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Selective / Touch-up Passivation of Stainless Steel
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Quickstart:
When stainless steel is fabricated it often gets exposed to tooling of one sort or another, and tiny fragments of plain carbon steel become embedded in it. Those fragments will rust and can act as initiation points for more serious corrosion. It is important to remove that source of corrosion via a process called 'passivation of stainless steel'.
Traditionally, passivation has been done by immersion in strong nitric acid, which simultaneously dissolves the plain steel and oxidizes the chrome on the surface of the stainless steel.
A safer & more environmentally-friendly alternate process is immersion in citric acid, which dissolves the plain steel particles, with subsequent exposure to the atmosphere producing the needed oxidizing conditions.
If, after passivation of the component has been done, an area of it is once again exposed to plain carbon steel, there may be a need to re-passivate just that selective area.
Touchup of Nitric Acid Passivation
Q. This may be a dumb question but is there any way to locally touch up passivation on a 17-4PH machined part without re-passivation of the entire part.
Rosti Sorochynskyjactuation systems - Pine Brook, New Jersey
2003
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A. Get in touch with "brush plating" ⇦ huh? companies. They might help. There are several in the US.
Guillermo MarrufoMonterrey, NL, Mexico
A. I don't believe that we are the only company in the aerospace industry to swab passivate. For 17-4, we use passivation solution directly from the Type VIII bath [a composition spec'd in SAE AMS 2700]. Yes, we have concerns that the temperature is at ambient rather than 130 °F, and that the time is usually less than the 30 minute minimum of immersion. Yet this is done as a rework operation, and we take steps to mitigate the problem, like using carbide tools or freshly dressed grinding wheels (which keep stray iron particles off the surface).
Lee Gearhart
metallurgist - E. Aurora, New York
Multiple threads merged: please forgive chronology errors and repetition 🙂
Q. Looking for help with a selective passivation issue. The parts are made of 15-5 PH [precipitation hardened] steel and get machined to approximately 1" square in size. They then get sent out for passivation. After machining and passivating, the parts get stocked and then assembled with various components as needed. After assembly, the (15-5) parts get laser marked with serial numbers. We require that the laser marking be passivated with a touch up kit. I've been experimenting with the Citric Gel [jellied citric acid] with inconsistent results.
The current process is as follows:
Can this process be adjusted to work every time without the use of harsh chemicals? Dan Mitchel
Engineering - New Haven, Connecticut
2006
A. Dan:
The use of citric acid gel or any passivation gel requires maintaining the gel fluid on the surface for the total treatment time. It should be agitated at least periodically and refreshed to insure it does not dry. When the gel dries on the surface, the iron that it has removed is precipitated back on the surface and will likely result in a failed salt spray test.
I would recommend using a commercial citric acid gel blend and maintaining it fluid on the surface for up to 2 hours. This alloy is not the easiest to attain a passive film.
Thanks,
Costa Mesa, California, USA
Daryl, thank you for that information, I will try that next.
Does anyone know of any other selective passivation kits that are available? Currently I have the Citric Gel. ⇦ Answer?
Thanks,
Engineering - New Haven
Multiple threads merged: please forgive chronology errors and repetition 🙂
Q. Is there a process for touch-up passivation for stainless steel?
John KENNELLY- La Crescenta, California, USA
May 22, 2013
A. Hi John. We appended your inquiry to a thread which indicates that touchup of both citric acid and nitric acid passivation processes is possible, and which offers some hints. Please feel free to flesh out your question to get personalized help with your situation. Good luck.
Regards,
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
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Q. I work at a small tubing manufacturing company. After machining, we send stainless parts to be passivated. it was discovered that when the parts were returned after passivation, one part had a very small dark pit that has revealed itself after nitric passivation. I do not know why it is black or if it some kind of contamination.
I would like to polish it out using a non ferrous media and Then do a local re-passivation.
Is there some kind of solution that we can use?
- Walnut, California, USA
April 23, 2019
A. James,
It might have been an inclusion in the original material that got selectively eaten away by the acid.
You can passivate locally with a paste product of either citric or nitric acid.
Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois
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