A chromium-free conversion coating of magnesium alloy by a phosphate–permanganate solution

A chromium-free conversion coating for AZ91D magnesium alloy has been obtained by using a phosphate–permanganate solution, which has been developed out without acid pickling. Examinations have been carried out on the conversion coatings for morphology, composition, adhesion force, and corrosion resistance. Results show that the conversion coatings are relatively uniform and continuous, with thickness from 7 μm to 10 μm. They demonstrate good adhesion to matrix and have some non-penetrating tiny holes on the surface. The main elements of the conversion coating of AZ91D alloy are Mg, O, P, K, Al and Mn by EPMA analysis. Results of corrosion resistance test indicate that the corrosion resistance of the conversion coating by phosphate–permanganate solution is matched to that of the conversion coating by traditional chromate solution, but for the corrosion resistance after painting, the former is better than the later.