The most used metallic coating for the corrosion protection of steel is zinc(galvanize). It offers a good combination of galvanic and barrier protection. Its excellent performance for many applications is well documented. One hot dip product that was developed is 55% aluminium zinc alloy coated steel. It was well received in the marketplace, particularly for metal building roofing. The coating is comprised of 55% aluminium, 43.5% zinc, and 1.5% silicon. Steel Sheet with this coating has proven to be an excellent product for long life building cladding, especially low slope roofing on industrial buildings. It has been widely applied as bare(unpainted)sheet with than coating being directly exposed to the atmosphere. The product is also used as a substrate for prepainted sheet and this use has also grown significantly.