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Natural Products for Materials Protection: Inhibition of Mild Steel Corrosion by Date Palm Seed Extracts in Acid Media SAVIOUR UMOREN, Zuhair M Gasem, and Ime B Obot Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie401737u • Publication Date (Web): 27 Sep 2013 Downloaded from http://pubs.acs.org on October 7, 2013

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Industrial & Engineering Chemistry Research

Natural Products for Materials Protection: Inhibition of Mild Steel Corrosion by Date Palm Seed Extracts in Acid Media

Saviour A. Umoren*, Zuhair M. Gasem , Ime B. Obot

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

*For correspondence: Email: [email protected] (S.A.Umoren); Phone: +966 3 860 7902; Fax: +966 3 860 3996.

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ACS Paragon Plus Environment


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ABSTRACT The corrosion inhibition effect of date palm (Phoenix dactylifera) seed extracts for mild steel in 1 M HCl and 0.5 M H2SO4 solutions was investigated by weight loss and electrochemical methods. Inhibition efficiency increased with increase in extract concentration and decreased with increase in temperature. Extract concentration of 2.5 g L−1 and 1.5 g L−1 gave maximum inhibition efficiency in 1 M HCl and 0.5 M H2SO4 respectively. Immersion time was also found to influence the corrosion inhibition effect in both acid media. Polarization curves indicate that the extract functions as a mixed inhibitor affecting both the anodic and cathodic partial reactions of the corrosion process. The adsorption of the extract onto mild steel surface followed Langmuir adsorption isotherm. The mechanism of physical adsorption has been proposed based on the trend of inhibition efficiency with temperature. The date palm seed extract is a better corrosion inhibitor for mild steel in HCl than in H2SO4 solution. .

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1. INTRODUCTION Some aqueous fluids used to stimulate oil and gas wells, clean surfaces or pickle steel include formulations that contain (HC1) used at up to 15% concentration 1. In addition, formulations of chelating agents such as EDTA, organic acids, concentrated solutions of sulphuric acid, and other mineral acids are also used. The major problem with hydrochloric acids and any other acids used in these processes is the enhanced corrosion rate of oil and gas well equipment during acid treatment. In order to control the corrosion rate of oil and gas well equipment during acid treatment, the acid must be inhibited. Among the several methods of corrosion control and prevention, the use of corrosion inhibitors is very popular in acidic medium. The majority of the wellknown inhibitors are organic compounds containing heteroatoms, such as oxygen, nitrogen or sulphur, and multiple bonds, which allow an adsorption on the metal surface 1

. Unfortunately, many common corrosion inhibitors that are still in use today are

hazardous to health 2. As a result of more stringent environmental regulations that have been implemented in recent years, production chemicals used in the oil and gas production industry are required to have an environmentally friendly profile. Production chemicals are required to be non bioaccumulative, biodegradable and have a low toxicity level 3. Researches nowadays are directed towards the development of environmentally compatible, nonpolluting corrosion inhibitors. Plant extracts viewed as an incredibly rich source of naturally synthesized chemical compounds can readily satisfy this need. In addition to being inexpensive, they are environmentally friendly and ecologically acceptable, renewable and readily available. They can also be extracted by simple procedures with low cost. In recent times, considerable effort has been devoted to study the corrosion inhibiting efficacy of some natural products, particularly biomass extracts of plant origin for mild steel in HCl and H2SO4 solutions. For instance Raja et al has reported the corrosion inhibition effect of Neolamarckia cadamba for mild steel in 1 M HCl 4. Also Oguzie and his co-workers have reported the corrosion inhibition of mild steel in 1M HCl 3



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and 0.5 M H2SO4 solutions by leaves extracts of Piper guineense

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5,6

, Aframomum

melegueta 7. Coffee senna 8 and seed extracts of Punica granatum 9. Schinopsis lorentzii extract has been reported as a green corrosion inhibitor for low carbon steel in 1 M HCl solution10. Also, corrosion inhibition effect of extracts of Artemesia pallens leaf

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leaves

11

, bamboo

, Ginkgo leaves13, garlic peel14, Osmanthus fragran leaves15, Salvia officinalis 16

, Oxandra asbeckii

17

, Argemone Mexicana

18

, Acalypha torta Leaf

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and

Caulerpa racemosa 20 for steel in HCl and/or H2SO4 have been reported in the literature. The date palm (Phoenix dactylifera) is a monocotyledonous woody perennial fruit species belonging to the Arecaceae family 21. It has long been one of the most important fruit crops in the arid regions of the Arabian Peninsula, North Africa, and the Middle East. Date palms produce many products that are useful to humans. The primary product is the date fruit, which can be eaten fresh, dried, or in various processed forms. The anticorrosive effect of date palm fruit juice on 7075 type aluminum alloy in 3.5% NaCl solution

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as well as leaf extract on steel, aluminium, copper and brass in hydrochloric

acid and sodium hydroxide solutions 23 has been reported. Date palm seeds ground into paste has been reported to be effective in treating ague 24. It is also used as feed for livestock or strung as beads for decoration 21. The date palm seeds apart from the above mentioned purposes have no commercial value and may be considered as waste products. Recently, corrosion inhibition effect of date palm seed extracts for mild steel in 2 M H2SO4 solution has been reported using weight loss and hydrogen evolution methods

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. However, there is no published report on the corrosion

inhibition effect of date palm seed extract for mild steel in HCl solution and electrochemical investigations highlighting the influence of date palm seed extract on the kinetics of anodic and cathodic partial reactions of the corrosion process. The present study was undertaken to evaluate the corrosion inhibition potential of date palm seed extract (DPSE) for mild steel in 1 M HCl and 0.5 M H2SO4 solutions using weight loss, linear polarization,

potentiodynamic polarization and electrochemical impedance

spectroscopy (EIS) measurements at 25 and 60 oC.

