Protein–surfactant aggregate as a potential corrosion inhibitor for mild steel in sulphuric acid: zein–SDS system

Abstract

The corrosion inhibition of mild steel in 0.5 M H₂SO₄ in the presence of zein (a biocompatible water insoluble corn protein) and sodium dodecyl sulfate (SDS, an anionic surfactant) is studied employing electrochemical techniques and gravimetric measurement. SEM and FTIR techniques are employed to observe the morphology of the corroded surface and the nature of the adsorbed layer. It is revealed that SDS micelles in high concentration exhibit low corrosion inhibition efficiency for mild steel in H₂SO₄, but inhibition efficiency increases drastically in a zein–SDS mixed system, reaching more than 90% in the presence of 4 mM SDS–500 ppm zein. The dependence of the inhibition efficiency on surfactant–protein ratio is explained by zein–SDS complex formation, followed by SDS induced conformational change of zein.

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Introduction

Corrosion inhibition of metals and alloys by suitable inorganic and organic inhibitors has been the focus of research for decades. In recent years, research on biocompatible green corrosion inhibitors has gained tremendous impetus primarily due to environmental concerns. Different biomolecules have now been tested for their potential anticorrosive properties towards various metals and alloys and in different corrosive environments.^1–14 The presence of heteroatoms like S, N, O, as well as aromatic groups and large molecular volumes makes biomolecules suitable to act as good corrosion inhibitors. In this paper, the corrosion inhibition properties of one of the most important industrial corn proteins, zein, in combination with an anionic surfactant, sodium dodecyl sulphate (SDS), have been assessed for mild steel in 0.5 M H₂SO₄. Zein has long been known for its use in fields like textiles, cosmetics, paints, medicines, etc.^15,16 The large amounts of hydrophobic residues in zein, such as leucine (20%), proline (9%), alanine (14%), and glutamine (20%) make it almost insoluble in aqueous solution.¹⁷ Under acidic conditions, zein is reported to carry positive charges, possibly due to the high glutamine residue exposure in its tertiary structure.¹⁸ Its solubility in water increases many fold in the presence of the anionic surfactant SDS, which provides a dielectric constant of the aqueous surfactant solution comparable to a water–alcohol mixture.^19,20 Anionic SDS, on the other hand, has versatile applications including as a corrosion inhibitor.^21,22 Above the critical micelle concentration (CMC), it forms spherical micelles in aqueous solution, with anionic head groups protruding outwards, while the aliphatic chains form the hydrophobic core.²³

In this paper, we have focused on the effect of the protein–surfactant interaction between zein and SDS, which is supposed to be initiated by electrostatic interaction (between the net positive charge of zein and negatively charged head groups of SDS), followed by protein–surfactant complex formation,^19,20 adsorption on a mild steel surface in acidic media, and subsequent inhibition of corrosion of the metal. Different electrochemical techniques, like potentiodynamic polarization (Tafel extrapolation), electrochemical impedance spectroscopy (EIS), as well as gravimetric measurements have been employed for this purpose.

Experimental

Metal coupon preparation and chemicals

Test specimens were cut from a commercially available mild steel rod (wt% composition: 0.24 C, 0.40 Si, 0.90 Mn, 0.07 Ni, 0.03 Cr, 0.01 P, 0.005 S and the remainder iron). The cross-sectional surface was ground with different grade emery papers (400, 600, 800, 1200 and 1600), washed with water and acetone, and used as the working electrode in electrochemical measurements. SDS (Sigma-Aldrich) and zein (Sigma) were used without any further purification. As the molecular weight of zein could not be determined accurately due to its inherent molecular complexity, the concentration of zein was expressed in terms of ppm by weight.

Electrochemical measurements

Potentiodynamic polarization and electrochemical impedance measurements were done using a conventional three-electrode system (model: Gill AC, ACM Instruments, UK) consisting of a mild steel working electrode (WE) with an exposed area of 0.25 cm², platinum as a counter electrode and a saturated calomel electrode (SCE) as a reference. Before electrochemical tests, the WE was kept in the test solution for sufficient time to attain a steady open circuit potential (OCP). The polarization experiment was done for the potential range of ±250 mV from OCP with a potential sweep rate of 30 mV per min. Corrosion current density (i_corr) was determined from the intercept of the extrapolated cathodic and anodic Tafel lines at the corrosion potential (E_corr). The values of inhibition efficiency, η_P (%) were calculated from the following equation:where i_corr and i_corr(inh) are the values of the corrosion current density of the uninhibited and inhibited specimens, respectively.

