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Citric acid passivation of stainless steel

 

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I am Senior Project Engineer for a generic pharmaceutical company on Long Island, NY. We currently do most of our passivation in house with a Nitric Acid bath. This is a safety concern as well as creating difficulties in neutralizing the strong acid after use.

I have noticed that the industry seems to be going more and more towards passivation via citric acid solutions. My management is concerned over this change. I am researching toxicity vs. nitric acid as well as functionality (does the citric acid get the job done). Any input you could provide, or if you could point me in a direction of where I could find the technical back up data to support such a change would be extremely appreciated.

Thanking you in advance.

Paul Diolosa

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I sent an inquiry to the Q&A page in one of the trade publications a year or two ago about this same issue (use of alternate passivation techniques) but never got a response. I would happily go over to a non-chromated passivation solution if I thought the people who require QQ-P-35 [link is to free spec at Defense Logistics Agency, dla.mil] Type II would let me. Anyone else have input? Our application is 303SS.

Bill Vins
microwave & cable assemblies
Mesa (what a place-a), Arizona


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It has been stated repeatedly in the literature, so it's probably true, that electropolishing is a superior passivation technique to passivation itself. It's more expensive, and may not be practical for all ware, but it would eliminate the hex chrome issue.

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


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Ted,

Since the company I work for does bulk passivating of small 303 stainless connector subcomponents (as well as other finishing processes like gold plating) I think the tradeoff to losing the hex chrome would not be worth it (huge rectifiers, very concentrated rinses, etc.). It is probably OK on bigger parts that people do on racks, but basketed stuff like ours would be a nightmare.

Bill Vins
microwave & cable assemblies
Mesa (what a place-a), Arizona 

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The QQ-P-35C is in the process of being replaced, and the replacement will allow Citric Acid Passivation. Before you jump into this process consider the effect of the Citric on your waste treatment facility. Citric Acid is a chelator in the purist sense and should not be added to a waste stream containing metals you wish to remove. There are distributors for a Citric Acid Passivation process popping up all over. Good luck, mike.

Mike McDonald
- Jefferson, Wisconsin


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I am just going to have to visit http://www.finishing.com/letters/ on a regular frequency. I keep seeing these interesting discussion in MF journal and am probably too late to contribute on a meaningful basis.

In years past, citric acid, among other complexants, has been used in the nuclear industry to decontaminate (chemically clean) piping, vessels, etc. The procedure for stainless is to go in with nitric acid to oxidize the chromium and then strip out the Fe/Ni with the citric acid. Normally it was recognized that after the procedure, the surface would be very reactive and susceptible to recontamination if not allowed to "heal." It was not used on carbon steel just because too much would be needed.

Thus the use of citric acid to "passivate" is interesting. Is something else added, Hydrogen Peroxideamazoninfo or some such? To passivate a surface, you need to raise the electrochemical potential up to the passive region. I find it difficult to believe that citric alone will do it.

As Ted pointed out, electropolishing gives you the best surface. But if you use phosphoric acid, I suspect there is residual trace of phosphate. As a point of interest, if you ever get a chance, look at new stainless and electropolished stainless under a scanning electron microscope. A world of difference.

James R Divine, PhD, PE
West Richland, Washington


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I have to concur with Dr Divine. Citric acid on its own is not an oxidising agent. Passivation tests after citric acid "passivation" will prove positive but I would prefer to believe that the this is because this acid has played its role as a chelating solution and thus the stainless steel has effectively auto passivated.


Joel Levinsohn
- Sydney, Australia


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I have been searching for information which describes what sodium dichromate specifically does in the QQ-P-35C type II solution. I have been told that it "retards" the etching process, however, I have not been able to confirm this. If you know or can help me find this information I would very much appreciate your help.

Larry Mager
- Origin Medsystems


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I believe that the function of dichromate in the passivation solutions is as an oxidizer. The oxidation acceleration due to the dichromate produces a more stable oxide passive layer more quickly and with less metal dissolution than a plain nitric acid solution.

hanke Larry Hanke
  materials testing laboratory
Minneapolis, Minnesota



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look I need a general information about passivation
then I want the magazine my address is the next

country= Mexico
state= coahuila
city= monclova
colonia= estancias de santa ana; cp= 25830

I'm a student of the ultimate degree in chemical eng.
thanks for this virtual space

Israel barron Garcia
- facultad de cienias quimicas


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I also agree with Dr Devine and Joel Levinsohn.

Whilst Citric Acid acts as a chelating agent to bind free iron contaminants, the subsequent (auto)passivation process with oxygen is the oxidising reaction which generates a passive film.

Use of nitric acid provides meaningful passivation, especially in high corrosion environments where auto passivation is interfered with by contaminants such as iron oxides, iron chlorides, chlorides, bromides, etc.

Metalurgical tests performed for us on stainless steel (304 and 316 grades) also indicate that auto-passivation in a clean room laboratory environment can take 1 to 2 days after grinding, and 6 to 24 hours following pickling (nitric acid/hydrofluoric acid).

Whilst the auto-passivation process commences immediately following chromium depletion, it does take time to regenerate a meaningful corrosion resistant passive film in fabrication environments and in installations subject to corrosion.

Whilst I agree Citric Acid cleaning can be useful following fabrication due to its ability to bind (chelate) free iron contamination, I still lean towards use of a nitric acid solution or nitric acid gel to chemically oxidize a chrome oxide passive film.

I hope I don't upset the Citric Acid devotees, however, it is only one of a number of weapons in the armoury and should not be seen as a means to an end. Yours in passivation, John Hill

John Hill
- NSW, AUSTRALIA


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I am some what new to the realm of passivation and still learning. One of our companies difficult spots to get passivated is melt thru on the back side of 304, while tig welding, where the gas shield is not present. Does any one have any advice? We also are just looking into Citric Acid and would this make the problem worse?

Mark Bandixen
- Mt. Carroll, Illinois, USA


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While placing a 416 Stainless Steel part in a nitric passivation bath an extraordinarily active reaction occurred which ate approximately .001" off a diameter of a Swiss screw fabricated part. We fabricate an overwhelming majority of brass parts at our facility, and I was curious if brass contamination in the bath from the parts would cause a reaction like this. Thanks in advance for any help in troubleshooting this problem.

Mark Imbimbo
- Boonton


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I would doubt it, Mark. See our FAQs on SST Passivation

pic of Ted Mooney Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey

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