Corrosion behaviour of chemical conversion treatments on as-cast Mg–Al alloys: Electrochemical and non-electrochemical methods

Magnesium alloys are often used in as-cast conditions. So, the aim of this work is to characterize the corrosion protection of as-cast AZ91D alloys coated with simple chemical conversion (phosphate–permanganate, and cerium-based coatings).With the two coatings, the electrochemical measurements show that the corrosion protection is due to both the inhibition of cathodic and anodic reactions, because of the presence of stable CeO2 or manganese oxides in basic pH.Nevertheless, the non-electrochemical tests of corrosion are required to bring to light the healing effect of phosphate–permanganate coating compared to Ce-coating and to describe the corrosion behaviour completely. Finally phosphoric and soda pickling associated to phosphate–permanganate conversion treatment or cerium coating are ecologically efficient alternatives to fluoride-based pickling and the chromating treatment.