Corrosion and Passivation Property of the γ_N Phase in a Boric Acid Buffer Solution

A single high-nitrogen face-centered-cubic phase (γ_N) layer formed on the plasma source nitrided AISI 316 austenitic stainless steel. The electrochemical corrosion behavior of the γ_N phase in boric acid buffer solution with a pH 8.4 was studied by anodic polarization and electrochemical impedance spectroscopy (EIS). The results show that the anodic polarization curve of the γ_N phase presented a typical transition course from spontaneous passivation into passivation-transpassive dissolution process. Compared with the original stainless steel, the E_corr of the γ_N phase layer increased 75 mV, the passivation current density deduced one order magnitude. The EIS of the γ_N phase passive film had a larger diameter of capacitive arc, higher impedance modulus|Z|, and wider phase degree range, compares with the original stainless steel film. Correspondently, the interfacial charge transfer resistance R_ct of the γ_N phase passive film increased to 1.064×10⁷ Ω·cm² and calculated double layer capacitance C_dl decreased to 65.4 μF/cm². With the immersion time increased, the interfacial charge transfer resistance R_ct of the γ_N phase passive film was stable to a magnitude of 10⁷ Ω·cm². The high insulation of the γ_N phase passive film with higher compactness led to an improved corrosion resistance of the original stainless steel.