高純度マグネシウム被覆とふっ化物被覆の複合表面改質処理によるＡＺ３１Ｂマグネシウム合金の耐食性向上

概要

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Surface modification by combining high purity (h.p.) magnesium coating intermediate layer and the fluoride treatment has been applied to AZ31B magnesium alloy. The h.p. magnesium coating on AZ31B alloy was formed by vacuum evaporation method. Specimens with and without the h.p. coating are dipped into molten NaBF4 at 693 K for 10.8 ks to form fluoride layer. Diffraction measurements and residual stress analysis were carried out at synchrotron radiation facility SPring-8, BL-24XU (Hyogo prefecture beam line). The surface layer after the fluoride treatment was composed of NaMgF3 and MgF2 phase. Compression residual stress was detected, and the h.p. intermediate layer effectively decreased the compression stress. The fluoride treatment layer on the h.p. magnesium showed high corrosion resistance. The combined surface modification with the h.p. coating and the fluoride treatment improved corrosion resistance by a factor of one half in 0.1 N HNO3 solution, and by a factor of two in 0.02 N HCl solution, comparing with the fluoride-treated AZ31B alloy without the h.p. magnesium intermediate layer. Because Al–Mn compound, which might precipitate in AZ31B alloy and disturb the formation of the surface fluoride layer, were covered by the h.p. magnesium layer, the corrosion resistance seemed to be improved by the combined surface modification.
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