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Long, small dia., passivated 316L SS tubing is corroding

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Q. I'm working with quite a mysterious corrosion issue on one of our components. Our tubing is made of citric acid passivated austenitic 316L SS tube, small diameter (< 0.25") and thin wall. Even though the working temperature is ~100, our system definitely does not have enough halides concentration or other corrosive agents to cause the issue.

So I suspect our main problem is either the surface finishing process or the material itself.
1. Have there been cases when the passivation layer does not form properly due to dimensional challenges? Our tube is very long and thin, approx. 50 ft.
2. Is it possible for etching to happen in this case - the inside surface of the tube looks like it has been etched? Is there a viable solution to stop this issue?
3. Regarding material, would it be even possible for the tube to be more ferrite inside than outside (that is what it seems like under our SEM scans). I assume this can happen during the annealing and cooling steps?

Q Huynh
- Manchester, New Hampshire
November 15, 2022

A. citric acid is a reducer, not an oxidizer, it merely "activates" the surface and depends on air for "passivation" (oxidation).
Nitric Acid is an oxidizer, that is the kind of "passivation" you need.
robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina

thumbs up sign I hate to see this citric acid passivation debate arise here again for the umpteenth time, but when the subject of discussion is the I.D. of a 50 foot long 1/4" dia. tube, with probably very limited exposure to air, the point admittedly sounds worth raising :-)
Luck & Regards,
pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey

Q. Robert: I see. We are looking to switch vendor that will be doing nitric acid passivation.

Ted: Some of the vendors we spoke with admitted that passivating a tube with such dimensions is a challenge, with setting up bath tanks and so on. But on the technical standpoint, does passivating such long, thin tube pose any issues with forming a proper oxide layer? I would assume that the dimension has a direct correlation with the time needed to submerge in Nitric acid bath.

Q Huynh [returning]
- Manchester, New Hampshire
November 23, 2022

A. Hi again. I don't think there is any technical issue related to the length of an object in general, the issue is that the I.D. of the tubing must of course be exposed to the passivating agent, not just sit, full of air, in the passivating tank.
Although I suppose it might be possible to hang the coil like a vertical coil spring and immerse it, better would probably be an arrangement to pump the passivating agent through the tubing, displacing the air.
Luck & Regards,
pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey

thumbs up sign Hi Ted,
Thank you very much for the advice. I have been trying to set up something like this and having a second sanity check from an experienced PE like you is always helpful.

Q Huynh [returning]
- Manchester, New Hampshire

A. Yes, obviously we must first recognize that small diameter tubing is difficult to work with in any event.

I might observe that 316 has very good corrosion resistance generally and ask exactly what your tubing is being exposed to. I would usually only expect corrosion to occur on 316 with sustained chloride exposure, yet you say your system does not have halides. In any event, anything that can cause corrosion on 316 is likely to do so regardless of whether or not a passivation treatment has been performed, rather the difference will be in whether the corrosion occurs sooner rather than later.

The theory of the tube interior not having the proper composition/crystalline structure is worth investigating. Certainly austenitic stainless is known to need annealing after cold work to restore a fully austenitic structure.

As far as oxidation, let us be clear on some things. All acids are oxidizers, as the H+ ion is an electron acceptor. The nitrate ion is also an oxidizer, via reduction of the nitrogen atom, but data suggests that the nitrate does not serve as an oxygen donor, as citric acid passivation creates better chromium oxide layer numbers than nitric acid. There's a terminology issue here, since oxidation means both the formation of an oxygen compound through loss of electrons to zero valance oxygen atoms, and loss of electrons in general. I.e., I expect a nitrate ion acting to oxidize a chromium atom is more likely to create a chromium ion and a nitrite ion, than it is to create chromium oxide.

Personally if I were concerned about metal oxide formation in a small stainless tube, I would flush it through with a peroxide solution after rinsing the acid out, rather than rely on air or nitric acid.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
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