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Stainless steel discoloration

The composition of the metal basket used in our wash system containing DI water heated to 120C only as follows:

The wash basket base material is stainless steel SS 304 . To improve the finishing, it is nickel plated and its quality will deteriorate after prolong use in elevated temp, 120C in aqueous cleaner.

1. We are seeing discoloration on the metal after several times of washing? Can you help figure out what is this phenomenon?

2. Would this be a sign oxidation to the metal, giving off other different type of contaminants and causing contamination to the aqueous cleaner?

3. Is the discoloration a concern at all?

Rubert [last name deleted for privacy by Editor]
consumer electronics mfg - Irvine, California, USA
2003

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You didn't mention pressure and I don't know how you get water to 120 °C without holding it under pressure, Rubert, but 304 stainless is not fully resistant to DI water. I would expect nickel to be fully resistant though. If you are monitoring the conductivity of the DI water, and metal ions were dissolving into it, I think you'd know it.

A photograph of the discoloration might help, but I don't have enough personal experience to help you with this without research. Maybe someone can chime in.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey