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Black spots on 304 Stainless Steel tubing

2007

We have 304 stainless steel tubing formed in a circular fashion to make a cooling coil.

We are seeing small areas or "Black spots" on the coil. I can scrape the black off but I don't have the magnification to see if there is a stress fracture in the SS coil.

These are for heat exchangers but have not been installed. Any suggestions, is this carbon from the SS and is it "rust"?

Rich Kiolbasa
QA - Schaumburg, Illinois, USA

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SS tubing should be cleaned prior to forming to avoid incorporating any oxide scale or foreign matter. After forming, the SS should be cleaned, given a stress relief heat treatment, pickled and passivated.
Use the low carbon grade, 304L (unless you need strength at very high temperatures).
Foreign substances (oxide scale, carbide, burnt lubricant?) cause premature failure and should have been removed earlier.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
2007

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Rich,
The same problem occurred once in the past with some hand-railings over-here. We never found the origin of the problem, but removed the black stains with simple passivating.

Marc Quaghebeur
- Brugge, Belgium
2007