Electrochemical impedance (EIS) measurements were performed in the frequency range 10 mHz to 100 kHz with an a.c. amplitude of ±10 mV (rms) at the rest potential. The Nyquist plots obtained showed only one time constant corresponding to one capacitive loop without any trace of an inductive loop at a low frequency range. These capacitive loops were depressed with the centre under the real axis, which may correspond to the microscopic roughness of the electrode surface and inhibitor adsorption on it.^24–27 Accordingly, these were fitted using an equivalent circuit consisting of a parallel combination of charge transfer resistance-constant phase elements, which was in series with the solution resistance, R_s[R_ct − CPE].^24–27 The impedance of CPE is given by

Z_CPE = Q⁻¹(iω)⁻ⁿ (2)

where Q is a proportionality coefficient, ω is the angular frequency, and n is a measure of surface irregularity. Q is an indicative parameter proportional to the capacitance of the double layer formed at the metal surface for 0 > n > 1. For whole numbers of n = 1, 0, −1, CPE is reduced to the classical lumped elements capacitor (C), resistance (R), and inductance (L), respectively. The goodness of the fit was assessed from chi-squared values, which were in the range 10⁻³ to 10⁻⁵. To correlate the charge transfer resistance (R_ct) and the double layer capacitance (C_dl) among the metal–solution interface, the latter had been recalculated using the equation^27,28

C_dl = (QR_ct¹⁻ⁿ)^1/n (3)

The percentage inhibition efficiencies η_Z (%) in terms of R_ct were calculated through the following equation:

All the experiments were carried out at around a room temperature of 30 °C.

Weight loss measurements

For weight loss measurements, polished, dried and accurately weighed rectangular mild steel coupons (2.5 × 2.5 × 0.1 cm³) were immersed in 0.5 M H₂SO₄ without and with inhibitor for a duration of 6 h at a room temperature of around 30 °C. Then, these were removed from the acid solution, scrubbed with a bristle brush, washed thoroughly with distilled water and acetone, dried in a vacuum desiccator, and weighed. The percentage inhibition efficiency, η_W (%) was calculated following the relation:where W₀ and W are the weight loss of the metal coupons in the acid medium without and with inhibitor.

Surface analysis

A scanning electron microscope (SEM, S-3000N, Hitachi) was used to study the surface morphology of the metal surface after immersion in 0.5 M H₂SO₄ without and with inhibitor for a duration of 6 h. The surface of the dried specimen was scratched with a knife and the resultant powder was used for FTIR studies (KBr pellet method, Thermo Nicolet, model iS10).

Results and discussion

Polarization measurements

Potentiodynamic polarization measurements were carried out to study the anodic and cathodic reactions occurring on the mild steel electrode in 0.5 M H₂SO₄ solution with addition of various concentration of SDS, in the absence and presence of different concentrations of zein (Fig. 1 and 2). The corresponding values of electrochemical parameters, such as corrosion potential (E_corr), corrosion current density (i_corr), anodic and cathodic Tafel slopes (b_a and b_c), as well as corrosion inhibition efficiency, η_P (%), are listed in Table 1. It is observed that the nature of the potentiodynamic polarization curves for mild steel in 0.5 M H₂SO₄ solution in the presence of SDS with a concentration range of 2 mM to 30 mM do not change to any reportable extent with respect to that of the blank solution. It may be mentioned here that the critical micelle concentration (CMC) of SDS in 0.5 M H₂SO₄ solution is reported to be 0.8 mM,²⁹ and hence the concentration range of SDS used in the present study is much higher than its CMC. Thus it may be concluded that SDS, at concentrations much higher than its CMC, cannot hinder the rate of either cathodic reduction (hydrogen evolution) or anodic reactions (metal dissolution) occurring at the cathodic and anodic reaction sites, respectively, on a mild steel surface in H₂SO₄ solution, to any appreciable extent.³⁰ This may be associated with the tendency of SDS micelles to remain in the solution phase more than be adsorbed on the metal surface.

Fig. 1 Potentiodynamic polarization curves for mild steel in 0.5 M H₂SO₄ in the presence of (a) no inhibitor, (b) 4 mM SDS, those with zein having conc. (c) 50 ppm, (d) 100 ppm, (e) 250 ppm, (f) 500 ppm.

