corrosion protection of mild steel in cooling water...

CORROSION PROTECTION OF MILD STEEL IN COOLING WATER

SYSTEMS WITH GREEN POLYMERIC INHIBITOR, TANNIC ACID

İpek Öztürk1, Esma Sezer

2

1 Kurita Turkey Kimya A.Ş., Tugayyolu Cad. Ofisim İstanbul Plazaları No:20 B Blok Kat 4

Daire 23 Maltepe Istanbul/Turkey, [email protected]

2 Istanbul Technical University, İTÜ Ayazağa Kampüsü 34469 Maslak Istanbul/Turkey,

[email protected]

Abstract

Tannin is one of the promising green inhibitor from natural polymers due to its natural and renewable

source and it has high biodegradability under aerobic and anaerobic conditions, which makes it safe

for disposal compared to inorganic corrosion inhibitors such as chromates, nitrites, zinc salts or oxides.

The present work is designed to investigate and gain further understanding of the inhibition

mechanism of tannin as green inhibitor for the corrosion of mild steel in acidic media (0.5 M H2SO4

and 1 M HCl solutions), decarbonized and sea water via electrochemical techniques.

In the first part, acidic medias are examined in absence of and with varying amount of tannin in order

to observe the optimum dosage of the inhibitor and adsorption isotherm plots were investigated in

order to gain information about corrosion protection mechanism.

In the second part, decarbonised and sea water are examined in absence and presence of tannin at

varying pH values in order to observe the optimum dosage of the inhibitor. Thin Layer

Choromotography (TLC) was conducted for the characterization of solutions and to examine the

structure effective for the corrosion inhibition.

In the last part, pilot plant studies, simulating closed cooling systems have been carried out to

investigate the behaviour of the tannin for corrosion inhibition under practice-like conditions.

Keywords : Mild steel, inhibitor, tannic acid, corrosion inhibition, electrochemistry

1. INTRODUCTION

Due to stringent environmental regulations and as well as human safety, inorganic corrosion

inhibitors such as chromates, nitrites, polyphosphates, zinc salts or oxides incorporated in

protective coatings for mild steel are being replaced by the so-called "Green inhibitors"

[1,2,3]. Tannin is one of the promising green inhibitor from natural polymers. Not only

investigation of corrosion protection of one of the most widely used material mild steel at

some severe corrosive media with "Green inhibitors" but also commercially availability of

these inhibitors are one of the current interests of both corrosion protection and environment

authorities and all the world for sustainability of the natural resources.

One of the perfectly meeting the criteria of green chemistry is Tannic acid because its source

is natural and renewable. It possesses a high LD (lethal dose) factor and was found to be

nontoxic in animals. Tannic acid is reported to have high biodegradability under aerobic and

anaerobic conditions, which makes it safe for disposal [4].

Tannic acid is commercial form of hydrolysable tannin. It is a polymer of gallic acid

molecules and glucose. The pure form of tannic acid is contained in roots, husks, galls and

leaves of plants. It is also found in bark of trees (oak, walnut, pine, mahagony), in tea, nettle,

wood, berries and horse chestnuts. Tannic acid has a stringent, antibacterial, antiviral and

antienzymatic properties. Tannic acid is used in tanning of leather, staining wood, a mordant

for cellulose fibres, dyeing cloth, disinfectant cleansers, pharmaceutical industry, food

additives, metal corrosion resistance as rust convertor, slime treatment of petroleum drilling,

paper, ink production and oil industry [5]. The use of tannins in corrosion protection has been

disclosed since 1936 in Great Britain patent no. GB450547. Development and industrial uses

of tannins as green corrosion inhibitors in the formulations of pigments, chemical cleaning

agents for removing iron-based deposits and oxygen scavengers for boiler water treatment

system has been reviewed [6].

The weak acidity of tannic acid is due to multiple phenol groups in its structure. It is a yellow

to light brown amorphous powder and is highly soluble in water [4]. Tannic acid is used as

conversion coating to prevent corrosion of iron, zinc, aluminium, copper and their alloys. The

(ortho) hydroxyls react with metals forming metal-tannic acid complexes, which protect metal

from rusting [7]. Due to the OH− groups in the ortho position on the aromatic rings, tannins

are able to form chelates with iron and other metallic cations (e.g., copper). When Fe3+ ions

react with OH− groups in the orthoposition in aerated aqueous solution, a highly insoluble

and blue-black complex (ferric tannate) is formed [8]. As tannins contain polyphenolic

moieties and these moieties have the ability to form tanninate salts with ferric ions, the

corrosion inhibition of tannins is due to the formation of a highly cross-lined network of ferric

tanninate salts that protect the metal surface [7]. Additionally, tannic acid was proven to be an

efficient scale inhibitor squeeze treatment [4].

While green inhibitors from natural polymers are serving a great alternative to existing

corrosion inhibitors, the biodegradability of them limits the storage and long-term usage of

natural polymers like tannin which is also the focus of this thesis. Therefore, the subject of the

thesis is the investigation of green corrosion inhibitors for mild steel in sea water,

decarbonized water and acidic media and study the commercially availability of the possible

mild steel corrosion inhibitors for cooling systems.

One of the corrosion inhibitor compounds, selected for the study is based on tannic acid and

the generic structure of the compound is presented in Figure 1.1.

Figure 1.1 : Structure of tannic acid [5]

Due to the OH− groups in the ortho position on the aromatic rings, tannins are able to form

chelates with iron and other metallic cations (e.g., copper). When Fe3+

ions react with OH−

groups in the orthoposition in aerated aqueous solution, a highly insoluble and blue-black

complex (ferric tannate, Figure 1.2) is formed [8].

Figure 1.2 : Structure of ferric tannate [9]

In this study, it is aimed to define a novel, environmentally friendly, effective and

commercially available corrosion inhibitor formulation for mild steel corrosion protection in

acidic, decarbonized and sea water. For this purpose, electrochemical methods like

polarization and impedance measurements is used and effectiveness with and without

inhibitors is evaluated. Additionally, ZSimpWin program is used to analyze impedance data

and form equivalent circuit models.

2. Experimental procedure

For electrochemical measurements, a glass cell of capacity 30 mL was used, which contained

three electrodes; steel as working, platinum as counter and silver/silver chloride (Ag/AgCl) as

reference electrodes.

