Influence of electrolyte constituents on corrosion behaviour of PEO coatings on magnesium alloys

AM50 magnesium alloy is treated by PEO process using four different electrolytes which are combination of K(3)PO(4) and Na(3)PO(4) plus two different hydroxides, i.e. KOH and NaOH. Basic properties of the electrolyte such as pH, conductivity and breakdown voltage were measured. Moreover, coatings were characterised by scanning electron microscopy for the surface morphology and cross sectional study, X-ray diffraction for determination of phase composition and electrochemical impedance spectroscopy (EIS) for evaluation of corrosion resistance. The results indicate that KOH solutions need higher breakdown voltage and result in higher thickness of coatings. The values of the latter parameters are in the range of 150-160 V and 14-17 mu m respectively. While those of NaOH solutions are in the range of 106-116 V and 5-6 mu m thick. The final results show that presence of KOH in the solutions leads to a better corrosion performance of the coating compared to NaOH.