Effect of microstructure on the zinc phosphate conversion coatings on magnesium alloy AZ91

► The decrease of β-phase causes decreasing of both coatings weight and etching weight. ► Microstructure plays important roles for formation and characteristics of the coating crystals. ► The β-phase becomes hydrogen evolution center during phosphating process. ► The coatings form two layers: inner layer and outer layer. ► The initial corrosion resistance of AZ91 is increased by phosphate coatings.