The chemical composition of commercially mild steel metal for working electrode with

exposed area of 0.5 cm2 is as follows (percentage by weight): C=0.35, Mn=0.65, Si=0.25,

P=0.035, S=0.035 and Fe to 100.

The measurements were carried out in different aerated solution qualities: 0.5 M H2SO4 and 1

M HCl solutions, decarbonised water and sea water. The solutions were freshly prepared from

analytical grade chemical reagents supplied from Merck using distilled water and used

without further purification.

All the tests were carried out at ambient temperature (25 0C), the solutions being in contact

with air.

2.1 Decarbonised Water Preparation

The decarbonised water advantageously is produced in deionised water by addition of two

basic solutions.

Decarbonised Water I

In 1 L deionised water:

17.64 g CaCl2 · 2H2O

16.24 g MgCl2 · 6H2O

92.00 g NaCl are dissolved.

Decarbonised Water II

In 1 L deionised water:

6.72 g NaHCO3 is dissolved.

By filling-up 5 ml decarbonised water I and 5 ml decarbonised water II with deionised water

to 1000 ml, 1l decarbonised water is obtained

Table 2.1: Chemical composition of the decarbonized water used during the experiments

The choice of this medium was based upon the following criteria:

(i) its’ low electrical conductivity is close to that encountered in natural waters,

(ii) its’ corrosivity is fairly high and

(iii) it is an easily reproducible baseline solution [10].

2.2 Acidic Water Preparation

About 0.5 M H2SO4 solution was prepared by dilution of 98% H2SO4 (Merck), 1 M HCl

solution was prepared by dilution of 37% HCl (Merck) using distilled water.

2.3 Sea Water Preparation

Through the experiments, artificial seawater is prepared [11]. For each run, a freshly prepared

solution was used. The approximate pH value of this formulation is 8 and conductivity is

60.000 µs/cm.

3. Methods

Because corrosion occurs via electrochemical reactions, electrochemical techniques are ideal

for the study of the corrosion processes [12]. Electrochemical measurements, including

potentiodynamic polarization curves, and electrochemical impedance spectroscopy (EIS),

were performed in a three-electrode cell using Gamry Reference 600 Model Potentiostat with

Ion Ca2+

Mg2+

Cl- HCO3

- pH Conductivity (µs/cm)

Concentration

in mg.L-1

24 9.7 350 24.4 7.0 1,100 -1,200

a software version 5.3. The potentiodynamic current−potential curves were measured at a

scan rate of 1 mV/s.

Tafel extrapolation were carried by using PowerCorr software and chi-square value adjusted

as approx.1 in order to obtain consistency between the results, minimize the error percentage

and standardize the extrapolation range.

Impedance measurements will be carried out using AC signals of amplitude of ±10 mV (peak

to peak) at open circuit potential in the frequency range from 10 mHz to 1 MHz. Prior to the

potential sweep, the electrode was left under open-circuit in the respective solution for ~1 h

until a steady free corrosion potential was recorded. The above procedures were repeated two

times with success for each water quality and pH value.

Through the study, ZSimpWin program is used to analyze impedance data and form

equivalent circuit models. TLC (Thin Layer Chromatography) was also conducted for the

characterization of solutions and to be able to examine the structure effective for the corrosion

inhibition. Additionally, adsorption isotherm plots were observed by using EIS data at

different media to define the adsorption mechanism of the inhibitor.

In the last part, pilot plant studies, simulating closed cooling systems have been carried out to

investigate the behaviour of the tannin for corrosion inhibition under practice-like conditions.

The inhibition efficiency (IE%) was calculated both from polarization and EIS measurements

as given below with Equation 4.1.

100)(100)(% xRpinh

RpcorrRpinhx

Icorr

IinhIcorrIE

(3.1)

4. Results and Discussions

4.1 Measurements in 0.5 M H2SO4 solution

In this part of the work, corrosion phenomena of carbon steel is examined by polarization

curves and EIS measurements in absence and presence of varying amount of tannic acid in

0.5 M H2SO4 solution in order to observe the effect of chemical structure and dosage amount

at corrosion inhibition of carbon steel in H2SO4 solution.

4.1.1 Measurements in the Absence and Presence of Inhibitor in 0.5 M H2SO4 solution

Figure 4.1 reports the Nyquist diagrams plotted at the corrosion potential for the different

inhibitor dosage values in 0.5 M H2SO4 solution in the absence and presence of tannic acid.

The impedance diagrams are characterized by a single time constant (a single loop). High

frequency intercept of semi-circle on the real axis yields the solution resistance (Rs) and low

frequency region yield the sum of Rs and polarization resistance (Rp). Rp values obtained from

Nyquist diagram were summarized in Table 4.1. The semicircles are generally associated with

the relaxation of the capacitors of electrical double layers with their diameters representing

the charge transfer resistance. Loop size of in case of 200 ppm is larger than the other dosage

values, which means highest polarization resistance and less corrosion rates.

The corresponding Ecorr, Icorr, anodic Tafel slopes (βa) and cathodic Tafel slopes (βc) at

different dosage values were summarized in Table 4.1.

Figure 4.1 : Nyquist plot for carbon steel in 0.5 M H2SO4 solution at 25 oC in absence and

presence of varying amount of inhibitors

Tafel extrapolation data were observed with approx.1 chi-square value in order to obtain

consistency between the results, minimize the error percentage and standardize the

extrapolation range.

With 200 ppm dosage, a shift of Ecorr in the anodic direction is observed and Icorr decreased in

comparison with the other dosage values. Icorr obtained from Tafel extrapolation and

polarization resistance (Rp) data obtained from EIS support each other and show smaller

corrosion rate as compare to the other pH values (Table 4.1)

Figure 4.2: Potentiodynamic polarization curves for carbon steel in 0.5 M H2SO4 solution at

25oC in absence and presence of varying amount of inhibitors.

Table 4.1 : Polarization parameters for carbon steel in 0.5 M H2SO4 solution at 25 oC in

absence and presence of inhibitors

Dosage

(ppm)

-Ecorr (mV vs.

