Effect of various additives on morphology and corrosion behavior of ceramic coatings developed on AZ31 magnesium alloy by plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) coatings were developed on AZ31 magnesium alloy using alkaline silicate with KOH as a base electrolyte system, and with the addition of sodium aluminate, sodium tetra borate, potassium titanium fluoride, tri sodium ortho phosphate and urea as additives. The phase composition and surface morphology of these multi-phase coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion behavior of the coated samples was evaluated by potentiodynamic polarization tests and electrochemical impedance spectroscopy (EIS) in 3.5 wt.% NaCl solution. The results showed that the anions namely, SiO32−, AlO2−, B4O72−, F− and PO43−, effectively participated in the coating formation influencing its chemical composition and surface morphology and thereby corrosion resistance. The mechanism of corrosion process of each coating was explained in detail with the help of Electrochemical Impedance Spectroscopy (EIS) analysis and equivalent circuit modeling. It was observed that the sample treated by PEO in the electrolyte solution containing sodium tetra borate as an additive showed higher corrosion resistance which could be attributed to its morphological characteristics.