green anticorrosive oilfield chemicals from seed and leave extracts of griffonia simplicifolia for...

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Journal of Chemistry and Materials Research

Vol. 5 (3), 2016, 4557

ISSN: 2381-3628

JCMR

Journal of Chemistry and

Materials Research

ORICPublications

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Original Research

Green Anticorrosive Oilfield Chemicals from Seed and Leave Extracts

of Griffonia Simplicifolia for Mild Steel

Ekemini B. Ituen1,2,

*, Onyewuchi Akaranta2,3

, Abosede O. James3, Shuangqin Sun

1

1 Materials Physics and Chemistry Research Laboratory, China University of Petroleum, Qingdao.

2 African Centre of Excellence in Oilfield Chemicals Research, Institute of Petroleum Studies, University of Port Harcourt,

Nigeria.

3 Department of Pure and Industrial Chemistry, University of Port Harcourt, Nigeria.

Received 29 April 2016; accepted 15 June 2016

Abstract

Extracts from Griffonia simplicifolia (EGS) were investigated as eco-friendly alternative oilfield chemical for protection of mild steel (MS)

surface in corrodible fluids associated with petroleum production. Corrosion rate was calculated in both 1M HCl and 15% HCl with and

without different concentrations of the seed (SEGS) and leave (SEGS) extracts using gravimetric and electrochemical measurements. The

extracts function as mixed type inhibitor and by spontaneous physical adsorption mechanism. Results from FTIR, UV-visible, SEM-EDS

support possible involvement of O and N sites in adsorption by formation of surface complex protective film of EGS molecules within 60 days

effective shelf life.

Keywords: Acid corrosion, Adsorption, Corrosion inhibitor, EIS, EFM, Griffonia simplicifolia, Oilfield chemicals, SEM-EDS.

1. Introduction

Production of hydrocarbons requires the use of a number of

chemicals referred to as oilfield chemicals. When existing

wells deplete, the use of chemistry to maintain production

through well stimulation and enhanced oil recovery operations,

becomes very crucial. Oilfield chemicals also include those

used as additives for the drilling mud, fluid loss additives, clay

stabilizers, lubricants, biocides, corrosion inhibitors, scale

inhibitors, gelling agents, filter cake removal agents, hydrate

control agents, cement additives, etc. Many fluids such as

fracturing, flooding, stimulation, and pickling contain acid

which stimulates corrosion of associated metallic materials.

* Corresponding author:

E-mail address: [email protected] (Ekemini B. Ituen).

All rights reserved. No part of contents of this paper may be reproduced or

transmitted in any form or by any means without the written permission of

ORIC Publications, www.oricpub.com.

Steel corrosion is a major industrial problem because steel is

used as structural material for production, transport, storage,

etc. Since corrosion gulps a major part of production cost in

the oil and gas industry, corrosion inhibitors are an important

class of oilfield chemicals [1]. The National Association of

Corrosion Engineers (NACE) estimated the direct cost of

corrosion in U.S.A. at $276 billion in 1998 which was

approximately 3.1 % of the gross domestic product (GDP) but

exceeded $1trillion in 2012 [2]. Globally, the annual cost of

corrosion worldwide has been estimated at $ 2.2trillion (2010),

believed to have been roughly 3% of world’s GDP [3].

According to Obot [3], such funds could have been channeled

to providing food for the poor as part of the Millennium

Development Goals (MDGs), hence the need for something to

be done to control corrosion and cut down the associated cost.

The use of corrosion inhibitors has been considered an

easy and cost effective approach to combating oilfield

corrosion. They can be used as additives in drilling, acidizing,

fracture, stimulation, and enhanced oil recovery fluids which

contain corrodible agents like acid (HCl), water and carbon

(iv) oxide [4] to reduce the rate of corrosion. In the selection

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46 Ekemini B. Ituen et al. / Journal of Chemistry and Materials Research 5 (2016) 45‒57

of the corrosion inhibitor, important factors such as sources

and cost of raw materials, its chemistry, and environmental

impacts, are considered. Plant biomasses afford cheap,

sustainable and eco-friendly source of corrosion inhibitors.

However, it has been claimed that they degrade easily at high

temperature and cannot be stored for long (i.e. they have short

shelf life) [5]. The effects of temperature on the effectiveness

of some corrosion inhibitors derived from plant materials have

been demonstrated in some reports [6‒13]. Nevertheless, the

shelf life within which these extracts can remain efficient has

not been reported in literature.

In this paper, we report results of our investigation of EGS

as alternative eco-friendly anti-corrosive oilfield chemical for

inhibition of mild steel corrosion for the first time. The

effectiveness and efficient shelf life of EGS is estimated.

