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The current strong interest in magnesium alloy die castings for automotive applications relies heavily on the “High Purity” concept. The basic knowledge of the detrimental effects of heavy element impurities on the general corrosion of magnesium alloys has been known for more than 60 years. However, it was not until the 1980's that the full potential of high purity alloys was recognized. The level of impurities (in particular, copper, nickel and iron) that can be tolerated in a die cast part exposed to a corrosive environment necessitates careful control throughout the complete manufacturing process, from ingot to finished product. In the present paper, some basic principles for the production of high purity alloys and the influence of subsequent melt handling practice in the die casting shop are discussed.

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.

Permanent mould cast medallions and 3mm die cast test plates of 47 different AZ91 based alloys covering 3-129 ppm Ni, 7-2850 ppm Cu, 87-1740 ppm Si and 0-100 ppm Co were produced. Medallions and plates were subjected to 72 hours immersion in 5% NaCl solution at 25C and to the 10 day ASTM-B117 salt spray test. The results include: a) for AZ91, the corrosion rate values anticipated from salt spray testing of die cast test plates can be calculated from the results of immersion tests on permanent mould cast medallions; b) the effect of Co on corrosion of AZ91 is 35-75 times more detrimental than Cu and thus similar to that of Ni; and c) Si showed almost no effect on the corrosion rates in the alloys examined.