Effects of current frequency on the structural characteristics and corrosion property of ceramic coatings formed on magnesium alloy by PEO technology

Plasma electrolytic oxidation (PEO) process was carried out on magnesium alloy with a pulsed direct current (dc) power source at high and low frequencies. The morphology and phase composition of the coatings were investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The corrosion performances of the different coatings were evaluated via a three-electrode system in a 3.5-wt.% NaCl solution. It was detected that the PEO coatings were mainly composed of MgF2 and Mg3(PO3)2 phase. Coatings produced at relative high frequency had smaller pore size and looked much denser than those at low frequency. As a result, they performed better corrosion resistance in the present corrosive solution. These experimental results could be ascribed to the formation mechanism of the PEO coatings: lower frequency provided longer time for one single pulse, which induced continuous discharge.