T CM

ro rH rH ro ^ 00 O tj> rg c~ in ro CO ro m O 00 t~ 'I' in rH i-H (N| ro "T

(7 ro 00 ro r-O ' '-' •* t-

in O in ro r-rH oj ro in >a

rH O lO CO lO

lO rH r~ O C~ 00 CO lO in ro

r^ o^ CO o (N O •>!• ^ CO

rH rH CTl r~ «*• 00 rH 'cO 00 VO

rH rH CM ro

Ol rH O rH ^

ro 00 vo m ro

ri lo in c •<}• (N ro in uD 00

ro -I 00 O fO

C vo a vo (N 00 >o t ro (N

00 r i [^ ts a> rH rv) CO r>) O in CO <^ Tf 't rH ff\ (j> r- in rH -H (N ro 'T

•X.

2 H rH m

— <

2

fH

o o o o o m o o o o o iJ in '^ M r J rH (J

< w

EH CO

O O O O O O 0 O O O 0 W in ^ ro <N rH s

M

H c o o o o m C O O O O W in ' I " ro (N rH 2;

< U)

o o o o o 0 0 O O O in TT ro (N rH

130

IE

100

80

60 f-

^0

20

0 40 50 ^

Temp. C

( a )

I

60

IE

100

80

60

AO

20

0 i.0 50

Temp. C

( b )

60

IE

100

80

60

AO

20

0 AG 5 0 ^

Temp . C

( c )

_ j

60

100

80

60

I.E AO

2

0 AO

J_

50 Temp'c

( d )

_J

50

Fig. S.*! THE VARIATION OF INHIBITION EFFICIENCIES WIT!! TEMP­ERATURE IN IN HCl 1, 500 ppm; 2, 400 ppm; 3, 300 ppm; 4 200 ppm.

a, SABT, b, SACLBT; c, 5AME0RT; d, S^MEBT

1 3 1

I.E.'

100 r

I 80

60 I.E

AG

20

0 AO

_L

(a)

50 Temp. C

( b )

_ j

60

100

. 80

60 E

AOh

20

0 AO

I 50

Temp C

(c)

60

)00

• 80

60

^0

20 f-

0 AG 5 0 ^

Temp. C

( d )

60

f i y - 3 ) . Z . 1 THF VARIATION OF INHIBITION E F F I C I E N C I E S WITH TEMP­ERATURE IN IN SULPHURIC ACID 1 , 5 0 0 ppm; 2 , 4 0 0 ppm; 3 , 3 0 0 p p n ; 4 , 2 0 0 p p n .

a , SART; b , SACLBT, c , SAMFOHT; c\ , SAriFRT

132

as compared to other anils.

This decrease in percentage inhibition efficiency indi­

cates that the inhibitor film,formed on the metal surface is

less protective in nature at higher temperatures. Most prob­

ably the desorption of the inhibitor molecule occurs at a

quicker rate from the metal surface at higher temperatures.

According to Putilova et al [40] behaviour of such inhibit­

ors can be compared to the unstable catalyst poisons where

adsorption falls appreciably with rise in temperature.

The performance of all the anils except parent anil

(SABT) has been found to be better in IN HCl. The order of

corrosion inhibition by different anils except SABT in both

the acids solutions is as follows:

SACLBT > SAMEOBT > SAMEBT

The higher inhibition efficiency of chloro derivative

may be attributed to its higher dipole moment than parent

anil (SABT). The better performance of methoxy anil compared

to methyl substituted anil can be explained on the basis of

Pearson's "HSAB" principle [23,24]. Since methoxy group can

be considered as a hard base, it forms a strong bond with

ferrous and ferric ions which leads to more adsorption and

more inhibition. In methyl substituted anil, the inductive

effect of methyl group makes nitrogen centre a softer base.

So, ferrous and ferric ions form a weak bond with soft base

methyl-anil (SAMEBT). This may lead to lesser adsorption and

lesser inhibition.

3.2.1.1 ADSORPTION ISOTHERM

The surface coverage values (6) were evaluated using

133

values of inhibition efficiency as reported earlier [33].The

surface coverage values {&) were tested graphically for

fitting a suitable adsorption isotherm. The plots of & vs

log C yields stright line in both the acids suggesting that

the adsorption of salicylideneaminobenzothiazole from both

the media on the mild steeel surface obeys "Tenkin's adsorp­

tion isotherm" which is shown in Fig.(3.2.2).

3.2.2 POTENTIOSTATIC POLARIZATION STUDIES

The anodic and cathodic polarization behaviour of mild

steel in IN H„SO. and IN HCl in the absence and presen c e of

anils at diffferent concentrations, carried out at 35 ± 2 C

are shown in Figs.(3.2.3,3.2.6). The various corrosion para­

meters such as corrosion potential (E ),corrosion current ^ ^ corr'

(I ), Tafel slopes (b and b ) and percentage inhibition corr c a

efficiency (IE) are given in Tables (3.2.4,3.2.5).

It is found from the tables that except parent anil,

other anils do not cause any appreciable change in ^rorr

values in both the acid solutions, suggesting that all the

anils (except parent) are of mixed type. Parent anil (SABT),

shifts the E to noble direction by 66 mv in IN HCl indi-corr ^

eating its predominant anodic behaviour. The anodic and cat­

hodic Tafel slope values for mild steel in IN HCl are 55 and

and 110 mV/dec respectively. Addition of anils do not change

these values significantly, indicating that all the inhibit­

ors block the active sites of the steel surface and the

mechanisms of both the anodic and cathodic reactions are not

affected. Similar behaviour has been observed during acid

134

0.98

0.96

0.92

0.88

0.8A -

0.82 J L 2.2 2.3

0.98

0.9A

f 0.86 -

0.78

0.70

0.66 - I I I

t i l l U

2./. 2.5

Log*C

( a )

2.6 2.7 2.8

2.2 - I 1 1 1 1 I I I ;

2,3 2.A 2.5 2.6 2,7 2.8 Log C

(b)

Fig.3.2,.2.TEriKIN ISOTHERM PLOTS FROM IN HCl AND IN SULPHURIC ACID AT 40°C. a, IN HC]^b, IN s u l p h u r i c ac id . 1,-SABT; 2,SACLBT; Sj-SAMEOBT; 4rSAMEBT

135

•410 1 IN HCl + 300 ppm SABT 2 IN HCl + 400 ppm SABT 3 IN HCl + 500 ppm SABT

o w> -510 M

> > E

z UJ •- -610 o 0.

-710 10

CURRENT DENSITY, OLA. Cm"^

Fig.3.1-3 POTKNTIOSTATIC POLARIZATION CUKVES OF MILDSTEEL IW IN (O.) HCl CONTAINING 2-SALICYLIDKNEAMINOBENZOTHIAZOLE HT

UIFFERENT COUCEWTKATIONS.

10*

-4M 1 IN H2S0^ + 300 ppm SABT

2 IN H^O^ + 400 ppm SABT

3 IN H2S0^ + 500 ppm SABT

-TSOL w to' to' CURRENT OeNSITY,Ai* cm'

Fiy.3.2-3 POTKNTIOSTATIC POLARIZATION CURVES OF MILDSTEEL IN iu (M SULPHURIC ACIU CONTAINING 2-SALICYLIUElJEAMINOBENZOTlliA-

ZOLE AT UIFFERENT CONCENTRATIONS.

136

-413

1 IN HCl + 300 ppm SACLBT 2 IN HCl + 400 ppm SACLBT " IN HCl + 500 ppm SACLBT

' -513

>

10

CURRENT DENSITY, i iA. Cm-^

Fig 3.2.4-POTNETIOSTATIC POLARIZATION CUKVES OF MlLUSTEEj. IN IN >C(\ HCl CONTAINING 2-SALICYLIDENEAMINO-6-CHLOKOBENZOTHIAZOLE

AT DIFFERENT CONCENTRATIONS.