Ag/AgCl) Icorr (µA.cm

-2) Ba (mV) Bc (mV) Rp (Ω.cm

2)

0 456.00 151.28 97.23 94.32 125

50 441.00 128.08 103.38 93.85 138

100 443.40 80.68 98.38 86.39 200

200 441.28 79.48 106.63 91.92 225

500 449.30 120.50 99.84 83.39 150

1000 444.45 113.70 85.37 87.45 135

2000 448.74 125.66 75.58 88.38 125

EIS data is also analyzed by fitting it to an equivalent electrical circuit model. As the Nyquist

plot obtained for all inhibitors present a depressed loop, such behavior is characteristic for

solid electrodes and often referred to as frequency dispersion, which can be attributed to the

surface heterogeneity [13-16].

Figure 4.3 depicts the equivalent circuits to model electrochemical behavior belonging to the

absence of the inhibitors after 1 hour immersion in 0.5 M H2SO4 solution. The simplified

Randles circuit with a constant phase element (CPE) is used to represent the corroding system

where Rs represents solution resistance, Rct charge transfer resistance, CPEdl a constant phase

element, non-ideal double layer capacitive element to give a more accurate fit [17].

Figure 4.3 : Values of the elements of equivalent circuit required for fitting the EIS

of carbon steel in 0.5 M H2SO4 solution in absence of inhibitors.

On the other hand, Figure 4.4 shows the equivalent circuits to model electrochemical behavior

in the presence of the inhibitors in case of dosage amounts pf 100, 200 and 500 ppm after 1

hour immersion in 0.5 M H2SO4 solution. The electrochemical circuit model is represented by

two time constants where the capacitance of the intact coating is represented by Cc. Its value

is much smaller than a typical double layer capacitance. Rpo (pore resistance) is the resistance

of ion conducting paths develop in the metal-tannic acid complexes. These paths may not be

physical pores filled with electrolyte. On the metal side of the pore, it is assumed that an area

of the coating has delaminated and a pocket filled with an electrolyte solution has formed.

This electrolyte solution can be very different than the bulk solution outside of the coating.

The interface between this pocket of solution and the bare metal is modeled as a double layer

capacitance in parallel with a kinetically controlled charge transfer reaction [17]. In case of

low dosage as 50 ppm and high dosage amount of 1000 and 2000 ppm, the simplified Randles

circuit with a constant phase element (CPE) is used due to the better fit to represent the

corroding system. The simplified Randles circuit fits better in case of 50 ppm dosage may be

a result of not forming a protective ferric tannate complex. It is observed that such a

protective complex is achieved after 100 ppm dosage amount. On the other hand, in case of

high dosage amounts like 1000 and 2000 ppm, ferric tannate complex may become soluble

and pass into the electrolyte.

It is observed that a reasonable accuracy of the fitting was obtained, as evidence by chi-square

in the order of 10-3

and 10-4

for all the experimental data.

Figure 4.4 : Values of the elements of equivalent circuit required for fitting the EIS of carbon

steel in 0.5 M H2SO4 solution in presence of varying amount of inhibitor.

Table 4.2 contains all the impedance parameters obtained from the simulation of experimental

impedance data, including Rs, Rct, Yo and n. In the Table 4.2, also the calculated ‘‘double

layer capacitance” values, Cdl, are shown using the equation 4.1.

nnRctYoCdl

/11).(

(4.1)

where Yo is the CPE constant, n is a CPE exponent which can be used as a gauge of the

heterogeneity or roughness of the surface. Inhibitor efficiency based on impedance data are

calculated (Equation 4.2) and listed at Table 4.2 [12, 18, 19].

)(

)(%

inhRct

RctinhRctIE

(4.2)

From Table 4.2, it is clear that the addition of inhibitors causes an increase in Rct in 0.5 M

H2SO4 solution as the Rct increases inhibitor efficiency increases and gets the highest value

with 200 ppm tannic acid dosage when compared with the other dosage amounts.

The value of the proportional factor Yo of CPE varies in a regular manner with inhibitor

concentration. The change of Rct and Yo values can be related to the gradual replacement of

water molecules by inhibitor molecules on the surface and consequently to a decrease in the

number of active sites necessary for the corrosion reaction.

Table 4.2 : Values of the elements of equivalent circuit required for fitting the EIS of carbon

steel in 0.5 M H2SO4 solution in absence and presence of varying amount of

inhibitor.

Circuit Model Dosage -

ppm Rs (Ω.cm

2)

Rct

(Ω.cm2)

CPEdl, Yo.105

(Ω-1

.sn.cm

-2)

ndl Cdl (µF.cm

-

2)

Cc (µF) IE% Rpore

(Ω.cm2)

R(QR) 0 6.10 12.77 38.40 0.88 185.95

R(QR) 50 10.45 133.10 35.96 0.91 266.24 90.41

R(C(R(QR))) 100 8.45 190.00 27.16 0.91 202.59 32.37 93.28 0.59

R(C(R(QR))) 200 7.02 210.75 31.80 0.87 212.34 4943 93.94 15.01

R(C(R(QR))) 500 1.76 144.80 30.92 0.78 128.75 54.11 91.18 2.77

R(QR) 1000 2.66 134.45 28.46 0.90 198.05 90.50

R(QR) 2000 2.99 123.95 29.66 0.90 205.48 89.70

The inhibition efficiency (IE%) can be calculated both from polarization and EIS

measurements as given below with Equation 4.3. Calculated IE% were given in Table 4.3.

Although values obtained from polarization and EIS have different values due to different

methods (i.e DC and AC current measurements), they have similar trends. The polarization

resistance (Rp) was calculated from the EIS data.

100)(100)(% xRpinh

RpcorrRpinhx

Icorr

IinhIcorrIE

(4.3)

IE(%) reaches a maximum value with 200 ppm dosage of tannic acid when compared with

other dosage amounts. At higher dosages like 2000 ppm, it is clear that no significant

corrosion inhibition is achieved.

Table 4.3: Polarization parameters and the corresponding inhibition efficiency for the

corrosion of carbon steel in 0.5 M H2SO4 solution in absence and presence of

varying amount of inhibitor

Dosage - ppm -Ecorr (mV vs. Ag/AgCl) Icorr (µA.cm-2

) Rp(Ω.cm2) IE% Icorr IE% Rp

0 456.00 151.28 125.00

50 441.00 128.08 137.50 15 10

100 443.40 80.68 200.00 47 60

200 441.28 79.48 225.00 48 80

500 449.30 120.50 150.00 20 20

1000 444.45 113.70 135.00 25 8

2000 448.74 125.66 125.00 17 0

4.2 Measurements in 1 M HCl solution


curves and EIS measurements in absence and presence of varying amount of tannic acid in 1

M HCl solution in order to observe the effect of chemical structure and dosage amount at

corrosion inhibition of carbon steel in HCl solution.

