Mill scale is the blue-grey oxide layer that forms on structural steel during hot-rolling. It looks clean and adherent, but it is electrochemically active and causes premature coating failure if not removed before painting. Shot blasting to Sa 2.5 is the only reliable method for complete mill scale removal.
Mill scale is a mixture of iron oxides (wüstite, magnetite and haematite) that forms on the surface of steel when it is hot-rolled at temperatures above 700°C. It is harder than the underlying steel and initially appears to be a well-adhered, protective layer. However, mill scale is cathodic to steel — when moisture is present, it accelerates corrosion of the underlying metal at any point where the mill scale is broken or porous.
Coatings applied over mill scale fail prematurely because: mill scale is poorly bonded to the steel and will eventually detach, taking the coating with it; mill scale provides no anchor profile for mechanical adhesion; and the electrochemical activity of mill scale accelerates under-film corrosion. All major coating manufacturers specify Sa 2.5 blast cleaning as a minimum for structural steel.
UK Shot Blasting Services blasts structural steel to Sa 2.5 on-site across the UK. Request a free site visit or call 07721 375756.
UK Shot Blasting Services — mill scale removal to Sa 2.5. Mobile UK-wide service. Free site visits.