Protective coatings on magnesium and its alloys: a critical review

摘要

The coating techniques discussed here show that it is possible to develop appropriate coating schemes for the protection of magnesium used in various applications. The coating schemes are complex, multilayer, systems that incorporate many different technologies and must be performed carefully to obtain optimum results. A number of new patents have been developed for the protection of magnesium and its alloys. Some of the coating technologies that have been tried on aluminium alloys have also been tried on magnesium in order to exploit the feasibility, though the chemistry of the aluminium is quite different from that of magnesium. Many factors have to be considered in developing a coating process for an industrial application, including capital investment, ease of manufacture, coating^ performance, and environmental issues. In the case of electrochemical plating, the capital investment is relatively low, but achieving uniform coatings on complex shapes can be extremely difficult, due to the uneven throwing power of the current required for metal deposition. Conversion coating also represents a minimum capital investment, but does not provide adequate corrosion and wear protection from harsh service conditions when used alone. This type of coating can, however, act as a good paint base for producing adherent organic coatings, and thereby to enhance corrosion resistance. Many patented coatings of this type have been developed. Anodizing is the most widely sought-after coating technology for magnesium and its alloys. The technique includes complex coating procedures, but any type of alloy can be coated. Anodizing involves more capital investment and power consumption, but coatings recently produced have withstood hard wear and corrosion tests, and are commercially successful. The use of gas-phase coating processes and laser-surface (wetting/alloying) cladding to modify the surface or create coatings on magnesium is an excellent alternative with respect to its environmental impact, but the capital investment is very high. By controlling the parameters and optimizing the process variables, very-hard and adherent coatings can be applied which have good wear and corrosion resistance. The organic coatings tried so far on magnesium alloys, such as polymer coating, sol-gel coating, and plasma polymerization, need surface pretreatments prior to deposition. More work is being done in this field, and silane based polymer coating and glycidoxyl-group epoxy primers are new developments. However, they are applied for purely decorative purposes or to enhance corrosion resistance of the overall coating systems. There are a large number of new coating technologies available for protecting ' magnesium and its alloys. However, widespread commercial use of the alloys in the automobile industry is still being assessed, and the materials will take some time to reach the market in a form able to withstand the harsh service conditions. A great deal of research is still to be done to develop cheaper coating techniques so that full advantage can be taken of the low weight and excellent mechanical properties of this metal. Table 4 (pages 160 - 162) summarizes the nature and effect of the films obtained from the different types of coatings considered in this paper.