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2. EXPERIMENTAL 2.1

Materials Preparation: The concentration of the blank corrodent was 1 M and 0.5

M prepared from AR grade 37 − 38% HCl (Merck) and 98% H2SO4 (Baker) respectively. All preparations were made using doubly distilled water. The tested extract was prepared by boiling 10 g of dried and ground date palm seed in one litre of 1 M HCl and 0.5 M H2SO4 respectively for 10 min. The solutions were cooled, filtered using Whatman filter paper No 1 and adjusted to one litre of volume to obtain the stock solution (10 g L−1). Different concentrations of the extract (0.5 – 2.5 g L−1) were prepared by diluting the stock solution with the respective blank corrodent of 1 M HCl and 0.5 M H2SO4. Mild steel specimens with the following chemical composition (weight percentage) were used in the experiments: 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 the balance Fe. Test coupons for weight loss and electrochemical measurements were cut into 3 cm × 3 cm × 0.25 cm dimensions. These coupons were abraded with silicon carbide abrasive paper (from grade #320 to #800), rinsed with distilled water, placed in an ultrasonic acetone bath for about 5 min to remove possible residue of polishing, rinsed with acetone, dried in warm air and then stored in moisture-free desiccators prior to use. 2.2

Weight loss measurements Weight loss measurements were conducted in glass vessels containing 250 mL of

test solution maintained at 25 and 60 oC with thermostated water bath. Tests were performed under total immersion in aerated and unstirred conditions in the absence and presence of the inhibitor. In each experiment, triplicate samples of the pre-cleaned and weighed mild steel coupons were freely suspended in the different test solutions. The test coupons were retrieved after 24 and 72 h immersion thoroughly cleaned using chemical method for cleaning rust products as previously reported

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and reweighed using digital

analytical balance with sensitivity 0.1 mg. The weight loss was taken as the difference between the weight at a given time and the initial weight of the coupon. The standard

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deviation values among parallel triplicate experiments were found to be smaller than 5%, indicating good reproducibility. From the weight loss values, corrosion rates in terms of thickness loss (mm y−1) were computed using the equation: C R (mm y −1 ) =

87600 × ∆W ρAT

(1)

where CR is the corrosion rate, W is the average weight loss (mg) , ρ is the density of mild steel specimen (g cm−3), A is the surface area of the specimen (cm2) and T is the exposure time (h). The inhibition efficiency (η%) of date palm seed extracts was evaluated from the following equation: η% =

C R(blank) − C R(inh) C R(blank)

× 100

(2)

where CR(blank) and CR(inh) are the corrosion rate in the absence and presence of the inhibitor respectively in 1 M HCl and 0.5 M H2SO4 at the same temperature.

2.3 Electrochemical Measurements All electrochemical experiments were performed in one-compartment cell with three electrodes connected to Gamry Instrument Potentiostat/ Galvanostat/ZRA (Reference 3000) with a Gamry framework system based on ESA410. Gamry applications include software DC105 for corrosion, EIS300 for electrochemical impedance spectroscopy (EIS) measurements and Echem Analyst 6.0 software package for data fitting. The mild steel was the working electrode with exposed surface area of 0.7855 cm2 in the corrosive environment, platinum wire was used as a counter electrode and saturated calomel electrode (SCE) as the reference electrode. All potentials were measured versus SCE reference electrode. Tafel curves were obtained by changing the electrode potential automatically from −250 to +250 mV versus open circuit potential (Ecorr) at a scan rate of 1 mVs−1. Linear polarization resistance (LPR) experiments were done from −20 to +20 mV versus Ecorr at the scan rate of 6


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0.125 mVs−1. EIS measurements were carried out under potentiostatic conditions in a frequency range from 100 kHz to 100 mHz, with an amplitude of 10 mV peak-to-peak, using AC signal at Ecorr. All experiments were measured after immersion for 30 min in 1 M HCl and 0.5 M H2SO4 with and without addition of inhibitor.

2.4 SEM Surface Morphology Morphological studies of the mild steel electrode surface were undertaken by SEM examinations of electrode surfaces exposed to diﬀerent test solutions using a JSM-5800 LV scanning electron microscope. Mild steel specimens of dimensions 3 × 3 × 0.25 cm were polished successively with silicon carbide abraded paper of different grades (#320−#800) and thereafter using cloth with 1µm diamond paste to near mirror finished surface. The precleaned coupons were immersed for 12 h in the blank solutions 1 M HCl and 0.5 M H2SO4 without and with 1.5 g L−1 date palm seed extract at 25 ±1 ◦C, and then washed with distilled water, dried in warm air, and submitted for SEM surface examination.

