Effect of potassium fluoride on structure and corrosion resistance of plasma electrolytic oxidation films formed on AZ31 magnesium alloy

Plasma electrolytic oxidation films on AZ31 magnesium alloy were prepared in silicate–KOH–glycol (base electrolyte) electrolyte with the addition of different KF concentration. The effect of KF on the characteristic of discharge in electrolytes was studied. The compositions, structures and morphologies of the oxide films formed in different KF concentration were determined by energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). Corrosion behavior of the oxide films was evaluated in 3.5 wt.% NaCl solution using potentiodynamic polarization tests, electrochemical impedance spectroscopy and potentiodynamic scanning tests. The films were mainly composed of Mg, Al, O, C, Si as well as a trace of Na and K. Major phases were MgSiO₃ and forsterite Mg₂SiO₄. The appearance of KF in the base electrolyte resulted in the decrease of the equilibrium current density and the appearance of MgF₂ in the films. Corrosion resistance depends on the amount of MgF₂ composition in the films. The films formed in the electrolyte containing 8.0 g/L KF exhibit the highest uniform corrosion resistance and the lowest pitting-corrosion tendency in NaCl solution.