Post-weld staining commonly comes from four sources: embedded free iron from carbon steel, unremoved heat tint/scale, crevice-forming weld geometry and insufficient alloy resistance to chlorides. “Stainless” depends on a chromium-rich passive surface; contamination or severe oxidation reduces local protection.
Values in this article support engineering screening during design review, RFQ and prototyping. They do not replace formal calculations, material data sheets, qualified procedures or contractual standards.
DIAGNOSE BY LOCATION
- Random red spots near grinding: suspect carbon-steel brushes, belts, bench dust or fork handling. - Along toes/heat tint: inadequate shielding/back purge or retained weld oxide. - At laps/sealant edges: stagnant water, oxygen depletion and chloride concentration create crevice attack. - Broad tea staining: coastal deposition, rough surface and inadequate washing; improve alloy, finish and maintenance together.
REPAIR AND PREVENTION
Determine whether the issue is surface iron staining or parent-metal pitting. Remove contamination/heat tint using an approved cleaning, pickling or electrochemical method, then rinse/neutralize and verify. ASTM A380/A380M-25 provides cleaning/descaling/passivation practices; ASTM A967/A967M-25 covers chemical passivation treatments and verification options. Pickling paste is highly corrosive and may contain fluorides; trained personnel must follow its SDS.
PROJECT CHECKLIST
Grade; shielding/back purge; heat input; dedicated tools/area; post-weld cleaning level; rinse water; crevices/drainage; service washing.