Fig. 2 Potentiodynamic polarization curves for mild steel in 0.5 M H₂SO₄ in the presence of (a) no inhibitor, 250 ppm zein with SDS having conc. (b) 2 mM, (c) 4 mM, (d) 10 mM, (e) 30 mM.

Table 1 Data from potentiodynamic polarization studies for mild steel in 0.5 M H₂SO₄ in various inhibitor systems

SDS (mM)	Zein (ppm)	−Ecorr (mV per SCE)	icorr (mA cm^−2)	βa (mV dec^−1)	−βc (mV dec^−1)	ηP (%)
0	0	517	1.70	92.8	114.3
2	0	498	1.44	80.6	104.5	15.3
	50	489	0.30	65.0	77.0	82.3
	100	514	0.28	76.0	94.6	83.5
	250	516	0.25	64.7	85.6	85.3
4	0	494	1.60	76.7	102.0	5.8
	50	510	0.33	75.6	101.5	80.5
	100	508	0.27	67.3	101.0	84.1
	250	503	0.20	65.4	104.2	88.2
	500	512	0.15	68.7	100.6	91.2
10	0	504	1.57	71.3	101.7	7.6
	50	504	0.37	65.7	101.0	78.2
	100	484	0.30	64.7	103.0	82.4
	250	495	0.22	67.5	101.0	87.0
	500	493	0.20	63.2	102.5	88.2
30	0	499	1.65	72.6	103.0	2.9
	50	497	0.41	65.4	100.0	75.9
	100	498	0.34	67.0	101.0	80.0
	250	469	0.32	57.7	93.8	81.2
	500	489	0.27	66.2	100.0	84.1

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Significant change in the potentiodynamic polarization curves is observed when zein is added into the acidic SDS solution (Fig. 1 and 2; Table 1). Both the rate of cathodic and anodic reactions are clearly seen to decrease significantly with the E_corr values remaining very close to that of the uninhibited sample. This indicates that when the protein–surfactant aggregate is formed and present on the mild steel surface, it acts as a mixed type corrosion inhibitor. For a particular SDS concentration, i_corr values are seen to decrease gradually with an increase in zein concentration.

It is worth mentioning that for a fixed zein concentration in the zein–SDS aggregate, changes in inhibition efficiency show a complex behaviour with an increase in SDS concentration. For 50 ppm zein, maximum inhibition efficiency is observed at 2 mM of SDS, thereafter it decreases slowly with SDS concentration. For higher concentrations of zein (100–500 ppm), maximum inhibition efficiency is seen at around 4 mM SDS. Thus, the corrosion inhibition effect of the protein–surfactant complex displays dependence on the protein–surfactant ratio. With a gradual increase in SDS–zein ratio, it is seen that the anodic polarization curves do not change significantly, but the cathodic polarization curves are modified, exhibiting an increase in the rate of the cathodic reduction reaction (Fig. 2). E_corr values also show a gradual shift towards the less negative direction. This indicates that after exhibiting maximum corrosion potentiality by the zein–SDS aggregate, if concentration of SDS are increased further, the overall rate of corrosion tends to increase, mostly due to the enhancement of the rate of the cathodic reduction reaction.³⁰

Electrochemical impedance measurements

Nyquist plots, derived from EIS experiments, for mild steel in H₂SO₄ solution in the presence of different concentrations of SDS and zein–SDS complexes with varied concentration ratios are shown in Fig. 3 and 4.

Fig. 3 Nyquist plots for mild steel in 0.5 M H₂SO₄ in the presence of (a) no inhibitor, (b) 4 mM SDS, those with zein having conc. (c) 50 ppm, (d) 100 ppm, (e) 250 ppm, (f) 500 ppm.

Fig. 4 Nyquist plots for mild steel in 0.5 M H₂SO₄ in the presence of (a) no inhibitor, 250 ppm zein with SDS having conc. (b) 2 mM, (c) 4 mM, (d) 10 mM, (e) 30 mM.

