What is mill scale?
Mill scale is the thin, blue-grey layer of iron oxide that forms on steel when it is rolled at high temperature. The hot steel reacts with oxygen in the air, and on cooling a hard, brittle oxide skin is left behind. You find it mainly on hot-rolled sections, plates and flats.
Why is mill scale a problem?
- Poor adhesion: differences in thermal movement and bending cause hairline cracks, and the scale flakes off in places.
- Under-rusting: where moisture reaches the steel through cracks, corrosion starts. Mill scale is more noble than steel, so exposed steel at defects can corrode faster.
- Paint does not last on it: a coating applied over mill scale fails as soon as the scale itself lets go.
In practice
- Before painting or powder coating, steel is blast cleaned to a cleanliness grade according to ISO 8501-1. Most paint systems on steel call for Sa 2½: virtually all mill scale, rust and contamination removed.
- Before hot-dip galvanising, steel is pickled in acid, which dissolves mill scale and rust.
- Reinforcing bar: in concrete, mill scale is not a problem, but loose rust flakes and dirt must be removed.
DIY tips
- Never paint directly onto shiny blue-grey mill scale. Have the steel blast cleaned, or remove the scale mechanically with a flap disc or wire brush down to bright metal.
- Prime promptly after cleaning, before flash rust forms.
- Order small parts “blasted and primed” from the steel supplier; it is often cheaper and better than grinding yourself.
- Check edges and welds: that is where scale and weld slag are most often left behind.
Related terms
- Hot-rolled steel
- Corrosion
- Grit blasting
- Cold-formed steel
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