Failure mode
Pitting & Crevice Corrosion
Small pits appear on an otherwise clean surface, or corrosion starts in a crevice — under a gasket, a bolt head, a lap joint — where the surface looked protected. Once started, the pit deepens instead of spreading, and it can perforate a thin section from a single point.
At a glance
| Grades affected | 304 · 316L · 2205 |
|---|---|
| Where it shows up | Stainless Steel Sheet & Plate · Stainless Steel Tube & Pipe · Stainless Steel Fittings & Flanges |
| Measured by | Pitting resistance is ranked by PREN, not by a single test. |
| Write into the order | State the chloride exposure · Pick the grade by PREN · Require pickling and passivation |
Mechanism
Why it happens
Four causes account for most of it. They are not equally common, and they are not equally within the mill's control.
Chlorides
Chloride ions break down the passive chromium-oxide film locally. The ocean, de-icing salt, chlorinated cleaners and even tap water in the wrong concentration all carry chlorides.
Stagnant crevices
A crevice traps chloride-rich water and starves the surface of oxygen, so the passive film cannot repair itself. The crevice is almost always where pitting starts.
Insufficient PREN
Pitting resistance is rated by PREN = Cr + 3.3 Mo + 16 N. 304 sits around 18–19, 316L around 24–26, 2205 around 34. The higher the chloride, the higher the PREN the part needs.
Surface contamination
Iron particles from fabrication tools or nearby carbon steel embed in the surface and rust, initiating attack that looks like the stainless itself is failing.
Verification
How it is measured
Pitting resistance is ranked by PREN, not by a single test. For a given environment, pick the grade whose PREN clears the chloride level; where the duty is severe, specify a duplex or super-austenitic. Crevice corrosion is avoided by design — eliminating crevices, or by pickling and passivating after fabrication.
Specification
How to specify around it
Most of these cost nothing to write into an order and are difficult to enforce afterwards. This is the part worth copying into your next RFQ.
1
State the chloride exposure
Coastal, marine, de-icing or chlorinated cleaning each need a different PREN. Tell the supplier which one the part faces.
2
Pick the grade by PREN
304 for general, 316L for coastal and mild chloride, 2205 for seawater and high-chloride duty.
3
Require pickling and passivation
After welding and fabrication, pickling removes the heat tint and passivation restores the passive film. Specify both, not one.
4
Avoid crevices by design
Where a crevice is unavoidable — a gasket or lap joint — say so, because it changes the grade decision.
For chloride-service enquiries we ask where the part lives — coastal, marine or process — before quoting, and we say if the requested grade is under-specified for the exposure. The grade recommendation is stated in the quotation, not buried in a later conversation.
We do not claim this never happens. Metal is rolled, shipped across an ocean and unloaded by someone else. What we do is ask the question before the order runs, put the answer in writing, and be the single party that answers for it afterwards.