Duplex-layered manganese phosphate conversion coating on AZ31 Mg alloy and its initial formation mechanism

A duplex-layered phosphate conversion coating was obtained on AZ31 Mg alloy by substituting NaF bath with a citric bath. The morphology, composition and corrosion resistance of the coating were investigated using SEM, EDS, SPM and electrochemical methods. A three-stage mechanism for initial formation of the coating was proposed: Dissolution of the loose oxide film and deposition of Mg-3(PO4)(2) and AlPO4, formation of a composite intermediate layer of Mg-3(PO4)(2), AlPO4 and Mg(OH)(2), and deposition of manganese phosphate nuclei followed by growth and lamination of the nuclei. The nuclei preferentially deposit at the Al-Mn phase surface and near the grain boundary. (C) 2013 Elsevier Ltd. All rights reserved.