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Passivation with citric acid +Sodium Dichromate
Q. I manufacture 316L SS components and am curious about passivation. I have heard of many formulations that are supposed to work. What I am using is 30% citric acid, 70% water. My temp. is 140 °F, and I run about 30 mins. I've heard that adding sodium dichromate (2% content) helps encourage the formation of thicker, longer lasting chromium oxide layers. Is this fact, or fiction? If so, does the sodium dichromate greatly affect the pH level of the bath? What pH should the bath be to start with, if I wish to add the sodium dichromate? Any help would be greatly appreciated.
Thanks in advance,
Jeff Swayzecustom body jewellery - Kelowna, B.C., Canada
2002
A. I don't know if mixing dichromate with your citric acid would improve it, but why would you want to do that? The citric acid passivation treatment is designed to eliminate chromic based passivation. Look at ASTM A967. I think citric works at least as well as chromic.
Peter Faxon- Oxnard, California
2002
A. Jeff:
We strongly advise against using dichromates in any formulation. The hexavalent chromium is a virulent carcinogen according to tests.
There are combinations of formulations that will give you much enriched chromium oxide layer that you seek without going to the old dichromate addition that is used in some nitric acid formulations. This is not typically used on 316 SS anyway. We have obtained up to 13:1 chrome oxide:iron oxide ratios with the correct conditions and proprietary formulations. TYPICAL chrome oxide ratios of 5 to 7 are easy to obtain, which far exceeds the SEMI standards on 316L.
Lee Kremer
Stellar Solutions, Inc.
McHenry, Illinois
2002
Thanks for the reply, Peter! I am just exploring to find a passivation solution that will encourage a very stable, abrasion and chemical resistant chromium oxide layer. citric acid works very well, but in an effort to provide my customers with the best possible results, I am always looking for an improved method.
Thanks again!
Jeff Swayze [returning]- Kelowna, B.C., Canada
2002
A. I think this idea of adding 2 percent dichromate idea came from an optional nitric acid passivation formula rather than from a citric acid formulation; it apparently has no role in citric acid passivation.
But I think that calling it a 'virulent carcinogen' is a bit overstated, Lee. The chromates do not become a part of surface as they do in a chromated aluminum or in a chromate passivated zinc plating, so there is no risk to the end user; and chromic acid concentrations 10 and 20 times as high are routinely and safely used in chrome plating and etching.
Avoid hexavalent chrome when it is unnecessary in the process; use it very judiciously when it will remain on the end product; but don't become paranoid about using it in the workplace.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2002
! Re: several comments above. Dichromate is probably carcinogenic by inhalation, but not by ingestion at reasonable levels. It did produce intestinal tumors in mice at levels well above the animals' natural reduction capacity from saliva and stomach. Mice are not an appropriate dichromate ingestion risk model for rats, let alone humans, because the retention time in their stomach is very short. E.g. rats did not get intestinal tumors in the same studies. Humans have greater reduction capacity and longer stomach retention time than rats.
Joseph CotruvoJoseph Cotruvo & Associates LLC - Washington, DC USA
October 24, 2016
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