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Metallurgical Engineering DISCUSSION
Some 316 stainless pipe fittings and brackets on a seawater intake show brown staining and small deep pits after about a year. The worst attack is hidden: under gaskets, beneath a pipe clamp and around a weld. Freely exposed surfaces in flowing water look almost new.
I thought 316 was the marine grade. Why does it corrode mainly in the hidden places, how do I compare grades for seawater service, and what should be used instead?
Stainless steel does not resist corrosion by being noble; it resists because a very thin chromium oxide film forms on the surface and re-forms when damaged, as long as oxygen is available. Chloride ions attack that film locally. Once a small area loses its film and cannot repair it, that spot becomes an anode surrounded by a large passive cathode, the solution inside turns acidic and rich in chloride, and the pit keeps growing downwards.
So the usual failure in seawater is localised pitting and crevice attack, not general thinning. 316 has far better chloride resistance than 304 because of its molybdenum, but in continuously wet seawater service it is near its limit.
The hidden locations are no coincidence. Inside a tight gap, under a gasket, clamp, deposit or marine growth, the water is stagnant and its oxygen is used up, so the film cannot heal there, while the open surface outside acts as the cathode. Crevice corrosion starts at lower chloride levels and lower temperatures than pitting on an open surface, which is why the clamp and gasket areas go first and the freely flowing surfaces look clean.
Design helps a great deal: avoid crevices where possible, use full-penetration welds instead of lap joints, keep the water moving, and let equipment drain rather than sit full of stagnant seawater.
To rank grades, use the pitting resistance equivalent number: PREN = %Cr + 3.3 × %Mo + 16 × %N. With typical compositions, 304 (18 Cr, no Mo) gives about 19. 316 (17 Cr, 2.1 Mo, 0.05 N) gives 17 + 6.9 + 0.8 = about 25. Duplex 2205 (22 Cr, 3.1 Mo, 0.17 N) gives 22 + 10.2 + 2.7 = about 35. Super duplex 2507 (25 Cr, 4 Mo, 0.27 N) gives 25 + 13.2 + 4.3 = about 42.
A PREN of 40 or more is the usual guideline for alloys expected to resist seawater at ambient temperature, which is the super duplex and 6 percent molybdenum austenitic class. PREN is a ranking from composition only, so treat it as a first screen.
The attack around the weld has its own likely causes. Heat tint, the coloured oxide beside a weld, sits on a chromium-depleted surface with much lower pitting resistance, and it should be removed by pickling or mechanical cleaning followed by passivation. If the material is standard 316 rather than low-carbon 316L, the heat-affected zone may also be sensitised: chromium carbides form at grain boundaries and the adjacent metal loses chromium.
A third cause is iron contamination from carbon steel tools, wire brushes or grinding dust, which rusts and starts pits underneath. Dedicated stainless tooling and a final passivation treatment prevent it.