3. RESULTS AND DISCUSSION 3.1 Corrosion rates and inhibition efficiency: The effect of addition of date palm seed extract tested at different concentrations on the corrosion of mild steel in 1 M HCl and 0.5 M H2SO4 solutions was studied by weight loss measurements at 25 and 60 oC after 24 h of immersion. The results of the weight loss determinations of mild steel in 1 M HCl and 0.5 M H2SO4 media without and with addition of different concentrations of DPSE are presented in Figure 1. Figure 1 shows the plot of (a) corrosion rate and (b) inhibition efficiency as a function of concentration of the extract at 25 and 60 oC for mild steel in 1 M HCl and 0.5 M H2SO4. These results show that the tested extract inhibits the corrosion of mild steel in 1 M HCl and 0.5 M H2SO4 at all concentrations used in this study, since there is a general decrease in the corrosion rates of mild steel at the end of the corrosion-monitoring process. Inspection of the plots show higher corrosion rates in 0.5 M H2SO4 and also revealed clearly that DPSE inhibited the corrosion of mild steel in 7



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both acid environments with pronounced effect observed in 1 M HCl. The corrosion inhibiting effect increases with increasing DPSE concentration indicating dependence of the inhibition process on the amount of inhibiting species present in the corrosive medium. Examination of the figures show that corrosion rates increased with increase in temperature both in the absence and presence of DPSE in both acid environments. Also inhibition efficiency is observed to decrease with increase in temperature. The trend of inhibition efficiency with DPSE concentration shown in Figure 1(b) for mild steel in 1 M HCl revealed an increase in inhibition efficiency with increase in extract concentration reaching a maximum value of 90.95% and 61.95% at 25 and 60 oC respectively. In 0.5 M H2SO4 however, maximum inhibition efficiency of 86. 25% was obtained at extraction concentration of 1.0 g L−1 at 25 oC and 42.14% at extract concentration of 1.5 g L−1 at 60 oC. The inhibitive action of DPSE toward the corrosion of mild steel in both HCl and H2SO4 can be attributed to the adsorption of the DPSE constituents onto the steel surface. Corrosion inhibition is initiated by the displacement of adsorbed water molecules by the inhibitor species leading to specific adsorption on the metal surface 27. Some of the important chemical constituents of date palm seed have been reported to include carbohydrate, alkaloids and tannins

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. These constituents have heteroatoms such as N and O which are

regarded as centers of adsorption in their molecules. The adsorption of DPSE constituents through these atoms, onto the metal surface creates barrier for mass and charge transfer and thus isolates the metal from further attack of the corrosive anions.

3.2 Effect of Immersion Time The corrosion behavior of mild steel in 1 M HCl and 0.5 M H2SO4 in the absence and presence of date palm seed extract was studied at 24 and 72 h in order to determine the influence of immersion time on the corrosion inhibition effect of DPSE using weight loss method. The results obtained are presented in Figure 3. Figure 3 shows the plot of (a) corrosion rate and (b) inhibition efficiency as a function of concentration of the extract at 24 and 72 h immersion for mild steel in 1 M HCl and 0.5 M H2SO4. It is observed from the plot that corrosion rate increased with increase in immersion time both in the absence and presence of low concentration of DPSE (0.5 and 1.0 g L−1) in HCl solution and a decrease in 8


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corrosion rate with increase in immersion time at high extract concentration. In H2SO4 solution, corrosion rate is observed to increase with immersion time both in the absence and presence of DPSE within the range of concentrations studied (except at 2.5 g L−1). Inspection of the figure further reveals a slight decrease in inhibition efficiency with increase in immersion time from 24 to 72 h at low extract concentration and an increase in inhibition efficiency with prolonged immersion as the extract concentration is increased in 1 M HCl solution. However, in 0.5 M H2SO4 solution, a decrease in inhibition efficiency with increase in immersion time from 24 to 72 h is observed from the plots at all DPSE concentrations with exception of 2.5 g L−1 extract concentration. The increase in inhibition efficiency with longer immersion time as noted in HCl solution can be attributed to the formation of a protective film which is time-dependent on the mild steel surface. It has been pointed out that stable, two-dimensional layers of inhibitor molecules are formed on metal surfaces after longer immersion time 28,29. On the other hand, according to Outirite et al. 30, the decrease in inhibition efficiency with prolonged immersion time can be due to instability of the protective layer as a result of desorption of the constituents of the DPSE and/or diffusion process through the interface protective layer.

3.3 Potentiodynamic polarization measurements Polarization measurements are suitable for monitoring the progress and mechanisms of the anodic and cathodic partial reactions as well as identifying the effect of an additive on either partial reaction 6. Potentiodynamic polarization experiments were undertaken to determine the effect of the anodic (Fe

Fe2+ + 2e) and cathodic (2H+ +2e

H2) partial reactions

of the corrosion process. Typical potentiodynamic polarization curves for the mild steel specimens in 1 M HCl and 0.5 M H2SO4 without and with DPSE are shown in Figure 3 (a) and (b) respectively. The polarization curves reveal that the mild steel specimen is seen to exhibit active dissolution with no distinctive transition to passivation within the studied potential range in both acid environments. The plots also show that the anodic and cathodic reactions in blank acid and on addition of DPSE follow Tafel’s law. Hence the linear Tafel segments of the anodic and cathodic curves were extrapolated to corrosion potential to obtain the corrosion current densities (icorr). 9



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The corresponding electrochemical parameters namely corrosion current densities (icorr), corrosion potential (Ecorr), the cathodic Tafel slope (βc) and anodic Tafel slope (βa) derived from the polarization curves are presented in Table 1. Results in the table indicate that corrosion current density decreases markedly in the presence of DPSE compared to the blank solution and also decreases with increasing concentration of the extract. It is also observed that the presence of DPSE does not shift Ecorr remarkably; therefore, DPSE could be regarded as a mixed-type inhibitor and the inhibition of mild steel corrosion by DPSE occurred by geometric blocking effect mechanism 31. The values of the corrosion current density in the absence (iocorr) and presence of inhibitor (icorr) were used to estimate the inhibition efficiency from polarization data as follows:

 i η% = 1 − corr o  i corr

  × 100 

(3)

The values obtained are listed in Table 1. From the table, it is seen that inhibition efficiency increases with increase in concentration of DPSE reaching a maximum value of 89.3% at the highest concentration (2.5 g L−1) of the extract studied in 1 M HCl. However, in 0.5 M H2SO4, inhibition efficiency increases with increase in concentration of the extract up to 1.5 g L−1 and thereafter decreases with further increase in concentration. Hence maximum inhibition efficiency of 87.3% was obtained at extract concentration of 1.5 g L−1. This result clearly demonstrates that DPSE is a better corrosion inhibitor in HCl than in H2SO4 environment.

