The layered iron oxide skin of wustite, magnetite and hematite that forms on steel as it is hot rolled. It looks tight and protective but is brittle and cathodic to the steel beneath, so it cracks with flexing and temperature change, coatings over it fail by flaking, and steel at the cracks pits. Abrasive blasting removes it; hand and power tool cleaning generally leave it behind.
Still blue mill scale in the corners of those beams, they need another blast pass before primer.
Tight mill scale fools new inspectors because it looks like sound blue-black steel. A blast that leaves it behind is not near-white.
Also heard as
- millscale
- scale
- hot-rolled scale
Related terms
Near-white blast
Blast grade allowing light shadows or stains on no more than 5 percent of the surface, the usual immersion standard.
White metal blast
Top abrasive blast cleaning grade: bare, uniform steel with no visible residue, rust or staining.
Galvanic corrosion
Accelerated corrosion of the more active metal when two dissimilar metals touch in a shared electrolyte.