Mill Scale Removal Shot Blasting

Mill scale is the blue-grey oxide layer that forms on structural steel during hot-rolling. It looks clean and adherent but 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.

What Is Mill Scale?

Mill scale is a mixture of iron oxides (wüstite, magnetite and haematite) that forms on steel during hot-rolling at temperatures above 700°C. It is harder than the underlying steel and initially appears protective. 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.

Why Mill Scale Must Be Removed

• Coating failure — 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.
• No anchor profile — mill scale provides no anchor profile for mechanical adhesion.
• Electrochemical activity — the electrochemical activity of mill scale accelerates under-film corrosion.
• Specification requirement — all major coating manufacturers specify Sa 2.5 blast cleaning as a minimum for structural steel.

Sa 2.5 — The Standard for Mill Scale Removal

ISO 8501-1 Sa 2.5 (near-white metal blast cleaning) removes all mill scale, rust and contamination and creates a 40–70 micron anchor profile. It is the standard specified by Jotun, Hempel, International Paint, Sherwin-Williams and all other major coating manufacturers.

UK Shot Blasting Services — mobile shot blasting across the UK. Free site visits. Call 07721 375756 or request a site visit online.