Experiments were simulated in acid solutions with different

corrosive strengths to estimate its suitability in different

oilfield fluids from the less aggressive to highly aggressive

acidizing and stimulation fluids. The corrosion process was

monitored by weight loss and electrochemical (EIS, PDP, LPR

and EFM) techniques; surface morphology of the protected

metal was characterized using SEM/EDS and the corrosion

products using UV/VIS and FTIR spectroscopy. Hydrochloric

acid is the most frequently used acid in the field, which

informed our choice. The choice of concentration of 15% was

particularly motivated by its application in acidizing fluids. In

most parts of Nigeria, Griffonia simplicifolia (see Fig. 1) is not

edible; hence using it for production of corrosion inhibitors

would not compete with food. The plant is rich in various

alkaloids like 5-hydroxytryptophan, melatonin, fluvoxamine,

amitriptyline, griffonin, clomipramine, 5-hydroxytryptamine,

etc [14‒16]. These alkaloids contain potential adsorption sites

like nitrogen, oxygen, multiple bonds and aromatic systems,

which are key active functionalities present in many efficient

industrial organic corrosion inhibitor molecules [17‒21].

2. Experimental procedure

2.1. Materials

Mild steel used for this study was purchased from

construction materials market in Uyo, Akwa Ibom state of

Nigeria. Mature seeds and leaves of the plant were harvested

in large quantities from a local forest in Ikot Ambon Ibesikpo,

Uyo, Nigeria. Some of the materials used in the study are

given in Table 1.

2.2. Preparation metal specimens surface

Mild steel sheet was mechanically press-cut into coupons

of sizes 4cm x 4cm and 1cm x 1cm for weight loss and

electrochemical measurements respectively. The surface was

treated as provided by NACE Recommended Practice RP-

0775 and ASTM G-1 & G-4 for surface finishing and cleaning

of weight-loss coupons. However, after rinsing the pretreated

Fig. 1. Images of mature Griffonia simplicifolia leaves and seeds

Table 1 Some of the materials used in the study

Materials/techniques Source/models

HCl, ethanol, acetone Analytical grade, BDH.

Weight measurements Sartorius CPA225D analytical balance, sensitivity = ±0.00001g.

Potentiostat/Galvanostat Gamry ZRA REF600-18042

Electrodes Reference – saturated calomel electrode (SCE); Counter – platinium electrode; Working

– mild steel

UV/VIS 756PG Spectrum, Shanghai Spectrum Instruments Co., Ltd

FTIR TENSOR II

SEM/EDAX AMETEX S4800 EDAX TSL

Ekemini B. Ituen et al. / Journal of Chemistry and Materials Research 5 (2016) 45–57 47

and weighed coupons in acetone and allowing to air-dry, they

were used immediately for experiments. In addition, coupons

for electrochemical study were abraded with different grades

of silicon carbide papers; one exposed surface area (about

1cm2) was finished to mirror surface using CC-3000 electro-

coated grade paper.

2.3. Preparation of EGS and test solutions

The corroding media were simulated using both 15% HCl

and 1M HCl prepared by diluting 37 % stock solution in

distilled water. The leaves and seeds of Griffonia simplicifolia

(Fig. 1) were washed convincingly in distilled water and air-

dried. The dried samples were grounded to powder and extrac-

ted in acetone by maceration, percolation and infusion [22].

Appropriate weights of the extract corresponding to

concentrations 100, 500 and 1000 ppm were prepared in 1M

HCl while 1000, 5000 and 10000 ppm were prepared in 15%

HCl. Distilled water was used for all solution preparations.

2.4. UV-visible spectroscopic technique

The UV-vis spectrum was first obtained using the solution

containing 1000 ppm SEGS in 1M HCl prior to immersion of

MS. Another spectrum was obtained using a solution resulting

from immersing MS in 1M HCl for 24 hours. The spectral

profiles were then compared.

2.5. FTIR spectroscopic technique

The spectrum of the pure sample and that of the film

formed on the mild steel surface after immersion (both mixed

with potassium bromide) were recorded.

2.6. SEM-EDS study

Mild steel coupons of size 1 cm x 2 cm were abraded to

mirror finish as described above. The SEM images were

recorded in the vacuum mode before and after immersion in

HCl. This was repeated with a coupon immersed in HCl

containing 1000 ppm EGS solution. The instrument operated

at 5 kV. EDS profiles of the surfaces were also recorded.

2.7. Electrochemical measurements

Electrochemical impedance spectroscopic (EIS) measure-

ments were conducted at frequency of 105 to 10

‒2 Hz with

amplitude AC voltage of 10 mV and open circuit delay for 30

minutes at 303 K. The voltage was set from ‒0.15 to +0.15 V

vs. EOC at scan rate of 1 mV/s at the same temperature for

potentiodynamic polarization (PDP) measurements. Linear

polarization resistance (LPR) was measured at ‒0.20 to +0.20 V

vs. EOC at 1 mV/s. Electrochemical frequency modulation

(EFM) measurements were conducted at 0.1 Hz and peak

voltage of 10 mV. Gamry Echem software package was used

for data fitting and analyses.

