Stainless steel can rust when the passive film is damaged or contaminated: free iron, chlorides, poor drainage, weld tint and inadequate cleaning are the usual causes.
Stainless steel resists rust because of a thin chromium-rich passive film, not because it cannot corrode. Rust appears when that film is damaged or cannot reform.
Common causes:
1) Free iron contamination — carbon steel tools, brushes, grinding dust, embedded particles from handling. The classic cause of "rust on new stainless".
2) Chlorides — salt spray, coastal atmosphere, chloride cleaners, pool water. Pitting and crevice corrosion start at weak points.
3) Weld heat tint — oxide scale from welding is chromium-depleted and less protective; it should be removed by pickling and passivation.
4) Poor drainage and crevices — water and dirt trapped under gaskets, in joints, or in low points promote localised corrosion.
5) Inadequate cleaning — deposits (dust, oil, process residue) keep the surface wet and oxygen-starved.
6) Wrong grade for the environment — 304 in coastal or high-chloride service will pit; 316L or duplex may be needed.
Prevention:
• Clean with dedicated stainless tools; avoid carbon steel contact.
• Pickle and passivate after welding; verify with a blue-dot test.
• Design for drainage and avoid crevices.
• Clean regularly, especially in coastal or polluted areas.
• Select the grade for the actual medium and temperature.
If rust appears, determine the cause before cleaning: contamination, pitting or crevice corrosion require different remedies.
