Every season we get products returned with rust bleeding out from under a fastener, accompanied by a reasonable question: this is stainless steel, so why is it corroding? The short answer is that stainless steel resists corrosion by holding a thin protective film on its surface, and that film needs oxygen to stay intact. Take the oxygen away and the metal loses the only thing protecting it.
Where the oxygen goes
Under a bolt head, inside a thread, beneath a strip of old tape, in the gap between a fitting and the surface it is mounted on: these are crevices. Moisture gets in, but fresh air does not circulate. Within days the trapped moisture becomes aggressive, the protective film breaks down locally, and the metal starts to corrode at a point rather than across a surface. That is why this kind of corrosion looks like a pinhole with a rust stain rather than a general orange bloom.
The three conditions
- A crevice. Any tight gap where moisture can sit without drying out.
- Salt or chemicals. Salt-laden air, cleaning agents or anything corrosive left on the surface.
- Stagnation. No air flow to bring oxygen back to the metal surface.
Remove any one of the three and the process stops. In practice the crevice is the one you can control at the design and installation stage, and stagnation is the one you control with maintenance.
What we do at the works
Our products are treated after machining to remove surface contamination picked up from tooling, then polished on the ranges where it matters. That finishing does two useful things: it removes a thin contaminated layer, and it smooths the microscopic peaks where corrosion likes to start. The difference shows clearly in accelerated corrosion testing.
What you can do
- Install fittings properly sealed. A well-bedded joint has no crevice; a poorly fitted one has a perfect crevice around every fastener.
- Rinse with fresh water regularly if the product lives in a harsh environment, and mean it: aim at the fittings themselves.
- Take threaded parts apart at annual service, clean the threads and reassemble with a suitable anti-seize compound.
- Remove old tape and coverings at least once a year. Tape holds moisture against the metal for months.
- Replace any part showing a pit deeper than it is wide. A pit weakens the part as well as marking a corrosion site.
None of this is exotic. It is the difference between a fitting that lasts fifteen years and one that surprises somebody at three.