Pitting and crevice corrosion: check chlorides, crevices, then surface
Pitting is common in chloride environments at weak points of the passive film; crevice corrosion concentrates under gaskets, overlaps and deposits. Check medium → geometry → surface → grade.
Pitting and crevice corrosion are the two most common forms of localised corrosion on stainless steel. A practical investigation order:
1) Medium: confirm chloride concentration, temperature, pH and oxidisers. Chlorides plus higher temperature sharply increase risk.
2) Geometry: look for crevices — under gaskets, in lap joints, bolt holes, trapped weld spatter or deposits. Crevices become oxygen-depleted and acidic.
3) Surface: check for free iron contamination, scratches, residual scale and poor passivation; blue-dot or iron-detection tests help.
4) Grade: assess whether PREN is sufficient. 304 is prone to pitting in high-chloride service; 316L or 2205 are better choices.
Remedies:
• Reduce crevices, improve drainage and avoid dead zones.
• Pickle and passivate thoroughly to remove embedded iron particles.
• Increase PREN for the medium and temperature, upgrading to duplex steel where needed.
When rust spots appear, do not just wipe the surface — first identify contamination, pitting or crevice corrosion, then treat the cause.