•410

1 IN H^SO^ + 300 ppm SACLBT

2 IN H2S0^ + 400 ppm SACLBT

3 IN H^SO^ + 500 ppm SACLBT

10

CURRENT DENSny,/jA cm

CURVES OF MILDSTEEL IN IN •SALICYLIDENEAMINO-6-CHLORO-

BENZOTHIAZOLE AT DIFFERENT CONCENTRATIONS.

W*

f'ig6-2.4 POTENTIOSTATIC POLARIZATION n^ SULPHURIC ACID CONTAINING 2

137

-420

CURRENT DENSITY, ji.A.Cm"2 Fig.3.2^POTENTIOSTATIC POLARIZATION CURVES OF MILDSTEEL IN IN (cO HCl CONTAINING 2-SALICYLIDENEAMINO-6-METHOXYBENZOTHIA-

ZOLE A DIFFERENT CONCENTRATIONS.

-450

K-550

i - 6 »

750

1 IN H^SO^ + 300 ppm SAMEOBT

2 IN H-SO. + 400 2 4

3 IN H2S0^ + 500 ppm SAMEOBT

5 to itf lo' CURRENT DENSITY, >JA cm'

f"ig.3.'Z-5POTENTI0STATIC POLARIZATION CURVES OF MILDSTEEL IN IN (b) SULPHURIC ACID CONTAINING 2-SALICYLIDENEAMINO-6-METHOXY-

BENZOTHIAZOLE AT DIFFERENT CONCENTRATIONS.

to*

138

•430

> E

- 6 3 0 -

CURRENT DENSITY, ixA. Cm~^ F ig .3 .2 .^POTENTIOSTATIC POLARIZATION CURVES OF MILDSTEEL IN IN

HCl COHTAINItlG 2-SALICYLTI)ENEAMINO-6-METHYLBENSOTHIAZOLE AT DIFFERENT CONCENTRATluNS.

(cc)

-425r

u-525 -

1 IN H SO^ + 300 ppm SAMEBT

2 IN H SO^ + 4 00 ppm SAMEBT

3 IN H^SO^ + 500 ppm SAMEBT

CURRENT DENSirY,>jA cm

f ig .S-Z-^POTENTIOSTATIC POLARIZATION CURVES OF MILDSTEEL IN IN n,) SULPHURIC ACID CJNTAINING 2-SAL1CYLIDENEAMIHO-6-HETHYL-

BENZOTHIAZOLE AT DIFFERENT CONCENTRATIONS.

139

Table 3.2.4 POTENTIOSTATIC POLARIZATION PARAMETERS IN IN HYDROCHLORIC

ACID AT 35 ± 2 C

Inhibitor

(ppm)

BLANK

SABT

500

400

300

SACLBT

500

400

300

SAMEOBT

500

400

300

SAMEBT

500

400

300

E corr (mV)

-576

-510

-574

-574

-563

-567

-563

-574

-576

-576

-568

-574

-570

I corr_ {fJk.cm }

350

10

30

35

7

10

12

10

10

13

12

15

20

Tafelslop (mV /dec. be 1

110

105

105

100

120

120

106

98

98

96

98

98

100

-es ) ba

55

45

45

40

45

42

45

45

45

46

45 _

48

50

I.E

(%)

97. 14

91.42

90.00

98.00

97. 14

96.57

97.14

97. 14

96.28

96.57

95.71

94.28

140

Table 3.2.5 POTENTIOSTATIC POLARIZATION PARAMETERS IN IN SULPHURIC ACID

AT 35 ± 2°C

Inhibitor

(ppm)

BLANK

SABT

500

400

300

SACLBT

500

400

300

SAMEOBT

500

400

300

SAMEBT

500

400

300

E corr (mV)

-581

-576

-572

-574

-568

-568

-570

-588

-588

-586

-591

-591

-581

I corr_2

(MA.cm )

320

12

18

20

13

20

25

20

20

30

20

35

70

Tafe; (mV be

100

96

92

90

104

100

90

95

92

90

98

95

95

Islcces dec". ) I ba

50

40

45

48

44

42

45

40

1 3

48

46 _

44

45

I.E

(%)

96.25

94.37

93.75

95.93

93 . 75

92.18

93.75

93.75

90.62

93.75

89.06

78.12

141

corrosion of iron and steel in presence of aliphatic sulphi­

des [41], thioureas [42] and thiosemicarbazide [43].

The values of corrosion current (I ) obtained by the corr ^

extrapolation method for the corrosion of mild steel in IN

HCl and IN H„SO in absence and presence of different anils

clearly bring out the fact that the addition of anils bring

down the values of I significantly at all the concentra-corr ^ -

tions. The maximum reduction of I values occurs at 500 corr

ppm concentration for each anil. It is also found that a decrease in the value of I in the presence of these anils

corr ^

is more in IN HCl ( except for parent anil, SABT ). These

observations clearly show that all the anils except parent anil are more effective inhibitors in IN HCl. Among the various anils except parent anil examined, the reduction in

I in both the acids follows the order : corr

SACLBT > SAMEOBT > SAMEBT

The values of inhibition efficiency obtained by weight loss

and polarization methods follow the same order as above.

The inhibition of corrosion of mild steel in acidic

solutions by the anils can be explained on the basis of

adsorption. These anils can adsorb on to the metal surface

due to following interactions :

i- Lone pairs of electrons of nitrogen and sulphur atoms can

interact with the positively charged metal surface {^^."i.

ii.n- electrons of azomethine group (-C=N-), benzothiazole

and benzene being can also interact with positively

charged metal surface [45].

iii.In aqueous acid solutions, benzothiazole gets protonated

142

at ring nitrogen. These protonated species can adsorb

on the negatively charged metal surface through electro­

static interactions.

3.2.3 AUGER ELECTRON SPECTROSCOPY

The Auger Electron Spectrum for mild steel in IN H.SO

containing 2-salicylideneamino-5-chlorobenzothiazole at a

concentration of 500 ppm in IN H^SO is shown in Fig.(3.2.7).

The appearance of peaks for N, S and CI atoms at 385,154 and

184 eV respectively supports the adsorption of this anil

over the metal surface.

A noteworthy feature of the investigation is that all

the anils (except parent anil, SABT) give better performance

in IN HCl than in IN H SO . This can be explained from the

fact that chloride ions being less hydrated than sulphate

ions, get strongly adsorbed on the metal surface and create

an excess negative charge towards the solution phase and

favour cooperative adsorption (synergistic) of anils on the

metal surface [30]. Similar synergistic mechanism has been

proposed by Granese et al [3] to explain the higher efficie­

ncy of some nitrogen containing heterocycles in acidic solu­

tion.

There is a fairly good agreement between the values of

inhibition efficiency obtained by weight loss method and

polarization method.

3.2.4 HYDROGEN PERMEATION STUDIES

Hydrogen permeation current versus time curves for mild

steel in IN HCl and IN H^SO. in absence and presence of =00

ppm of each anil are shown in Figs.(3.2.8,3.2.9 ) . The values

143

8

8

> a> ^

_ > O o

2 ° > * UJ

z UJ

z o

8 ^ UJ _ j

UJ

8

-8 ro

-8 I N

_8

ti- 1 o z

o la u u !2 tf Ifi td fH cr: <: P-

c 2- W H

• o <

ri H i-* g X a: S ^ "^ O I

N a< o z J fJ y D H CO Ul 5 O

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144

25

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TIME (Min)

IN HCl

IN HCl + S^MEBT

IN HCl + SAMEOBT

IN HCl + SABT

IN HCl + SACLBT

20 25 30

Fig.3.2.8HYDROGEN PERMEATION CURVES AT A CONCENTRATION OF 50C ppm IN IN HCl At 35+2°C

145

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IN

12 16 TIME(Min.)

