Effect of sulfur-containing amino acids on the corrosion of mild steel in sulfide-polluted sulfuric acid solutions

The influence of cysteine (RSH) and cystine (RSSR) on the corrosion behavior of mild steel in sulfide-polluted H₂SO₄ solutions was studied by potentiodynamic polarization methods and AC impedance technique. The results show that S²⁻ accelerates the corrosion process markedly, especially the anodic dissolution of iron. Tafel polarization curves show that RSH and RSSR act mainly as anodic-type inhibitors without affecting the mechanism of the hydrogen evolution reaction or iron dissolution. Adsorption of RSH and RSSR in most sulfide-polluted H₂SO₄ solutions obeys Temkin’s isotherm. Impedance studies indicate that in the inhibited and uninhibited solutions, charge transfer controls the corrosion process either at E_corr or at 30 mV vs E_corr. Potentiodynamic anodic polarization curves show that RSSR effectively inhibits the steel dissolution both in the active and passive states and greatly reduces the current oscillations observed in the passive region.