2.8. Weight loss technique

Pre-weighed mild steel coupons were immersed in the test

solutions in the absence and presence of the EGS for required

time interval maintained at 303 K in a water bath. They were

retrieved, washed in mild detergent solution and distilled

water, rinsed in acetone, air-dried and weighed to determine

the weight loss. By denoting the initial and final weights of the

coupons as w1 and w2 respectively, corrosion rate of iron,

percentage inhibitor effectiveness (inhibition efficiency), WL

and degree of surface coverage (θ), were calculated as follows:

At

wwR

)(6.87 21 (1)

b

ibWL

R

RR )(100

(2)

inh 01.0 (3)

where bR and iR are the corrosion rates (mmpy) in the

absence and presence of the inhibitor, is the density of iron

and A is the surface area (cm2) of the metal specimens.

3. Results and discussion

3.1. Weight loss measurement

The effectiveness of an anti-corrosive additive can be

influenced by the medium of application. For crucial jobs like

stimulation, cooling systems, drilling muds, pipelines

protection, refinery units, oil storage tanks, production units

and decaling treatments, the key factors that influence inhibitor

performance include concentration, pH, temperature, contact

time, shelf life of inhibitor and grade/metallurgy of steel

material. Since it is difficult to reproduce the exact field

conditions in the laboratory, weight loss experiment were used

to probe these factors. In this report, we consider the

influences of concentration, shelf life, and contact time of the

inhibitor as well as concentration/pH of aggressive fluid. The

effects of temperature and grade of steel are under

investigation.

3.1.1. Effects of extract source

Same concentrations of both SEGS and LEGS were invest-

tigated under the same experimental conditions. SEGS provided

better inhibitive effect than LEGS as can be seen in Table 2. In

literature, EGS has been reported to be effective for folk

medicine and health products because of the active ingredient,

5-hydroxytryptophan, 5-HTP [23,24]. Chromatographic assay


has also revealed that naturally occurring 5-HTP is more

abundant insides and leaves extract [25‒27]. The better

performance exhibited by SEGS can be attributed to the higher

5-HTP composition than in LEGS.

3.1.2. Effects of extract concentration

The effect of amount of the inhibitor solution was also

investigated. Results obtained using SEGS are shown in Table

3. Corrosion inhibition efficiency increased with increase in

inhibitor concentration. The 1000 ppm sample was about 34

and 28 % more efficient than 100 and 500 ppm respectively.

This implies that the performance of EGS can be improved by

increasing the concentration. Apart from 5-HTP, other alkaloid

compounds like amitriptyline, fluvoxamine, clomipramine,

phenelzine, melatonin, imipramine, hydritioerectine, 5-hydrox-

ytryptamine, griffonin, trigonneline, etc, are also present in

EGS [14]. The anti-corrosive effect of EGS can also be

attributed to the presence of these compounds because they

possess electron donor functionalities (see Fig. 2) which are

potential active sites for adsorption on the steel surface. The

relative influence of each of these compounds on the EGS

inhibitive effect is still under investigation.

3.1.3. Effects of corrodible medium aggressiveness

Many field operations employ HCl of concentration up to

15 % hence the effect of concentration of the aggressive fluid

was also investigated. Results (SEGS only) reveal that the

extract was effective in both 1M HCl and 15 % HCl, with

better effectiveness in 1M HCl. This can be attributed to

sensitivity of the phyto-compounds to changes in pH. As

concentration of the acid increases, less phyto-compounds of

SEGS are capable of displacing initially adsorbed chloride

ions and/or water molecules on the steel surface resulting in

reduced inhibition efficiency. The 1000 ppm extract was about

28% less effective in 15 % HCl than in 1M HCl. To optimize

this performance, its concentration was increased by ten folds

and this resulted in an increase in inhibition efficiency by

about 54 % as shown in Table 4. This demonstrates that SEGS

can be an effective alternative corrosion inhibitor additive in

well acid treatment fluids.

Table 2 Weight loss results for mild steel corrosion inhibition by SEGS and LEGS

Test

solution

Initial weight (g) Final weight (g) Weight loss (g) Corrosion rate (mpy) Inhibition efficiency (%)

1M HCl 9.83725 9.64504 0.19221 26.72 -

1000ppm SEGS 9.70883 9.65699 0.01584 2.21 91.73

1000ppm LEGS 9.87204 9.83306 0.03898 5.43 79.68

Table 3 Effect of concentration on the inhibition of mild steel corrosion by SEGS

Test

solution

Initial weight (g) Final weight (g) Weight loss (g) Corrosion rate (mpy) Inhibition efficiency (%)

1M HCl 9.83725 9.64504 0.19221 26.72 -

100ppm SEGS 9.81401 9.75305 0.06096 8.49 68.23

500ppm SEGS 9.76215 9.73783 0.02432 3.39 87.31

1000ppm SEGS 9.70883 9.65699 0.01584 2.21 91.73

Table 4 Effects of corrodible fluid concentration on performance of SEGS as mild steel corrosion inhibitor

Corroding

fluid

Amount of Inhibitor Initial weight

(g)

Final weight

(g)

Weight loss (g) Corrosion rate

(mpy)

Inhibition

efficiency (%)

1M HCl

0ppm SEGS 9.83725 9.64504 0.19221 26.72 -

100ppm SEGS 9.81401 9.75305 0.06096 8.49 68.23

500ppm SEGS 9.76215 9.76215 0.02432 3.39 87.31

1000ppm SEGS 9.70883 9.70883 0.01584 2.21 91.73

15% HCl

0ppm SEGS 9.84207 8.73695 1.10512 153.94 -

1000ppm SEGS 9.76483 9.16840 0.59643 82.78 46.22

5000ppm SEGS 9.89142 9.49468 0.39674 55.27 64.10

10000ppm SEGS 9.77614 9.46063 0.31551 43.95 71.45


3.1.4. Effect of contact time

The performance of a corrosion inhibitor can be influenced

by the duration of contact with the system. To determine this

influence, the immersion tests were conducted in 1000 ppm

SEGS at different contact intervals of 1 h, 5 h, 12 h and 24 h.

