Porosity and corrosion resistance of plasma conversion coatings on magnesium alloys

The topologies of oxide-ceramic conversion coatings on magnesium alloys which had been produced in an electrolyte plasma were investigated using a visual scanning technique. In addition, the porosities of these coatings were evaluated by two independent methods, i.e. by dipping and via electrochemical deposition. The corrosion resistance was assessed from measurements of the corrosion current in polarisation experiments which were performed in 0.3% HCl solution and in 0.3% NaCl solution, respectively. It was found that craters resulting from an extinction of some of the discharge channels of the electrolyte plasma did not always extend down to the base metal and therefore in these cases could not be considered as real pores. On the other hand, pores were detected which were located close to the crater areas and, evidently, had been caused by high thermal stresses occurring during the plasma treating process. Another finding was, that the degree of corrosion resistance provided by the coatings investigated here is in acid environments mainly determined by the chemical composition of the substrate alloy whereas in neutral environments the porosity of the coatings plays the dominating role.