3.4

Electrochemical Impedance spectroscopy Measurements Electrochemical impedance spectroscopy (EIS) experiments were undertaken in order to understand the characteristics and kinetics of the electrochemical processes on the mild steel in 1 M HCl and 0.5 M H2SO4 solutions and how they were modified by DPSE. Figure 4 shows the impedance spectra represented in panels a and b as Nyquist and Bode plots respectively in the absence and presence of different concentrations of DPSE for mild steel corrosion in 1 M HCl. Similar plots for mild steel corrosion in 0.5 M H2SO4 is depicted in Figure 5(a) and (b) respectively. In HCl solution, the complex impedance diagram recorded at Ecorr in the absence and presence of different concentrations of DPSE has similar 10


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shape, indicating that almost no change in the corrosion mechanism occurred as a result of DPSE addition. The Nyquist plots show single semicircles for all systems over the frequency range studied, corresponding to one time constant in the Bode plots. The high frequency intercept with the real axis in the Nyquist plots is assigned to the solution resistance (Rs) and the low frequency intercept with the real axis ascribed to the charge transfer resistance (Rct). However, in H2SO4 solution, the Nyquist plots in the presence of DPSE are characterized by a large capacitive loop at high to medium frequency and inductive loop at low frequency. The capacitive loop at high frequencies represents the phenomenon associated with the double electric layer. It arises from the time constant of the electrical double layer and the charge transfer in corrosion process 32, 33. The presence of inductive loop may be attributed to relaxation of adsorption species like Hads+ and SO42− on the electrode surface, exhibiting negative change in the surface coverage with potential on the surface of the metal

34, 35

.

Inspection of Figures 4 and 5 reveal an increase in size of the semicircle in Nyquist plots in the presence of DPSE compared to the blank solutions which is an indication of the inhibition of the corrosion process. Also, the sizes of the semicircles increase with increase in concentration of DPSE particularly in HCl solution. In H2SO4 environment, however, the sizes of the semicircles increase with increase in concentration of DPSE up to 1.5 g L−1 and thereafter decrease. It is observed that the semicircles are not perfect but depressed with centre under the real axis. This depressed form of semicircles has been attributed to surface inhomogeneity of structural or interfacial origin such as those found in adsorption processes36. The impedance spectra were analyzed by fitting to the equivalent circuit model shown in Figure 6 (a) and (b) for mild steel corrosion in 1 M HCl and 0.5 M H2SO4 respectively. The circuit consists of the solution resistance Rs, the charge transfer resistance, Rct, the constant phase element, CPE for HCl corrosive medium and in addition the inductive elements, RL and L for H2SO4 corrosive medium. For the description of a frequency independent phase shift between an applied AC potential and its current response, a constant phase element (CPE) is used which is defined in impedance representation as 37: Z CPE = Y0 −1 ( jω )

−n

(4) 11



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where Y0 is the CPE constant, n is the CPE exponent which can be used as a guage of the heterogeneity or roughness of the surface

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, j2 = −1 is an imaginary number and ω is the

angular frequency in rad s−1. Depending on n, CPE can represents a resistance (ZCPE = R, n = 0); capacitance (ZCPE = C, n = 1), Warburg impedance (ZCPE = W, n = 0.5) or inductance ((ZCPE = L, n = −1). The values of the electrochemical parameters derived from Nquist plots for mild steel corrosion in 1 M HCl and 0.5 M H2SO4 in the absence and presence of different concentrations of DPSE are listed in Tables 2 and 3 respectively. Results presented in the tables reveal that the DPSE extracts increased the magnitude of Rct, with corresponding decrease in the double layer capacitance (Cdl). The increase in Rct values in inhibited systems, which corresponds to an increase in the diameter of the Nyquist semicircle, confirms the corrosion inhibiting effect of the DPSE extracts. The observed decrease in Cdl values, which normally results from a decrease in the dielectric constant and/or an increase in the doublelayer thickness, can be attributed to the adsorption of the extract organic matter (with lower dielectric constant compared to the displaced adsorbed water molecules) onto the metal/electrolyte interface, thereby protecting the metal from corrosive attack 5. The decrease in Cdl is in accordance with Helmholtz model given by the following equation 39: C dl =

εε 0 A d

(5)

where d is the thickness of the protective layer,ε is the dielectric constant of the medium, ε0 is the vacuum permittivity and A is the effective area of the electrode. The double layer capacitance, Cdl values were obtained at the frequency at which the imaginary component of the impedance is a maximum from the equation 40:

C dl =

1

(6)

2πf max R ct

Inhibition efficiency from the impedance data was computed by comparing the values of the charge transfer resistance in the absence and presence of DPSE as follows: η% =

R ct − R oct × 100 R ct

(7)

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where Roct and Rct are the charge transfer resistances in the absence and presence of the inhibitor, respectively. The values of inhibition efficiency given in Tables 2 and 3 for mild steel corrosion in 1 M HCl and 0.5 M H2SO4 respectively show that inhibition efficiency increases with increase in concentration of DPSE in 1 M HCl, while in 0.5 M H2SO4, inhibition efficiency increases with increase in concentration of the extract up to 1.5 g L−1 and thereafter decreases with further increase in concentration. Linear polarization resistance (LPR) measurements give a virtual instantaneous measurement enabling the corrosion rate and performance of the inhibitors to be monitored with time. The most important electrochemical parameter derived from LPR measurements is the polarization resistance (Rp) which is defined as the slope of the potential-current density (E vs i) curve at the corrosion potential,Ecorr. The polarization resistance values without and with different concentrations of DPSE for mild steel corrosion in 1 M HCl and 0.5 M H2SO4 as well as corrosion inhibition efficiency obtained by LPR method are given in Table 4. Inhibition efficiency from the linear polarization data was estimated by comparing the values of the polarization resistance in the absence and presence of DPSE as follows:

