Passivation behavior of S136H steel in neutral electrolytes composed of NaClO₃ and NaNO₃ and its influence on micro electrochemical machining performance

To investigate the influence of passivation behavior on electrochemical machining (ECM) performance, electrochemical behavior and passive film properties of S136H in neutral solutions composed of NaClO₃ and NaNO₃ were investigated by potentiodynamic measurements, electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, and auger electronic spectrometry. The results demonstrated that the corrosion resistance of the passive film increased with increasing NaNO₃ concentration. The passive film formed in NaNO₃ was more stable than that in NaClO₃, which was attributed to the larger thickness, higher Fe²⁺/Fe³⁺ ratio and higher Cr₂O₃/Cr(OH)₃ ratio of the passive film formed in NaNO₃. From the ECM experimental results, the material removal rate and radial overcut area both decreased with increasing NaNO₃ concentration because the area of the broken-down passive film was smaller than that in NaClO₃. Additionally, a larger local corrosion area was formed on the workpiece in NaNO₃ because some pores were generated in the passive film instead of the decomposition of the passive film. Therefore, compared with the NaNO₃ electrolyte, using a NaClO₃ electrolyte can improve the material removal rate and profile accuracy of microstructures on S136H steel.