Mechanism of the formation of stannate and cerium conversion coatings on AZ91D magnesium alloys

The characteristics of the formation of conversion coatings on an AZ91D Mg alloy were detailed. Because the galvanic effect prevailed around the eutectic α + β, alkaline stannate conversion coatings nucleated first near the β phase where plenty Mg2+ were present. Conversely, the formation of cerium conversion coatings largely relied on an increase in the interfacial pH. Intense dissolution of the primary α phase in acid cerium nitrate solution caused hydrogen bubble evolution, which, in turn, resulted in blisters. It is evident that the galvanic corrosion routes on the AZ91D depend on the pH of the conversion coating solution.