H2SO4

HjSO^ + SAMEBT

H2S0^ + SAMEOBT

H2SO4 + SACLBT

H2SO4 + SABT

20 U 26

Fig .3.'2;5 HYDROGEN PERMEATION CURVES AT A CONCENTRATION OF 500 ppm IN IN SULPHURIC ACID At 35+2°C

146

of hydrogen permeation current for different anils are given

in Table (3.2.6). All the four anils reduce the permeation

current in both the acids. The extent of reduction is consi­

derably higher in IN HCl than in IN H-SO (except parent

anil, SABT), the percentage reduction in permeation current

in both the acids follows the order :

SACLBT > SAMEOBT > SAMEBT

Table 3.2.6

PARAMETERS OF HYDROGEN PERMEATION CURVES AT A CONCENTRATION

OF 500 PPM AND TEMPERATURE 35 ± 2°C.

IN HCl IN H.,S0 2 4

Inhibitor Permeation Percentage Permeation Percentage current (pA) reduction current (/JA) reduction

BLANK SABT SACLBT SAMEOBT SAMEBT

24.0 10.5 9.0 12.5 16.0

56.6 62.5 47.9 33.3

12.5 4.5 7.0 8.0 9.5

64 44 36 24

The reduction in permeation of hydrogen in presence of

these anils can be explained due to

i. Formation of surface compounds on the metal surface by

adsorption.

ii.The inhibitors may retard the discharge of hydrogen ions

and lower the coverage of metal surface by hydrogen atoms.

iii.The formation of hydrides by freely dissolving metals

may also create a barrier for the transfer of hydrogen

ions into the metal.

Fairly good correlation has been observed between order

of inhibition efficiency and reduction in permeation current

by these anils in both the acids.

147

3.2.5 ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY

One of the anils (SAMEBT) has been investigated by AC

impedance technique.

Impedance diagrams obtained for the frequency range 60

kHz to 1 mHz at the open circuit potential for mild steel in

both the acids are shown in Fig.(3.2.10). It is seen from

these figures that impedance diagrams are not a perfect sem­

icircle and the difference has been attributed to frequency

dispersion [46,47]. The charge transfer resistance R values

are calculated from the differences in impedance at lower

and higher frequencies as suggested by Tsuru and Haruyama

[48].These values of R have been substituted in Stern-Geary

equation to obtain the corrosion current. To obtain the dou­

ble layer capacitance (C,,), the frequency at which the

imaginary component of impedance is maximum (-Z ) is

found. max 2TIC,,R. dl t

Table 3.2.7

IMPEDANCE PARAMETERS IN IN HCL AND IN

SAMEBT

(ppm)

BLANK

500

400

300

IN HCl

R. I C,, R. t corr dl t -2 -2 -2 (ohm cm ) (mA cm )(F cm )(ohm err

17.0 0.94 765 13.0

143.0 0.09 222 137.0

133.0 0.10 280 105.0

121.0 0.11 343 84.0

H SO^ OF SAMEBT

IN H-SO, 2 4 I corr

-2 -2 1 )(mA cm

1.11

0.11

0.13

0.16

C ., al

)(Fcn~^)

366

185

2G0

255

Table (3.2.7) gives values of R^, 1^^^^ and C^^for nild

steel in IN HCl and IN H2S0^ alone and in the presence of

different concentrations of SAMEBT. It is seen from mis

148

Z {ohm cm' in IN h y d r o c h l o r i c a c i d .

(CO IN HCl + 400 ppni

20 40

risi3-l.te-

,;)

Dedance

IN HjSC,

60 80 100

Z'lohm cm'^)

if IN s u l p h u r i c a c i d , s ^ e c t r u n in_].N s n i p ^ ^^^ ^^.^^^.^

b + 400

IN H^SO^

oan SA-' IN H^SO,, 50 0

149

table that I and C,, values are more for HCl than H_SO., ^ w J. 1. \JL ^ T

The presence of SAMEBT is found to enhance the R values in

both the acids. But C,, values are brought down considerably

in both the acids, because of its stronger adsorption on the

metal surface, which in turn leads to a decrease in the

values of I corr

3.11 CONCLUSIONS

1. These anils have been found to perform well as corrosion

inhibitors in both sulphuric and hydrochloric acids but a

better performance is noticed in the case of hydrochloric

acid.

2. The mechansim of the inhibition of corrosion of mild

steel in the presence of these inhibitors in both IN HCl

and IN H_SO. is under mixed control except the parent co­

mpound which behaves predominantly as anodic.

3. All these anils have been found inhibit the corrosion of

mild steel in acidic solutions by getting adsorbed on the

active sites of the steel surface through lone pair of

electrons, n-electrons of the benzothiazole ring and

azomethine (-C=N-) group. Except parent compound showed

predominantly anodic behaviour.

4. All the anils bring down the permeation current consider­

ably in both the acids.

5. The adsorption of anils on the mild steel surface from

both the acids obey Temkin's adsorption isotherm.

SECriOM -

AMIMOf>HEMYir}-IIAZOlE AMD ITS AMILS AS ACID CORROSION INHIBITORS

150

In view of the reported high performance of the conden­

sation products of amines and aldehydes [37-39, 49] a few

anils have been synthesized by condensing 2-amino-4-phenyl-

thiazole (APT) and different aldehydes such as cinnamalde-

hyde, vanillin and salicylaldehyde. The influence of these

anils on the corrosion of mild steel in IN H^SO, and IN 2 4

HCl has been investigated by weight loss determinations,

electrochemical methods and surface characterization by Sca-

anning Electron Microscope and Auger Electron Spectroscopy

of APT. The performance of the anils as corrosion inhibitors

was compared with that for APT. Molecular structures and

other details of the inhibitors selected for the present

investigations are given in Table (3.3).

The important structural feature of all these compounds

is that they contain thiazole heterocyclic ring with isolat­

ed benzene ring at 4th position of the thiazole ring. The

inhibitors studied in the earlier section (I) possess benze­

ne ring fused with thiazole ring.

The weight loss experiments were conducted using IN HCl

and IN H„SO at different temperatures ranging from 40 -60 C

using different concentrations of all the inhibitors. Polar­

ization experiments were carried out at 35 ± 2 C . The Auger

Electron Spectroscopy (AES) has also been used to examine

the adsorption of aminophenylthiazole on the mild stesi

surface in IN HCl.

3.3.1 WEIGHT LOSS STUDIES

The various corrosion parameters obtained from weighr

loss method, such as percentage inhibition efficiency (I.E,,

151

z o >-< o > ^ u '^ a D CD CD <

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l/l

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152

corrosion rate for mild steel in IN HCl and IN H„SO in the 2 4

absence and presence of 2-ainino-4-phenylthiazole (APT) and

different anils at different concentrations and temperatures

40 -60 C are summarized in Table (3.3.1-3.3.4). It is seen

from these tables that the inhibition efficiency for all the

compounds increases with increase in concentration. APT

gives maximum value of efficiency at 500 ppm concentration

and all the anils show their maximum value of efficiency at

300 ppm. All the anils give high protection towards corro­

sion of mild steel ( 90%) even at concentrations as low as

25 ppm in both the acidic solutions.

The various corrosion parameters at 50 and 60 C are

summarized in Tables (3.3.1-3.3.4) and Figs.(3.3,3.3.1) show

variation of inhibition efficiency with temperature from 40

to 60°

It is also observed from these tables and figures

that inhibition efficiency for all the compounds decreases

with increase in temperatures from 40 -60 C in both the

acidic solutions except CAPT, indicating its stronger

adsorption over the metal surface. This decrease in percent­

age inhibition efficiency indicates that the inhibitor film

formed on the metal surface is less protective in nature, at

higher temperature. Most probably the desorption of the

inhibitor molecules occurs at a faster rate from the meta

surface at higher temperatures. According to Putilova, the

behaviour of such inhibitors can be compared to the unstable

catalyst poisons whose adsorption falls appreciably with

rise in temperature [40].

