Journal of Industrial and Engineering Chemistry 21 (2015) 239–247

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Journal of Industrial and Engineering Chemistry

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Utilization of watermelon rind extract as a green corrosion inhibitor

for mild steel in acidic media

N.A. Odewunmi *, S.A. Umoren, Z.M. Gasem

Center of Research Excellence in Corrosion, Research Institute, King Fahd University of Petroleum and Minerals, 31261, Saudi Arabia

A R T I C L E I N F O

Article history:

Received 17 December 2013

Accepted 17 February 2014

Available online 26 February 2014

Keywords:

Watermelon

Acid corrosion

Corrosion inhibition

Mild steel

Biomaterials

A B S T R A C T

Electrochemical techniques were used to investigate the efficacy of watermelon rind extract (WMRE) as

corrosion inhibitor for mild steel in HCl and H2SO4 solutions. The inhibition efficiency increased as the

concentration of the extract increases. Potentiodynamic polarization results indicate that WMRE act as a

mixed-type inhibitor. The values of effective capacitance obtained by utilizing the impedance

parameters in both acidic media are in the range of double layer capacitance. Corrosion inhibition effect

of WMRE can be attributed to the adsorption of constituents of WMRE onto the mild steel surface which

can be approximated by Temkin adsorption isotherm model.

� 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

1. Introduction

Mild steels are frequently used for a wide range of applicationsin construction industry due to their very good mechanicalproperties and low cost compared with many metallic materials.The poor corrosion resistance of mild steel in acid [1–3] is a majorconstraint in its applications. The use of inhibitors in controlling itscorrosion processes has proven to be efficient and most practicable[4,5]. However, most of the compounds that constitute theseinhibitors are toxic to human and its environments. The toxicity ofthese organic and inorganic inhibitors has paved way to explorethe use of non-toxic natural products inhibitors [6–9] that areenvironmentally friendly. The use of extracts of some plants toinhibit the corrosion of steel in acid media has been reported in theliterature [10–27].

Watermelon is vine-like flowering plant which belongs to thefamily of Cucurbitaceae with varieties of seeded and seedlessspecies. It is widely grown due to its large edible fruits that containa hard green rind and a watering reddish, yellowish or pink pulp. Itis an essential agricultural product utilized in the production ofmany food products such as fruit cocktails, juices and nectars[28,29]. The most useful part of the watermelon in the foodindustry is its pulp. Watermelon, Citrullus lanatus has beenreported by Johnson et al. [30] to contain valuable amount of

* Corresponding author. Tel.: +966 13 860 7895; fax: +966 13 860 3996.

E-mail address: adewalen@kfupm.edu.sa (N.A. Odewunmi).

http://dx.doi.org/10.1016/j.jiec.2014.02.030

1226-086X/� 2014 The Korean Society of Industrial and Engineering Chemistry. Publis

anti-nutrients namely saponin, alkaloids, hydrogen cyanide,tannins, phytate, phenol, oxalate and flavonoids in fresh and driedwatermelon peel (WMP), watermelon pulp (WMPu) and water-melon rind (WMR). In addition, WMR being an agricultural wasteproduct has been reported to be a natural source of citrulline, anon-essential amino acid [31]. The chemical structure of citrulline(major component of WMRE) is shown in Fig. 1. These compoundscontain heteroatoms (N, O) and aromatic ring which are regardedas centers of adsorption hence the extracts obtained fromwatermelon waste products could function as a potential corrosioninhibitor. There are few reports on the use of watermelon parts ascorrosion inhibitor for metals in some corrosive environments. Forinstance watermelon peel extract (WMPE) and watermelon leafhave been used for corrosion protection of zinc in natural sea water[32]. However, there is no report to the best of our knowledge onthe use of watermelon rind as corrosion inhibitor for any metal inany corrosive environment. Therefore our main objective in thepresent work is to utilize extract of WMR as a corrosion inhibitorfor mild steel in acidic media (HCl and H2SO4) using electrochemi-cal impedance spectroscopy (EIS), potentiodynamic polarization(PDP) and linear polarization (LPR) methods.

