The role of anions in the formation and corrosion resistance of the plasma electrolytic oxidation coatings

Combination of KOH with each of Na2SiO3, Na3PO4 and NaAlO2, formed three different coating solutions to produce plasma electrolytic oxidation (PEO) coatings on the surface of AM50 magnesium alloy. The surface morphology, cross section, chemical composition, corrosion resistance and structure of each of the coatings were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and electrochemical impedance spectroscopy (EIS). The results showed that different anions, i.e., SiO32−, PO43− and AlO2−, influence the coating characteristics such as thickness, chemical composition and coating structure. The results showed that thicknesses of the Si-, P- and Al-coatings are 8, 4 and 1 µm, respectively. Moreover beside MgO existing in structure of all three coatings, specific phases namely Mg2SiO4, Mg3(PO4)2 and MgAl2O4 were formed in the structure of the Si-, P-, and Al-coatings, respectively. It was revealed that usage of SiO32− instead of PO43− or AlO2− led to formation of a coating layer with better corrosion protection properties. The better performance of the Si-coating compared to P- or Al-coatings is considered to be due to the fact that the thickness, the number of open pores and the resistance of the barrier layer are formed under such optimum conditions which result in a higher corrosion resistance.