Corrosion inhibition of mild steel in sulfamic acid solution by S-containing amino acids

R_p, potentiodynamic polarization curves and EIS techniques were applied to study the effect of five S-containing amino acids on the corrosion of mild steel in 5% sulfamic acid solution at 40 °C. The compounds are effective inhibitors and the inhibition efficiency follow the order: N-acetylcysteine (ACC) > cysteine (RSH) > S-benzylcysteine (BzC) > cystine (RSSR) ≅ methionine (CH₃SR). The inhibitors affect the anodic dissolution of steel by blocking the anodic sites of the surface. EIS measurements indicated that charge transfer is the rate determining step in the absence and presence of the inhibitors and the steel/solution interface can be represented by the equivalent circuit R_s(R_ctQ_dl). Adsorption of RSH, CH₃SR and RSSR follows the Langmuir model while the Temkin isotherm describes the adsorption of ACC and BzC. From the application of the Flory–Huggins isotherm, the number of water molecules displaced by the adsorbing inhibitor molecules was estimated. The potential of zero charge pzc of mild steel without and with the inhibitors is calculated and the mechanism of corrosion inhibition is discussed in the light of the molecular structure.