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60

Fi i .S-S'^^^ variation of inhibition efficiencies with temperature in 1, 500 ppm; 2, 400 ppm, 3, 1, 300 "ppm; 2, 200 ppm, 3,

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IN sulphuric acid. For APT : 1, 500 tjjm,- 2, 400 ppn, ' ppm; 4, 200 ppm. For Anils : ppn; 4, 50 ppm. a, APT; b, CAPT; c, VAPT; d, SAPT

60

3, 300 300 ppm; 2, 200 ppn, 3, 100

159

The inhibition efficiency values for APT and different

anils in IN HCl and IN H^SO^ at 40°C follows the order

CAPT > VAPT > SAPT > APT

The excellent performance of CAPT in both the acids can be

explained in the following way :

i. It possesses additional conjugate n-electrons through

which it interacts with the metal surface. This favours

more adsorption of this compound on the metal surface

leading to more inhibition,

ii. Lone pair of electrons present on nitrogen and sulphur

atoms may also lead to more interaction with t he metal

surface leading to more adsorption, thereby more inhibi­

tion.

VAPT gives better inhibition efficiency than that for

SAPT because it contains -OCH_ group which is absent in the

case of SAPT. Methoxy group makes the compound as a hard

base which forms a strong bond with ferrous and ferric ions

and gets strongly adsorbed on the metal surface leading to a

high value of inhibition efficiency.

SAPT inhibits the corrosion of mild steel in both the

acids due to the following interactions with the metal

surface,

i. n-electrons of the benzene, thiazole ring and azoraethine

(C=N-) group.

ii.Lone pair of electrons of nitrogen and sulphur atoms of

the thiazole ring.

iii.The electron releasing inductive effecr of -OH group

attached with benzene ring of this compound also leads

160

to adsorption and thereby the inhibition of corrosion.

The presence of an extra benzene ring and -OH group in

SAPT makes it more effective in corrosion inhibition than

APT. The absence of methoxy group in this compound makes it

less inhibitive than VAPT.

The adsorption of APT on the metal surface occurs due

to the following factors.

i.n-electrons in the benzene ring and thiazole ring inter­

acts with the positively charged metal surface leading to

adsorption and inhibition.

ii.Lone pair of electrons of the nitrogen and sulphur atoms

also leads to the interaction with the positively charged

metal surface, which causes the adsorption of the compound

on the metal surface and inhibition,

iii. -NH_ group of this compound can form protonated species

in acidic solutions which interacts with the negatively

charged metal surface, and leads to adsorption and corro­

sion inhibition.

iv.The absence of extra benzene ring, -OH group and -CCH

group makes it less effective than othe r compounds -jnder

study.

The better performance of 2-amino-4-phenylthiazole APT)

in hydrochloric acid than in sulphuric acid can be explained

by similar reasonings as discussed in section (II) for anin-

obenzothiazoles. These anils can also get protonared at

nitrogen atoms of the thiazole ring [25]. These prctcna^sd

anils can electrostatically interact with negatively cnargsd

161

metal surface, leading to their its adsorption on the metal

surface. The higher values of inhibition efficiency for

anils in IN HCl than in IN H SO can also be explained on

the basis of similar reasonings.

3.3.1.1 ADSORPTION ISOTHERMS

The plots of surface coverage (©) versus logC for all

the inhibitors at 40 C are shown in Fig.(3.3.2). Almost a

straight line is obtained indicating that all the compounds

follow Temkin's adsorption isotherm.

3.3.2 POLARIZATION STUDIES

The anodic and cathodic polarization behaviour for mild

steel in IN HCl and IN H_SO in the absence and presence of

2-amino-4-pheny 1 thiazole (APT) at different concentrations;

and for different anils at optimum concentrations(300 ppm)is

shown in Figs.(3.3.3,3.3.4). Various corrosion parameters

such as corrosion potential (E ),corrosion current(I ) ^ ^ corr' ' ^ corr'

and percentage inhibition efficiency (IE) of APT and differ­

ent anils are given in Tables (3.3.5).

It is found from these tables that APT and different

anils do not cause any appreciable change in E values in

both the acid solutions, thereby suggesting that all t he

inhibitors are of mixed type.

It is also observed that APT causes maximum decrease in I at a concentration of 500 ppm. In case of anils the corr • ^^

maximum reduction of I values occurs at a concentration corr

of 300 ppm.

The effect of KI on the polarization of mild steel in

IN H SO and IN HCl containing 300 ppm of the 2-amino-4-

0.99 r

0.37 -

4 0.95 -

e 0-93

a9I -

0.89

0.87

085

1 6 2

- J I 1 < i L . J L.

lA 2.6 2.8

\.t. 1.6 t.8 2,0 22 IM 2.6 2.8

logC

(b)

Fig.3.3.2, TEMKIN ISOTHERM PLOTS FROM IN HCl AND IN SULP^'URIC ACID AT 40°C. a, IN HCl; b, IN Sulphuric acid 1,-CAPT; 2r-VAPT; 3jSAPT; 4, APT

163

- 4 2 0

-5J0 -

1-620 -

O a.

-720 to' 10

CURRENT DENSITY >j A cm'

Fig.33.3PO'rENTIOSTATIC POLARIZATION CUKVES OF MILDSTEEL IH liJ (a_) HCl CONTAINING 2-AHINO-4-PHENYLTH1AZOLE AT UlEEEHEirr

CONCENTRATIONS.

- 4 0 9

-509

> E

-609

-7O9L-

1

2

3

4

IN

IN

IN

IN

H.SO. 2 4 H.SO,

2 4 H-SO, 2 4 H^SO^

300 APT

W 10" W CURRE DENSITY / l A c m '

Fiu,3.i-3POTENTIOSTATIC POLARIZATION CURVES OF MILDSTEEL IN lU o) SULPHURIC ACID CONTAINING 2-AMINO-4-PHENYLTHIAZOLE AT

DIFFERENT CONCENTRATIONS.

164

-410

1 IN HCl 2 IN HCl + SAPT 3 IN HCl + VAPT 4 IN HCl + CAPT

-510

-610

-710

CURRENT DENSITY, >jA cm

F i g . 3 3 . 4 POTENTIOSTATIC POLARIZATION CURVES OF MILDSTEEL IN IN /^^ HCl CONTAINING VARIOUS ANILS OF 2-AMINO-4-PHENYLTHIAZOLE ^ '' AT 500 ppm CONCENTRATION.

-*u

« - 5 U -

z

2 - tu -

-7U

-

-

1

2

3

4

IN H-SO. 2 4 IN H^SO. + SAPT 2 4 IN H2S0^ + CAPT

IN H2S0^ + VAPT

1

>o\3

iS\

10 10 K) CURRENT DENSITY, >j A c n i '

Fig.3.3.4 POTENTIOSTATIC POLARIZATION CURVES OF MILDSTEEL IW IN (j SULPHURIC ACID CONTAINING VARIOUS ANILS OF 2-AMINO-4-

PHENYLTHIAZOLE AT 500 ppm CONCENTRATION.