4.2.1 Measurements in the Absence and Presence of Inhibitor in 1 M HCl Solution

Figure 4.5 reports the Nyquist diagrams plotted at the corrosion potential for the different

inhibitor dosage values in 1 M HCl solution in the absence and presence of tannic acid. The

impedance diagrams are characterised by a single time constant (a single loop). High

frequency intercept of semi-circle on the real axis yields the solution resistance (Rs) and low

frequency region yield the sum of Rs and polarization resistance (Rp). Rp values obtained from

Nyquist diagram were summarized in Table 4.4. The semicircles are generally associated with

the relaxation of the capacitors of electrical double layers with their diameters representing

the charge transfer resistance. Loop size of in case of 200 ppm is larger than the other dosage

values, which means highest polarization resistance and less corrosion rates.

Figure 4.5: Nyquist plot for carbon steel in 1 M HCl solution at 25 oC in absence and

presence of varying amount of tannic acid.

Tafel-extrapolation measurements werecarried out in the potentials region 250 mV from

corrosion potential, Ecorr. Figure 4.6 shows the steady-state current voltage curves obtained in

1 M HCl solution in absence and presence of varying amount of inhibitors. The corresponding

Ecorr, Icorr, anodic Tafel slopes (βa) and cathodic Tafel slopes (βc) at different dosage values

were summarised in Table 4.4.

Figure 4.6: Potentiodynamic polarization curves for carbon steel in 1 M HCl solution at 25oC

in absence and presence of varying amount of tannic acid.

Table 4.4: Polarization parameters for carbon steel in 1 M HCl solution at 25 oC in absence

and presence of tannic acid.

Dosage -Ecorr (mV vs.


-2) Ba(mV) Bc (mV) Rp (Ωcm

2)

0 441 69.00 77.10 91.80 190.00

200 435 26.40 81.20 81.20 340.00

500 449 49.20 82.10 85.50 260.00

1000 448 45.00 86.80 87.80 310.00

2000 459 41.40 95.90 79.30 315.00

With 200 ppm dosage, a shift of Ecorr in the anodic direction is observed and Icorr decreased in

comparison with the other dosage values. Icorr obtained from Tafel extrapolation and

polarization resistance (Rp) data obtained from EIS support each other and show smaller

corrosion rate as compare to the other dosage values (Table 4.4).

EIS data is also analyzed by fitting it to an equivalent electrical circuit model. As the Nyquist

plot obtained for all inhibitors present a depressed loop, such behavior is characteristic for

solid electrodes and often referred to as frequency dispersion, which can be attributed to the

surface heterogeneity [13-16].

Figure 4.7 depicts the equivalent circuits to model electrochemical behavior belonging to the

absence of tannic acid after 1 hour immersion in 1M HCl solution. The simplified Randles

circuit with a constant phase element (CPE) is used to represent the corroding system where

Rs represents solution resistance, Rct charge transfer resistance, CPEdl a constant phase

element, non-ideal double layer capacitive element to give a more accurate fit [17].

Figure 4.7: Values of the elements of equivalent circuit required for fitting the EIS of carbon

steel in 1 M HCl solution in absence and also presence of tannic acid.

On the other hand, Figure 4.7 also represents the equivalent circuits to model electrochemical

behavior in the presence of tannic acid in case of dosage amounts of 200, 500, 1000 and 2000

ppm after 1 hour immersion in 1 M HCl solution.

Table 4.5 contains all the impedance parameters obtained from the simulation of experimental

impedance data, including Rs, Rct, Yo and n. In the Table 4.5, also the calculated ‘‘double

layer capacitance” values, Cdl, are shown using the Equation 4.2.

nnRctYoCdl

/11).(

(4.2)

where Yo is the CPE constant, n is a CPE exponent which can be used as a gauge of the

heterogeneity or roughness of the surface. Inhibitor efficiency based on impedance data are

calculated (Equation 4.3) and listed at Table 4.5 [13, 18, 19].

)(

)(%

inhRct

RctinhRctIE

(4.3)

From Table 4.5, it is clear that the addition of inhibitors causes an increase in Rct in 1 M HCl

solution as the Rct increases inhibitor efficiency increases and gets the highest value with 200

ppm tannic acid dosage when compared with the other dosage amounts.

The value of the proportional factor Yo of CPE varies in a regular manner with inhibitor

concentration. The change of Rct and Yo values can be related to the gradual replacement of

water molecules by inhibitor molecules on the surface and consequently to a decrease in the

number of active sites necessary for the corrosion reaction.

Table 4.5 : Values of the elements of equivalent circuit required for fitting the EIS of carbon

steel in 1 M HCl solution in absence and presence of varying amount of tannic

acid.

Circuit

Model 1 M HCl Rs (Ω.cm

2)

Rct

(Ω.cm2)

CPEdl, Yo.105

(Ω-1

.sn.cm

-2)

ndl Cdl (µF.cm-2

) IE%

R(QR) blank 1.00 197.45 19.52 0.90 135.91

R(QR) 200 ppm 1.05 349.60 15.18 0.91 113.53 43.52

R(QR) 500 ppm 0.96 269.80 12.20 0.90 83.49 26.82

R(QR) 1000 ppm 0.99 320.20 12.48 0.90 87.27 38.34

R(QR) 2000 ppm 3.02 328.00 12.52 0.87 77.70 39.80

The inhibition efficiency (IE%) that calculated both from polarization and EIS measurements

Equation 4.1 were given in Table 4.6. Although values obtained from polarization and EIS

have different values due to different methods (i.e DC and AC current measurements), they

have similar trends. The polarization resistance (Rp) was calculated from the EIS data.

IE(%) reaches a maximum value with 200 ppm dosage of tannic acid when compared with

other dosage amoounts. At higher dosages like 2000 ppm, it is clear that increase in inhibitor

dosage does not result in increase in inhibition efficiency. It can be concluded that 200 ppm

tannic acid is an effective dosage amount for corrosion protection of carbon steel in 1 M HCl

media.


corrosion of carbon steel in 1 M HCl solution in absence and presence of

varying amount of tannic acid.