The variation of inhibition efficiency and corrosion rate with

contact time is shown in Fig. 3. The highest inhibition

efficiency (which corresponds to lowest corrosion rate) was

obtained at 5 hrs. Variation of inhibition efficiency with

contact time followed the trend 24h < 1h < 12h < 5h. This

implies that SEGS performs optimally within 5-12 hours of

contact. It may be inferred from the results that contact time

less than 5 hours could be insufficient for the phyto-

compounds to migrate from the bulk phase and adsorb on the

mild steel surface. Above this time, the adsorbed phyto-

compounds may have lost some of their affinities for the

surface perhaps due to constant sweeping by the contacting

bulk phase or desorption. In many field operations, contact

time of acids with metal surface range between 8-12 h [17]

which is comparable to that obtained for SEGS.

3.1.5. Inhibitor shelf life

The performance of 1000 ppm SEGS was evaluated at

different storage durations by comparing inhibition efficiency

of the freshly prepared extract with those prepared, stored and

used later. The powdered extract was stored in laboratory

cupboard inside light-proof container, wrapped completely

with aluminum foil to exclude the effect of light. Preliminary

results (Fig. 4) reveal that the extract considerably sustained its

efficiency for 60 days after which slight decline was observed

(at 90 days and 120 days).Variation in inhibition efficiency

became significantly different with weight loss and EIS measu-

Fig. 3. Effects of contact time on the inhibition of mild steel corrosion by

SEGS in 1M HCl at 303K.

Fig. 4. Variation of EGS inhibition efficiency with its shelf life

rements after 60 days. Such observation could be attributed to

microbial or biochemical decomposition of the extract.

i

ii iii

iv

v

vi

Fig. 2. Molecular structures of some alkaloids in EGS: (i) amitriptyline (ii) 5-hydroxytryptophan (iii) 5-hydroxytryptamine (iv) fluvoxamine (v) phenelzine

and (vi) griffonin.


3.1.6. Electrochemical measurements

The efficiency of SEGS and LEGS were also determined

using EIS, PDP, LPR and EFM techniques. EIS is an easy and

time conserving technique for monitoring corrosion process.

The results are reliable because the measurements are

performed close to the corrosion potential. EIS is also a non-

destructive test because the magnitude of potential applied is

small. Information derived can be related to the kinetics of the

electrode process, mechanism and surface properties at the

metal/solution interface.PDP was carried out to investigate the

effect of EGS on the anodic and cathodic reactions occurring

in the system. EFM was carried out to compare the corrosion

current density (Icorr), Tafel constants (βa, βc) and Ecorr with

PDP results as well as to obtain the causality factors (CF-2 and

CF-3). Results obtained from these measurements are

summarized below.

3.2. Electrochemical Impedance Spectroscopy

measurement

Nyquist and Bode plots shown in Fig. 5 (SEGS only) were

obtained for the different concentrations of SEGS. The sizes of

diameters of semicircles in Nyquist plot are influenced in the

presence of SEGS from that of the free acid solution

demonstrating the influence of SEGS on corrosion rate due to

inhibition. The diameter increases as SEGS concentration

increases (d1000ppm > d500ppm > d100ppm > d0ppm) which corresponds

with the trend of inhibition efficiency. The Nyquist plot also

produced imperfect single depressed semicircles which can be

attributed to surface roughness of the mild steel [28]. The single

capacitive loop obtained indicates that the corrosion process is

mainly controlled by charge transfer process. The shapes of the

plots were similar without and with SEGS demonstrating that

the presence of the extracts does not change the steel corrosion

Fig. 5. Nyquist and Bode plots for inhibition of mild steel corrosion in 1M HCl by SEGS at 303K using EIS measurement

Fig. 6. Electrochemical equivalent circuit model used for data fitting

Table 5 Some parameters obtained from EIS technique used to monitor the inhibition of mild steel corrosion by SEGS and LEGS

