Inhibition by amino acids of the corrosion of steel in acid

Electrochemical measurements and corrosion tests were performed at room temperature using mild steel specimens immersed in 1 N H₂SO₄ solution in the presence and absence of cysteine, cystine and methionine. Inhibition by the three amino acids increased with increasing additive concentration. More than 60% inhibition was achieved with an amino acid concentration as low as 10^-3 M. Polarization curves indicated that these amino acids act as mixed-type inhibitors, i.e. both the cathodic and anodic curves were affected. Adsorption of the amino acids on the steel occurs in two steps: a monolayer of the adsorbate is formed on the metal surface, and this is followed by the deposition of a second adsorbate layer.