Manifestations in corrosion prophase of ultra-high strength steel 30CrMnSiNi2A in sodium chloride solutions

The corrosion behaviors of ultra-high strength steel 30CrMnSiNi2A in sodium chloride solution were studied by weight loss and electrochemical methods. The morphology of corrosion products was observed using scanning electron microscopy(SEM) and the composition was analyzed using an energy dispersive spectroscopy(EDS) and X-Ray diffraction (XRD). The experimental results showed that the corrosion came from pitting corrosion and the rust layer was composed of outer rust layer γ-FeOOH and inner rust layer Fe₂O₃ with a little β-FeOOH. The correlation between corrosion rate and test time accorded with exponential rule. The corrosion current measured by polarization methods was higher than that calculated by weight loss method after a long-time immersion, the main reason was that β-FeOOH and γ-Fe₂O₃ transformed by γ-FeOOH led to overestimating corrosion rate. The processes of corrosion prophase were obtained from XRD and EIS results. The corrosion product, Fe(OH)₂ formed at the initial stage stayed at a non-steady state and then consequently transferred to γ-FeOOH, γ-Fe₂O₃ or β-FeOOH.