Investigation on corrosion of magnesium and its alloys

Magnesium alloys are well known for their high specific strength. There is growing interest in the alloys as structural materials in the automotive, aerospace and electronic industries. However, the corrosion performance of most magnesium alloys is not satisfactory for some practical applications. The Cooperative Research Centre for Cast Metals Manufacturing (CAST) has been investigating the corrosion behaviour of magnesium and its alloys since 1995. This paper presents some of the major achievements in this area recently made by CAST. The following issues are addressed: Negative Difference Effect (NDE), role of alloying elements, effect of microstructure and galvanic corrosion. It was found that: (a) the NDE is a common phenomenon for all alloys investigated: (b) alloying with aluminium and zirconium can stabilise the matrix phase of magnesium alloys in corrosion; (c) in the aluminium containing alloys the β phase appears to have a beneficial effect on the corrosion resistance especially where a fine grain size is produced, but in some cases, its presence in an alloy accelerates the corrosion of the alloy; (d) galvanic corrosion of magnesium alloys strongly depends on the cathode material and the manner by which the galvanic couples are exposed to the environment