Magnesium alloys were one of most favoured structural metals, because they had a number of excellent physical and mechanical properties, such as low density, high strength and high stiffness. Unfortunately, the poor corrosion resistance of magnesium alloys greatly limited their further application. Thus, the adoption of surface treatment is needed to improve their corrosion resistance. Plasma electrolytic oxidation (PEO) was thought as one of most effective, economical and popular surface protective treatments for magnesium alloys,.
In order to have magnesium alloy do not corrode at the beginning of PEO process, five solutions used for pretreatment were chose. Microstructure morphologies of magnesium alloy after treated in the five solutions. And microstructure of PEO films on magnesium alloy oxidized respectively under low voltage before sparking and high voltage with continuous sparking were also discussed. Finally the most suitable pretreatment solution would be found. The results indicated PEO films of magnesium after pretreated in 5-10% (volume percent) HF solution were denser and had better protective properties.
In order to choose best PEO solution, composition concentration and temperature of PEO solutions must be discussed .Five solutions containing different additives , solutions with adding same electrolyte of different concentration and solution of same composition with different temperature were chose . The results indicated that the PEO films oxidized in solution of 20~30g/L Na2SiO3 + 2g/L NaOH + 5~8g/L KF + 5g/L other additives with temperature controlled between 20-50℃ had denser structure and better protective properties.
In order to clarify the optimized PEO process chose above best, the microstructure and phase composition of PEO films were investigated using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The corrosion resistance of AZ91D magnesium alloy with PEO films were evaluated by potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS) and neutral salt spray test. PEO films oxidized by Dow 17 and U.S. Patent process were also tested by potentiodynamic polarization curves for comparing. The results indicated that Icorr of the magnesium alloy oxidized by optimized process was decreased by three to four orders of magnitude compared with that oxidized by Dow 17 process, and the passivity property was enhanced simultaneously. The phase composition of PEO films oxidized by optimized process were MgO, Mg2SiO4, Mg5F2[SiO4]2 and MgAl2O4. Moreover the PEO films oxidized by optimized process were denser . Mg5F2[SiO4]2 that was rich in inner layer near to the metal was beneficial to form denser films and the existence of MgAl2O4 resulted in enhancing passivity of PEO films.
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