EIS Parameters SEGS LEGS

0ppm 100ppm 500ppm 1000ppm 100ppm 500ppm 1000ppm

Rct (Ωcm2) 102.30 385.50 1089.00 1144.00 304.20 911.30 977.00

Rs (Ωcm2) 1.035 0.999 0.873 1.138 0.804 0.823 0.806

Y0 (10‒6

) (Ω‒1

sncm

‒1) 157.70 81.62 80.50 60.20 77.52 78.00 67.90

α (10‒3

) 898.50 888.60 859.90 845.00 889.30 858.40 821.10

Goodness of fit (x10-6

) 351.80 683.10 999.00 720.40 347.90 894.70 38.50

n 0.572 0.566 0.547 0.538 0.566 0.546 0.523

Cdl (10‒10

F) 12.770 6.221 0.999 0.338 6.814 1.036 0.202

εWL (%) - 73.46 90.61 91.06 66.37 86.80 87.68


mechanism. The equivalent circuit shown in Fig. 6 best fitted

the data and the associated parameters obtained are given in

Table 5. The goodness of the fits was in the order of 10‒4

indicating good correlation with the )( CPEcts RR equivalent

circuit model used. The surface rough-ness of the steel was

compensated by introduction of a non-integer element depen-

dent on frequency called constant phase element, CPE [29]

with magnitude given by 0Y and n , related to impedance by:

nCPE jwYZ )()( 1

0 (4)

where CPEZ is the impedance of the CPE, 0Y is the CPE

constant, is the angular frequency, j is an imaginary complex

number, )1( 2 j is the phase angle of CPE and n = 2α⁄(π )

is the CPE exponent.

The value of n indicates deviation of the CPE and can be

used to predict the degree of roughness or inhomogeneity of

the mild steel surface. This value decreased on addition of the

extracts suggesting that the surface roughness of the mild steel

is increased by adsorption of inhibitor molecules on steel

surface active sites. It also indicates that there is relative

and/or integrated influence on the CPE: not just a single

resistance, capacitance or inductive element. This parameter

can also be used to evaluate phase shift or deviation from ideal

behavior: decrease in n on addition inhibitor indicates

insulation of the metal/solution interface by formation of a

surface film. Formation of this film could also be responsible

for the increase in charge transfer resistance in the presence of

inhibitor than the free acid. The charge transfer resistance

increased with increase in inhibitor concentration showing that

the ‘blanketing’ property of the film improved as SEGS

concentration increased.

The inhibition efficiency was calculated from the charge

transfer resistances using eq. 5 below:

RctI

RR ctBctIEIS

)(100

(5)

where ctBR and ctIR are charge transfer resistances in the

absence and presence of inhibitor respectively. The inhibition

efficiency obtained increased with increase in inhibitor

concentration, following the same trend as weight loss results

(SEGS > LEGS).

Increase in peak heights of the Bode plots suggests more

capacitive response of the interface due to the presence of

adsorbed inhibitor layer. This capacitive response can result

from formation of an electrochemical double with a capacit-

ance, and its magnitude ( dlC ) was estimated using Eq. 6 below:

nnctdl RYC

1

10 )( (6)

The dlC values decreased in the presence of EGS, similar to

results obtained by Shaban and coworkers using vanillin

cationic surfactants [30], attributed to decrease in the local

dielectric or an increase in the thickness of the double layer or

both, caused by the adsorbed protective film.

3.3. Potentiodynamic Polarization measurement

The corrosion of mild steel in 1M HCl involves at least one

oxidation and one reduction process. For iron, typical

reactions at the electrodes are:

Anode: Fe(s) Fe2+

(aq) + 2e (7)

Cathode: 2H+

(aq) + 2e H2(g) (8)

While partial anodic oxidation of the iron results in its

dissolution (corrosion), hydrogen gas is liberated at cathode.

The sum total of both cathodic and anodic processes were used

to obtain the compromise or free corrosion potential (Ecorr) and

the corresponding current density (Icorr). Tafel cathodic and

anodic constants (βa, βc) were obtained from the slope of the

plots in Fig. 7 (SEGS only) with the obtained x2 values

indicating good linear fits. PDP parameters obtained are

shown in Table 6. The Icorr values decreased more with

increase in inhibitor concentration due to formation of

adsorbed protective film. Shift in Ecorr to more positive values

in the inhibited solutions compared to the free acid solution

was also observed.

Usually, cathodic corrosion inhibitors shift the corrosion

potential in the negative direction while anodic inhibitors

displace the potential in the positive direction. Inspection of

the Tafel plot quickly reveals that the inhibitors displace the

corrosion potential to the positive direction suggesting that

EGS has domino influence on partial anodic reaction.

However, the highest shift from that of the free acid (δEcorr)

was 17 mV and 31 mV obtained with 1000 ppm SEGS and

LEGS respectively which is less than 85 mV and not sufficient

to categorize the inhibitor is cathodic or anodic type. Similar

observations have been reported [31] and the inhibitors were

regarded as mixed type. A mixed type inhibitor acts by

blocking of some active anodic and cathodic sites of the metal

without changing its dissolution mechanism. In doing this,

SEGS inhibits both the iron dissolution and hydrogen evolu-

tion processes but more actively inhibiting iron oxidation.

Fig. 7. Tafel plots for inhibition of mild steel corrosion in 1M HCl by SEGS

at 303K using PDP measurement.