2. Experimental

2.1. Materials preparation

1 M HCl and 0.5 M H2SO4 solutions as corrosive media wereprepared with A-R grade 37% HCl (Sigma–Aldrich) and 98% H2SO4

hed by Elsevier B.V. All rights reserved.

[(Fig._1)TD$FIG]

Fig. 1. Chemical structure of citrulline.

N.A. Odewunmi et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 239–247240

(Baker), respectively, and double distilled water. Extract of theWMR was prepared by weighing 5 g of dried grounded WMR in 1 Lof 1 M HCl and 0.5 M H2SO4 and boiled for 10 min. The solutionswere cooled and later filtered with sintered glass Buchner funnelNo. 36060 to obtain 5 g/L stock solution. Different concentrationsof the extract (0.1–2.0 g/L) were prepared from the stock solutionby dilution with the respective corrodents (1 M HCl and 0.5 MH2SO4).

The chemical composition (weight %) of the mild steel sampleused in the experiment is: C, 0.15; Mn, 1.26; V, 0.017; Si, 0.035; S,0.008; Cr, 0.036; Ni, 0.03; Al, 0.083; Cu, 0.038 and Fe (balance). Themild steel was cut into coupons of 3 cm � 3 cm � 0.25 cmdimension, abraded with silicon carbide abrasive papers ofdifferent grades (# 120–800), and rinsed with distilled water.The residues of polished particles were removed by sonication inethanol bath for 10 min. The cleaned mild steel specimens werekept in the desiccator prior to use for corrosion test usingelectrochemical measurements.

2.2. Electrochemical measurements

The electrochemical measurements were performed in a singlecompartment electrochemical cell designed for mounting varietiesof flat samples for electrochemical test at 25 8C with threeelectrodes system. Gamry potentiostat/galvanostat/ZRA (Refer-ence 3000) embedded with Gamry framework system composed ofESA410. Gamry software applications include EIS300 for theelectrochemical impedance spectroscopy (EIS) measurements,DC105 for the potentiodynamic polarization (PDP) and linearpolarization (LPR) measurements and Echem Analyst 6.0 for datafittings. The exposed area of the working electrode (mild steel) tothe corrosive acidic medium was 0.7855 cm2; graphite rod and thesaturated calomel electrode (SCE) were used as the counter andreference electrodes, respectively.

Tafel curves were obtained from the potentiodynamic polari-zation measurements between the potential range of +250 mV and�250 mV against the open circuit potential (Ecorr) at scan rate of1 mV s�1. A scan rate of 0.125 mV s�1 was used to generate a linearpolarization resistance (LPR) at +20 mV and �20 mV against theEcorr. EIS measurements were done at frequencies of 100 kHz to100 mHz with an alternative (ac) amplitude of 5 mV r.m.s.(7.07 mV peak-to-peak) signal from the Ecorr. The open circuitpotential (Ecorr) was allowed to attain a stable state which wasachieved after 1800 s before the electrochemical measurementswere carried out both in the absence and presence of the inhibitor(WMRE).

3. Morphological studies

A JSM-5800 LV scanning electron microscope (SEM) was used toexamine the surface morphology of the mild steel specimen withand without WMRE in the acidic media (1 M HCl and 0.5 M H2SO4)after 24 h exposure. Prior to the exposure in different test solutions,the mild steel coupons of dimensions 3 cm � 3 cm� 0.25 cm wereground with silicon carbide abraded papers of different grades(# 120–800) followed by polishing with a cloth using 1 mm diamond

paste to near mirror surface finish. The same procedure for removingthe residue of polishing by sonication in ethanol bath was employed.The cleaned mild steel specimens were immersed in both 1 M HCland 0.5 M H2SO4 without and with 2.0 g/L WMRE. The coupons wereretrieved after 24 h, rinsed with distilled water, dried and submittedfor SEM examination.

4. Results and discussion

4.1. Open circuit potential

The variation of open circuit potential (Eocp) with time for mildsteel in 1 M HCl and 0.5 M H2SO4 without and with addition ofdifferent concentrations of WMRE is shown in Fig. 2. It could beobserved from the figure that a stable OCP values were attainedafter 1800 s immersion both in the absence and presence of theinhibitor. It is also observed that on introduction of WMRE into thecorrosive medium, the potentials shift to noble values than thoseobserved in the blank acid solution. The positive shift of corrosionpotential was found to be concentration dependent. This suggests apossible influence of the extracts on both anodic and cathodicpolarization.

