Non-Chromate Conversion Coatings for Magnesium Die Castings

Hexavalent chromium-containing baths have traditionally been used as pre-treatments on magnesium alloys. Environmentally friendly alternatives to these baths are being investigated. One alternative seems to be a phosphate-permanganate treatment consisting of a mixture of sodium dihydrogenphosphate and potassium permanganate. The phosphate-permanganate treatment has shown equivalent performance to a standard chrome pickle, both as stand-alone corrosion protection and as a base for subsequent coating. Phosphate-permanganate treatments must be combined with a deoxidizing process to be successful. Another alternative is a fluorozirconate treatment. The paint base properties of fluorozirconate are quite close to what is obtained using a chrome pickle.