Alloying for Corrosion Resistance

Alloying magnesium does not lead to any improvements in its corrosion resistance and in some cases can lead to a severe degradation in corrosion resistance. The effects of various alloying elements on the uniform corrosion rates of magnesium are shown in Figure 37.

Iron, with a concentration of ≥ 0.017%, and also Ni, Co, and Cu significantly increase the corrosion rate of Mg. Aluminum improves the strength and hardness of Mg without seriously degrading its corrosion resistance. A content of 6% Al provides the best combination of strength and ductility. Although it may lead to an increase in corrosion rate, zinc is second to aluminum in its strengthening effect on Mg. It is used in combination with small amounts of other elements such as zirconium and the rare earths to produce a precipitation hardenable alloy. Zn also counters the corrosion effects of Fe and Ni contaminants found in Mg alloys. Manganese has been found to slightly increase resistance to salt water environments by interacting with iron and other heavy metal elements in the alloy. Mn has a low solubility in Mg and thus is used in small amounts, about 1.5% maximum and 0.3% with Al. Some common magnesium alloys with their respective applications are listed in Table 39.