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

The effect of the microstructure, particularly of beta-Mg17Al12 phase, on the formation and growth of zinc phosphate conversion coatings on magnesium alloy AZ91 (AZ91) was studied. The zinc phosphate coatings were formed on AZ91 with different microstructures produced by heat treatment. The effect of the microstructure on the zinc phosphate coatings were examined using optical microscope (OM), X-ray diffraction (XRD), coatings weight and etching weight balances, scanning electron microscopy (SEM) and salt immersion test. Results showed that as-cast AZ91 contained a high volume fraction of the beta-Mg17Al12 phase and it was dissolved into alpha-Mg phase during heat treatment at 400 degrees C. The beta-phase became center for hydrogen evolution during phosphating reaction (cathodic sites). The decreased volume fraction of the beta-phase caused decreasing both coatings weight and etching weight of the phosphating process. However, it increased the crystal size of the coatings and improved corrosion resistance of AZ91 by immersing in 0.5 M NaCl solution. Results also showed that the structure of the zinc phosphate conversion on AZ91 consisted of two layers: an outer crystal Zn-3(PO4)(2)center dot 4H(2)O (hopeite) and an inner which was mainly composed of MgZn2(PO4)(2) and Mg-3(PO4)(2). A mechanism for the formation of two layers of the coatings was also proposed in this study. (C) 2012 Elsevier B.V. All rights reserved.