Effects of C6H18O24P6 on properties of micro arc coatings formed on AZ91HP magnesium alloy in Na2SiO3 electrolyte

In a basic solution containing 18 g/L Na2SiO3, the anodizing of AZ91HP magnesium alloy was investigated with and without addition of phytic acid （C6H18O24P6）. The thickness, composition and morphologies of anodic coatings formed at different concentrations of C6H18O24P6 were determined. The corrosion resistance of anodized samples was evaluated by salt spray test in 5% （mass fraction） NaCl solution. The results show that C6H18O24P6, which is harmless and environmentally friendly, greatly affects the properties of anodic coatings. Under the same electric parameters, the final voltage increases with the concentrations of C6H18O24P6. The coating thickness slightly increases from 8 μm formed in the base electrolyte without C6H18O24P6 to 9 10μm in C6H18O24P6 solution. The P content and color of anodic coatings separately increases and darkens with the addition of C6HIsO24P6. After the anodizing solutions were changed from 0 to 16 g/L addition of C6H18O24P6, the largest size of micropores decreases from 4 μm to 3 μm, while the number of micropores per area on coating surface decreases from 0.076 to 0.047μm 2. The salt spray test shows that C6H18O24P6 can improve the corrosion resistance of the anodic coatings and the coating formed in the electrolyte containing 12 g/L C6H18O24P6 exhibits the highest corrosion resistance.