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Pitting of 316 and 316L Stainless Steel

I'm having a problem that just started up on a product made from 316 Stainless Steel. We have been making this product for 20 years plus. The product is for the filling of beverages and does get cleaned on a daily bases with a mild bleach solution at 185 degree temperature. The material has been purchased from the same material suppliers. The manufacturing process has not changed. The only thing that has changed is the company that performs the passivation process. Now we are seeing pitting in the product after short usage times. Can you please educate me?

Michael Brown
Engineer - Baltimore, Maryland, USA
2007

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If your interpretation of the situation is correct, Michael, then the plating shop is not passivating the parts properly. But unfortunately I don't think there is much chance of you, based on an internet posting, being able to educate a plating shop on how to fix their electropolishing techniques. If you know that the process can be done successfully, all you can do is send the components to a shop which can do it.

Still, the purpose of passivating is to chromium enrich the surface, and I am not convinced that it is a powerful weapon against pitting.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2007

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2007

Michael,

Our engineers can give the following recommendations: If your bleach contain ClO- or ClO ions, then you have accumulation of results of their reduction: Cl- - ions that cause pitting. There is a special ratio of Cl-/ClO- or ClO, when pitting is most likely to happen. If ratio is lower or higher, possibility of corrosion decrease. Maybe composition of bleach, or storage conditions have been changed, thus moving to area of dangerous ratio, when pitting starts.
We suggest to stop using bleach based on chlorine as cleaning solution for food appliances. Instead use cleaner where hydrogen peroxide or ammonium polysulfide is used.

We have developed passivating-sterilization solution for food industry applications that can be done on regular basis after cleaning. The solution is very cheap and safe. It does not cause pitting, but increase corrosion protection. This method was designed especially for milk industry, but can be used in other applications.

For small fee we provide complete technological know-how to set this process in-house.

Anna Berkovich
Russamer Lab

Pittsburgh, Pennsylvania

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I agree with Anna that they should stop using bleach if at all possible. This is very bad for stainless steel.
Just because you purchased the steel from the same supplier does not mean that the steel is the same. Your supplier could be purchasing from a number of manufacturers. There are many steel problems out there today.
If the steel is good and is passivated correctly before installation you should have no problems unless bleach is continued.

Lee Kremer
Stellar Solutions, Inc.

McHenry, Illinois

January 2, 2008

Our plant has developed a cleaning process for a 316L SS vessel includes 1-2 hrs (room temp) treatment with 0.1N NaOH ⇦ this on eBay or Amazon [affil links] and 0.05% sodium hyochlorite (via Clorox Ultra Bleach), followed by a triple rinse. Is the chloride contact time and concentration cause for concern? Should this be mitigated in any way? One suggestion has been to follow with an acid wash step.

Craig Morton
- Branchburg, New Jersey, USA
October 1, 2010