A Mechanistic Study of the Oxidative Addition Reaction of Benzyl Thiocyanate at Iron, Nickel, and Zinc Surfaces in Sulfuric Acid

Reactions of benzyl thiocyanate at Fe, Ni, and Zn surfaces in sulfuric acid were studied in relation to the corrosion inhibition behavior of benzyl thiocyanate. Corrosion inhibition efficiency obtained by the electrochemical polarization method was extremely high for Fe and Ni but modest for Zn. Benzyl thiocyanate decomposed at these three metal surfaces, and the decay kinetics, which was first order in benzyl thiocyanate, was slow on Fe and Ni but fast on Zn. Formation of a multiple-layer film on the metals was indicated from the amount of benzyl thiocyanate consumed when the metals contact with the benzyl thiocyanate solution. Thiocyanate ion, benzaldehyde, toluene, and benzyl mercaptan were detected as the reaction products. Benzaldehyde was mainly formed on Fe and Ni, while toluene and benzyl mercaptan were the major products on Zn. Conclusions on the reaction mechanism deduced from these results are as follows: oxidative addition of benzyl thiocyanate to the surface metal atoms forms a multiple-layer film on the surface which is resistive against the metal corrosion; the reaction products on Fe and Ni consist of an j/3-benzyl-metal 7r-binding, whereas the product on Zn is composed of an j^-ben-zyl-Zn (T-bonding; and the high stability of the ir-bonding of benzyl ligand with Fe and Ni gives the excellent corrosion protection ability.