Corrosion resistance of calcium-modified zinc phosphate conversion coatings on magnesium–aluminium alloys

• Zn–Ca–P coatings have been prepared on as-extruded AZ31 plates, AZ31 pipes and AM30 pipes.
• The Zn–Ca–P coatings markedly enhance the corrosion resistance.
• The coating formation is affected by microstructure and composition of the substrates.
• The proposed model elucidates the coating formation of AM30 alloy.

The influence of the microstructure of the calcium-modified zinc phosphate (Zn–Ca–P) conversion coatings of Mg–Al alloys on corrosion resistance was investigated using OM and SEM and hydrogen evolution tests. The results demonstrated that the Zn–Ca–P coatings markedly enhanced the corrosion resistance of the alloys. The microstructure and chemical compositions of the alloys exerted a significant influence on the corrosion resistance of their coatings. A model was proposed to elucidate the formation mechanism of the porous Zn–Ca–P coating on the AM30 alloy.