Effect of CaO Additive on the Properties of Plasma Electrolytic Oxidation Coatings on AZ31 Mg Alloy

Magnesium alloys are considered a suitable candidate for body implants due to their favorable properties such as non-toxicity and favorable mechanical properties. The limitation is their high corrosion rate in the physiological environment of the body. In the present study, PEO coating was applied from a phosphate-based bath with and without CaO nanoparticles additive on AZ31 Mg alloy using unipolar and bipolar waveforms and soft sparking waveforms. The cathodic cycle led to the entry of more calcium, and a coating was formed with higher bioactivity and smaller pore size. However, the incorporation of calcium showed negative effects on the corrosion performance of the coatings in the short-term immersion (3 h) in simulated body fluid (SBF) due to the creation of a thinner coating and more micro-cracks. However, these coatings were able to show a higher barrier performance against the corrosive solution in long-term immersion (longer than 1 day), due to the higher ability of calcium phosphate formation and blocking of coating defects. By increasing the cathodic width in the bipolar waveform, high-intensity cathodic discharges happened on the coating surface, which damaged the coating and created spalling that deteriorated the coating barrier performance.