η% = 1 −

Rp R 0p

× 100

(8)

where Rop and Rp are the polarization resistances in the absence and presence of inhibitor (DPSE) respectively. The results obtained show that polarization resistance increased on introduction of DPSE into the corrosive media. Also, the polarization resistance increases with increase in concentration of DPSE in HCl medium. In H2SO4 medium, the polarization resistance increases with increasing DPSE concentration up to 1.5 g L−1 and thereafter decreases. The increase in Rp values in inhibited systems also confirms the corrosion inhibiting effect of the DPSE extracts. Table 5 shows comparative values of corrosion inhibition efficiency of DPSE with other plant based extracts for mild steel in acidic media at ambient temperature. Results in the table show that the values of inhibition efficiency obtained from DPSE compares favourably with other plants extracts reported in the literature. Comparison of some corrosion parameters and inhibition efficiencies values obtained from the three electrochemical methods (EIS, PDP and LPR) used in the present study are 13



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given in Table 4. From the table, it could be observed that the values of the parameters obtained from the three techniques are in good agreement. The corrosion inhibition efficiencies followed the same trend. DPSE is observed to be a better corrosion inhibitor in HCl than in H2SO4 environment. Comparative studies of corrosion inhibition of organic species in chloride (HCl) and sulphate (H2SO4) containing solutions has shown that the inhibiting effect of organic species in HCl is much higher than that in H2SO4 acid solutions 41−43

. It has been suggested that chloride ions being less hydrated than sulphate ions has a

stronger tendency to adsorb than do sulphate ions on the metal by creating an excess negative charge towards the solution phase, which favours synergistic adsorption on the metal surface 43

3.5

may be the reason for an increased protective effect in the chloride containing solution.

Adsorption Isotherms The general consensus in corrosion inhibition literature is that the primary step in the action of inhibitors in acid solutions is believed to be adsorption onto the metal surface. This involves the assumption that the corrosion reactions are prevented from occurring over the area of the metal surface covered by adsorbed inhibitor species, whereas these corrosion reactions occurred normally on the inhibitor-free area 44. To clarify the nature of adsorption, the degree of surface coverage (θ) is very important and its value for the different concentrations of DPSE at the two temperatures studied were computed from the weight loss measurements as follows: η% = θ × 100 (assuming a direct relationship between surface coverage and inhibition efficiency) and then fitted theoretically to different adsorption isotherms. The value of correlation coefficient (R2) was used to determine the best fit isotherm. Best result was obtained with Langmuir adsorption isotherm model. Figure 7 shows the plot of C/θ against C. Linear plots were obtained which shows that the adsorption of constituents of DPSE onto mild steel surface follow Langmuir adsorption isotherm. Langmuir adsorption isotherm is characterized by: C 1 = +C θ K ads

(9)

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where θ is the surface coverage, C is the concentration, Kads is the equilibrium constant of adsorption process and is related the standard Gibbs free energy of adsorption by the equation:

K ads =

 − ∆G oads 1 exp 55.5  RT

  

(10)

where 55.5 is the concentration of water expressed in mol dm−3, R is the molar gas constant and T is the absolute temperature. Adsorption parameters derived from the plots are listed in Supporting Information (Table S1. Results in the table indicate that Kads values whose values indicate the binding power of the inhibitor to the metal surface

45

is seen to decrease with increasing temperature. Such

behaviour can be interpreted on the basis that increases in temperature results in desorption of some adsorbed constituents of the extracts on the metal surface and is consistent with the proposed physisorption mechanism. The values of free energy of adsorption which lies between –9.0 and –18.72 kJ mol–1 (in both environments), indicate that the extracts function by physically adsorbing on the metal surface. Generally, values of free energy of adsorption up to –20 kJ mol–1 are consistent with electrostatic interaction between charged molecules and a charged metal (which indicates physical adsorption) while those more negative than – 40 kJ mol–1 involves charge sharing or transfer from the inhibitor molecules to the metal surface to form a co-ordinate type of bond (which indicates chemsisorption). The data in Supporting Information (Table S1) also show considerable deviation of the slope from unity for mild steel corrosion in 0.5 M H2SO4 indicating that the isotherm may not be strictly applied. The deviation may be explained on the basis of the interaction among the adsorbed species on the surface of the metal. The differences in corrosion behavior for mild steel in 1 M HCl and 0.5 M H2SO4 in the presence of DPSE confirms the vast variety of corrosion systems and the specificity of action of most acid inhibitors.