4.2. Electrochemical impedance spectroscopy (EIS) measurements

EIS measurements were taken to characterize the kinetics of theelectrochemical processes and the capacitive behavior of WMRE onmild steel. Figs. 3 and 4 show the Nyquist and Bode (modulus andphase angle) plots of WMRE in 1 M HCl and 0.5 M H2SO4 withoutand with different concentrations of WMRE respectively. TheNyquist plots in both media exhibited a semi-circle (one shoulder)and a single relaxation process (one time constant) over the rangeof the studied frequencies as shown by the Bode phase angle plots.The number of distinguishable maxima or related shoulder is anindication of number of relaxation (time constant) taking place inelectrochemical system as reported by Beverskog et al. [33]. Also,Figs. 3c and 4c revealed a phase angle maxima smaller than 908. Itis observed from the plots (Figs. 3a and 4a) that the semi circles arenot perfect but depressed with the centers under the real axis.Some reasons advanced to explain this behavior include thesurface roughness, frequency dispersion of time constants as aresult of local inhomogeneity in the dielectric material, relaxationand porosity in mass transport effects [34]. To account for theseeffects, a constant phase element (CPE) must be introduced to theequivalent circuit to estimate the EIS experimental results [35,36].Fig. 5 shows the equivalent circuit used to accurately fit theexperimental data obtained from EIS measurements for mild steelin (a) 1 M HCl and (b) 0.5 M H2SO4 in the absence and presence ofWMRE. The circuit comprised of the CPE in parallel with the chargetransfer resistance (Rct) and solution resistance (Rs) in HCl mediumand additional inductive elements (L and RL) in H2SO4 medium.

In HCl solution, there is no change in the corrosion mechanismin spite of the addition of different concentrations of WMRE as theshape of the impedance plots largely remain the same as shown inFig. 3, however, the diameter of the semi-circle (Fig. 3a) and thereal impedance value (Fig. 3a and b) increased as the extractconcentration increases from 0.1 g/L up to 1.5 g/L and a lowerdiameter as the concentration was raised to 2.0 g/L. The solutionresistance (Rs) and the Rct are assigned as the high and lowfrequencies intercept on the real impedance axis, respectively.However, for mild steel corrosion in 0.5 M H2SO4 the impedance ischaracterized by a large capacitance loop at high-mediumfrequency region and an induction loop at the low frequency(LF) in the presence of WMRE (Fig. 4a). The high frequencycapacitive loop could be attributed to double layer capacitance inparallel with the Rct but the phenomenon of the LF inductive loop

[(Fig._2)TD$FIG]

Fig. 2. OCP plots for mild steel in (a) 1 M HCl and (b) 0.5 M H2SO4 in the absence and presence of different concentration of WMRE.

N.A. Odewunmi et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 239–247 241

have been reported by several authors in the literature andrecently by Umoren et al. [37] to be probably originated from theadsorption relaxation of intermediates such as H+

ads and SO42–. The

inductive loop could also be as a result of the re-dissolution of thepassivated surface at the LF region [38]. The electrochemicalparameters and inhibition efficiency from the impedance mea-surements for mild steel in HCl and H2SO4 in the absence andpresence of WMRE are listed in Tables 1 and 2, respectively. Theresults in the table show that inhibition efficiency increased withincrease in concentration of the extract reaching the maximumvalues of 83.35% and 77.34% at extract concentration of 1.5 g/L and2.0 g/L in HCl and H2SO4, respectively. The corrosion inhibitioneffect could be attributed to the adsorption of some components ofthe extract onto the mild steel surface. Literature reports [30,31]

indicated that WMR majorly contain citrulline (Fig. 1) in additionto alkaloids, saponnins, tannins, flavonoids and polyphenols. Theadsorption of these compounds through heteroatoms (N, O) andaromatic ring onto the mild steel surface create barrier for massand charge transfer thus isolating the metal from attack of thecorrosive ions present in the solution.

