Chrome-free surface treatments for magnesium alloy

Conversion coatings for magnesium have traditionally been based on immersion treatment in a solution containing hexavalent chromium compounds. However, the need for a replacement surface treatment has been strongly emphasized by the present environmental drive to eliminate hexavalent chromium. The development of permanganate bath chemical conversion coatings for magnesium alloys was studied, using mass gain measurements, measurements of corrosion potential, X-ray diffraction, X-ray photoelectron spectroscopy and microscopic examination. The chemical conversion coating obtained from a permanganate bath consists of a film, composed mainly of manganese oxides and magnesium oxide or hydroxide. Such films were found to be amorphous composite coatings. The corrosion resistance of permanganate chemical conversion coatings was comparable with that of chromating.