The characteristics of the formation of conversion coatings on an AZ91D Mg alloy were detailed. Because the galvanic effect prevailed around the eutectic alpha+beta, alkaline stannate conversion coatings nucleated first near the beta 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 alpha 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. (C) 2013 Elsevier B.V. All rights reserved.