The authoritative public forum
  for Metal Finishing since 1989
-----

Cause of Black Discoloration of Stainless Steel?

2003

Our company recently completed a scrubber installation at a client's facility. The scrubber uses sodium hypo-chloride to scrub odors from a fish meal process. We reused the original stainless steel stack for the project. Material is either 304 or 316, probably 304. After a few months of operation the client has started to complain that his stack is turning black in color. I assume is is from the chlorine, but I can't find anything that clearly relates the black discoloration to the chlorine. Any assistance or explanation of what is happening would be greatly appreciated.

Thanks,

Wayne LaRochelle
- Fredericton, NB, Canada

---

The sodium hypochlorite is definitely the culprit in your problem. It is one of the worst things that you can get on stainless steel, even good grades like 304 & 316. The black color could be several things. Without knowing the exact process it is hard to tell for sure. If you are spraying the bleach solution onto the steel it will certainly cause corrosion and then the surface is open to attach by whatever chemicals are given off in the fish meal process. I would strongly suggest that something other than bleach be used as the scrubbing agent, as good as it might be in eliminating the odors. The steel will eventually corrode very badly.

Lee Kremer
Stellar Solutions, Inc.

McHenry, Illinois

2003