Kinetics of Passive Film Growth on 304 Stainless Steel in H2SO4 Pickling Solution under Chemical Oxidation

Abstract

H2SO4-H2O2 mixtures are a promising and environmentally friendly passivation medium for the stainless-steel pickling process. The corrosion behavior of stainless steel is highly dependent on the kinetics of passive film growth. Long-term electrochemical measurements, including polarization resistance, open circuit potential (OCP), and electrochemical impedance spectroscopy (EIS) measurements were performed to investigate the evolution of the passive state of 304 stainless steel. According to the OCP results, an active-passive transition takes place in 10 ks in 0.5 M H2SO4 solution containing 0.005 M to 0.3 M H2O2. Polarization resistance results indicate that the passive film thickness keeps growing after OCP stabilization in the presence of H2O2. Electrochemical impedance spectroscopy (EIS) results confirmed that the growth of the passive film in H2SO4-H2O2 solutions takes about 9 h. Additionally, according to the Point Defect Model (PDM) and Mott–Schottky analysis, the semiconductor properties of the passive film on 304 stainless steel in H2SO4-H2O2 solution were studied. The results indicate that the passive film is an n-type semiconductor. The donor density is in the range of 1.6 × 10−21 cm−3 to 24 and decreases exponentially with increasing film formation potential (this potential coincides with the final OCP in the corresponding H2SO4-H2O2 solutions). By postulating that most donors are oxygen vacancies, the point defect properties including diffusivity and electrical field strength are obtained.