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Passivation of 303SS with Nitric Acid Q&A, Problems & Solutions

Q. We've just ran into an issue with passivating a job that we have ran for years. Literally thousands of parts without any issue previously. The material is 303 stainless per ASTM 582, condition A. the passivation requirement is QQ-P-35, 24 hour test not required. The result on our last batch of approximately 170 pcs was a dark etch like finish all over. We sent a 2nd batch to another passivate house and received similar results. The plating houses said there was a "sulfer type smell" during the process. We then sent the material out to a testing lab and it came back acceptable to the specification. Is there something we could be doing during the machining process that is causing this? we haven't changed our process in any way over the years, yet these are the results. Any ideas or suggestions? we just ordered replacement material from a different supplier, but now I'm not sure that it will make any difference.

Ken Gerhart
- El Cajon
September 7, 2021

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⇩ Related postings, oldest first ⇩

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Q. We have a machining supplier that has not followed the procedure on our drawing calling for sodium dichromate to be added to the nitric acid bath for passivating our 303 St Stl part. If sodium dichromate is left out of the bath, will this effect the corrosion resistance of the part?

Becky Stufft
- Plymouth, Minnesota USA
2000

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Q. The passivation spec AMSQQP35 [canceled] gives an option for PH materials of using type II or type VIII passivation. The type II uses sodium dichromate and a weaker solution of nitric acid. Type VIII uses only a stronger nitric acid solution for a longer time.

Is one method better than the other?

Thank you for the help,

Jay Shapiro
- Valencia, California, USA
2001

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Multiple threads merged: please forgive chronology errors :-)

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Etching of 303 SST during Passivation

Q. I am a quality engineer assigned to the purchasing function of a OEM company. Recently we have been experiencing a, for lack of better terms, etching of parts during the passivation process. The material is 303 SST machined bar stock and it has happened on more than one occasion, with different parts, all the same supplier. The problem is not consistent and does not effect all the parts on a "load". Fallout has been anywhere from 20 to 50 percent of the lot. The appearance looks like the parts were actually sand blasted, extremely rough. Both inside and outside features have been affected.

The supplier has made the claim it is our (customer) issue. Needless to say we disagree.

Has anyone out there experienced the same issue? If so what was done to correct?

Thanks

Thomas J Luniewski
OEM - MPLS, Minnesota, USA
2005

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A. Do a search of the letters at this site. There are a couple with lots of answers that cover nearly all of the possibilities. Are you using the correct passivation for 303? I will bet that it is 303Se or 303S and not plain 303. It makes a huge difference. James Watts - Navarre, Florida 2005 A. Erratic results during passivation of t303 is always a problem, and it happens just as you describe - a few parts in one lot, perhaps more or less parts in the next. The parts which are attacked are usually damaged beyond use, while other parts in the same lot may be fine. This is caused by sulfur added to make the T303 easier to machine. If you change to T304, the problem will go away. The price you pay is somewhat more difficult machining. Jeffrey Holmes, CEF Spartanburg, South Carolina 2005 A. By which method were the parts passivated? Nitric acid has a tendency to react with the sulfides added into 303 to ease machining. Once a reaction occurs a granular attack like you describe takes place. There are methods and solutions employed to "minimize" this but potential for attack remains. If nitric acid per QQ-P-35C (canceled) [link is to free spec at Defense Logistics Agency, dla.mil] was used, was a type II performed as recommended? P.S. Look for the link to "frequently asked questions" and you will find more info! Bill Grayson - San Jose 2005

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Multiple threads merged: please forgive chronology errors :-)

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How do I remove sulfur smell after passivation of 303SS?

Is there a chemical or process for removing the sulfur smell after "gassing" has resulted from passivating (type 2) 303ss?

Tomas Mahar
- Bainbridge, New York
2005

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A. 303 stainless steel contains sulfur and therefore sulfides. When treated in acid, the sulfides are decomposed to form sulfurous gases. To avoid the smell, try dipping the workpiece in dichromate; this will oxidise the sulfurous material and reduce the dichromate to trivalent chromium. Trevor Crichton R&D practical scientist Chesham, Bucks, UK 2005 A. You need to soak the parts in alkaline solution, preferably pH above 12. This should take care of it. Lee Kremer Stellar Solutions, Inc. McHenry, Illinois 2005

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Multiple threads merged: please forgive chronology errors :-)

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Explanation for reaction of 303SS to nitric acid passivation?

Q. 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 sulfur 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 incident 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 involved? 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, Connecticut, USA
June 5, 2008

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Multiple threads merged: please forgive chronology errors :-)

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Type 2 or type 7 passivation for 303 stainless?

September 13, 2011

Q. I have a question in regards to Passivation. I am using 303 Stainless Steel for a small electrical connector housing that is machined. I wish to have the housing passivated per QQ-P-35C.
I am a bit confused whether use Type 2 or Type 7. I understand the difference is the Type 2 uses the sodium dichromate and Type 7 does not.
Can you offer up a suggestion which Type would be advantageous to 303 and why.

Barry Rupert
Designer - Illinois