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

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.