10

165

T a b l e 3 . 3 . 5

POTENTIOSTATIC POLARIZATION PARAMETERS IN IN HYDROCHLORIC AND IN SULPHURIC ACIDS AT 35 ± 2*C

IN HCl KI=0.25%

IN H2S0^

I n h i b i t o r E (ppin) c o r r

(mv)

c o r r

(A^A.cm )

IE

(%)

c o r r

(mv)

c o r r IE

(/JA.cm ^) (%)

BLANK (IN HCl) APT

500 400 300 CAPT

500 400 300 VAPT

500 400 300 SAPT

500 400 300 APT

300 300+KI

-576

-580 -578 -578

-570 -573 -570

-569 -571 -571

-576 -572 -573

-578 -574

350

70 110 130

120 130 170

160 170 180

170 200 200

130 45

-

80.0 68.57 62.85

65.71 62.85 51.42

54.28 51.42 48.57

51.42 42.85 42.85

62.85 87.14

-581

-575 -577 -579

-569 -576 -576

-582 -585 -579

-582 -585 -589

-579 -569

320

100 110 130

100 120 150

130 150 170

160 •190 250

130 30

-

71.87 65.62 59.37

68.75 62.50 53.12

59.37 53.12 46.87

50.00 40.62 21.87

59.37 90.62

166

- *o»

-5oe -

•-u»

10' » ' CURRENT DENSITY ,/J A cm'

Fig .3 , .3 . rPOTENTIOSTATIC POLARIZATION CURVES OF MILDSTEEL IN IN (a)' ' ' HCl CONTAINING 2-AMINO-4-PHENYLTHIAZOLE AND K I .

-*15

1 IN H2S0^

2 IN H2S0^ + 300 ppm APT

Itf 10' CURRENT DENSITY ./u A. cn i '

Fig-SSsfPOTENTIOSTATIC POLARIZATION CURVES OF MILDSTEEL IN (b) SULPHURIC ACID CONTAINING 2-AMINO-4-PHENYLTHIAZOLE AND

IN KI.

167

phenylthiazole (APT) is shown in Fig.(3.3.5). Various electr­

ochemical parameters obtained from polarization curves are

sumiT.arized in Table (3.3.5). It can be clearly seen fron the

table that the values of I have been brouaht down from corr

- 2 - 2 . - 2 . 130 MA.CM t o 30 /^A.cm m IN H_SO. and t o 45 /Jk.cm i n A 4

in IN HCl, when a combination ot 300 ppm APT and 0,25% KI is

used. This significant decrease in I value caused by KI

can be explained on thebasisof synergistic mechansin as

discussed in section (I). The synergistic model can be shown

as follows:

/ \ H H

<s:

© 0/" C H

- y ^ 6 0/

^ H H

> ^ 0 0/'

AT) I-- N---<-, J H H

3.3.3 SCANNIr G ELECTRON MICROSCOPY (SEM)

To ascertain the inhibition of corrosion of mild steel

in IN HCl by APT and different anils, SEM photographs were

taken in absence and presence of these compounds Fig.(3.3.6)

for mild containing 500 ppm of APT ar.- 300 ppm of each anil.

It is found from these figures that the inhibition of corro­

sion of mild steel in the acidic solution by r.hese compounds

is due to the formation of a surface film on the metal surf­

ace which acts as a barrier, preventing the corrosive attack

bv the acid on steel surface.

168

r-^i-p:

't-»

'i

x600 0007 ISkV 50»Jm

(oO

^mi^

Fiy- JS-6 SURFACE STUDY OF MILD STEEL (MS) BY SEM

a, Polished MS; b, MS in IN Hcl; c, MS in IN HCl+SOOppm CAPT; d, MS in IN Hcl+300 ppm VAPT; e, MS in IN HCl + 500 ppm. APT.

169

'§

8

> a

in tii

O

U z tn

Di

H

• a

Sit!

8 ro

O O CM

8

z o K H U w w o

c o in

bu O

Z O H

< :

£->

z

D J < H 2

<

c

z K

a I

I o z H

I

u X

(0 en

E a a

z UJ

170

3 . 3 . 4 AUGER ELECTRON SPECTROSCOPY

The Auger Electron Spectrum of APT in IN HCl is shown

in Fig.(3.3.7).The appearance of peaks of S and N at 156 and

390 eV respectively confirms the adsorption of this compound

through S and N atoms on the metal surface.

3.3.5 CONCLUSIONS

1. The 2-amino-4-phenylthiazole (APT) and different anils

have been found to perform well in both sulphuric and

hydrochloric acids. The inhibition efficiency of all the

anils have been found to be better than APT.

2. The performance of all the inhibitors has been found to

be better in hydrochloric acid.

3. The mechanism of the inhibition of the corrosion of mild

steel in presence of these compounds in both IN HCl and

IN H_SO. is found to be under mixed control.

4. All these compounds have been found to inhibit the

corrosion of mild steel in acidic solution by getting

adsorbed on the metal surface through lone pair of

electrons of N and S atoms,n -electrons and the protonat-

ed species.

5. The adsorption of APT and different anils on the mild

steel surface from both the acids obey Temkin's adsorpt­

ion isotherm.

SECnOM - lY

AZATHIOMcS AS ACID CORROSION IMHISITORS

171

Ayres [50] have reported that compounds containing

atoms of both nitrogen and sulphur in their molecule give

good inhibition efficiency. This work has aroused our inter­

est to synthesize some azathiones which contain both N and

S atoms in the same molecule. In this section the influence

of azathiones on the corrosion of mild steel in IN H„SO. and 2 4

IN HCl has been investigated. The molecular structures and

other details of azathiones selected for the present invest­

igations are given in Table (3.4).

3.4.1 WEIGHT LOSS STUDIES

The corrosion rate and the percentage of inhibition

efficiency (IE), obtained from weight lossnethod at differ­

ent concentrations, at a temperature of 35 ± 2 C in IN HCl

and IN H^SO are given in Table (3.4.1). From the weight

loss values, the inhibition efficiency and surface coverage

for each concentration were calculated. All the inhibitors

gave their maximum efficiency at a concentration of 500 ppm.

A further increase in the concentration of the compounds

does not cause any change in their performance. It is found

from these tables that inhibition efficiency increases with

increase in the concentration of azathiones. It is also

observed that all the azathiones inhibit the corrosion

in both the acid solutions. The inhibition efficiency values

for azathiones follows the order :

CPTAT > EMTAT > DMTAT

The difference in the order of their inhibitive action

can be explained on the basis of their molecular area.

Trabanelli [51] has reported that inhibition efficiency can

1 7 2

z o

a

y-

< \— CL O

(— < h-V

Q

o

CO

rH < <

cr

o o

<

z

z — z

^ > Ln

Z — 2 X X

CO

Ixl

o

1

_J >-h-

z UJ 0.

o _J

o >

1

< N <

1

o (r o > X < cr V-LL)

u

en

z o

( J

U z o

1

- J >-X H-

LU J

Q

< N <

O Q: Q >-I < X V-LJJ I—

<

UJ

-U1

to

z

X LP

o

UJ

J > X H-

UJ y

1

_ j

>-X h-UJ

o X \—

< Nl <

1

o cr Q >-X < Q: 1— UJ \—

u

+1

CO

M Q

CO U

W O w x: O

H -- K ••

3

o a u,

u

w

< a, 2 o CO o cr K: O u

01

M oV>

- 0 0

• H U QJ

a c 0 H

w l-l Q)

E E H-)

to w 0

^ •u

x: C T ' -

-^ cp 0) E

S —

. ^ e y-/ a 0 a

— •

C M 0 U

• H 0 4J 4-1

m -^ ^J XJ ^ - H

c r QJ C U - H

c 0 H-l

U 0

tNJ 00

U3

O rg

a

n in 00 n vo • * 00 (N

ui r~ O in

O (N •* <N

CTi <Nj CT> m r~ r~ in «t

VD VO CTi O rH O (N O

m O o '-' r-l (N fM TT

o o o o o o o o m •<«• m ( N CiJ

O --I o> o .-H •!}• CO r-

O j-i en m

n ^ CO ^

<N r~ r~ r-( vo in 'T Tf

O n in CO •H p» VD (M

r~ O r- <N r^ en m •^

< O O H o o s n (N Q

173

vo CT\ r^ a^ (N CO rn (N

i-i CM rr vo

O t7\ rj- (M

00 r-l lo CT> m in «* n

<N (N a\ in n r- ID CO

O 't CO n n m m f

O O O O O O O O in »}• n (N

O

fl f-l

• i r

> n . t

V t-t 12 ITJ

H

(N) + 1 i n M

w M M Q

D (-> U\

W M U W 3

O 0 J JZ

E-i n

X • • O M T3 U 0 3 - H

Ui

S 0) O Ou X u. c

0 W - H OS W

u u E-> Q)

w e 2; s

a < a

z o M

cn O a: « o u

0) JJ (TJ l-l

c 0

• H

(/) 0

u u 0

u

CiJ

« M

w w 0

,-{

•p

x: cr

• H 0)

3

14-1

0

c 0

• H *J

fl

^ >. a E

E > — •

v-».

