Why it forms
Freshly cleaned steel has no protective oxide, and a blasted surface offers a large reactive area. Once water and oxygen reach it, rust starts almost at once, and any soluble salts left on the surface accelerate it. Flash rust is expected after waterjetting, wet abrasive blasting and water washing to remove salts. It also appears on dry-blasted steel when condensation forms, which is why steel temperature relative to the dew point is monitored during blasting and application.
How it is graded
For waterjetted steel, SSPC-VIS 4/NACE VIS 7 and ISO 8501-4 describe three flash rust grades, paraphrased here:
| Grade | Appearance |
|---|---|
| Light (L) | Thin, tightly adherent yellow-brown rust through which the original steel surface can still be seen |
| Moderate (M) | A layer of yellow-brown rust that obscures the original steel surface and marks a cloth wiped over it |
| Heavy (H) | A heavy red-brown rust layer that hides the surface, is loosely adherent and leaves significant marks on a cloth |
The exact wording and the reference photographs are in the standards themselves; wet abrasive blasting standards define their own flash rust grades.
How specifications handle it
- Dry abrasive blasting. A grade such as SSPC-SP 10/NACE No. 2 describes the surface at the moment of coating. If flash rust appears before the primer goes on, the surface no longer meets the grade and is re-blasted. The NACE No. 2 surface preparation guide covers this rule.
- Wet methods. The specification states the maximum flash rust grade allowed at the time of coating, normally matched to what the primer data sheet says it tolerates.
- Rust inhibitors. Inhibitors added to wash or blast water are used only when the coating manufacturer approves them, since residues can affect adhesion.
Keeping it under control
Blast only the area that can be primed while conditions hold, keep the steel above the dew point with the margin the specification sets, and remove soluble salts before final preparation. Steel blasted on a previous shift is re-inspected before coating, not assumed to still meet the grade.