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topic 13080

Nitric vs Phosphoric Acid Based 316L SS Passivation


I write specs for electrical equipment. I read an interesting series of replies to letter 778 about Pickling and Passivation. I have an offshore gas platform in the North Atlantic where we have typically specified 316L SS. The posts on letter 778 prompted me to request a pickling and passivation routine similar to the guidelines given. A reply from a Vendor requested authorization to use a phosphoric acid passivation bath rather than a nitric acid based bath. This was based by concerns within the industry for not using the nitric acid based baths out of concern for potential carcinogenic side effects for those in the industry using it.

Any comments?

Ken Almon
- Halifax, NS, Canada


I have not heard of any specific results using phosphoric acid. Generally, companies that wish to switch away from using nitric acid for whatever reason, usually go to citric acid, as it has an A.S.T.M.# A-379. The results have been the same or better than nitric acid, especially with 316 L SS. Citric acid can also be quite volatile, so the usual precautions should be taken.

Good luck,

Jeff Swayze
- Kelowa, B.C., Canada


Hello Ken,

Phosphoric acid passivation is not a recognized passivation technique according to any of the standards that I am familiar with.

The cancer concern for nitrates in humans is more of a theoretical concern for nitrates in drinking water. It would not be a concern for the workers handling the chemicals. You consume a large amount of nitrate from the food you eat without it being a cancer risk. The biggest concern in the environment is for blue baby syndrome which can be caused by high levels of nitrate in drinking water.

For more information try a search on the web. I found this site of some interest there are lots of others.

I use nitric acid for passivation every day but if you are looking for a more environmentally friendly solution to passivation try citric acid. There is no reason to use one of the proprietary mixes as perfectly ordinary citric works just as well.

Even if you use citric acid you should not just dump it down the sewer when you are done. You must dispose of it in conformance with local ordinances.

Hope this helps.

John Holroyd
- Elkhorn, Wisconsin

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