The inhibition efficiency (Eimp) for the impedance data wasestimated by comparing the charge transfer resistances (Rct) in theabsence and presence of WMRE as follows.

Eim p ¼Rct � R�ct

Rct� 100 (1)

where Rct and R8ct are the charge transfer resistance values in thepresence and absence of WMRE, respectively.

[(Fig._3)TD$FIG]

Fig. 3. EIS plots for mild steel in 1 M HCl in the absence and presence of different

concentration of WMRE exemplified as: (a) Nyquist, (b) bode modulus and (c) bode

phase angle plots.

[(Fig._4)TD$FIG]

Fig. 4. EIS plots for mild steel in 0.5 M H2SO4 in the absence and presence of different

concentration of WMRE exemplified as: (a) Nyquist, (b) bode modulus and (c) bode

phase angle plots.

N.A. Odewunmi et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 239–247242

The transfer function with a simple Faradiac reaction withoutdiffusion in terms of CPE had been expressed to show theproportionality factor (Y) as it deviated from the ideal capacitanceby n < 1. The cause of CPE behavior in a blocking electrode is

independent of frequency and characterized by a phase anglemaxima smaller than 908 [39].

In a surface time constant distribution when n < 1, Brug et al.[40] formula have been developed using numerical simulation for

[(Fig._5)TD$FIG]

Fig. 5. Equivalent circuit for the EIS data fittings: (a) WMRE in 1 M HCl and (b)

WMRE in 0.5 M H2SO4.

N.A. Odewunmi et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 239–247 243

the influence of geometry induced current distribution [41] toEq. (2) and proven to be a more accurate way to determine effectivecapacitance (Ceff) in a CPE behavior impedance.

Ce f f ¼ Y1=nRð1�nÞ=ns (2)

The values of effective capacitance obtained by utilizing theimpedance parameters (Rs and Yo) in both acidic media are in therange of double layer capacitance. The double layer capacitance Cdl,values (Tables 1 and 2) obtained at the maximum frequency (fmax)of the imaginary component of the impedance was observed todecrease with the introduction of the extract into the acidic media.The decrease in Cdl resulted from the increase in double layerthickness that will lead to decrease in dielectric constant as a resultof adsorption of extract (organic matter) on to the surface of themild steel by displacing the adsorbed water molecule [42], therebyprotected it from acid attack. This behavior is in conformity withthe mathematical expression of Helmholtz model [43] that relatesthe dielectric constant of the medium (e), vacuum permittivity (e0),exposed area (A) and the thickness of the protective layer (d).

Table 1EIS data of mild steel in 1 M HCl in the absence and presence of different concentratio

WMRE (g/L) Rs (V cm2) Rct (V cm2) Yo (mV Sn cm�2) Ceff (mF

1 M HCl 4.13 73.14 154.60 68.30

0.10 5.02 179.70 89.43 38.00

0.50 5.06 253.20 94.14 40.30

1.00 6.63 339.90 78.92 34.10

1.50 3.56 439.30 97.50 40.30

2.00 4.23 439.20 94.89 39.90

Table 2EIS data of mild steel in 0.5 M H2SO4 in the absence and presence of different concent

WMRE (g/L) Rs (V cm2) Rct (V cm2) L (V m2) RL (V cm2) Yo (mV Sn cm

0.5 M H2SO4 4.67 50.91 – – 148.80

0.10 5.23 67.61 4.94 604.10 148.90

0.50 5.34 135.70 1707.0 1870.0 116.80

1.00 4.98 137.80 560.60 1482 145.30

1.50 4.86 195.30 4229.0 0.049 119.20

2.00 4.93 224.70 1810.0 3530 113.40

4.3. Potentiodynamic polarization (PDP) measurements

PDP curves for mild steel samples in the presence and absenceof WMRE in 1 M HCl and 0.5 M H2SO4 are shown in Fig. 6a and b,respectively. The anodic and cathodic polarization curves for theWMRE in HCl medium exhibited a well-defined Tafel regions(Fig. 6a) while the anodic region of the H2SO4 are not distinct(Fig. 6b). This behavior is attributed to the formation of non-passive film from the corrosion product or the impurities in themild steel [2]. The electrochemical parameters such as corrosionpotential (Ecorr), corrosion rate (Crate), corrosion current density(Icorr), anodic Tafel slopes (ba) and cathodic Tafel slopes (bc)extrapolated from the curves in both media are given in Table 3.Results in the table for both media indicate that the Icorr decreasesin the presence of WMRE when compared with blank acidsolutions. Further inspection of the table revealed that Icorr