The values of βc and βa obtained changes with

concentration of EGS but with no definite trend. Olasunkanmi

and coworkers also obtained a similar trend [32] and the

quinoxaline derivatives were regarded as mixed type inhibitors

with cathodic predominance because the magnitude of the net

change compared to the free acid solution was higher in βc and

βa. In this study, larger Tafel constant values were obtained

from the anodic slopes than cathodic slopes supporting that the

EGS is mixed type inhibitor with anodic predominance. The

inhibition efficiency ( PD ) was calculated from corrosion

current density values as described in eq. 9 and results are

presented in Table 6.

bcorr

icorr

PDI

I1100 (9)

where bcorrI and i

corrI are the corrosion current densities in the

absence and presence of the inhibitor respectively. The calcul-

ated inhibition efficiency increased with increase in concentr-

ation of the inhibitor, similar to weight loss and EIS results

3.4. Linear polarization resistance

Linear polarization resistance was also measured for

uninhibited and inhibited solutions and the parameters

obtained are shown in Table 7. The values of PR were calcul-

ated from the slope of the linear portions of the plots in Fig. 8

(SEGS only) and increased with increase in inhibitor concen-

tration which agrees with results of previous measurements.

Corrosion rate decreased in the presence of inhibitor indicating

that EGS inhibited the mild steel corrosion process. Inhibition

efficiency was calculated from PR values using Eq. 10.

Fig. 8. LPR curves obtained for inhibition of mild steel corrosion in 1M HCl

by SEGS at 303K.

Pi

PbPiRP

R

RR )(100

(10)

The trends of inhibition efficiency, Icorr, and Ecorr are

consistent with those obtained using the other methods. The

differences in values of inhibition efficiencies may be due to

reaction time difference because weight loss gives an average

corrosion rate over a considerably longer immersion time than

the other measurements [33].

3.5. Electrochemical frequency modulation (EFM)

A portion of the current-frequency spectral chart obtained

from EFM measurements in the absence and presence of

different concentrations of EGS is shown in Fig. 9 (SEGS

Table 6 Some parameters obtained from PDP technique used to monitor the inhibition of mild steel corrosion by SEGS and LEGS at 303 K

PDP Parameters SEGS LEGS


βa (mV/decade) 95.40 65.90 107.50 104.40 77.40 81.90 97.80

βc (mV/decade) 64.70 100.30 98.70 92.70 72.30 69.50 86.00

Icorr(μA) 969.30 48.84 15.40 14.00 104.85 81.40 38.90

Ecorr (mV) -499.0 -487.0 -483.0 -482.0 -477.0 -475.0 -468.0

Corrosion rate (mpy) 65.74 15.41 4.84 4.42 42.04 16.08 7.80

x2 (x10

-3) 635.60 28.04 723.60 98.55 469.30 56.88 154.50

εPDP - 94.96 98.41 98.55 89.18 91.60 95.99

Table 7 Some parameters obtained from LPR technique used to monitor the inhibition of mild steel corrosion by SEGS and LEGS at 303 K

PDP Parameters SEGS LEGS


Icorr(μAcm-2

) 418.40 114.70 54.14 47.72 136.24 101.40 78.87

Icorr(mV) -465.6 -478.5 -486.0 -486.9 -472.7 -471.2 -468.0

RP (Ωcm2) 62.27 227.20 481.20 546.03 212.16 365.80 421.60


εLPR - 72.59 87.06 88.60 70.64 82.98 85.23


only). The spectra were analyzed to calculate Icorr, βa, βc and

the causality factors (CF-2 and CF-3) which are listed in Table

8. It can be seen from the table that corrosion current densities

decrease with increase in SEGS concentration supporting the

trend obtained from PDP and LPR. The change in magnitudes

of βa and βc values was also small indicating that the presence

of SEGS does not change the corrosion mechanism. The

causality factors, CF-2 and CF-3, obtained were close to

theoretical values of 2 and 3 respectively showing that the

obtained data are reliable [31]. However, CF-2 for 1000ppm

LEGS was slightly higher than the ideal value due to influence

of electrochemical noise. Deviation of causality factors from

their ideal values can be associated with too small perturbation

amplitude, bad frequency resolution or not too good

performance of the inhibitor [34]. The EFM results were also

comparable with results from other techniques.

Fig. 9. EFM spectra obtained for inhibition of mild steel corrosion in 1M HCl

by SEGS at 303K.

Fig. 10. UV-vis absorption spectra for SEGS in 1M HCl before and after

immersion of mild steel

3.6. UV-visible spectroscopy

The absorption spectra shown in Fig. 10 reveal that before

immersion, the band was obtained in the UV-visible region

which can be attributed to * and *n electronic

transitions and considerable charge transfer properties. After

immersion, the bands positions shifted to higher absorbance

with reduced resolution at lower wavelengths of absorption.

This behavior demonstrates possible chemical interaction betw-

een Fe2+

and inhibitor compounds in the inhibited solution [32].

Since SEGS is a composite of many compounds, it is difficult

to determine what type of complex is formed or to assign

electronic transitions.