3.6

Effect of Temperature

In order to evaluate the effect of temperature on the corrosion inhibition effect of DPSE, the weight loss experiments were undertaken at 25 and 60 oC for 24 h immersion time. Results 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 43

obtained as illustrated in Figure 1 for mild steel dissolution in 1 M HCl and 0.5 M H2SO4 respectively show that corrosion rates increases with increase in temperature in the absence and presence of the extract as well as a decrease in inhibition efficiency with temperature rise in both acid environments. Some authors have opined that an increase in temperature will enhance the rate of H+ diffusion to the metal surface as well as ionic mobility

46

. At

lower temperatures, the adsorbed hydrogen atoms block the cathodic area, while the increase in the solution temperature causes desorption of hydrogen. Such hydrogen desorption leads to an increase in the cathodic area and consequently increases the corrosion rate. A decrease in inhibition efficiency with rise in temperature is often attributed to physisorption of inhibitor species on the corroding metal surface7,

47

. The physisorption

process is hindered by the enhanced rates of hydrogen gas evolution at higher temperatures, which increasingly agitates the interface and also promotes dispersal of adsorbed inhibitor 7. Accordingly, an increase in temperature is accompanied by desorption of those constituents of DPSE extract that are attached to the metal surface via physical adsorption, leading to reduced surface coverage, hence lower inhibition efficiency. The activation parameters for the corrosion process were calculated from Arrheniustype plot according to the following equation:

log

Ea  1 1  CR2  −  = CR1 2.303R  T1 T2 

(11)

where CR1 and CR2 are the corrosion rates at temperature T1 and T2 respectively, and R the molar gas constant. An estimate of heat of adsorption was obtained from the trend of surface coverage with temperature as follows48:   θ Qads = 2.303R log 2  1−θ2

  θ   T T   − log 1  x 1 X 2 kJmol −1   1 − θ 1   T2 − T1 

(12)

where θ1 and θ 2 are the degrees of surface coverage at temperatures T1 and T2, The calculated values for both parameters are given in Supporting Information (Table S2). Inspection of the data show that the activation energy is higher in the presence of extracts compared to the blank solutions. Similar observation has been reported for 16


Page 17 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


corrosion inhibition effect of steel in 0.2 M HCl using date palm leaves and henna extracts 23

. This result, in addition to the observed decrease in inhibition efficiency with increase in

temperature, suggests that DPSE did not alter the mechanism of the corrosion process

49

.

Rather, corrosion inhibition occurred primarily through a geometric blocking effect of the inhibitor on the mild steel surface, reducing access of the aggressive corrosive media. The negative Qads values indicate that the degree of surface coverage decreased with rise in temperature, supporting the earlier proposed physisorption mechanism for DPSE.

3.7

Surface Morphology The surface morphologies of exposed mild steel to 1 M HCl and 0.5 M H2SO4

solutions in the absence and presence of 1.5 g L−1 DPSE after 12 h immersion at 25 oC were examined using SEM. The results shown in Fig. 8(a) and (c) reveal that the mild steel specimens underwent active dissolution as indicated by the roughness of the surface in the absence of DPSE in both acid environments as expected due to corrosive attack of the acid solutions. The attack was uniform with no evidence of selective corrosion (localized attack). In the presence of DPSE (Fig. 8 (b) and (d) in 1 M HCl and 0.5 M H2SO4 respectively) the rough surface is seen to reduce indicating an inhibiting effect of DPSE on the surface of mild steel due to formation of protective layers that create a barrier for charge and mass transfer and there is slight evidence of the adsorbate presence on the metal surface.

CONCLUSIONS The following conclusions can be drawn from the work. 1. DPSE is found to inhibit the corrosion of mild steel in 1 M HCl and 0.5 M H2SO4 solution. Inhibition effect is concentration dependent. Extract concentration of 2.5 g L−1 and 1.5 g L−1 gave maximum inhibition efficiency in 1 M HCl and 0.5 M H2SO4 respectively. 2. Corrosion inhibition effect was also found to be influenced by immersion time. An increase in inhibition efficiency with prolonged immersion as the extract concentration is increased in 1 M HCl was observed while in 0.5 M H2SO4 solution, a decrease in inhibition efficiency with increase in immersion time was observed with increasing extract concentration up to 2.0 g L−1 17



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Page 18 of 43

3. DPSE offers better corrosion protection for mild steel in 1 M HCl than in 0.5 M H2SO4 solutions 4. Corrosion inhibition is afforded by virtue of adsorption of DPSE components onto mild steel surface which can be approximated by Langmuir adsorption isotherm. 5. The results of potentiondynamic polarization measurements indicate that AFE behaved as a mixed inhibitor affecting both cathodic hydrogen evolution and anodic mild steel dissolution reactions. 6. Impedance spectra show a high frequency capacitive loop related to the charge-transfer process of the metal corrosion and the double layer behaviour. 7. The phenomenon of physical adsorption is proposed from the trend of increase in inhibition efficiency with increase in temperature which is corroborated by the values of kinetic parameters obtained from the experimental data.

ACKNOWLEDGEMENTS The authors gratefully acknowledged King Fahd University of Petroleum and Minerals (KFUPM) for providing the facilities for the research.

SUPPORTING INFORMATION AVAILABLE The parameters obtained from Langmuir adsorption isotherm and activation (Ea and Qads) parameters data are given here. This material is available free of charge via the Internet at http://pubs.acs.org.

18


Page 19 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FIGURES LEGEND

Figure 1:

Plot of (a) corrosion rate and (b) inhibition efficiency against inhibitor

concentration at different temperatures for mild steel in 1 M HCl and 0.5 M H2SO4 from weight loss measurements Figure 2:

Plot of (a) corrosion rate and (b) inhibition efficiency against inhibitor

concentration at different immersion times for mild steel in 1 M HCl and 0.5 M H2SO4 from weight loss measurements Figure 3:

Potentiodynamic polarization curves for mild steel in (a) 1 M HCl and (b) 0.5 M

H2SO4 in the absence and presence of different concentrations of DPSE. Figure 4:

Impedance plots for mild steel in 1 M HCl in the absence and presence of

different concentrations of DPSE exemplified as (a) Nyquist (b) Bode plots Figure 5:

Impedance plots for mild steel in 0.5 M H2SO4 in the absence and presence of

different concentrations of DPSE exemplified as (a) Nyquist (b) Bode plots Figure 6:

Equivalent circuit diagrams used to fit impedance data in the absence and

presence of DPSE in (a) 1 M HCl and (b) 0.5 M H2SO4. Figure 7:

Langmuir adsorption isotherm for DPE on mild steel in (a) 1 M HCl and (b) 0.5

M H2SO4 at 25 and 60 oC.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 8:

SEM images for exposed mild steel in (a) 1 M HCl solution (b) 1 M HCl

containing 1.5 g L−1 DPSE , (c) 0.5 M H2SO4 solution and (d) 0.5 M H2SO4 containing 1.5 g L−1 DPSE at 25 oC for 12 h.

