Corrosion and Surface Protections

Magnesium and its alloys are the lightest, but also the most basic, of light-alloy constructional materials. The current challenge involves the increasing use of light alloys in high-technology sector constructional materials, with the aim of both reducing mass and of saving energy. Typical areas of application include vehicle construction, aeronautics and the space sector, along with the mechanical engineering and electrical industries. The use of magnesium-based materials delivers, on one hand, the important advantages of reduced mass, while offering improved mechanical and physical properties. However, the characteristics of magnesium-based materials also make them highly susceptible to corrosion. Indeed, the existence of unfavourable ambient conditions (such as those caused by the spreading of salt on the roads during the cold months of the year), and the effect of airborne pollutants such as SO2, mean that the use of magnesium-based materials continues to be regarded with a great degree of scepticism. The problem here resides not only in the corrosion of the magnesium in a general sense, but also in the consequences of galvanic corrosion (also known as contact corrosion). Magnesium is normally used in so called "mixed-construction" applications. Differences in potential arise here due to the contact between noble components (e.g., steel) and the base-metal component provided by the magnesium-based material. The presence of the ambient conditions described above (saltwater, etc.) causes galvanic corrosion, which in turn leads to the magnesium base-metal acting as an anode and wearing away. The following sections of this chapter show how the carefully targeted surface treatment of magnesium-based material can provide effective protection against corrosion.