3.7. FTIR spectroscopy

The FTIR spectrum of the pure SEGS obtained was

different from that of the surface film formed on the mild steel

as can be seen in Fig. 11. The peaks around 1350, 2600, 3400,

and 3650 cm‒1

may be assigned to C‒O (alcohol, ether, ester,

carboxylic, etc) or C‒N (amines), S‒H (mercapto), N‒H

(amines or amides stretching) and O‒H (alcohols or aromatic)

functional groups respectively. These peaks were very

prominent in the profile of pure SEGS but disappeared or

became less prominent in the spectrum of the corrosion

product. This indicates that the adsorption of SEGS may be

facilitated by these groups, which coheres with popular

opinion that adsorption is facilitated by the presence of oxygen

and nitrogen atoms. The peak around 1700 ‒ 1600 cm‒1

representing C=O (carbonyl), C=C or C=N absorption also

diminished after adsorption, pointing to possible involvement

of any or more of these functionalities in surface adsorptive

interaction.

Fig. 11. FTIR spectral profile for pure SEGS (black) and surface film formed

(red) by 10000 ppm SEGS on mild steel surface in HCL after 24 hours of

immersion


3.8. EDS study

To further establish the reliability of inferences drawn from

FTIR results, EDS profiles of the surfaces of mild steel was

obtained without and with formation of adsorbed film. The

spectral profile of pure mild steel surface before immersion

(Fig. 12a) shows presence of Fe, C and small O. On immersion

in 1M HCl, Fe slightly decreased, O slightly increased and Cl

ions were detected (Fig. 12b). This surface behavior could

suggest interaction of Fe with O and Cl to form oxides and

chlorides in the presence of water and acid molecules

respectively. However, the surface profile of the steel sample

with the adsorbed EGS film (Fig. 12c) revealed the presence

of increased amount of O and decreased amount of Fe and Cl.

The profile also showed N atoms. A summary of observations

Table 8 Some parameters obtained from EFM technique used to monitor the inhibition of mild steel corrosion by SEGS and LEGS

EFM Parameters SEGS LEGS


Icorr(μA) 162.60 64.97 63.63 41.74 66.72 68.46 49.23

β1 (mV/decade) 64.64 87.52 90.55 92.34 78.16 79.24 77.87

β2 (mV/decade) 107.80 96.41 97.88 100.20 101.30 111.70 109.46


CF-2 1.966 1.898 1.487 1.706 1.856 1.743 2.017

CF-3 2.358 2.362 2.206 2.240 2.284 2.349 2.969

ΕEFM - 60.04 60.87 74.33 58.97 57.90 69.72

Table 9 Summary of atoms obtained on pure mild steel surface (case I), corroded mild steel surface (case II) and inhibited mild steel surface (case III) by

EDS

Element Case I Case II Case III

Wt % At% Wt% At% Wt% At%

Fe 89.85 64.17 80.24 49.28 68.77 50.54

C 09.07 29.90 09.79 30.69 14.32 29.84

O 01.04 04.91 07.96 17.07 11.26 12.34

N - - - - 03.81 04.78

Cl

Si

-

00.04

-

01.02

02.01

-

02.96

-

01.80

00.04

01.48

01.02

a b c

Fig. 12. Shows a: Case I-EDS profile of pure abraded mild steel surface; b: Case II-EDS profile of mild steel surface after immersion in 1M HCl and c:

Case III-EDS profile of mild steel surface after immersion in IM HCl containing 1000ppm SEGS.

Fig. 13. Left to Right: SEM micrographs of mild steel surface before immersion, after immersion in 1M HCl and IM HCl containing 1000ppm SEGS.


from EDS profiles is given in Table 9.Thus, adsorption of the

inhibitor on the surface involved mainly N and O atoms, and

resulted in decreased iron exposure due to protective effect of

the inhibitor. This result is consistent with observations using

FTIR.

3.9. SEM and surface morphological study

Micrographs of abraded mild steel coupons prior to

immersion and those immersed in 1M HCl with and without

1000 ppm SEGS for 24 hours were recorded (Fig. 13) by

scanning electron microscopy. Results reveal that the surface

of the abraded steel coupon (left) was considerably smooth

with minimal undulation. The surface of the coupon immersed

in the free acid solution (middle) experienced severe damage

due to corrosive attack. However, the surface of the coupon

immersed in the solution containing 1000 ppm SEGS was

relatively smoother compared to the free acid solution. These

results demonstrate that the addition of SEGS reduced

corrosive pitting which was characteristic of the free acid

solution [35]. The extent of protection can be considered to be

high since pitting decreased. The protective layer formed by

SEGS was not evenly distributed over the metal surface hence

some portions of the surface were smoother than others. Thus,

the active sites on the mild steel surface might not be

equivalent or possess similar affinity for active molecules of

SEGS. It could also be that adsorption of some molecules on

the surface blankets the acid from attacking that portion by

steric hindrance or micelles-like conformation of adsorbed

molecules as described by [36].

3.10. Adsorption behavior at metal-fluid interface –

Adsorption isotherms

In other to further explain the nature of interactions

between the anti-corrosive oilfield chemical (corrosion

inhibitor) and mild steel surface, adsorption isotherms were

employed. Fractional surface coverage (θ) was calculated from

inhibition efficiency using WL data for SEGS in both 1M HCl

and 15% HCl according to Eq. 11. Temkin, Flory-Huggins,

Freundlich, and El-Awady adsorption isotherm models were

used to fit the coverage data. Langmuir isotherm which

assumes homogeneous surface of the metal was not tested

because EIS results already provided insights that like any

other surface, the steel surface was not homogeneous. Best fits

were obtained with Temkin (Eq. 12-13) and Freundlich (Eq.

