The entrance mechanism of calcium and phosphorus elements into micro arc oxidation coatings developed on Ti6Al4V alloy

In a solution containing 15 g/L phytic acid and 10 g/L KOH, the influences of processing factors including EDTA-Na2 concentration, Ca(CH3COO)2 concentration, current density and treating time on the amounts of calcium and phosphorus elements in anodic coatings obtained by microarc oxidation (MAO) on Ti6Al4V alloy were investigated by an orthogonal experiment of four factors with three levels. Surface morphology, chemical composition and structure of anodic coating were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier infrared spectrum (FT-IR) and X-ray photoelectron spectroscopy (XPS). Calcium and phosphorus elements of MAO coatings exist mainly as CaTiO3, PO43 − and HPO42 −. The affecting sequence of factors on the calcium content in MAO coatings is Ca(CH3COO)2 concentration > current density > EDTA-Na2 concentration > treating time, indicating that the entrance mechanism of calcium element includes diffusion and electromigration. The influencing order on the phosphorus amount is Ca(CH3COO)2 concentration > treating time > current density > EDTA-Na2 concentration, suggesting that at a fixed concentration of phytic acid, the phosphorus amount of anodic coatings strongly depends on Ca(CH3COO)2 concentration.