Investigation on Passivation and Transient Electrochemical Behavior of Fe-Cr-Ni Based Alloy in Micro ECM

Passivating electrolyte and pulse power supply are usually adopted in micro electrochemical machining (ECM). For Fe-Cr-Ni based alloys, metal oxides generate and cause significant passivation effects. Meanwhile, with short pulse-on time (T _on), current densities are time-dependent. These passivation and transient effects are beneficial to improved machining localization, but they also result in very slow metal dissolution and even suspension of micro ECM processes. To further reveal the micro ECM mechanism, the electrochemical behavior of Fe-Cr-Ni based alloys was investigated. The equivalent circuit was proposed to represent passivation and transient effects, while resistances (R _ox, R _dis) represent metal oxidation and metal dissolution reactions, and capacitance (C) represents the double layer. The material removal rate (MRR) was calculated from circuit elements and processing parameters. Electrolyte constituent and T _on were deduced as the main influencing factors. Taking 18CrNi8 steel as a case verification, specific circuit elements were analyzed by chronoamperometry tests. MRRs with different electrolytes and T _on were quantitatively predicted. The experimental effects of electrolyte and T _on on MRR agree with the predicted results. Therefore, the equivalent circuit is a practical approach to representing the electrochemical behavior of Fe-Cr-Ni based alloys. Combined with circuit elements analysis, the MRR can be predicted and improved.