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

• Published in: Applied surface science 2013-01, Vol.264, p.70-78
• Main Authors: Van Phuong, Nguyen, Moon, Sungmo, Chang, Doyon, Lee, Kyu Hwan
• Format: Article
• Language: English
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Conversion coating; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Heat treatment; Hopeite; Magnesium; Physics; Zinc phosphate
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/j.apsusc.2012.09.119

► 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. The effect of the microstructure, particularly of β-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 β-Mg17Al12 phase and it was dissolved into α-Mg phase during heat treatment at 400°C. The β-phase became center for hydrogen evolution during phosphating reaction (cathodic sites). The decreased volume fraction of the β-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 0.5M NaCl solution. Results also showed that the structure of the zinc phosphate conversion on AZ91 consisted of two layers: an outer crystal Zn3(PO4)2·4H2O (hopeite) and an inner which was mainly composed of MgZn2(PO4)2 and Mg3(PO4)2. A mechanism for the formation of two layers of the coatings was also proposed in this study.