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Why stainless steel rusts, and what to do about it

Stainless steel is not a metal that cannot rust. It is a metal that repairs itself, and everything that goes wrong is something that stopped it repairing.

The passive layer, and what it needs

What makes stainless stainless is a chromium oxide film a few atoms thick. Scratch it and it reforms, provided two things are present: chromium in the alloy, and oxygen at the surface.

Take away the oxygen and the repair stops. That is why rust appears under a gasket, under a deposit, inside a dead leg, and in the crevice behind a badly fitted clamp — all places where the surface is shielded from air while staying wet.

Chlorides

Chloride ions attack the film directly, and they concentrate as water evaporates. A coastal site, a chlorinated sanitiser, or a wash-down that is allowed to dry rather than being rinsed will each put chloride on the surface and leave it there.

316 resists this better than 304 because of its molybdenum, and a better surface finish resists it better than a rougher one because there is less for the chloride to sit in. Neither is immunity. A 316 valve on a coastal line still needs rinsing.

Free iron

The most common rust on a new installation is not the stainless at all. It is carbon steel picked up from a grinding wheel, a wire brush, a scaffolding pole or a tool, sitting on the surface and rusting there.

It looks like the valve is failing. It is not — but it will become that, because the rusting particle holds moisture against the surface and starves the film of oxygen underneath it. This is what passivation removes, and it is why a passivation pass after installation is worth the afternoon it costs.

What to do

Rinse after any chlorinated clean and do not let it dry on. Keep separate tools for stainless. Passivate after installation and after any significant mechanical work. Check under gaskets at the intervals the rest of the line is checked, because that is where it starts and where nobody looks.

And when rust does appear, find out whether it is the steel or something on the steel before replacing anything. Most of the time it is something on the steel.

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