Electrochimica Acta 53 (2007) 909–914 Effect of 2-amino-3-mercaptopropanoic acid (cysteine) on the corrosion behaviour of low carbon steel in sulphuric acid E.E. Oguzie a,b,1 , Y. Li a,∗ , F.H. Wang a a State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China b Electrochemistry and Materials Science Research Laboratory, Department of Chemistry, Federal University of Technology Owerri, PMB 1526, Owerri, Nigeria Received 3 June 2007; received in revised form 23 July 2007; accepted 31 July 2007 Available online 15 August 2007 Abstract The effect of cysteine (cys) on the corrosion of low carbon steel (LCS) in sulphuric acid solution was investigated using electrochemical and scanning electron microscopy (SEM) techniques. Electrochemical impedance spectroscopy (EIS) results reveal that the presence of cys at low concentrations (0.1–0.5 mmol L -1 ) promoted the LCS corrosion process, whereas an inhibiting effect was observed at higher concentrations (1.0–5.0 mmol L -1 ), which was enhanced on deaeration of the test solution. Polarization results revealed that cys actually inhibited the cathodic process at all concentration but exerted a stimulating effect on the anodic metal dissolution reaction. Despite the cathodic inhibiting effect, the polarization resistances at low cys concentrations were less than that in the blank acid. This suggests that the anodic reaction was the predominant influence determining the corrosion rates in the presence of cys. This has been discussed vis-` a-vis the catalytic effect of the Fe–cys complex, which turns the Fe surface more electrochemically active. © 2007 Published by Elsevier Ltd. Keywords: Low carbon steel; Corrosion; Cysteine; Anodic reaction; Cathodic reaction 1. Introduction The corrosion of metals at the solid–liquid interface by acids is a major dilemma encountered in numerous industrial pro- cesses. A significant method to protect the metals from corrosion is addition of species to the solution in contact with the surface in order to inhibit the corrosion reaction and reduce the corro- sion rate. To this end, the use of organic compounds containing nitrogen, oxygen, or sulphur as inhibitors to reduce corrosion attack has received detailed attention [1–5]. These compounds act at the interphase created by corrosion product between the metal and aqueous aggressive solution and their interaction with the corroding metal surface, usually via adsorption, often leads to a modification in either the mechanism of the electrochemi- cal process at the double layer or in the surface available to the ∗ Corresponding author. Tel.: +86 2423925323/2481546907. E-mail address: liying@imr.ac.cn (Y. Li). 1 ISE member. process. Subsequent categorization of cathodic or anodic inhibi- tion is derived from experimental evidence of the predominant influence of an additive on one of the two partial reactions that constitute the corrosion phenomenon. A number of studies have been undertaken to relate the modes inhibitor adsorption with various molecular parameters [6,7]. In most cases parameters connected with the chemical and electronic structures of the molecule often act simultaneously to influence the adsorption process; making it difficult to indisputably isolate the predominant effect. Some fundamental correlations can however be proposed from examination of available literature. For instance it has been shown that N-containing compounds tend to preferentially adsorb on cathodic sites on Fe in acidic solution whereas S-containing compounds mainly function by chemisorption on anodic sites [8]. Interestingly the presence of functional groups containing both N and S in a compound has been shown to often result in enhanced inhibition effectiveness. This has been attributed to an adsorption mechanism involving both the N and S atoms of the molecule and has been highlighted in studies 0013-4686/$ – see front matter © 2007 Published by Elsevier Ltd. doi:10.1016/j.electacta.2007.07.076