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.