^0

' ^

> - K

a E

E a a

• * — '

w IJ 0 4J • H

U XI p '( C £ QJ

u c 0 u

c H

14-1

0

m

<• CO y j 00 ^ CO <7 f*l

(N fM CM m

CO ' ^ CO ^ (N i n O c^

ro f o ^ ^

r - (N vD O i n rvj .-1 r -

n ' T i n lO

(N O <r rn

(N oi 00 in CO r* r~ (^

O O

r- 'f vo I" r-l in (71 (N

n in in CO

U

z • J ra

rH (NJ n O

VD "T O •'I' r» (^ r 1X3

r- vo (^ Tt vO O CT^ vo

r~ CT\ r-( VO rt -H r\) <M

f-< H 0< U

H <

O 0 O 0 t^ 0 0 0 OZ i n ^ f i (N UJ

H <

O O O O h O 0 O O X i n ' f m fNj Q

O O O O 0 0 O O in ^ ro rsl

O (N •* (N

^ CTl ( N r- l r - i£i ID i n

(N ^ CO CO (N r- r~ O

CTv (N r- vD r-" (M r\J ro

174

be increased by increasing the molecular area of the compou­

nd. Since CPTAT has more molecular area than other azathion-

es, it gives highest inhibition efficiency among the studied

azathiones. Increased area causes larger coverage of metal

surface and this leads to higher inhibition efficiency. As

azathiones contain both nitrogen and sulphur atoms they

exhibited satisfactory performance as inhibitors for the

corrosion of mild steel in IN HCl and IN H SO .

3.4.1.1 ADSORPTION ISOTHERMS

Surface coverage(S) values have been obtained from

weight loss measurements for various concentrations of azat­

hiones. It is found from Fig.(3.4) that a plot of (©) versus

logC give a straight line in both the acids suggesting that

adsorption of azathiones on mild steel/acidic solution

interface obeys Temkin's adsorption isotherm.

3.4.2 POLARIZATION STUDIES

The Potentiostatic Polarization studies were carried

out at 35 ±2 C to study the anodic and cathodic polarization

behaviour of mild steel in IN H_SO. and IN HCl in the 2 4

absence and presence of azathiones at a concentration of 500

ppm, and are shown in Fig.(3.4.1). Various corrosion parame­

ters such as corrosion potential (E ), corrosion current ^ ^ corr^'

(I ) and percentage inhibition efficiency (IE) in IN H_SO. corr f ^ 1 \ I 2 4 and IN HCl are. given in Table (3.4.2). E values do not ^ ^ ' corr

change appreciably in the presence of azathiones suggesting

that these compounds are of mixed type and inhibit the corr­

osion of mild steel in IN HCl and IN H SO^ controlling both

the anodic and cathodic reactions.

0.90 175

f 0-80

e

0.70

060

050 -

0.80 r

f 0.70-

e

0.60

0.50

0.A0

J L -I 1 L 2.2 2.3 2.4 2.5 2.6 2.7

Log. C

Ca)

I \ . L_u. J I I u

2.A 2.5 2.6 2.7 Log. C •

(b)

2.8

F i g . 3 - 4 TEMKIN ISOTHERM PLOTS FROM IN HCl AND IN SULPHURIC ACID AT 35+2-C. a . I N HCIJ b , IN s u l p h u r i c a c i d . 1,-CPTAT; 2rEMTAT; 3jDMTAT

176

> e

i 5

-711

CURRENT DENSITY,>uA cm*

Fig.3.4.1 POTENTIOSTATIC POLARIZATION CURVES OF MILDSTEEL IN IN /() HCl CONTAINING VARIOUS AZATHIONES AT 50 0 ppm CONCENTRA­

TIONS.

-413

I n - 5 1 3

> E

< z o a

- 6 1 3 -

-713

1

2

3

4

IN

IN

IN

IN

H.SO, 2 4 H-SO. + DMTAT 2 4 H-SO. + EMTAT

H-SO. + CPTAT 2 4

1

3 , , . , — •

• — — " T

i ^ ' ^ ^ ^ ^ ^

w 10 K) n ' CURRENT DENSITY, >iA cm'

Fig.3.4-1 POTENTIOSTATIC POLARIZATION CURVES OF MILDSTEEL IN IN /n SULPHURIC ACID CONTAINING VARIOUS AZATHIONES AT 500 ppm ^ ^ CONCENTRATION.

177

Table 3.4.2 POTENTIOSTATIC POLARIZATION PARAMETERS IN IN HYDROCHLORIC AND

IN SULPHURIC ACIDS AT 35 ± 2°C

Inhibitor

(ppm)

BLANK

CPTAT

500

400

300

EMTAT

500

400

300

DMTAT

500

400

300

E corr (mV)

-576

-588

-582

-582

-583

-582

-576

-578

-580

-570

IN HCl

I corr_2

(/JA.cm )

350

150

185

250

170

220

310

170

224

313

IE

(%)

57.14

47.14

28.57

51.42

37.14

11.42

51.42 -

3 6.00

10.57

IN

E corr (mV)

-581

-573

-575

-575

-583

-577

-581

-590

-581

-585

H2SO,

I corr _^ (/uA.cm )

320

47

100

115

55

99

114

117

130

170

IE

(%)

85.3

68.7

64.0

82.8

69.0

64.3

63.4

59.3

46.8

178

It is evident from the Table (3.4.2) that azathiones

bring down I values in both the acids. Phis behaviour ^ corr

indicates that azathiones act as inhibitors for the corros­

ion of mild steel in both the acids. Among the various

azathiones examined, the decrease in I in Doth the acids corr

follows the order :

CPTAT > EMTAT > DMTAT

The plausible mechanism of corrosion inhibition of mild

steel in IN HCl and IN H_SO can be explained on the basis

of adsorption. In acidic solutions azathiones can exist as

cationic species, similar to other amino compounds [52].

Thes cationic species may adsorb on the cathodic sites of

the mild steel surface and decrease the evolution of

hydrogen. The adsorption of molecules of the azathiones on

the anodic sites through lone pairs of electrons of nitrogen

and sulphur may decrease anodic dissolution of mild steel.

A characteristic feature of the present investigations

is that azathiones give better performance in IN HCl than in

IN H SO . This can be explained on the basis of synergistic

mechanism, [53] according to whi -h the chloride ions and mo­

lecules of azathiones can jointly adsorb on the steel surfa­

ce giving higher inhibition efficiency.The synergistic model 0 0

can he shown as follows: /j0(-, -[N.SHHCS]

/

/^c i q-H H

/

A /

/ / /

0 0 Cl- [NtSH.tCj

H

•x-y

0 0/ -—Cr- H \ H

^ 0 0

179

Rengamani et ai [54] have proposed a similar synergistic

model to explain the higher efficiencies of some nitrogen

containing compounds in hydrochloric acid solutions.

CONCLUSIONS

1. Azathiones act as mixed type of inhibitors for the corro­

sion of mild steel in IN HCl and IN H_SO..

