Abstract: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 mm formed in the base electrolyte without C₆H₁₈O₂₄P₆ to 9−10 mm in C₆H₁₈O₂₄P₆ solution. The P content and color of anodic coatings separately increases and darkens with the addition of C₆H₁₈O₂₄P₆. After the anodizing solutions were changed from 0 to 16 g/L addition of C₆H₁₈O₂₄P₆, the largest size of micropores decreases from 4 mm to   3 mm, while the number of micropores per area on coating surface decreases from 0.076 to 0.047 mm⁻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.

Key words: AZ91HP magnesium alloy; anodic coatings; electrolyte