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Rust on 300 series SS

Simplified, I have a 303 SS plate with a 6mm hole drilled in it, and a 6mm 316 SS dowel pressed into it. Both pieces were independently passivated. The plate is coated with zirconium nitride before the dowel is pressed in. After perhaps a month, significant rust appears at the joint. The piece does see occasional water (not continuous flow) as well as a mild alkaline wash. Some of these rust, the rest don't. The ones that do rust do so severely. Materials come from the same lot.

Any ideas as to the cause?

My guess is that crevice corrosion is taking place within the crevice formed at the joint; stainless steels rely on oxygen being present to form a Chromium Oxide film for corrosion protection. In crevice conditions where moisture is present, as the oxygen is used to create the film, a corrosion cell will be set up between the low oxygen condition in the crevice, and the oxygenated condition externally. Also, as metal loss occurs, a low pH condition - acidic - develops and becomes autocatalytic.
No galvanic effect is necessary for crevice corrosion.
However, why this should happen selectively to only some items is a bit of a mystery; perhaps in time all would do the same.
It is possible that the likely differing clearances at the joint also play a part.
Regards,

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