2. The presence of lone pair of electrons on the nitrogen

and sulphur atoms of the inhibitor molecules and the

formation of cations of azathiones in acid solutions

favours more adsorption on the mild steel surface which

leads to more inhibition.

3. The adsorption of azathiones on the mild steel surface

from both the acids obeys Temkin's adsorption isotherm.

180

REFERENCES

1. M.M. Osman, E. Khamis and A. Michael, Corros. prevent, and control (1994) 60.

2. E. Stupnisek-Lisac and M. Metikos-Hukovic, Br. Corros. J., 28 (1993) 74.

3. S.L. Granese, B.M. Resales, C. Oviedo and J.O. Zerbino, Corros.Sci., 33 (1992) 1439.

4. F. Zucchi and G. Trabanelli, Proc. 7th Eur. Symp. Corros. inhibitors Annali Univ., Ferrara, Italy N. 3. Sez. V. Suppl. (1990) 339.

5. G. Subramaniam, K. Balasubramanium and P. Shridhar Corros. Sci. 30 (1990) 1019.

6. S.N. Banerjee and S. Mishra, Corrosion, 45 (1989) 780.

7. A.B. Tadros and B.A. Abd-El-Nabey, J. Electroanal. Chem., 246 (1988) 433.

8. B.A. Abd-El-Nabey and A. EL-Toukhy, M. El-Gamal and G. Mahgoob. Surf.& Coat. Technol., 27 (1986) 325.

9. G. Kaur, Ph.D. thesis, Delhi Univ., India (1984).

10. E. Stupnisek-Lisac, M. Metikos-Hukovic, D. Lencic, J. Vorkapic-Furac and K. Berkovic, Corrosion, 48 (1992) 924.

11. S. Hettiarachchi, Y.W. Chan, R.B. Wilson Jr. and V.S. Agarwala, Corrosion, 45 (1989) 30.

12. T. Kobayashi, Corros., Eng., 13 (1964) 246.

13. V.P. Dobrovolskaya, V.P. Barannik and T.G. Nezmahova, Ivz. Khim. Tech., 9 (1966) 339.

14. N.K. Patel, M.M. Patel and K.C. Patel, Chem. Era, 10 (1975) 24.

15. N.K. Patel, M.M. Patel and S.H. Mehta, Chem. Era, 12 (1976) 46.

16. I. Singh and T. Banerjee, Corros. Sci., 12 (1972) 503.

17. I. Singh, A. Laniri and V. Altekar, Proc. 5th Inter. Cong. Met. Corros.,Tokyo, Japan (1972) 570.

18. M.A. Wajid Khan, M.Phil, dissertation, Aligarh Muslim University, Aligarh, India (1992).

19. M. Ajmal, A.S. Mideen and M.A. Quraishi, Corros. Sci., 36 (1994) 79.

181

20. Z. Szklarska- Smialowska, M. Kaminsko, Proc. 5th Inter. Cong, on Met. Corros. Tokyo, Japan (1974) 555,

21. A.G. Gad-Allah, M.M. Abou-Ronia and H.H. Rehan, Ind. J. Technol., 30 (1992) 252.

22. S. Rangamani, S. Muralidhran, M. Ganesan and S.V.K. Iyer, Ind.J. Technol., 1 (1994) 168.

23. R.G. Pearson, J. Am. Chem. Soc, 85 (1963) 3533.

24. R.G. Pearson, J. Chem. Edu., 45 (1968) 581 and 643.

25. R.K. Bansal, " Heterocyclic Chemistry ", Wiley Eastern Ltd., New Delhi (1990) 345.

26. M.A.V. Devanathan, Trans. Far Soc, 50 (1954) 373.

27. T.N. Anderson and JO'M. Bockris, Electrochim. Acta, 9 (1964) 347.

28. A.N. Frumkin, Vestn. Mosk. Gos. Univ., 9 (1952) 37.

29. Z.A. lofa, Vestn. Mosk. Gos. Univ., 12 (1956) 139.

30. Z.A. lofa and G.N. Tomashova, Zh. Fig. Khim., 34 (1960) 1036.

31. T. Murakawa and N. Hackerman, Corros. Sci., 4 (1964) 387.

32. S. Rengamani, S. Muralidharan, M. Anbukulanathainathan and S.V.K. Iyer, J. Appl. Electrochem., 24 (1994) 355.

33. S. Syed Azim, S. Muralidharan and S.V.K. Iyer, J. Appl. Electrochem., (in Press).

34. A. Lamberto, R. Felipe and Abersturi, "Steel Embrittlem-ent in Sulphuric Acid Pickling", Inst. Hierro Acero, 15 (1962) 226.

35. A. Cizek, Mat. Perform., 33 (1994) 56.

36. G. Schmitt, Br. Corros. J. 19 (1984) 165.

37. E.G. Turbina, N.E. Bredkhina, V.V. Pikulev and T.R. Chelyabinsk, Politekh. Inst., 91 (1971) 16.

38. M.N. Desai, M.B. Desai, C.B. Shah and S.M. Desai, Corros. Sci., 26 (1986) 827.

39. S. Muralidharan, M.A. Quraishi and S.V.K. Iyer, Proc. 184th Meeting Electrochem. Soc, 93-2 (1993) 185.

182

40. I.N. Putilova, S.A. Balezin and V.P. Barannik, "Metallic Corrosion Inhibitors " , Pergamon Press, New York, (1960) 31.

41. H. Brandt, M. Fischer and K. Schwabe, Corros. Sci., 10 (1970)631.

42. R. Agarwal, T.K.G. Namboodhiri, Corros. Sci., 1 (1988) 37.

43. B.A. Abdel-El-Nabey, E.Khamis and M.A.E. Shaban ,Surf. & Coat Technol., 28 (1986) 67.

44a.N. Hackerman and A.C. Makrides, Ind. Eng. Chem., 46 (1954) 523.

44b.H. Kaesche and N. Hackerman, J. Electrochem. Soc, 105 (1958) 192.

45a.H. Tanamura, 0. Ikeda and K. Kataoka, Soviet Electrochen. , 14 (1978) 605.

45b.V.M. Gerovich, R.I. Kaganovich and E. Protokaya, Sovie* Electrochem., 15 (1979) 91.

46. K. Juttner, Electrochem. Acta, 35 (1990) 1501.

47. F. Mansfeld, Corrosion, 37 (1981) 301 / 38 (1982) 570.

48. T. Tsuru and S. Haruyama, Boshoku Gijutsu, J. Japan Soc. Corros.Eng., 27 (1978) 573.

49. M.A. Quraishi, M.A. Wajid Khan, M. Ajmal, S. Muralidharan and S.V.K. Iyer, Proc. 185th Meeting Electrochem. Soc, 94-1 (1994) 76.

50 J.A. Ayre, Corrosion Aspects of Reactor Decontamination and Corrosion of Reactor Materials, Inter Atomic Agenc/, Wien (1962) 199.

51. G. Trabanelli, Corrosion/89 (1989) 133.

52. G. Banerjee and S.N. Malhotra, Corrosion, 48 (1992) 14.

53. W.J. Lorenz, Z. Phys. Chim., 65 (1970) 244.

54. S. Rengaraani, S. Muralidharan, M. Ganesan and S.V.K. Iyer, Indian J. Chem. Technol., 1 (1994) 168.

SUMMARY iii

The work embodied in the present thesis deals with the

study of some nitrogen and sulphur containing heterocyclic

compounds as corrosion inhibitors for mild steel in IN HCl

and IN H SO . The performance of these compounds as corrosi­

on inhibitors has been investigated using the Weight Loss

studies and Potentiostatic Polarization technique.

Hydrogen Permeation, AC Impedance techniques; Auger

Electron Spectroscopy and Scanning Electron Microscopy have

also been used to examine the inhibitive performance of some

selected compounds.

