2008 Volume 49 Issue 5 Pages 1042-1047
Two techniques for improving corrosion resistance in magnesium alloys were combined, coating with high purity magnesium and treating with fluoride. The specimens used were commercial AZ31B alloy. A vapor deposition technique was applied for the coating. Temperatures of an evaporation source were varied in the range of 823 to 973 K, while a temperature of the substrate, the specimen, was kept at 523 K. Then the specimen was immersed into molten NaBF4 kept at 693 K for various durations. The corrosion resistance of the specimen was evaluated by immersion testing using a 0.02N HCl solution and also by salt spray tests. Fluoride-treated specimens coated with high purity magnesium showed superior corrosion resistance, even in the acidic solution. The fluoride films formed on the substrate were extracted and observed with a TEM. The films were composed of small grains about 0.5 to 1 μm in diameter, and no defects, voids, cleavages, or cracks were observed. Most of the diffraction patterns were indexed as an MgF2 phase, although the fluoride layer is composed of MgF2 and NaMgF3 layers.