The steel for our structural steelwork has been delivered with what appears to be a very hard, smooth, bluish coating. Can we apply the coating system directly over it?
No! The bluish layer is called millscale, and is a type of iron oxide that is formed on the surface of the steel during the hot-rolling process. The very high surface temperature combined with high roller pressures result in a smooth, bluish grey surface. Whilst millscale appears to be sound and firmly adhering, the underlying steel will corrode preferentially to the millscale, causing delamination, taking the protective coating with it. The only way to prepare steel prior to applying a protective coating is to remove all surface contaminants including millscale, and create a surface profile on the steel to achieve maximum bonding between the steel and the first coat. The preferred way to achieve this is abrasive blast cleaning according to AS1627.4 Class 2.5 (SA 2 _). Read more.
