48970

Explanation for reaction of 303SS to nitric acid passivation? [Connecticut] 

June 5, 2008

My question concerns an event that occurred about two years before I began working with this company so I only have a description based upon accounts for those I interviewed. I am investigating this because I'm a QA manager and need to ensure that this doesn't happen again.

The job consisted of small parts about three eighths dia. and an inch and a half long. The print only specified a 300 series stainless. The material we used was 303 Stainless Steel that was being passivated in a nitric acid bath. (I'm guessing a similar concentration to what it is now of about 20-40 percent nitric @ 70-90 degrees F)

When two or three parts were tested there were no problems, when a full tray of 20 pieces was processed however, it was reported that the acid began to bubble almost as if it were boiling, accompanied by the smell of sulfer and a black scum that formed on the surface of the tank. To me this sounded like a dangerous situation as the smell from the reaction probably contained some form of nitric oxides. When removed, the parts had turned black. Now, two years later the part I have from that inceident is a dark grey covered with a rust colored oxidized layer. The pieces appear rather rough to the touch where they were initially machined smooth.

My question is what kind of reaction was taking place and did it present a danger to the people invovled? My guess would be that the acid reacted with the sulfur and phosphorus content in the 303 grade SS but I'm not exactly sure what took place. There were no known contaminants and the specs on the steel were verified, a subsequent material from a different lot resulted in the same reaction. Since then, they have switched to 304 SS and have no problems with that.

My intent is to document the incident as part of a validation of the passivation process used here so that it can be understood what took place and avoid a repeat of similar issues.

Vernon Lilly
Quality Assurance - Westbrook, CT, USA

Hi, Vernon. Years ago a theory was well presented on this board that certain types of oils on the parts, and not fully removed, are the culprit. I think I believed the theory at the time it was presented, but I am more skeptical now because I haven't seen any supporting evidence over the years. But if two or three of your parts processed fine and then a larger batch presented a problem, I suppose it's possible that the larger quantity of parts, or some dirtier parts, introduced enough oil to cause this problem. Anyway, review the FAQ on Stainless Passivation and make of it what you will. I'd love to see someone put a drop of oil in a beaker of nitric and see if it can wreck a part. Good luck.

Regards,

Ted Mooney, P.E.  finishing.com Brick, New Jersey

Thank you for directing me to the FAQ page. I found the explanation offered by Mitchell G. Lee of Robertshaw Controls to be particularly appropriate. I replicated his explanation; albiet on a far less technical scale, and was able to determine that the machining oil did indeed react with the nitric acid resulting in the symptoms reported to me. While I was already vaguely aware of this fact and had included a note about it in our documentation it was my investigation into this event which allowed me to put two and two together and determine that this experiance we had was an actual example of the oil mixing with the nitric solution to create an organic acid and the resulting reaction.

It will now be included in our quality documentation so it can be avoided in the future.

Thank you for providing such a valuable forum for the industry.

Vernon Lilly
- Westbrook, CT, USA