20


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FIGURES (DPSE)

160

o

HCl (25 C) o HCl (60 C) o H2SO4(25 C)

(a)

Corrosion rate (mm y−1

140 120

o

H2SO4(60 C)

100 80 60 40 20 0 0.0

0.5

1.0

1.5

2.0

2.5

−1

Extract concentration (g L )

(b)

90 80

Inhibition efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70 o

60 50

HCl (25 C) o HCl (60 C) o H2SO4(25 C) o

H2SO4(60 C) 40 30 20 0.5

1.0

1.5

2.0 −1


Figure 1

29


2.5


36 32

HCl (24 h) HCl (72 h) H2SO4(24 h)

28

H2SO4(72 h)

Corrosion rate (mm y−1

(a)

24 20 16 12 8 4 0 0.0

0.5

1.0

1.5

2.0

2.5

−1


(b) 90


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

85

80

75

70

HCl (24 h) HCl (72 h) H2SO4(24 h)

65

H2SO4(72 h)

60 0.5

1.0

1.5

2.0 −1


Figure 2

30


2.5

Page 30 of 43

Page 31 of 43

-0.2

1.0 M HCl −1 0.5 g L DPSE −1 1.0 g L DPSE −1 1.5 g L DPSE −1 2.0 g L DPSE −1 2.5 g L DPSE

E (V) /SCE

-0.3

-0.4

(a)

-0.5

-0.6

-0.7

-0.8 -6

-5

10

10

-4

-3

10

10

-2

10

−2

log i (A cm )

0.5 M H2SO4

-0.2

(b)

−1

0.5 g L DPSE −1 1.0 g L DPSE −1 1.5 g L DPSE −1 2.0 g L DPSE −1 2.5 g L DPSE

-0.3

-0.4

E (V) /SCE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.5

-0.6

-0.7

-0.8 -6

10

-5

10

-4

-3

10

10

-2

10

−2

log i (A cm )

Figure 3

31



-450

1 M HCl −1 0.5 g L DPSE −1 1.0 g L DPSE −1 1.5 g L DPSE −1 2.0 g L DPSE −1 2.5 g L DPSE

(a)

-400 -350

2

Zimg (Ω cm ))

-300 -250 -200 -150 -100 -50 0 0

50

100

150

200

250

300

350

400

450

2

Zreal (Ω cm )

450

1 M HCl −1 0.5 g L DPSE −1 1.0 g L DPSE −1 1.5 g L DPSE −1 2.0 g L DPSE −1 2.5 g L DPSE

(b)

400 350 2

Zreal (Ω cm ))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

300 250 200 150 100 50 0 -50 0.1

1

10

100

1000

10000

Log f (Hz)

Figure 4

32


100000

Page 32 of 43

Page 33 of 43

-200

0.5 M H2SO4

(a)

−1


2

Zimg (Ω cm ))

-160

-120

-80

-40

0 0

40

80

120

160

200

2

Zreal (Ω cm ) 200

0.5 M H2SO4

(b)

180

−1


160 140 2

Zreal (Ω cm ))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


120 100 80 60 40 20 0 0.1

1

10

100

1000

10000

Log f (Hz)

Figure 5

33


100000


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

CPE

(a)

Rs

Rct

CPE

(b)

Rs

Rct

RL

L

Figure 6

34


Page 34 of 43

Page 35 of 43

o

4.0

25 C o 60 C

(a)

3.5

−1

C/θ (g L )

3.0 2.5 2.0 1.5 1.0 0.5 0.5

1.0

1.5

2.0

2.5

2.0

2.5

−1

C (g L )

7

o

25 C o 60 C

(b)

6

C/θ (g L )

5 −1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

3

2

1

0.5

1.0

1.5 −1

C (g L )

Figure 7

35



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

Page 36 of 43

(b)

30µm 30µm (c)

(d)

30µm

30µm

Figure 8

36


Page 37 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TABLES (DPSE)

Table 1: Potentiodynamic polarization parameters for mild steel in 1 M HCl and 0.5 M H2SO4 solutions in the absence and presence of DPSE. Acid medium

HCl

H2SO4

Extract concentration (g L-1) 1 M HCl

Ecorr (mV/SCE)

icorr (µA cm–2)

βc (mV dec–1)

βa (mV dec–1)

η%

−511.6

303.3

97.3

76.5

−

0.5

−495.0

49.9

108.8

81.5

83.6

1.0

−502.8

48.3

105.8

88.6

84.1

1.5

−500.4

40.7

103.2

85.9

86.6

2.0

−497.7

36.5

109.2

77.8

87.9

2.5

−499.6

32.5

104.7

79.0

89.3

0.5 M H2SO4

−517.9

836.9

117.7

111.6

−

0.5

−479.3

170.1

126.4

62.5

79.67

1.0

−488.8

116.7

119.9

69.9

86.1

1.5

−479.0

106.6

124.3

62.9

87.3

2.0

−480.7

119.3

130.1

65.9

85.8

2.5

−471.8

175.8

137.6

52.2

78.9

37



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Page 38 of 43

Table 2: Electrochemical impedance parameters for mild steel in 1 M HCl solution in the presence of DPSE

n

Rct (Ω cm2)

