The pitting corrosion behaviour of mild steel in Na2HPO4 solutions contains chloride ion as an aggressive ion and serine and methionine as inhibitors were investigated using open-circuit potential (OCP), potentiodynamic polarization measurements, and pitting corrosion current measurements; both inhibitors shift the potential in the positive direction. The corrosion rate of the mild steel was measured in the absence and presence of the inhibitors, and the inhibition efficiency of the amino acids at a concentration of 0.02？M was calculated. The pitting corrosion current shows that increasing concentration of the inhibitor causes a decrease in pitting current density, and inhibition efficiency increases with increasing concentration of the inhibitors. The adsorption of these inhibitors on the mild steel surface obeys Langmuir isotherm, and the calculated adsorption free energy (Δ ) for the inhibitors on the mild steel in 0.1？M (Na2HPO4？+？NaCl) solutions was found to be (？24.61, ？29.34)？kJ/mol for serine and methionine, respectively, which reveals strong physical adsorption of the amino acids molecules on the mild steel surface. 1. Introduction Mild steel is one of the major construction materials, which is extensively used in chemicals and industries [1, 2]. Pitting corrosion in the presence of aggressive chloride ions Cl？ is the most frequently encountered cause of failure of mild steel. It is generally accepted that pitting proceeds by destruction of the protective oxide by adsorption of that subsequently passes into solution [3]. Compounds that retard or stop this process when present in aggressive medium are prospective corrosion inhibitors. Its protection against pitting corrosion has attracted much attention. One of the available methods is the use of soluble inhibitors; the use of inhibitor is one of the most practical methods to protect metals from corrosion, especially in aggressive media in particular, in chemicals, and petrochemical and oil industries [4, 5]. Unfortunately, many of the inhibitors used are inorganic salts and organic compounds, with toxic properties or limited solubility [6]. Protective action of inorganic inhibitors is related to the formation of oxide film or hardly soluble salt on the metal surface. One the other hand protective action of organic inhibitors comes from the adsorption on the oxide films. Increasing awareness of the health and ecological risks has drawn attention to finding more suitable inhibitors, which are nontoxic. Amino acids inhibitors fall into this category since they are cheap; most of them are soluble
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