Dosage -Ecorr (mV vs. Ag/AgCl) Icorr (µA.cm-2

) Rp(Ωcm2)

IE%

Icorr IE% Rp

0 441 69.00 190

200 435 26.40 340 61.74 44

500 449 49.20 260 28.70 27

1000 448 45.00 310 34.78 39

2000 459 41.40 315 40.00 40

4.3 Measurements in the Absence and Presence of Inhibitor in Seawater


curves and EIS data in absence and presence of tannic acid at different pH values from 2 to

9.5 in seawater.

4.3.1 Measurements in Seawater in the Absence of Inhibitor

Figure 4.8 shows the Nyquist diagrams obtained after 1h immersion in absence of inhibitors at

different pH values. When the diameter of semicircle obtained at different pH compared with

each other, significant changes in the case of alkaline pH values over 9 (Figure 4.8) were

observed in decarbonized water.

Figure 4.8: Nyquist plot for carbon steel in seawater at 25 oC in absence of inhibitors at

varying pH values.

Anodic and cathodic polarization curves obtained at different pH in seawater were given in

Figure 4.9. The polarization resistances (Rp) were obtained from the EIS measurements and

compared with polarization measurements. Data collected from these curves were

summarized in Table 4.7. It can be seen that anodic and cathodic slopes are increasing

towards high pH values. Additionally, at pH 9.5, Rp has the highest value as compared to the

other pH values suggested more stable behaviour of carbon steel.

Figure 4.9: Potentiodynamic polarization curves for carbon steel in sea water at 25 o

C in

absence of inhibitors at varying pH values.

Table 4.7: Polarization parameters for carbon steel in seawater at 25 oC in absence of tannic

acid.

BLANK -Ecorr (mV vs.



2)

pH=2.0 563 32.00 59.60 95.70 400

pH=7.5 620 26.80 260.70 11220.00 1000

pH=8.5 582 9.70 81.40 503.90 1500

pH=9.5 527 14.00 110.80 683.80 3000

0 1 0 0 0 2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0 6 0 0 0

0

5 0 0

1 0 0 0

1 5 0 0

2 0 0 0

Zim

(o

hm

s)

Z re (o h m s )

p H = 2 b la n k

p H = 7 ,5 b la n k

p H = 8 ,5 b la n k

p H = 9 ,5 b la n k

4.3.2 Measurements in Seawater in the Presence of Inhibitors at Constant pH and

Varying Dosage Values

pH was stayed constant and tannic acid was added in seawater in order to observe the effect of

dosage amount and polarization and EIS measurements were performed.

Figure 4.10 shows both Nyquist diagrams and the steady-state current voltage curves obtained

in seawater in absence and presence of varying amount of tannic acid. The corresponding

Ecorr, Icorr, anodic Tafel slopes (βa) and cathodic Tafel slopes (βc) at different dosage values

were summarized in Table 4.8.

The impedance response of carbon steel in seawater has significantly changed after the

addition of inhibitor. The diameter of loop is the largest at an alkaline pH value of both 8.5

and 9.5 in case of 500 ppm tannic acid dosage (Figure 4.10.g). Measurement at pH 10.5 could

not be conducted due to the coagulation occurred with the dosage of tannic acid in seawater.

a b

c d

Figure 4.10: Nyquist plot and potentiodynamic polarization curves for carbon steel in

seawater at 25 oC in presence of tannic acid at different pH values a,b) pH=2

c,d) pH=7.5 e,f) pH=8.5 g,h) pH=9.5

Table 4.8: Polarization parameters for carbon steel in seawater at 25 oC in presence of

tannic acid at varying pH values.

pH = 2



-2) Ba (mV) Bc (mV) Rp (Ωcm

2)

0 563 32.00 59.60 95.70 400

200 567 43.00 110.40 204.00 600

500 547 37.40 108.50 190.30 700

pH = 7.5




2)

0 620 26.80 260.70 1122 1000

200 635 13.28 104.40 824.00 1100

500 720 10.20 73.70 269.00 1500

e f

g h

pH = 8.5




2)

0 582 9.70 81.40 503.90 1500

200 731 3.40 83.80 109.00 2000

500 717 2.00 50.40 95.70 4500

pH = 9.5




2)

0 527 14.00 110.80 683.80 3000

200 494 2.44 41.80 211.30 4500

500 439 5.06 58.90 621.80 5000

IE% were calculated and given in Table 4.9. With 500 ppm dosage, a significant decrease in

Icorr is observed at pH 8.5 and 9.5 in comparison with the other dosage and pH values which is

also complies with EIS measurement.

IE(%) reaches a maximum value with 500 ppm dosage of tannic acid at pH 8.5 which is

followed by the efficiency at 200 ppm dosage at pH 9.5 value according to the polarization

measurements. These results suggest that chelate formation is favorable at this pH and this

allow the better coverage of surface with increasing amount of inhibitor.


corrosion of carbon steel in seawater in absence and presence of varying

amount of tannic acid.

pH = 2


) Rp(Ωcm2) IE% Icorr IE% Rp

0 563 32.00 400

200 567 43.00 600 -34.38 33

500 547 37.40 700 -16.88 43

pH = 7.5



0 620 26.80 1000

200 635 13.28 1100 50.45 9

500 720 10.20 1500 61.94 33

pH = 8.5



0 582 9.70 1500

200 731 3.40 2000 73.81 88

500 717 2.00 4500 84.59 94

pH = 9.5



0 527 14.00 3000

200 494 2.44 4500 82.57 50

500 439 5.06 5000 63.86 67

4.3.3 Measurements in Seawater in the Presence of Inhibitors at Constant Dosage and

Varying pH Values

Dosage amounts stayed constant and, inhibitors added in seawater in order to observe the

effect of pH and polarization and EIS measurements were performed.

Figure 4.11 shows both Nyquist diagrams and the steady-state current voltage curves obtained

in seawater in absence and presence of varying amount of inhibitors.

The impedance response of carbon steel in seawater has significantly changed after the

addition of inhibitors. The capacitive loop has the largest shape at an alkaline pH value of 9.5

(Figure 4.11.c).

Figure 4.11: Nyquist plot (a,c)and potentiodynamic polarization curves (b,d) for carbon steel

in seawater at 25 oC in presence of 200 and 500 ppm of tannic acid.

Ecorr, Icorr, anodic Tafel slopes (βa) and cathodic Tafel slopes (βc) obtained from the

polarization curves at different dosage values, in seawater in absence and presence of varying

amount of tannic acid were summarised in Table 4.10.

