Investigations on the passive and pitting behaviors of 17–4 PH martensitic stainless steel containing Al₂O₃ inclusions in chlorine environment

The passive and pitting behaviors of 17–4 PH martensitic stainless steel containing Al₂O₃ inclusions in chlorine environment were investigated. The electrochemical behavior was evaluated by electrochemical impedance spectroscopy (EIS), current-time transient measurements, potentiostatic pulse test (PPT), potentiodynamic polarization, and Mott-Schottky carried out in solutions containing 0.5 wt%, 3.5 wt%, and 5 wt% NaCl. The results show that pitting corrosion resistance decreased with an increase in NaCl concentration. It was suggested to draw a line between pitting potentials and logarithm of the Cl^- concentration. The pits initiate at the microcrevices around the Al₂O₃ inclusions. The acceptor and donor values of the passive films behaved as p and n-type bilayer were in the order of 10²¹–10²² cm^-3. Furthermore, the constitution change in passive films was investigated via X-ray photoelectron spectroscopy (XPS). The lower resistance of the passive film to pitting corrosion may be attributed to the increased hydroxides and decreased oxides. Furthermore, the stability of passive film is greatly influenced by the content of CrO₃, bound water, and the Cr/Fe ratios.