Structure and corrosion behavior of conversion coatings on magnesium alloys

We studied various types of surface treatments for magnesium alloys that involved the application of chromium or manganese by measuring their corrosion potential using X-ray diffraction, X-ray photoelectron spectroscopy, auger electron spectroscopy, and microscopic examination. The chromium-conversion coating is composed primarily of amorphous trivalent chromium oxide (or hydroxide) and a fluoride of magnesium and aluminum. The permanganate-conversion coatings, we obtained by adding HF to the immersion bath, consisted of a film that was composed primarily of manganese oxides and magnesium fluoride. We found that these films formed an amorphous composite coating. In addition, we examined the corrosion resistance of AZ91D magnesium alloy die-cast panels with various surface treatments. The exposure tests, we performed in Miyakojima, indicated that the anodic oxidation coating and the MX1-type surface treatment containing chromate, which result in relatively thick coatings, are superior in corrosion resistance. The corrosion resistance of permanganate chemical conversion coatings is comparable with that of chromium conversion coatings.