Cdl (µF cm2)

α2 × 10−4

η%

3.249

Yo (Ω sn cm−2) × 10−6 139.7

0.90

54.72

122.7

0.78

−

0.5

1.884

75.2

0.89

312.4

3.9

2.34

82.48

1.0

3.607

67.8

0.88

325.2

3.6

2.60

83.17

1.5

3.587

66.4

0.89

369.9

2.7

2.67

85.21

2.0

4.694

65.9

0.89

387.5

2.5

0.92

85.88

2.5

3.707

66.3

0.89

414.0

2.2

1.08

86.78

Extract concentration (g L−1) 1 M HCl

Rs (Ω cm2)

38


Page 39 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3: Electrochemical impedance parameters for mild steel in 0.5 M H2SO4 solution in the presence of DPSE n

Rct (Ω cm2)

L H cm2)

RL (Ω cm2)

Cdl (µF cm2)

α2 × 10−4

η%

3.84

Yo (Ω sn cm−2) × 10−6 165.2

0.89

28.2

−

−

257.3

1.08

−

0.5

5.62

131.4

0.91

117.6

875.4

1199

27.2

1.99

76.0

1.0

6.32

112.3

0.87

147.3

71.1

2107

17.1

2.06

80.9

1.5

5.74

99.9

0.87

177.5

2021.0

3220

11.8

2.68

84.1

2.0

5.99

101.2

0.87

164.8

1401.0

2875

13.6

2.73

82.9

2.5

5.39

146.2

0.88

113.0

431.2

2519

29.3

3.35

75.1

Extract concentration (g L−1) 0.5 M H2SO4

Rs (Ω cm2)

39



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Page 40 of 43

Table 4: Comparison of inhibition efficiencies from the three electrochemical techniques Acid medium

Extract Concentration (g L−1)

EIS

Rct

PDP

η (%)

icorr

(Ω cm−2)

HCl

H2SO4

LPR

η (%)

(µA cm–2)

Rp

η (%)

(Ω cm−2)

1 M HCl

54.72

−

303.3

−

44.41

−

0.5

312.4

82.5

49.9

83.6

249.1

82.2

1.0

325.2

83.2

48.3

84.1

281.8

84.2

1.5

369.9

85.2

40.7

86.6

360.0

87.7

2.0

387.5

85.9

36.5

87.9

354

87. 5

2.5

414.0

86.8

32.5

89.3

414.4

89.4

0.5 M H2SO4

28.2

−

836.9

−

32.54

−

0.5

117.6

76.0

170.1

79.67

107.7

69.8

1.0

147.3

80.9

116.7

86.1

154.8

78.9

1.5

177.5

84.1

106.6

87.3

168.1

80.6

2.0

164.8

82.9

119.3

85.8

158.2

79.4

2.5

113.0

75.1

175.8

78.9

94.4

65.7

40


Page 41 of 43

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 5: Comparison of corrosion inhibition efficiency of DPSE with other plant based extracts for mild steel in acidic media at ambient temperature. Plant extract

Acid medium

Maximum concentration

Immersion time (h)


References

Date palm seed

1 M HCl

2.5 g L−1

24

90.95a

0.5 M H2SO4

1.0 g L−1

Present study

Garlic peel

1 M HCl

1000 mg L−1

24

96.0a

(18)

Argemone Mexicana

1 M HCl

600 mg L−1

5

92.53a

(22)

Schniposis lorentzii

1 M HCl

2000 ppm

−

66.0b , 63.0c

(14)

Ginkgo leaves

1 M HCl

100 mg L−1

6

94.8a

(17)

86.50a

82.0a

0.5 M H2SO4 Acalypha torta

1 M HCl

1000 ppm

−

89a

(23)

Neolamarckia cadamba

1 M HCl

5 mg L−1

0.5

88.0b, 84.0c

(8)

Piper guineense

1 M HCl

900 mg L−1

24

96.5a

(9)

97.0a

0.5 M H2SO4 Aframomum melegueta

1 M HCl

800 mg L−1

3

1 M HCl

(11)

55.0a

0.5 M H2SO4 Coffee senna

96.1a

1000 mg L−1

3

76.6a

(12)

82.4a

0.5 M H2SO4 1 M HCl

400 mg L−1

0.5 M H2SO4

600 mg L−1

Artemesia pallens

1.0 M HCl

400 mg L−1

24

98a

(15)

bamboo leaf

1.0 M HCl

200 mg L−1

6

89.0a

(16)

Punica granatum

24

92.0a

(13)

95.0a

79.4a

0.5 M H2SO4 Osmanthus fragran

1.0 M HCl

0.34 g L−1

1

94.1b, 95.3c

(19)

Salvia officinalis

1.0 M HCl

2.0 g L−1

1

96.6a

(20)

Oxandra asbeckii

1.0 M HCl

100 mg L−1

3

87b, 92c

(21)

41



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Caulerpa racemosa

1.0 M HCl

100 ppm

2

a

Page 42 of 43

83a

(24)

values from weight loss method; b values from potentiodynamic polarization method, c values from electrochemical impedance spectroscopy method

42


Page 43 of 43

FOR TOC ONLY

-400

Extract -300

2

Zimg ( cm ))

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2 g/L DPSE

-200

-100

Blank 0 0

100

200

300 2

Zreal ( cm )


400

Natural Products for Material Protection: Inhibition ... - ACS Publications

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