mobile?Metal finishing Q&As since 1989
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Chemical passivation methods for Inconel alloy
Q. I am a tube manufacturer and we are processing inconel 625 tubes. After the final stage of sizing, the tubes when done Positive Material Inspection, show low chromium content around 15-16%, but when the tube is ground on the surface the chromium level shows above 20%.
Now the tubes are to be inspected with a TPI and these tubes will be rejected for low chromium level. Could someone help me understand the problem and a solution on how to get higher levels of chromium shown on the tube surface during PMI.
- Mumbai, Maharashtra
May 13, 2024
A. Hi Hemendra,
Is it possible that passivation could remove enough iron to bring the chromium content back to its normal level?
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Q. Hi,
Thank you for your quick reply. Can you suggest a passivation bath formula, so i can take a trail and let you know if it really worked?
Thank you
- Mumbai, Maharashtra
May 21, 2024
A. Hi Hemendra,
Your inquiry is on a page where a number of people have said there is no formula specific to Inconel, and I assume they're right. Citric acid, preferably a proprietary from Stellar Solutions, or a QQ-P-35C (canceled) [link is to free spec at Defense Logistics Agency, dla.mil] type 6 as suggested by Lee Gearhart are the only ideas I can echo.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
⇩ Related postings, oldest first ⇩
Q. Passivating type 625 Inconel. I have a print callout to passivate 625 Inconel. AMSQQP35 [canceled] does not address Inconel materials. It's purpose statement and tables recommending solution type only calls out stainless steels. Are there any detrimental effects that can result from passivating these parts? Kevin Evers
- New Port Richey, Florida
2002
A. Kevin:
I do not have any personal experience with using nitric acid with Inconel. We cannot verify that with data on nitric, but we have had success with proprietary formulations of citric acid.
adv.
Contact us if we can help.
Lee Kremer
Stellar Solutions, Inc.
McHenry, Illinois
2002
A. Kevin, when we have a make-to-print job telling us to passivate any of the Inconel alloys, and it's too much trouble to tell the customer it's a worthless operation, we just do it in Type 6 solution. Doesn't hurt, doesn't help, satisfies the print. Good luck!
Lee Gearhart
metallurgist - E. Aurora, New York
2002
Q. There are many methods for the chemical passivation of stainless steel. But, I'm finding difficulty in finding chemical passivation method for Inconel 690. Can anybody help me regarding this?
Pradeep Samantarayresearcher - Chennai, INDIA
March 22, 2010
A. You can treat it the same way that you would stainless. If you have a slight amount of scale, then you should pickle it instead (which will also passivate it) in a solution containing nitric acid at around 40% by volume and hydrofluoric acid at around 3% by volume. The temperature could be ambient. The immersion time should be kept to a minimum which will depend on what you are trying to do. If it is clean non-scaled Inconel, 10 minutes is just fine.
Jon Barrows, MSF, EHSSC
GOAD Company
Independence, Missouri
A. Of course Inconel by itself doesn't need a treatment to be passive, as is generally true of non-ferrous alloys. However the parts may be susceptible to surface iron contamination, which can lead to surface rust. Concern over actual or potential iron contamination is what leads you to need a passivation treatment.
Since stainless steel passivation (nitric or citric acid based) is all about removing iron, the same procedures apply. Let us know if we can help.
Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois
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