The protective performance of a Mg-rich primer on AZ91D magnesium alloy was greatly improved by addition of aluminum tri-polyphosphate (SAP) pigment into the primer. The effects of SAP in the Mg-rich primer were investigated. In the first stage, the pigment SAP prolongs the cathodic protection of the Mg-rich primer for AZ91D alloy by a pH buffering effect. Then, the corrosion products of the magnesium particles, composed of phosphates and oxides of magnesium, form in the primer, which enhances the barrier effect of the primer. Finally, a protective layer composed of magnesium phosphates and oxides forms on the substrate surface. (C) 2013 Elsevier Ltd. All rights reserved.