The aim of this research is to study the effect of Cu and Ni on the electrochemical properties and hydrogen induced cracking (HIC) phenomenon of the pressure vessel steel (ASTM A516) in the sour environment containing hydrogen sulfide (H2S). The electrochemical properties were investigated by means of the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The current density and the polarization resistance were measured as a function of immersion time and the various kinetic parameters were determined by the curve fitting to the polarization test results based on the Wagner-Traud equation. The results indicate that the Cu and Nicontaining steel (steel A) shows higher corrosion and HIC resistance. It is ascribed to the formation of thin and protective sulfide film on the surface of steel A. Since the protective nature of the Cu and Ni containing sulfide film reduces the anodic dissolution and the hydrogen reduction reaction of the steel, the value of Icorr, crack length ratio (CLR) and diffusible hydrogen contents of steel A were lower than those of steel B.
The HIC phenomenon in the pressure vessel steel used widely in the field of petrochemical industry is caused by the diffusion of hydrogen atoms produced by the electrochemical corrosion reaction between the steel and aqueous H2S environment. Since the H2S gas suppresses the recombination reaction of hydrogen atoms to the molecular hydrogen, the hydrogen atoms can be easily diffused into the steel matrix and results in the cracking problem. Especially, in the wet sour environment, the electrochemical corrosion reaction of the steel is critical to the vessel safety since the wall thickness of the steel can be reduced by the anodic dissolution of the steel and the generation of the hydrogen can be increased by the cathodic reaction on the steel surface.