Fitting these plots using the equivalent circuit, as described in the electrochemical measurements section, the corresponding fitting parameters and the inhibition efficiencies, η_Z (%), obtained from the R_ct values are tabulated in Table 2. It is observed that the diameter of the capacitive loops for mild steel in H₂SO₄ solution in the presence of different concentrations of SDS are almost the same as that of the blank solution. This indicates that SDS at concentrations much higher than its CMC cannot effectively cover the reaction sites on the metal surface, and therefore is unable to change the charge transfer resistance along the metal–solution interface.

Table 2 Impedance parameters for the corrosion of mild steel in 0.5 M H₂SO₄ in various inhibitor systems

SDS (mM)	zein (ppm)	Rct (Ω cm^2)	Q (μΩ^−1 s^n cm^−2)	n	Cdl (μF cm^−2)	ηZ (%)
0	0	3.7	740	0.82	203.0
2	0	4.7	605	0.84	198.0	21.3
	50	30.0	155	0.83	51.6	87.6
	100	37.5	125	0.83	41.7	90.1
	250	47.0	95	0.84	33.9	92.1
4	0	4.2	623	0.83	184.0	11.9
	50	28.6	144	0.83	46.7	87.0
	100	42.6	120	0.84	43.9	91.3
	250	59.0	100	0.85	40.4	93.7
	500	68.6	89	0.86	38.8	94.6
10	0	4.3	600	0.83	177.0	13.9
	50	28.0	167	0.84	60.1	86.7
	100	34.0	138	0.84	49.7	89.1
	250	51.5	102	0.86	43.4	92.8
	500	64.8	83.4	0.87	38.2	94.3
30	0	4.0	686	0.85	24.2	7.5
	50	23.0	200	0.82	61.4	83.9
	100	29.3	146	0.85	55.7	87.3
	250	39.5	120	0.86	50.2	90.6
	500	52.0	102	0.87	46.6	92.8

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The diameter of the capacitive loops, however, is seen to increase gradually with zein concentration in the acidic SDS micellar solution. Thus, it may be concluded that protein–surfactant aggregates efficiently adsorb on the mild steel surface in acidic media and block the reaction sites on the metal surface, thereby increasing the charge transfer resistance and the corrosion inhibition efficiency.^{8–14,24–28} At higher SDS–zein ratios, however, charge transfer resistance tends to decrease. This suggests that any possible structural change in the zein–SDS aggregate compels the aggregate to desorb to some extent from the metal surface, leaving the reaction sites easily accessible to the corrosive environment.

Adsorption isotherm

The Langmuir adsorption isotherm model, which is based on the assumption that all the adsorption sites are energetically equivalent, has been applied to the adsorption of the zein–SDS aggregate on the mild steel surface in H₂SO₄ solution. As per this model, the degree of surface coverage θ (θ = η_Z (%)/100) is related to the concentration of the inhibitor (C) following the relation:³¹

C/θ = 1/K_ads + C (6)

where K_ads is the constant of adsorption. As SDS itself does not show any appreciable corrosion inhibition in the present study, only the concentration of zein present in the different micellar solutions is considered during fitting of the experimental data as per the above equation. Good linear fits (correlation coefficient, R² = 0.999) are obtained for all the micellar solutions with slope values very close to 1 (Fig. 5). From the values of the adsorption constant, K_ads, the standard free energy of adsorption (ΔG⁰_ads) for all the inhibitor systems is determined using the following equation:²⁸

ΔG⁰_ads = −RT ln(1 × 10⁶K_ads) (7)

where 1 × 10⁶ is the concentration of water molecules expressed in mg L⁻¹, R is the universal gas constant and T is the temperature (here, 303 K) (Table 3). It is seen that in the presence of higher concentrations of SDS, K_ads for adsorption of zein on mild steel in an acid medium is lower. As the ΔG⁰_ads value corresponds to an equilibrium state, based on this one cannot draw any definite conclusion regarding whether adsorption takes place through physical (i.e., charge sharing between the inhibitor and metal) or chemical (i.e., complete charge transfer between the inhibitor molecule and metal) means. But, as a rough estimate and comparing the values given in the literature, it may be said that for the present system with a ΔG⁰_ads value of around −30 kJ mol⁻¹, physical adsorption (i.e. electrostatic interaction) is predominant over chemical adsorption.^10–14

Fig. 5 Langmuir adsorption plot for mild steel in 0.5 M H₂SO₄ in the presence of different combinations of SDS and zein.

