Characteristics of Phosphate Conversion Coatings on AZ91D Magnesium Alloy

The characteristics of phosphate conversion coatings and corrosion resistance of the coatings on magnesium alloy AZ91D were studied. Three phosphatation solutions containing phosphoric acid, phosphate ions, nitrates and nitrites added with or without zinc and fluorides were used. The morphology and the coating composition were analyzed by scanning electron microscopy (SEM), Energy Dispersive Spectrometry (EDX), X-ray diffraction (XRD) and transmission electron microscopy (TEM). No crystalline structure coatings are formed and visible cracks in the chemical conversion surface layer are observed for bath 1. Part of crystallized particles present lamelated for bath 2 and the crystallized coating is flower-like or small particles for bath 3. The phosphate film for bath 1 solution is composed of amorphous Mg₃(PO₄)₂ and Mg₁₇ Al₁₂, and the phosphate films for bath 2 and bath 3 are mainly composed of tetra-hydrated zinc phosphate. The corrosion potential is ranked in the following decreasing order: E _{bath 2} > E _{bath 3} > E _{bath 1} > E _AZ921D . The coating in the phosphate solution with zinc ions has better corrosion resistance than those in the other solutions.