Elsevier

Corrosion Science

Volume 48, Issue 11, November 2006, Pages 3668-3674
Corrosion Science

The effect of cysteine on the corrosion of 304L stainless steel in sulphuric acid

https://doi.org/10.1016/j.corsci.2006.02.003Get rights and content

Abstract

The effect of cysteine on the corrosion of 304L stainless steel in 1 mol l−1 H2SO4 was studied using open-circuit potential measurements, anodic polarization curves, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). All the electrochemical measurements obtained in the presence of low cysteine concentration (10−6–10−5 mol l−1) presented the same behaviour as those obtained in the absence of cysteine, a passivated steel surface. However, for higher cysteine concentrations (10−4–10−2 mol l−1), a different behaviour was observed: the corrosion potential stabilized at a more negative value; an active region was observed in the anodic polarization curves and the electrochemical impedance diagrams showed an inductive loop at lower frequencies and a much lower polarization resistance. These results show that the presence of cysteine at high concentration turns the surface of 304L stainless steel electrochemically active, probably dissolving the passivation layer and promoting the stainless steel anodic dissolution. SEM experiments performed after immersion experiments at corrosion potential were in good agreement with the electrochemical results.

Introduction

Amino acids form a class of organic compounds, completely soluble in aqueous media, and produced with high purity at reduced costs. These properties would justify their use as corrosion inhibitors. Some studies involving amino acids on the corrosion of iron [1], steel [2], [3], [4], [5], aluminium [6], nickel [7] and copper [8], [9] were reported.

There are few works in the literature that study amino acids as corrosion inhibitors of stainless steel [2], [3], [4], [5]. The corrosion inhibition of steel by aspartic acid was investigated by Kalota and Silverman [2]. The inhibition effect was found to be strongly dependent on pH. At a pH lower than the ionization constant, the aspartic acid appeared to accelerate the corrosion. Over that pH, it acted as a corrosion inhibitor for steel.

Madkour and Ghoneim studied the corrosion inhibition of 16/14 austenitic stainless steel by methionine, citrulline, alanine, glycine and hidroxyproline in hydrochloric acid solution, using potentiodinamic, Tafel extrapolation and polarization resistance methods [3]. Polarization curves indicated that the transpassive region shifts towards more positive potentials in the presence of the studied amino acids, thus, leading to the conclusion that, in the presence of the amino acids, the passivation film is more stable. The corrosion parameters were dependent on the percentage of tungsten in the alloy composition as well as on the nature of the tested amino acids.

Gomma studied the inhibitory action of glycine, alanine, glutamine, lysine and p-amino benzoic acid on steel corrosion in 1 mol l−1 H2SO4 [4]. The anodic and cathodic site protection was determinated, and found to be affected by the structure of the amino acid bounded to the steel. Corrosion rates were determinated extrapolating the cathodic Tafel line to the rest potential and by linear polarization. Maximum protection efficiency was obtained with glutamic acid, with other compounds showing different degrees of inhibition.

Morad et al. studied the effect of some amino acids containing sulphur: cysteine, N-acetylcysteine, methionine and cystine, on the corrosion of mild steel in phosphoric acid solutions polluted with chloride, fluoride and Fe3+ ions near and at corrosion potential [5]. Using polarization resistance method and electrochemical impedance spectroscopy the authors showed that both cysteine and N-acetylcysteine present higher inhibition efficiency than methionine and cystine. Adsorption of methionine onto a mild steel surface obeys the Frumkin adsorption isotherm and has a free energy of adsorption value lower than those obtained in the presence of cystine, cysteine and N-acetylcysteine whose adsorption isotherms follow that of Langmuir. Both F and Fe3+ ions stimulate mild steel corrosion while Cl ions inhibit it. The binary mixtures of methionine, cysteine or N-acetylcysteine with Cl or F ions are effective inhibitors (synergism) while the combinations of the amino acid with Fe3+ or the ternary Cl/F/Fe3+ mixture have low inhibitive action (antagonism).

Imamura et al. studied the adsorption behaviour of amino acids on a stainless steel surface at 30 °C and over a 3–10 pH range. Acidic and basic amino acids, except histidine, adsorbed remarkably well at pH 3–4 and 7–10, respectively, and showed Langmuir type adsorption isotherms [10]. Fourier-transform infrared spectroscopic analysis and semiempirical molecular orbital calculations were carried out to analyse the ionization states and configuration of amino acid adsorption on a stainless steel surface. These investigations suggest that the acidic and basic amino acids adsorb via the electrostatic interaction of two ionized groups in the amino acid with a stainless steel surface.

Cysteine (HSCH2CHNH2COOH) is a very interesting amino acid that contains three dissociable protons (pKCOOH = 1.91, pKNH3+=8.16 and pKSH = 10.25). In aqueous solutions, ionization of amino acids depends upon pH. At the isoelectric pH, the molecules present no net charge. The zwitterionic structure is dominant in the range between 1.91 and 8.16. Below or above these pH, the molecules are cationic or anionic, respectively [9].

The electrochemical behaviour of cysteine has been studied by cyclic voltammetry using solid electrodes. This molecule undergoes oxidation to cystine and, in the presence of stronger oxidants, to the cysteic acid, according to the following sequence [11], [12]:RSHRSSRRSO2HRSO3H.In previous work we have showed using stationary platinum disk electrode that the cysteine oxidation begins at 100 mV/SSE in 1 mol l−1 H2SO4 solution containing 10−2 mol l−1 of cysteine [9]. Cysteine has been found to form weak complexes with various metal ions such as zinc, copper, cobalt and iron in biological systems [13].

There is no work in the literature using cysteine as a corrosion inhibitor of stainless steel, but we have observed, in a previous work, its inhibitor behaviour in the copper dissolution in 1 mol l−1 H2SO4 [9]. The aim of the present work is to study the effect of cysteine on the corrosion of 304L stainless steel in 1 mol l−1 H2SO4 by open-circuit potential measurements, anodic polarization curves, electrochemical impedance measurements and scanning electron microscopy images.

Section snippets

Experimental

It was used a conventional electrolytic cell of three electrodes that was built with boron silicate glass and had a capacity of 100 ml. The cell top had entry ports that provided space for the electrodes and one entry to pass N2 gas through the solution.

The working electrode was a rotating disk electrode (RDE) consisting of a cylindrical 304L stainless steel rod (Villares Metals) with a 0.2 cm2 cross-sectional area. The surface was polished with silicon carbide emery cloth (grade 1000), then

Results and discussion

Table 1 shows the steady-state corrosion potential measurements of the stainless steel RDE at 1000 rpm in 1 mol l−1 H2SO4 in the absence and presence of different cysteine concentrations. In the absence and presence of low cysteine concentration (10−6 and 10−5 mol l−1), the corrosion potential stabilizes at around 360 mV. At higher concentration of cysteine (10−4–10−2 mol l−1), the corrosion potential stabilizes at more negative potentials than those obtained without cysteine. These results suggest

Conclusions

In accordance with the results of the electrochemical impedance obtained at the present time, and corroborated by the other techniques investigated, cysteine in the concentration ranges between 10−4–10−2 mol l−1 turns the surface of 304L stainless steel electrochemically active, probably dissolving the passivation layer by the cysteine complex formation with metallic ions present in this film. In low concentrations (10−6–10−5 mol l−1), the presence of cysteine does not affect the electrochemical

Acknowledgements

The authors thank FAPERJ and CNPq for the financial support and CNPq for research fellowship.

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