decreased as the concentration of WMRE increased up to 1.5 g/Land thereafter, slightly increased as the concentration wasincreased to 2.0 g/L. It could also be observed from the table thatthere is no significant change in the Ecorr values in the presenceand absence of the extract. This shows that the inhibition of mildsteel corrosion by WMRE occurred by the geometric blockingeffect mechanism [44] hence WMRE can be regarded as a mixed-type inhibitor. The inhibition efficiency (Epdp) estimated from thevalues of corrosion current density (Icorr) in the presence and inthe absence (I8corr) of the inhibitor using Eq. (7) are listed inTable 3.

E pd p ¼I�corr � Icorr

I�corr

� 100 (7)

It is seen in the table that inhibition efficiency increases withincrease in concentration of WMRE in both acid media reachingmaximum values of 80.71 and 79.90% at the extract concentrationof 1.5 g/L and 2.0 g/L in HCl and H2SO4, respectively. The results arein close agreement with the ones obtained from EIS measurements.

4.4. Linear polarization resistance (LPR) measurements

Fig. 7 shows linear polarization resistance plots for mild steel inthe absence and presence of different concentrations of WMRE in(a) 1 M HCl and (b) 0.5 M H2SO4 solutions. The slope of thepotential (E) versus current density (i) curve measured at thecorrosion potential (Ecorr) was used to estimate the polarizationresistance (Rp) values obtained from the LPR measurement. Theinhibition efficiency (Elpr) estimated by comparing the Rp values

ns of WMRE.

cm�2) Cdl (mF cm�2) n u Eimp (%) x2�10�4

87.19 0.90 – – 0.24

55.85 0.89 0.59 59.30 10.51

50.85 0.87 0.71 71.11 5.74

47.10 0.86 0.78 78.48 9.66

57.26 0.87 0.83 83.35 5.00

57.28 0.87 0.83 83.35 3.54

rations of WMRE.

�2) Ceff (mF cm�2) Cdl (mF cm�2) n u Eimp (%) x2�10�4

66.10 98.60 0.92 – – 1.41

67.80 74.14 0.89 0.25 24.70 1.78

51.50 58.99 0.88 0.62 62.48 2.04

65.10 72.84 0.87 0.63 63.05 1.76

52.10 81.98 0.89 0.74 73.93 1.97

49.40 57.06 0.88 0.77 77.34 2.53

[(Fig._6)TD$FIG]

Fig. 6. PDP curves for mild steel in 1 M HCl in the absence and presence of different

concentration of WMRE: (a) 1 M HCl and (b) 0.5 M H2SO4.

N.A. Odewunmi et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 239–247244

obtained in the presence and absence of WMRE using Eq. (8) isgiven in Table 4.

El pr ¼R p � R� p

R p� 100 (8)

where Rp and R8p represent the polarization resistance in thepresence and absence of WMRE, respectively. It can be observedfrom the table that the Rp values increases as the concentration of

Table 3PDP data of mild steel in 1 M HCl and 0.5 M H2SO4 in the absence and presence of diff

Medium WMRE (g/L) Ecorr (mV/SCE) Icorr (mA cm�2)

HCl 1 M HCl �487.60 226.00

0.1 �503.60 106.70

0.5 �508.50 68.01

1.0 �509.80 63.77

1.5 �498.20 43.59

2.0 �504.40 45.51

H2SO4 0.5 M H2SO4 �491.60 360.60

0.1 �489.10 347.90

0.5 �484.20 142.00

1.0 �477.90 98.35

1.5 �467.00 88.08

2.0 �473.30 72.51

the extract increases in the H2SO4 medium. However, the Rp valuesincreases up to 1.5 g/L in the HCl medium and thereafter decreasesto confirm its optimum concentration in the medium.

