Flash rust
Flash rust turns up in 3 fields: corrosion, thermal spray, powder coating. Here is what it means to the people in each one.
Flash rust in corrosion
Thin, quick layer of rust that forms on freshly blasted or water-washed steel within minutes to hours.
Light orange rust that blooms on bare steel soon after blasting, waterjetting or washing, especially in humid weather or where chlorides remain. Specs grade it as light, moderate or heavy, and many coatings tolerate light flash rust on waterjetted steel while immersion linings do not. Fast or heavy flash rusting is usually a clue that soluble salts are still on the surface.
"Flash rust showed up within an hour of washing, test for chlorides before anybody primes over it."
Flash rust in thermal spray
A thin rust bloom that forms on freshly blasted steel when it sits in humid air before coating.
Light orange or brown oxidation that appears on newly blasted steel within minutes to hours, especially when humidity is high or the steel is near the dew point. Spraying over it weakens adhesion and starts undercutting corrosion, so field specs require coating before any visible rust and often a steel temperature at least 3 °C above the dew point. Steel that flash rusts has to be re-blasted.
"Humidity's at 85 and we're getting flash rust on the far end, blast smaller sections and shoot each one right away."
Flash rust in powder coating
A thin orange rust bloom that forms within minutes on wet, freshly cleaned steel.
Clean bare steel rusts fast when water sits on it, especially if the rinse is warm, the line stops or the dry-off is slow. Flash rust shows as a light orange or brown haze, often in streaks where water ran. Powder will cover it but adhesion and corrosion resistance suffer. Prevention is fast drying, a proper conversion coating and not leaving parts parked wet.
"Line was down 20 minutes with racks in the rinse, now every part has flash rust in the corners."