Corrosion Resistant Magnesium Alloys

Significant improvements in the corrosion resistance of commercially cast magnesium-aluminum alloys have been achieved during the past decade by reducing the heavy metal impurity level, alloying and heat treatment. The importance of these factors has been reviewed in order to indicate possibilities for further improvements in the corrosion resistance. The detrimental effect of iron impurities in magnesium is related to the precipitation of iron containing phases acting as cathodes in the corrosion process. The electrochemical behavior of these particles is determined partly by the Fe/Mn ratio in the alloy, a parameter used for assessing the corrosion resistance of AM and AZ series alloys. For optimum corrosion resistance, it is more important to reduce the Fe concentration rather than increasing the Mn content in order to satisfy the Fe/Mn ratio specification limit. The corrosion behavior of Mg-Al alloys is negligibly affected by zinc, whereas rare earth and silicon additions are beneficial, indicating that the corrosion performance of high purity magnesium alloys is maintained also in recycled metal containing these elements. The influence of casting method and heat treatment on corrosion resistance of aluminum rich alloys such as AZ91 is related to the distribution of aluminum in solid solution and in the β (Mg₁₇Al₁₂) phase, which can be utilized to increase the corrosion resistance further.