Table 4.10: Polarization parameters for carbon steel in seawater at 25 oC in presence of

different amount of tannic acid.

BLANK -Ecorr (mV vs. Ag/AgCl) Icorr (µA.cm-2

) Ba(mV) Bc (mV) Rp (Ωcm2)

pH=2 563 32.00 59.60 95.70 400

pH=7.5 620 26.80 260.70 11220.00 1000

pH=8.5 582 9.70 81.40 503.90 1500

pH=9.5 527 14.00 110.80 683.80 3000

200 ppm -Ecorr (mV vs. Ag/AgCl) Icorr (µA.cm-2


pH=2 567 43.00 110.40 204.00 600

pH=7.5 635 13.28 104.40 824.00 1100

pH=8.5 731 3.40 83.80 109.00 2000

pH=9.5 494 2.44 41.80 211.30 4500

d c

a b

500 ppm -Ecorr (mV vs. Ag/AgCl) Icorr (µA.cm-2


pH=2 547 37.40 108.50 190.30 700

pH=7.5 720 10.20 73.70 269.00 1500

pH=8.5 717 2.00 50.40 95.70 4500

pH=9.5 439 5.06 58.90 621.80 5000

The inhibition efficiency (IE%) calculated from polarization curve and EIS data with

Equation 4.1. were given in Table 4.11.

With 200 ppm dosage, a significant decrease in Icorr is observed at pH 9.5 value in comparison

with the other dosage and pH values.


followed by the efficiency at 500 ppm dosage at pH 8.5 value according to the EIS

measurements. This can be attributed to the much more ionization of tannic acid at alkaline

pH values which results in better ferric tannate complex.

Table 4.11: Polarization parameters and the corresponding best inhibition efficiency for the

corrosion of carbon steel in decarbonised water in absence and presence of

tannic acid.

pH = 8.5



-2) Rp (Ωcm

2) IE% Icorr IE% Rp

0 582 9.70 1500

200 731 3.40 2000 73.81 88

500 717 2.00 4500 84.59 94

pH = 9.5



-2) Rp (Ωcm

2) IE% Icorr IE% Rp

0 527 14.00 3000

200 494 2.44 4500 82.57 50

500 439 5.06 5000 63.86 67

Figure 4.12 defines the equivalent circuits to model electrochemical behavior in absence and

presence of tannic acid at pH 2 after 1 hour immersion in seawater. The electrochemical

circuit model is represented by the capacitance of the intact coating is represented by Cc. Rpo

(pore resistance) is the resistance of ion conducting paths the develop in the coating. These

paths may not be physical pores filled with electrolyte. On the metal side of the pore, it is

assumed that an area of the coating has delaminated and a pocket filled with an electrolyte

solution has formed. This electrolyte solution can be very different than the bulk solution

outside of the coating. The interface between this pocket of solution and the bare metal is

modeled as a double layer capacitance in parallel with a kinetically controlled charge transfer

reaction [17]. Additional circuit elements (Cc and Rpo as) and increase in Rp values in

accordance with bare electrode (well agreement with Randles circuit) support the surface

coverage with inhibitor.

Figure 4.12: Values of the elements of equivalent circuit required for fitting the EIS

of carbon steel in seawater in absence and presence of tannic acid at pH 2.

Moreover, Figure 4.13 presents the equivalent circuits to model electrochemical behavior both

in absence and presence of tannic acid at pH 7.5, 8.5 and 9.5 after 1 hour immersion in

seawater. The electrochemical circuit model includes a CPE which is used to represent the

corroding system where Rs represents solution resistance, Rct charge transfer resistance,

CPEdl a constant phase element, non-ideal double layer capacitive element to give a more

accurate fit. Rpo (pore resistance) is the resistance of ion conducting paths developing in the

coating. At higher pH values the diameter of loop increases (Figure 4.11). Better fit of

equivalent circuit model to experimental data when CPE is used instead of Cc, indicates that

the paths between solution and the bare metal is longer due to thicker coverage of surface.

All the experimental data was observed by a reasonable accuracy of the fitting by chi-square

in the order of 10-4

.

Figure 4.13: Values of the elements of equivalent circuit required for fitting the EIS

of carbon steel in seawater in absence and presence of tannic acid at pH 7.5,

8.5 and 9.5.

Table 4.12 contains the impedance parameters of optimum corrosion control conditions

obtained from the simulation of experimental impedance data, including Rs, Rct, Yo, n and

also the calculated ‘‘double layer capacitance” values (Cdl) are shown, using the Equation

4.1.

From Table 4.12, it is clear that the addition of inhibitor causes an increase in Rct in seawater.

Table 4.12: Values of the elements of equivalent circuit required for fitting the EIS of carbon

steel in seawater in absence and presence of varying amount of tannic acid.

4.4 Measurements in the Absence and Presence of Inhibitor in Decarbonized Water


curves and EIS data in absence and presence of inhibitor at different acidic pH values 2, 4 and

6 in decarbonised water.

4.4.1 Measurements in Decarbonized Water in the Absence of Inhibitor

Figure 4.14 shows the Nyquist diagrams obtained after 1h immersion in absence of inhibitors

at different pH values. When the diameter of semicircle obtained at different pH compared

with each other, significant changes in the case of alkaline pH values over 9 (Figure 4.14)

were observed in decarbonized water.

Figure 4.14: Nyquist plot for carbon steel in decarbonised water at 25 oC in absence

of tannic acid at varying pH values.

Circuit Model seawater Rs

(Ω.cm2)

Rct

(Ω.cm2)

CPEdl. Yo.105

(Ω-1

.sn.cm

-2)

ndl Cdl

(µF.cm-2

)

Cc

(µF) IE%

CPEc.