14-15) models as shown in Fig. 14.The free energy change of

the adsorption process ( adsG , (kJmol‒1

)) was calculated from

adsorption-desorption equilibrium constant ( adsK , (103 M

‒1))

at T = 303K using equation 16. The obtained adsorption

parameters are displayed in Table 10.

100

WL (11)

CKe adsa 2 (12)

adsKa

Ca

ln2

1ln

2

1 (13)

Table 10 Adsorption parameters for inhibition of mild steel surface corrosion by SEGS in both 1M HCl and 15% HCl

Acid

Solution

Temkin Freundlich

α adsK adsG

n

1 adsK adsG

1M HCl -4.587 0.441 -8.055 0.133 0.275 -6.863

15% HCl -4.808 0.193 -5.967 0.192 0.124 -4.849

Fig. 14. Adsorption isotherms for coverage of SEGS molecules on mild steel surface in both 1M HCl and 15% HCl solution at 303K.


n

adsCK /1 (14)

Cn

Kads log1

loglog (15)

adsads KRTG 5.55ln (12)

where R is the molar gas constant, α is the molecular

interaction parameter which can be negative or positive, C is

the mass concentration of inhibitor (in ppm), andn

1 is a

constant used to describe the ease of adsorption. Usually, when

11

0 n

adsorption is believed to be easy and moderate or

difficult when 11

n or 1

1

n respectively.

The negative values of a obtained indicate that repulsion

takes place in the adsorbed layer and increases with corrodent

concentration. The values of n

1indicate that adsorption of

SEGS on mild steel surface is easier in 1M HCl than 15%

HCl. The adsorption of SEGS in both media was spontaneous

and physical in nature as indicated by negative free energy

change. It can also be observed from the table that both

adsK and adsG values from Temkin and Freundlich are in the

same order indicating coherency of results.

3.11. Mechanism of adsorption

A mixed type inhibitor protects the metal surface by

adsorption and film formation. The adsorption mechanism may

be physisorption or chemisorption. PDP results revealed that

the inhibitor is mixed type with anodic dominance. The

mechanism was predicted from this inference based on basic

polarization principles. During cathodic polarization, the metal

surface is negatively charged due to discharge of cations on its

surface. The reverse happens during cathodic polarization.

Since SEGS showed predominant anodic behaviour, the metal

surface can be considered to be positively charged with

negatively charged species from the inhibitors adsorbed on its

surface. This generates electrostatic force which binds the

adsorbed layer on to the steel surface by physisorption.

However, the strength of binding is not as strong as those of

chemisorptions. Since EDS results reveals the presence of

chloride ions on the steel surface, it can be considered that the

chemisorption also took place simultaneously. Thus, at the

anodic sites where SEGS influence is higher, positively

charged inhibitor anions in the presence of negatively charged

chloride ions can be adsorbed on the positively charged steel

surface. The Chloride ions act as a bridge between the

inhibitor molecules and metal surface - a kind of synergistic

phenomenon caused by sharing of charge or charge transfer

between inhibitor species and metal surface. Some of the

compounds in SEGS can form both positive and negative ions,

which can result in a highly complex film that cannot be

explained from available results. However, investigation is

ongoing to elucidate the nature of this film formed using some

of the individual active alkaloids isolated from the extract.

4. Conclusion

Seed extract (SEGS) shows high to moderate inhibition

efficiency of 91.73% and 71.45% for mild steel corrosion

in 1M and 15% HCl respectively at 1000ppm

concentration.

Inhibition is facilitated by adsorption of active phyto-

molecules of EGS on mild steel surface by means of

oxygen and nitrogen functionalities.

SEGS sustains its effectiveness throughout 60 days

storage and provides optimum efficiency within a contact

time of 5 hours.

About 1000ppm SEGS solution forms efficient unevenly

distributed protective film on the mild steel surface at

303K than LEGS.

Inhibition efficiency of both LEGS and SEGS increases

with increase in their concentrations.

Adsorption of EGS on mild steel surface is spontaneous

with simultaneous physisorption and chemisorption

mechanism.

EGS which functions as a mixed type inhibitor on mild

steel surface.

Both LEGS and SEGS could serve as alternative eco-

friendly anti-corrosive additive in well acidizing fluids.

Acknowledgement

The authors acknowledge support from World Bank

through the Robert S. McNamara Fellowship programme to

conduct laboratory work abroad, Materials Physics and

Chemistry Department, China University of Petroleum

Qingdao for providing facilities for carrying out this research

and African Centre of Excellence in Oilfield Chemicals

Research for their support. EI is grateful to Prof A. P. Udoh,

Prof. Lu, Dr. Li, Dr. Wang, Ubong Jerome, Chen, Chao, Xiang

and Min in UPC for their assistance.

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