Table 3 Calculated parameters from the Langmuir adsorption isotherm

Inhibitor system	Slope	R^2	Kads (L mg^−1)	ΔG^0ads (kJ mol^−1)
2 mM SDS and zein	1.07	0.9999	2.77 × 10^−1	−31.5
4 mM SDS and zein	1.04	1	2.0 × 10^−1	−30.7
10 mM SDS and zein	1.04	0.9999	1.52 × 10^−1	−30.1
30 mM SDS and zein	1.06	0.9999	1.27 × 10^−1	−29.6

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Weight loss measurement

We have verified the results obtained from electrochemical measurements with gravimetric analysis. The variation of the weight loss of the mild steel in 0.5 M H₂SO₄ solution in the presence of different concentrations of zein (50 ppm to 500 ppm) together with various concentrations of SDS in the range of 2 mM to 30 mM are shown in Fig. 6. It is seen that the η_W (%) is always higher with a higher concentration of zein for all the concentrations of SDS used. It is also seen that with 50 ppm zein, a maximum η_W (%) is obtained in the solution having 2 mM SDS and thereafter it declines slowly. While, for higher concentrations of zein, the solution with 4 mM SDS provides the maximum η_W (%). Thus, it is confirmed that the surfactant–protein ratio plays some important role for the adsorption of the protein–surfactant complex on mild steel and its subsequent corrosion inhibition in an acid medium. From the observed variation of η_W (%) against immersion time using 4 mM SDS with 500 ppm zein, it is seen that maximum inhibition efficiency is achieved within 6 h of exposure (inset of Fig. 6) and the inhibitor system exhibits appreciable corrosion inhibition properties for nearly 48 h.

Fig. 6 Variation of inhibition efficiency of different concentrations of SDS (2 mM to 30 mM) with zein having conc. (a) 50 ppm (b) 100 ppm (c) 250 ppm (d) 500 ppm. Figure in the inset shows the variation of inhibition efficiency of 4 mM SDS with 500 ppm zein with immersion time.

Surface analysis

Scanning electron micrographs (SEM) of the surface of mild steel immersed in 0.5 M H₂SO₄ solution without and with the inhibitors are shown in Fig. 7(a) and (b). With 4 mM SDS in the acid medium, the mild steel surface shows a very rough surface (Fig. 7a) similar to that observed for the blank solution, indicating the very low corrosion inhibition potential of SDS for the present system. In the presence of 4 mM SDS together with 500 ppm zein, a cleaner and smoother metal surface can be observed (Fig. 7b). This shows the efficacy of the zein–SDS complex system as a corrosion inhibitor for mild steel in H₂SO₄ solution.

Fig. 7 SEM images of mild steel after immersion in 0.5 M H₂SO₄ having (a) only SDS (b) SDS with zein.

In order to determine the functional groups responsible for binding on the metal surface, we have examined and compared the FTIR spectra of native zein, native SDS, surface adsorbed SDS and the surface adsorbed zein–SDS system (Fig. 8a–d). Among various bands observed for the native zein sample, the main bands are amide I (mainly stretching vibration of the C=O group, 1657 cm⁻¹), amide II (in-plane N–H bending, 1532 cm⁻¹), the deformation vibration of C–H from the CH₂ of the protein (1444 cm⁻¹). Bands in the range of 2920 cm⁻¹ to 2850 cm⁻¹ are due to aliphatic C–H stretching vibrations.^18,32,33 SDS, on the other hand, shows characteristic bands at (Fig. 8b) 1472 cm⁻¹ (bending mode of CH₂–), doublet at 1254 and 1218 cm⁻¹ (asymmetric S–O stretching), 1080 cm⁻¹ (symmetric S–O stretching).³⁴ In the case of surface adsorbed SDS (Fig. 8c), all these bands are observed with much lower intensity and shifted towards the low wavenumber region (the 1080 cm⁻¹ band shifts to 1017 cm⁻¹). This clearly suggests that SDS adsorbs on the mild steel surface through the anionic head group (i.e. –O–SO₃⁻), which is also reported by others.^21,29 In the surface adsorbed zein–SDS aggregate, the amide I band of zein is seen to have a prominent red shift at 1636 cm⁻¹ (Fig. 8d). Any possible shifts of the other bands are too weak to resolve. Thus, it may be concluded that the head groups of the SDS micelle and amide linkages present in the protein chain are mostly responsible for the adsorption of the zein–SDS aggregate on the metal surface.

