Aluminum Coating Type

Two kinds of aluminum coating are produced. Type 2 is a thicker coating (typically 30 to 50 pm) that is applied by dipping in an unalloyed aluminum bath. This product is used for outdoor construction applications such as roofing, culverts, and silos that require resistance to atmospheric corrosion and have limited formability requirements.

As shown in Fig. 11, the microstructure of a Type 2 coating consists of an aluminum overlay and a thick iron-aluminum intermetallic layer. The formability and adhesion of the coating are limited by the poor ductility of this intermetallic layer.

Fig. 11 Type 2 aluminum coating microstructure. Scanning electron microscope cross section

The outdoor durability of the Type 2 aluminum coating is excellent in most atmospheres because of the good barrier properties of the thick coating and its relatively low corrosion rate, as given in Fig. 8. However, except in severe salt environments, the aluminum coating does not provide sacrificial protection to steel. As a result, Type 2 tends to show rusting at cut edges and rust staining of adjacent surfaces in rural and industrial environments. In marine environments, the aluminum coating is susceptible to crevice corrosion.

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