Yo.105 (Ω

-

1.s

n.cm

-2)

Rpore

(Ω.cm2)

nc

R(C(R(QR))) pH=2

blank 5.40 403.70 12.38 0.83 67.02 1.24

1.69

R(C(R(QR))) pH=2

500 ppm 4.52 681.50 11.04 0.80 57.82 1.03 40.76

2.06

R(Q(R(QR))) pH=7.5

blank 4.90 1327.50 19.90 0.83 151.51

40.54 25.25 0.79

R(Q(R(QR))) pH=7.5

500 ppm 4.58 967.50 45.08 0.71 321.21

-37.21 21.50 371.35 0.84

R(Q(R(QR))) pH=8.5

blank 4.90 2021 59.64 0.63 665.53

62.44 7.51 0.75

R(Q(R(QR))) pH=8.5

500 ppm 4.90 2025.50 38.34 0.63 330.49

78.45 17.64 941.50 0.86

R(Q(R(QR))) pH=9.5

blank 5.29 2810.00 57.78 0.59 809.18

12.46 1015.00 0.82

R(Q(R(QR))) pH=9.5

500 ppm 5.67 1739.50 32.76 0.90 307.76

-61.54 14.10 3541.00 0.78

Anodic and cathodic polarization curves obtained at different pH in decarbonized water were

given in Figure 4.15. The polarization resistances (Rp) were obtained from the EIS

measurements and compared with polarization measurements. Data collected from these

curves were summarized in Table 4.13. It can be seen that anodic and cathodic slopes are

increasing towards high pH values. Additionally, at pH=10.5, Rp, βa and βc have the highest

values as compared to the other pH values suggested more stable behaviour of carbon steel.

Figure 4.15: Potentiodynamic polarization curves for carbon steel in decarbonised

water at 25 oC in absence of inhibitors at varying pH values.

Table 4.13: Polarization parameters for carbon steel in decarbonised water at 25 oC in

absence of tannic acid.

BLANK -Ecorr (mV vs. Ag/AgCl) Icorr (µA.cm-2

) Ba(mV) Bc (mV) Rp(Ωcm2)

pH=2 518.00 35.40 100.60 142.50 500

pH=4 481.00 23.00 124.70 >1000 1000

pH=6 476.00 3.88 64.00 99.00 1000

pH=7.5 671.71 14.66 86.81 468.95 725

pH=8.5 479.61 8.65 92.72 202.10 1375

pH=9.5 532.00 19.44 144.33 756.28 1500

pH=10.5 481.69 26.06 268.37 1813.96 2100

4.4.2 Measurements in Decarbonized Water in the Presence of Inhibitors at Constant

pH and Varying Dosage Values

pH stayed constant and inhibitors added in decarbonized water in order to observe the effect

of dosage amount and polarization and EIS measurements were performed.

Nyquist diagrams and the steady-state current voltage curves were obtained in decarbonised

water in absence and presence of varying amount of tannic acid at acidic pH values. The

corresponding Ecorr, Icorr, anodic Tafel slopes (βa) and cathodic Tafel slopes (βc) at different

dosage values were summarised in Table 4.14. The inhibition efficiency (IE%) calculated

from polarization curve and EIS data with Equation 4.1. were given in Table 4.15.

Table 4.14: Polarization parameters for carbon steel in decarbonised water at 25 oC in

presence of tannic acid at varying acidic pH values.

pH = 2




2)

0 518 35.40 100.60 142.50 500

200 527 57.00 135.30 141.80 400

500 527 60.20 114.80 133.30 375

pH = 4




2)

0 481 23.00 124.70 >1000 1000

200 707 20.40 164.00 702.50 1300

500 719 145.20 2371.00 4802 1550

pH = 6




2)

0 476 3.88 64.00 99 1000

200 709 28.00 197.00 2158 750

500 731 4.24 60.70 101 1050

It can be concluded from Table 4.15. that at acidic pH values at decarbonized water, corrosion

inhibiton efficiency is not obtained with 200 ppm and 500 ppm dosages of tannic acid. Tannic

acid is an effective corrosion inhibitor in decarbonized water at more alkaline pH values like

9.5 and 10.5.



varying amount of inhibitor

pH = 2


Ag/AgCl)

Icorr

(µA.cm-2

) Rp (Ωcm

2) IE% Icorr IE% Rp

0 518 35.40 500

200 527 57.00 400 -61.02 -25

500 527 60.20 375 -70.06 -33

pH = 4


Ag/AgCl)

Icorr

(µA.cm-2

) Rp (Ωcm

2) IE% Icorr IE% Rp

0 481 23.00 1000

200 707 20.40 1300 11.30 23

500 719 145.20 1550 -531.30 35

pH = 6


Ag/AgCl)

Icorr

(µA.cm-2

) Rp (Ωcm

2) IE% Icorr IE% Rp

0 476 3.88 1000

200 709 28.00 750 -621.65 -33

500 731 4.24 1050 -9.28 5

4.4.3 Measurements in Decarbonized Water in the Presence of Inhibitors at Constant

Dosage and Varying pH Values

Dosage amounts stayed constant and tannic acid was added in decarbonized water in order to

observe the effect of pH and polarization and EIS measurements were performed. The

corresponding Ecorr, Icorr, anodic Tafel slopes (βa), cathodic Tafel slopes (βc) and IE% obtained

from polarization at different dosage values were summarised in Table 4.16.

With 200 ppm dosage, a significant decrease in Icorr is observed at pH 9.5 and 10.5 values in

comparison with the other dosage and pH values.


followed by the efficiency at 200 ppm dosage at pH 10.5 value.

Table 4.16: Polarization parameters and the corresponding best inhibition efficiency for the


tannic acid

Decarbonised water -Ecorr (mV vs. Ag/AgCl) Icorr (µA.cm-2

) IE% Icorr

blank - pH=9.5 532.00 19.44

200 ppm - pH=9.5 724.57 9.14 52.98

500 ppm - pH=9.5 775.54 5.04 74.07

blank - pH=10.5 481.69 26.06

200 ppm - pH=10.5 525.00 7.62 70.76

500 ppm - pH=10.5 702.00 12.76 51.04

4.5 Adsorption Mechanisms

Electrochemical impedance spectroscopy provides a new method to characterize the film

coverage on the electrode, which is related to charge transfer resistance (Rct). The interface

capacitance can also be used to determine the film quality. It is known that the coverage of an

organic substance on the metal surface depends not only on the structure of the organic

substance and the nature of the metal, but also on the experimental conditions such as

immersion time and concentration of adsorbent [14].

The adsorption isotherms can provide basic information on the interaction of inhibitor and

metal surface [20].

It is known that the adsorption isotherms are very important for the understanding of the

mechanism of corrosion inhibition [21]. The most frequently used isotherms are Langmuir,

Freundlich, Temkin and Frumkin equations. Because impedance measurements are based on

small amplitude perturbations, they are non- destructive and well suited to continuous

monitoring of the corrosion [22].

