Inhibition of corrosion mechanism of Cl− in 304 stainless steel by different SO42− concentration at 30 °C

The development of passivation layer and characterization of pitting formation on the surface of 304 stainless steel (SS) in solutions containing different concentrations of 0.38 M Cl⁻ were investigated. Electrochemical methods, Mott-Schottky analysis, XPS, pitting growth analysis and surface morphology characterization were used in the study. The results show that an increase in the concentration of SO₄²⁻ at 0, 0.097 M, 0.106 M, and 0.113 M leads to an increase in corrosion potential, breakdown potential, and a decrease in corrosion current density, whereas a change in the concentration of SO₄²⁻ inhibits the corrosion reaction and pitting growth of Cl⁻ on stainless steel. In the presence of sulfate ions, a sulfate film forms on the top of the passivation film due to the interaction between SO₄²⁻ and the metal cations. The adsorption of SO₄²⁻ on the surface of the sample replaces some of the Cl⁻, reducing the oxygen vacancies available for the adsorption of Cl⁻ and lowering the carrier density. At the bottom of the crater, the large amount of SO₄²⁻ adsorption prevents the penetration of Cl⁻, thus inhibiting the growth of the pitting craters.