Sound technical content, curated with aloha by
Ted Mooney, P.E. RET
Pine Beach, NJ
The authoritative public forum
for Metal Finishing since 1989
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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"?
QA - Schaumburg, Illinois, USA
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.
- 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
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.
- Brugge, Belgium
2007
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