In this work, the influence of concentration on the surface coverage in 0.5 M H2SO4 and 1 M

HCl solutions and decarbonized water with tannic acid was carried out. Therefore, EIS

measurement data were used to evaluate the surface coverage (θ), which was given by

Equation 4.3.

)(

)(%

inhRct

RctinhRctIE

(4.3)

It is assumed that the adsorption of these inhibitors follows the Langmuir adsorption isotherm

model, and can be described by the following Equation 4.4 [20].

cKads ..1

(4.4)

The plot of C/Ѳ versus C (Equation 4.6) yields a straight line with correlation coefficients of 1

and 0.977 for 0.5 M H2SO4 and 1 M HCl, respectively, providing that the adsorption of tannic

acid in these solutions on the carbon steel surface obeys Langmuir adsorption isotherm, which

is presented by Equation 4.5.

cKads

c

1

(4.5)

where C is inhibitor concentration, Ѳ is the degree of coverage on the metal surface and Kads

is the equilibrium constant for adsorption-desorption process.

From the intercepts of the straight lines on the C/Ѳ axis, Kads can be calculated that relates to

the standard free energy of adsorption, ΔG0

ads has given by the Equation 4.6.

)5.55ln(0

KadsRTGads

(4.6)

Free energies (ΔG0

ads) were calculated to be -39 and -31 kJ/mol for tannic acid in 0.5 M

H2SO4 and 1 M HCl and -33 and -32 kJ/mol for tannic acid in decarbonized water at pH 9.5

and 10.5 respectively; the negative value of ΔG0

ads indicates spontaneous adsorption of these

inhibitors on the mild steel surface and also the strong interaction between inhibitors

molecules and metal surface. Generally, values of ΔG0

ads up to -20 kJ/mol are consistent with

physisorption, while those around -40 kJ/mol or higher are associated with chemisorptions as

a result of the sharing or transfer of electrons from organic molecules to the metal surface to

form a co-ordinate.

The basic character of inhibitors affects the adsorption of cation on the surface of carbon steel

(electrostatic attraction). In the presence of Cl- and SO4

-2 which are strongly adsorbed on the

metal surface, the metal surface becomes negatively charged hence favored the adsorption of

cation type inhibitors. Thus, tannic acid adsorbed through electrostatic interactions between

the positively charged molecules and negatively charged metal surface [21].

Thus, the value of ΔG0

ads for tannic acid in 0.5 M H2SO4,1 M HCl and in decarbonized water

at alkaline pH values like 9.5 and 10.5 on carbon are in the range of -30-40 kJ/mol indicated

that it is adsorbed by mixed mode (physisorption and chemisorptions) of adsorption on the

metal surface [21, 23].

4.6 Thin Layer Choromotography (TLC)

Thin layer chromatography is used to separate mixtures of substances into their components.

All forms of chromatography work on the same principle. They all have a stationary phase (a

solid, or a liquid supported on a solid) and a mobile phase (a liquid or a gas). The mobile

phase flows through the stationary phase and carries the components of the mixture with it.

Different components travel at different rates. In order to gain some ideas about the reaction

in basic medium, TLC in presence and absence of tannic acid were obtained.

In this work, as stationary phase, TLC aluminium foil covered with silica gel plates from

Merck KGaA is used. The mobile phase is toluene/acetome/formic acid:60/60/10 (v/v)

suitable liquid for hydrolysable tannins [24]. The distance travelled relative to the solvent is

called the Rf value. For each compound it can be worked out using the formula:

Rf = distance travelled by compound / distance travelled by solvent (4.7)

Rf values are listed at Table 4.17 and close values were obtained for gallic acid and tannic

acid after polarization which supports tannic acid degredation and gallic acid formation

during corrosion inhibition.

Table 4.17: Rf values belonging to pyrogallic, gallic and tannic acid in decarbonized water at

pH 10.5 at 25 .

Solvent Pyrogallic Acid Gallic Acid Tannic Acid

Tannic Acid after

polarization

Distance (cm) 10 8 5 4 4.8

Rf

0.80 0.50 0.40 0.48

4.7 Pilot Test System

Accelerated corrosion tests have been carried out to simulate the conditions of real industrial

water circuits. First, a set of experiments was run on the laboratory scale in a circulation

device in case of untreated decarbonized water at pH 9.5 and also decarbonized water treated

with 500 ppm tannic acid at pH 9.5. For this, the steel coupons were mounted in a

miniaturized cooling circuit with a test rack according to ASTM 2688 [25]. Decarbonized

water was then circulated from a reservoir at 600 L/h for the predefined exposure time.

Figure 4.16 shows a schematic representation of the apparatus. Visual inspection and weight

loss evaluation (after pickling, rinsing and drying) were then used to evaluate inhibitor

performance. Corrosion protection factors were calculated from the weight loss measurements

with untreated water as the reference system.

Figure 4.16: Scheme and picture of the laboratory recirculating test rig with the test coupons

Corrosion rate results obtained from steel coupons were listed at Table 4.18 after 58 days

immersion in the circulation system. Corrosion rates were calculated to be 16.76 and 5.31

mpy for untreated decarbonized water and treated decarbonized water with 500 ppm tannic

acid at pH 9.5 respectively.

Table 4.18: Corrosion coupon analysis results

Coupon Total Surface

Area (cm2)

Metal Weight Loss Duration Corrosion Rate

Density (g) Time (day) (mpy)

Blank -

C 1010-087 16.432 7.87 0.8747 58 16.76

500 ppm Tannic

Acid C 1010-085 16.432 7.87 0.2770 58 5.31

Figure 4.17 shows the trends of online corrosion rates and pitting corrosion rates of untreated

and treated media were measured by obtaining linear polarization resistances by using

Rohrback 9000 Plus Corrater Instrument. Corrosion inhibition efficiency results observed by

both weight loss measurements and online linear polarization measurements were stated at

Figure 4.18.

Figure 4.17: Online and pitting corrosion rate development during lab scale test

From Figure 4.19, in decarbonised water at pH 9.5, tannic acid performs good corrosion

inhibition efficiency and this can be observed by both weight loss measurements and also

online linear polarization measurements. The results were also complying with the lab

measurements listed at Table 4.18. It can be also concluded from Figure 4.18 that tannic acid

is especially efficient towards pitting corrosion in decarbonised water.

Figure 4.18: Online and pitting corrosion rate development during lab scale test

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