Comparison of the inhibition efficiencies from EIS, PDP and LPRtechniques shown in Fig. 8 show that 1.5 g/L of the inhibitor(WMRE) was the optimum concentration in all the electrochemicalmethods. The values obtained from all the techniques were in closeagreement in both the HCl and H2SO4 environments.

Results obtained in the present work shows that WMRE is abetter corrosion inhibitor in HCl than H2SO4 solution. Reports inthe literature on the comparative studies of corrosion inhibition oforganic species in HCl and H2SO4 solutions indicate that theinhibiting effect of organic species in HCl is much higher than thatin H2SO4 acid solutions [45–48]. This observation has beenattributed to the influence of chloride and sulphate anions. Ithas been suggested that chloride ions being less hydrated thansulphate ions has a stronger tendency to adsorb on the metalsurface than do sulphate ions thereby creating an excess negativecharge toward the solution phase, which favors synergisticadsorption of the organic species on the metal surface [46]. Thismay be the reason for an increased protective effect in the HClsolution.

4.5. Adsorption isotherm

The EIS experimental data was utilized to obtain usefulinformation on the nature of adsorption of WMRE on the mildsteel surface via adsorption isotherms. Langmuir isotherm which isderived for a monolayer adsorption without the consideration ofinteractions between the adsorbed species was first considered.Fig. 9 (a) shows the plot of the ratio of surface coverage (u) andextracts concentration (Cinh) against the WMRE concentration. Astraight line graphs were obtained with a correlation coefficient(R2) of 0.9987 and 0.9925 in HCl and H2SO4, respectively, but theslopes were greater than unity. The deviation of the slopes fromunity indicates that there are interactions between the adsorbedspecies in both media. Langmuir isotherm is characterized by:

Cinh

u¼ 1

Kadsþ Cinh (9)

where u is the surface coverage, Cinh is the concentration and Kads isthe equilibrium constant of the adsorption process. Kads is alsorelated to the standard Gibbs energy (DG8ads) of adsorption by theequation:

DGads ¼ �RT lnð55:5� KÞ (10)

where R is the molar gas constant, T is the absolute temperature inKelvin and 55.5 as the concentration of water in mol dm�3.

erent concentrations of WMRE.

ba (mV dec�1) bc (mV dec�1) Crate (mpy) Epdp (%)

97.05 125.00 131.50 –

114.30 109.70 62.07 52.79

121.60 115.10 39.56 69.91

135.70 121.50 37.10 71.78

128.70 108.20 25.36 80.71

120.30 109.00 26.47 79.86

77.93 134.50 209.80 –

77.91 145.80 202.40 3.52

63.41 129.50 82.58 60.62

55.54 126.60 57.21 72.73

48.54 128.60 51.24 75.57

53.39 123.10 42.18 79.90

[(Fig._7)TD$FIG]

Fig. 7. LPR plots for mild steel in (a) 1 M HCl and (b) 0.5 M H2SO4 in the absence and

presence of different concentration of WMRE.

[(Fig._8)TD$FIG]

Fig. 8. Bar chart for the comparison of the inhibition efficiencies for the three

electrochemical techniques: (a) 1 M HCl and (b) 0.5 M H2SO4.

N.A. Odewunmi et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 239–247 245

Temkin adsorption isotherm model was tested to account forthe observed interactions between the adsorbed species fromLangmuir isotherm model. It is given by the equation:

u ¼ 1

2aln K þ 1

2aln Cinh (11)

Table 4LPR data of mild steel in 1 M HCl and 0.5 M H2SO4 in the absence and presence of

different concentrations of WMRE.

Medium WMRE (g/L) Rp (V cm�2) Elpr (%)

HCl 1 M HCl 75.95 –

0.1 156.90 51.70

0.5 287.00 73.54

1.0 363.70 79.12

1.5 428.20 82.26

2.0 403.00 81.15

H2SO4 0.5 M H2SO4 58.16 –

0.1 69.75 16.62

0.5 144.40 59.72

1.0 144.50 59.75

1.5 182.80 68.18

2.0 219.20 73.47

where ‘‘a’’ is the lateral interaction term and K is the adsorption–desorption equilibrium constant. A linear plot with R2 of 0.9872and 0.9545 for mild steel in 1 M HCl and 0.5 M H2SO4 in thepresence of WMRE were obtained from the Temkin plot (u vs ln C)as shown in Fig. 9(b). The adsorption parameters such as Kads, DGads

