Correlation between current frequency and electrochemical properties of Mg alloy coated by micro arc oxidation

The present work investigated the electrochemical behavior of Mg alloy subjected to micro arc oxidation coating for 120 s with respect to current frequencies from 60 Hz to 2000 Hz. The microstructure and chemical compound of thin coating layers with a thickness of ~ 3 μm were analyzed using scanning electron microscopy and X-ray photoelectron spectroscopy. The microstructural observations on the surface and cross sections revealed that both the size of pores and the number of discharge channels decreased significantly as the current frequency increased, resulting in a compact coating layer. This was primarily attributed to the transition time of the alternating electrical wave, which was determined by the current frequencies tested. Based on potentiodynamic polarization tests, the sample coated at a frequency of 2000 Hz demonstrated the highest polarization resistance of 6.37 × 105 Ω cm2, implying that the corrosion resistance was superior to that obtained under other conditions due to its condensed structure. This electrochemical response was also interpreted in relation to the equivalent circuit model.

► The population of discharge channels decreased with increasing current frequency. ► The amount of MgO increased with increasing current frequency. ► High current frequency of 2000 Hz led to high corrosion resistance of Mg alloy.