An orange mark on stainless steel is most often iron that has landed on it, not the steel itself. A carbon-steel tool, wire brush or grinding dust leaves iron particles on the surface, and they rust as soon as they get wet (BSSA). What protects stainless steel is an oxide layer that forms on its surface by itself, and on a clean surface with access to oxygen that layer renews itself.
How the mark behaves tells you the cause. A mark that wipes off, with smooth metal underneath, is transferred iron. A mark that will not wipe off and leaves a pit is pitting corrosion, and chlorine is its most common source: according to BSSA, household bleach spilled on a 304 sink and not rinsed off at once can pit it overnight.
- Photograph the mark and wipe it with a non-abrasive cloth and a cream cleaner suited to stainless steel.
- If a chlorine product touched the surface, rinse thoroughly and dry it.
- Keep wire brushes and carbon-steel tools away.
- If you suspect contamination, the ferroxyl test (ASTM A380) shows free iron as a blue stain.
If a pit remains after wiping, the damage is in the metal; on a food-contact surface a pit also makes cleaning harder.
