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

• A chromium-free conversion coating was obtained in two ingredients phosphate bath.
• Citric acid was used as an accelerating agent instead of NaF in phosphate bath.
• A duplex-layered manganese phosphate conversion coating was obtained.
• Initial formation mechanism of conversion coating on AZ31 Mg alloy is discussed.
• Schematic of initial formation process is proposed.

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 Mg3(PO4)2 and AlPO4, formation of a composite intermediate layer of Mg3(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.