and ‘‘a’’ values derived from Temkin isotherm plot are presented inTable 5. The larger the value of K, the better the inhibitor efficiency[2]. The higher values of K in HCl medium indicate strongeradsorption of WMRE in HCl than H2SO4 solution. The existence oflateral forces of attraction between the adsorbent molecule in theadsorption layer was revealed by the positive values of ‘‘a’’ [49].Also, the spontaneity and the stability of the adsorption process arededuced from the negative values of DG8ads. The values of DGads inboth media lies between �10.69 and �15.92 kJ mol�1, whichindicates that the WMRE is physically adsorbed on the mild steelsurface. Generally, the values of DG8ads of up to �20 kJ mol�1 areconsistent with electrostatic interaction between charged inhibi-tor molecules and the mild steel surface (physical adsorption) andthose more than �40 kJ mol�1 involves charge sharing or transferfrom the inhibitor molecule to mild steel surface to form acoordinate bond (chemisorption) [50,51].

4.6. Morphological study

The surface analysis of the mild steel was done after exposure to1 M HCl and 0.5 M H2SO4 solutions in the absence and presence of

[(Fig._9)TD$FIG]

Fig. 9. Adsorption isotherms for WMRE in 1 M HCl and 0.5 M H2SO4 on mild steel:

(a) Langmuir isotherm and (b) Temkin isotherm.

Table 5Adsorption isotherms for WMRE in 1 M HCl and 0.5 M H2SO4 on mild steel.

Isotherms Medium Slope R2 Kads DG8ads a

Langmuir HCl 1.1564 0.9987 11.22 �15.92 –

H2SO4 1.1633 0.9925 3.41 �12.97 –

Temkin HCl – 0.9872 2.23 �11.92 5.92

H2SO4 – 0.9545 1.36 �10.69 2.92

[(Fig._10)TD$FIG]

Fig. 10. SEM images of mild steel exposed in 1 M HCl and 0.5 M H2SO4 in the absence an

1 M HCl with 2.0 g/L WMRE and (d) 0.5 M H2SO4 with 2.0 g/L WMRE.

N.A. Odewunmi et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 239–247246

2.0 g/L concentration of WMRE for 24 h at 25 8C. The resultsindicated that the surface of the mild steel were severely damagedas shown in Fig. 10a andb due to the roughness of the surface asreveal by SEM when the mild steel was immersed in HCl andH2SO4, respectively, without WMRE. The corrosion was relativelyuniform with no evidence of localized attack. However, in thepresence of WMRE, the SEM images revealed a reduced surfaceroughness compared with the acidic media. The corrosioninhibition effects observed in Fig. 10c and d in the presence ofWMRE in HCl and H2SO4, respectively, on the mild steel was due tothe formation of a protective film.

5. Conclusions

Based on the experimental results, the following conclusionscan be drawn from the study:

(1) W

d pre

MRE inhibits corrosion of mild steel in 1 M HCl and 0.5 MH2SO4 solution at 25 8C. The corrosion inhibition was found tobe concentration dependent with an optimum concentration of1.5 g/L in 1 M HCl medium.

(2) W

MRE was found to be more effective toward corrosionprotection in the 1 M HCl medium than in 0.5 M H2SO4.

sence of WMRE for 24 h at room temperature: (a) 1 M HCl, (b) 0.5 M H2SO4, (c)

N.A. Odewunmi et al. / Journal of Industrial and Engineering Chemistry 21 (2015) 239–247 247

(3) T

he values of effective capacitance obtained by utilizing the Rs

and Yo for both acidic media are between 20 and 70 mF cm�2

which are in the range of double layer capacitance.

(4) P DP results indicate a mixed inhibitor effect of WMRE affecting

both the anodic dissolution of mild steel and cathodic hydrogenevolution.

(5) W

MRE was found to be physically adsorbed on the mild steelsurface in both acid media following Temkin adsorptionisotherm model.

Acknowledgments

The support of the Center of Research Excellence in Corrosion,King Fahd University of Petroleum and Minerals is gratefullyacknowledged.

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