Effects of passivation behavior on micro-electrochemical machining (ECM) performance of stainless steels with different metallographic phases in NaNO₃ and NaClO₃ solutions

To investigate the passivation behavior of stainless steels with various metallographic phases and their impact on micro-electrochemical machining (micro-ECM) performance, the electrochemical behavior of representative dual-phase stainless steel (SUS329J1), martensite stainless steel (SUS440C), ferritic stainless steel (SUS430), and austenite stainless steel (SUS316L) in NaClO₃ and NaNO₃ solutions were examined using potentiodynamic measurements and electrochemical impedance spectroscopy (EIS). EIS results indicate the charge-transfer resistance of the ferritic and martensite phases in the NaClO₃ is higher than that in NaNO₃, suggesting a stable passive film formation in the NaClO₃ solution. Conversely, for the austenite phase, the passive film formed in NaNO₃ is more stable than that formed in NaClO₃. Experimental results from ECM reveal a different trend in material removal rates (MRRs) with the use of NaNO₃ electrolyte SUS440C > SUS430 > SUS316L, contrasting with their respective MRR trends observed using NaClO₃. Additionally, dual-phase steel SUS329J1 demonstrates a higher MRR than both SUS430 and SUS316L stainless steel. This study investigates the relationships between MRRs and metallographic phases of stainless steels, providing practical insights for optimizing electrolyte composition when machining different stainless steels.