Fig. 8 FTIR spectra of (a) native zein, (b) native SDS, (c) surface adsorbed SDS, and (d) surface adsorbed zein–SDS.

Discussion

The novelty of the work lies in the fact that as zein is insoluble in aqueous solution, and SDS with a concentration of 2 mM and higher does not show any significant corrosion inhibition for mild steel in H₂SO₄ solution, thus the profound corrosion inhibitory action observed for zein in the presence of SDS is mostly due to the effect of a protein–surfactant aggregate (Scheme 1). From the electrochemical measurements it is observed that SDS micelles at a concentration range much higher than its CMC adsorb on the mild steel surface in an acid medium to a small extent and cannot alter the rate of either the cathodic reduction reaction or anodic metal dissolution reaction significantly. This may be attributed to possible electrostatic repulsion between the positively charged metal surfaces with a large number of Na⁺ counter ions associated with SDS micelles at high concentration. But, for the zein–SDS aggregate, which is seen to behave as a mixed type inhibitor, we can find a substantial increase in inhibition efficiency. It is reported that when a polymer wraps the micelles forming a polymer–surfactant aggregate (necklace-bead model, Scheme 1A), some of the counter ions, as well as water molecules, are replaced from the Stern layer of the micelle, and this makes the Stern layer more hydrophobic.³⁵ Thus, in the zein–SDS aggregate, SDS micelles have more affinity for adoption on the steel surface than the bare SDS micelles. Also, zein itself has amide linkages in its backbone chain, which also get involved in adsorption on the metal surface through the lone pairs present on the O and N heteroatoms. It may thus be presumed that in the case of the protein–surfactant aggregate, both the protein and micelles cooperatively or synergistically influence each other in the overall adsorption process. Such cooperative interaction between a polymer or polysaccharide (a biopolymer) with a surfactant towards inhibition of corrosion is already reported.^36,37

Scheme 1 SDS dependent conformational change of zein and possible mode of adsorption of zein–SDS system on mild steel surface.

The observation that when the surfactant–protein ratio increases, the inhibition efficiency begins to fall, may be interpreted in terms of surfactant induced conformational change of the protein. Zein itself is insoluble in aqueous media due to its globular structure. With a gradual increase in the concentration of the SDS, a polymer–surfactant aggregate (necklace-bead model, Scheme 1A) forms and zein becomes soluble. Formation of such an aggregate is initiated from the electrostatic attraction between the partial positive charge of zein and the negatively charged polar head groups of the SDS micelles.^19,20 This aggregated state is mostly responsible for showing a high inhibition efficiency (Scheme 1A), as discussed above. For further increases in SDS concentration, when all the positive charge of the zein is neutralized, the hydrophobic SDS chain interacts with the hydrophobic backbone of the protein and thus the zein becomes completely unfolded and surrounded by SDS molecules.^19,20 This gradually hinders the direct interaction between the zein molecules and the mild steel surface (Scheme 1B). At this stage, mostly the cathodic sites on the metal surface become exposed to the acid solution (this may be due to the electrostatic repulsion between the negatively charged cathodic sites and negatively charged surfactant head groups), thereby enhancing the rate of the cathodic reduction reaction, and hence, the overall rate of corrosion.

Conclusions

SDS with concentration much higher than its CMC does not show any appreciable effect on the rate of corrosion of mild steel in 0.5 M H₂SO₄ solution. When water insoluble zein is added into the SDS micellar system, a soluble zein–SDS aggregate is formed, which is initiated by electrostatic attraction and subsequent wrapping of the micellar surface by the protein chain (necklace-bead model). The zein–SDS aggregate shows a strong affinity for adsorption on the mild steel surface, thereby enhancing the resistance towards charge transfer reactions occurring there. The zein–SDS aggregate acts as a mixed type corrosion inhibitor decreasing the rate of both the cathodic and anodic reactions. While SDS is adsorbed on the mild steel surface through the –O–SO₃⁻ group, zein uses mostly the amide linkages present in its chain for such purposes. The extent of corrosion inhibition is seen to depend on the surfactant–protein ratio, as observed from adsorption isotherm studies. This has been explained in terms of the SDS induced conformational change of zein.

Acknowledgements

PI thanks Department of Science and Technology, Govt. of India for supporting a research project under Fast Track Scheme for Young Scientists (no. SR/FT/CS-110/2010, dt. 20.09.2011).

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