Effect of KOH concentration on the microstructure and electrochemical behavior of ceramic coatings formed by plasma electrolytic oxidation method on AZ31B Mg alloy

In this study, ceramic coatings were prepared on AZ31B Mg alloy by plasma electrolytic oxidation technique in electrolytes containing sodium fluoride (KF), sodium silicate (Na2SiO3.5H2O) and different potassium hydroxide (KOH) concentrations. The electrochemical behavior of AZ31B alloy and plasma electrolytic oxidation coatings was studied by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and Mott-Schottky analysis. Also, the surface morphologies and chemical composition of the coatings have been characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. Generally, the plasma electrolytic oxidation coatings improved passive behavior of AZ31B magnesium alloy. In plasma electrolytic oxidation processes, increase in the concentration of KOH as a result of increase in the electrolyte electrical conductivity leaded to a reduction in sparking voltage. This improved the quality and the passive behavior of ceramic coatings up to the concentration of 1.5 M. The results indicated that the plasma electrolytic oxidation coatings showed its best protective behavior when KOH concentration was equal 1.5 M, and concentrations higher or lower than this value decreased the protective properties of MAO coating.