Metallic Coatings

For most of the nearly 200-year history of lightweight steel cladding, the protective metal coating has been made from zinc (usually with minor additions of other metals), and this is called galvanised steel. It works by the zinc sacrificing preferentially to the steel.

In the second half of the 20th century, research looked for metallic coatings which would provide longer life. Aluminium was tried as a coating material because of its passive surface, but it was not satisfactory on its own. However, aluminium alloyed with zinc and other metals produced more corrosion-proof products than any metal on its own. (See Barrier Protection.)

We now have two groups of metallic coatings for steel cladding products — zinc-dominant coatings, which primarily provide sacrificial protection; and aluminium-dominant coatings, which primarily provide a barrier protective coating of aluminium oxide. Coatings containing both aluminium and zinc are now the preferred coating for roof and wall cladding products, although zinc-based coatings continue to predominate for various other products.

The composition and weights of these coatings are described in detail in AS 1397:2011. The following sections discuss metallic coatings in the order in which they appear in AS 1397, not their rate of use in the market.