The compounds examined in the present investigations are

listed in Table (1-4). Their inhibiting action has been

discussed in the following four series.

1. 2-aminobenzothiazole and its substituted analogues.

2. 2-salicylideneaminobenzothiazole and its substituted

analogues.

3. 2-amino-4-phenylthiazole and its anil derivatives.

4. Azathiones.

The results of the present investigations reveal the

fact that aminobenzothiazole and its derivatives inhibit the

corrosion of mild steel effectively in both the acid

solutions at all the studied concentrations (100-500 ppm).

Maximum value of inhibition efficiency is achieved at a

concentration of 500 ppm. The order of inhibition efficiency

has been found as follows :

ACLBT > ABT > AMEOBT > AMEBT

The higher inhibition efficiency of ACLBT may be

attributed to its high dipole moment. The better performance

of methoxy derivative as compared to methyl derivative has

Tab ] cj

SI No N A M f

2 - AMINO BENZOTHIAZOLE

2 - A M I N O - 6 - CHLORO

B E N Z O T H I A Z O L E

2 - AMINO - 6 - METHYL -

B E N Z O T H I A Z O L E

2 - A M I N O - 6 - METHOXY

B E N Z O T H I A Z O L E

SI.No N A M E

2- S A L I C Y L I D E N E AMINO

B E N Z O T H I A Z O L E

2 - S A L I C Y L I D E N E A M I N O -

B-CHLORO- BENZOTHIAZOLE

s T p J c 1 u (' r

^^ NH-

^ NH2

H^CO

^ NH-j

X V

ABBREVIATION USED

A B T

A C L B T

A M E B T

AMEOBT

T a b l e Z

S T R U C T URE ABBREVIATION

USED

OH

V N = C H - ( ^ \ \ SABT

2-SALICYLIDENE AMINO-

6-METHYL-BENZOTHIAZOLE

2-SAL i CYLIDENE AMINO-

G-METHOXY- BENZOTHIAZOLE

Cl'

OH

V N ' - C H ^ ^ SACLBT

OH

" V N ^ C H Y / \ \ SAMEBT

H3CO

OH

> N : C H ^ " A S A M E O B T

T a b i c 3

SI No I (' U C I U R L ABBRC VIATION

USED

2 - A M I N O - L - P H E N Y L

THIAZOLE

'ly

iL. A P T - 5 - NH^

10 2 - C I N N A M A L I D E N E AMINO-

U- P H E N Y L - THIAZOLE

C A PT

11

12

2 - V A N I L L I D E N E AMINO

U- P H E N Y L - T H I A Z O L E

• N ^ A

OCH3

:A».CH/\ VAPT

OH

2-SALICYLIDENE AMINO

^ - P H E N Y L - TH IAZOLE

OH

T a b l e i!

SI No. N A M E S T R U C T U R E ABBREVIATION

USED

13 CYCLO PENTYL -

TETRAHYDRO - AZ A - T H i O N E

1^ DIMETHYL -

TETRAHYDRO- A Z A - T H I O N E

15 ETHYL - METHYL-

TETRAHYDRO - AZ- ' H I O N E

HN X H

1 HN NH

11 S

HN ' ' ^ NH 1 1

HN ^..^--NH

S

Hc,C2 CH3

HN NH 1 1

HN NH

Y c

CPTAT

DMTAT

EDTAT

vx

been explained on the basis of Pearson's HSAB principle.

The inhibition of corrosion by 2-aininobenzothiazole and

its derivatives may be explained on the basis of adsorption

of these compounds on the metal surface in terms of the

following interactions, a) lone pair of electrons of N and

S atoms of the benzothiazole ring can interact with metal

surface; b) 7T~electrons of benzothiazole ring can interact

with positively charged metal surface; c) protonated amino-

benzothiazoles can also interact, with negatively charged

metal surface.

All the anils derived by the condensation of salicylal-

dehyde and aminobenzothiazoles are found to give better per­

formance than the corresponding amines. Their enhanced inh­

ibition efficiency may be attributed to the presence of an

additional7T-bond of the azomethine group, 7T-electrons of

the benzene ring and an electron releasing -OH group.

The inhibition efficiency values of 2-amino-4- phenylth-

iazole and its condensation products, anils in acidic

solutions follow the order :

CAPT > VAPT > SAPT > APT

The higher value of inhibition efficiency of SAPT than

APT can be explained due to the presence of an additional

azomethine double bond, benzene ring and an electron relea­

sing -OH group. VAPT gives better inhibition efficiency than

SAPT because it contains an additional -OCH_ group. The

highest inhibition value obtained by CAPT can be explained

due to the presence of an additional 7T-bond in its molecules

which is absent in the case of VAPT, hence it gives the best

performance among the investigated anils.

VIX

The inhibition efficiency of azathiones follow the order:

CPTAT > EMTAT > DMTAT

The highest inhibition efficiency achieved by CPTAT

among azathianes has been explained on the basis of its

larger molecular area.

The order of performance of various classes of organic

compounds examined in the present investigations is :

Anils > Amines > Azathiones.

The lower inhibition efficiency of azathiones can be

explained due to the absence of aromatic character in

these compounds.

All the compounds investigated have shown good

inhibition efficiency at the studied temperatures ranging

from 40 to 60 C. The inhibition efficiency of the

investigated compounds decreased at lower concentrations

except ACLBT and all the anils which showed nearly 90%

inhibition efficiency even at 60 C. All the inhibitors

are found to obey Temkin's adsorption isotherm.

The Potentiostatic Polarization studies were carried out

at 35±2 C. The polarization behaviour of different series of

compounds in both hydrochloric and sulphuric acids were

studied. All the compounds are found to be mixed inhibitors

except aminobenzothiazoles which are predominantly cathodic

and SABT predominantly anodic in hydrochloric acid.

The interesting feature of the investigation is that the

inhibition efficiency of all the amino compounds (ABT and

APTs) enhanced significantly on the addition of Potassium

Iodide (KI). This has been explained on the basis of the

VXIX

synergistic model given below : H

^ H H

0 © /H I Q-r

H H

- < -

< : • ! :

0 0 /"

H H

All the investigated coinpounds are found to reduce the

perineation of hydrogen through steel surface effectively in

both the acid solutions.

The decrease in double layer capacitance values as

evident from AC impedance study in presence of SAMEBT

supports the adsorption of SAMEBT inhibitor on the steel

surface. The results of Auger Electron Spectroscopy studies

show that the adsorption of heterocyclic compounds on the

metal surface occurs through N and S atoms. The better

appearance of mild steel surface in inhibited acid solutions

than in plain acid solutions as evident from Scanning

Electron Microscopic (SEM) studies, further supports the

fact that inhibitor molecules are adsorbed over the steel

surface and prevent the attack of corrosive solution on the

surface.

IX

CONCLUSIONS

All the compounds studied perform well as inhibitors in

hydrochloric acid and in sulphuric acid. They are found

to be more effective in hydrochloric acid than in

sulphuric acid.

All the compounds examined are found to be mixed

inhibitors in both the acids but aminobenzothiazoles and

its derivatives show predominantly cathodic behaviour

in hydrochloric acid where as the salicylideneamino-

benzothiazole shows predominantly anodic behaviour.

A good correlation has been observed in the values of

inhibition efficiency among the different techniques

adopted for the investigations.

A fairly good agreement is observed between the

corrosion inhibition by the compound and the reduction

in hydrogen permeation.

Anils of aminophenylthiazole has shown nearly 90%

inhibition efficiency at the concentrations even as low

as 211 ppm.

All the Amino compounds are found to exhibit synergistic

action in presence of potassium Iodide.

2-Araino-6-chlorobenzothiazole and anils of 2-Ainino-4-

phenylthiazole are found to show good inhibition effici­

ency even at a temperature of 60 C.