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Passivating Stainless Steel with Citric Acid [Massachusetts] 

September 28, 2007

What are the risks of using Citric acid rather than Nitric acid on Stainless Steel manufactured products? Oxidation was mentioned? Also does Citric Acid remove less than Nitric Acid? Would a fall out occur because of this?

Ellyce Chrisbaie
QC Manager - Westwood, MA, USA

Personal opinion. Citric and nitric are a "wash" on 300 series SS. On 400 series, the citric will blow nitric out of the water. That is assuming that you are using a citric from a good company and not a home brew. The major advertiser of citric at this site has a product that I like, and I am not on their payroll.

James Watts
- FL

Passivation is the controlled "oxidation" of the nickel on the surface of stainless steel".

Citric Acid removes free iron, PERIOD.

Nitric Acid (and sometimes with dichromate depending on the alloy) also removes the free iron BUT then control oxidizes the surface to prefent further oxidation (unless exposed to a reducing liquid or chloride).

Robert H Probert Robert H Probert Technical Services Garner, NC, USA Editor's note:        Mr. Probert is the    author of Aluminum How-To

Quite the contrary, the risks are in using nitric. Safety risks, risks to surrounding equipment from fumes, risk of etching the surface of your parts.

Citric removes "less" than nitric in the sense that it removes the iron only and leaves behind the chromium and other metals. This is beneficial both from a waste disposal standpoint and from a corrosion resistance standpoint, as the citric leaves a deeper chromium-enriched layer than nitric.

And yes, the chrome oxide layer is formed not in the bath but in the air afterwards. This does not prevent citric from delivering equivalent or even superior corrosion resistance results.

Passivation of stainless steel as defined by ASTM A-967 [link is to spec at TechStreet] is the removal of the free iron from the surface, which enables the passive chrome oxide layer to form. Though yes, from a general chemist's perspective, the passivation is the formation of the unreactive oxide layer itself.

Ray Kremer Stellar Solutions, Inc. Algonquin, IL, USA

Regarding ASTM A 967. In between "free iron from the surface --- and which enables" ALSO appears what was left out, namely "with a mild oxidant". Citric Acid is not an oxidizing agent, nitric acid at the recommended concentrations for the various alloy IS an oxidizing agent. Let's quote the whole sentence.

Robert H Probert Robert H Probert Technical Services Garner, NC, USA

It's certainly not wrong to call attention to a misquote or selective quote, Robert, but after 12 years of countless postings on the subject here, I admit I too am weary of this "debate" between proponents and opponents of citric acid passivation.

A decade ago I asked for references to independent studies showing that citric acid "worked", and I immediately received them. I don't feel the shoe is on their foot anymore, but on the other foot; that those who remain opposed to citric acid passivation should quote some recent independent studies showing that it wasn't a satisfactory substitution.

For my part I've had too much experience with parts being destroyed in nitric acid, hazardous plant evacuations when steel parts were accidentally put into passivating tanks, and harassment from regulators with their nonsense that passivation shops are nitrate factories, to want to continue to use nitric acid for passivation unless there is actual evidence that it remains necessary.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

Not to belabor the point with petty bickering, but while the citrate ion is not an oxidizer, hydrogen ions, and by extension every type of acid, are. Iron atoms are oxidized in order to remove them from the metal surface as iron ions, by both citric and nitric acid. The oxidation of the chromium occurs in the air later on, which is also explicitly stated in A967.

Regardless of the exact mechanism, if it didn't work, people wouldn't be using it.

Ray Kremer Stellar Solutions, Inc. Algonquin, IL, USA

Citric acid is the reducer and promote de-passivation of the surface. Passive film formated after citric acid treatment is very thin. Further increase in thickness of such passive film take place during rinsing and drying. Such operations are hard to control. Therefore citric acid is very effective in removing free iron, but not sufficient enough in actual passivating. We recommend that after citric acid treatment stainless steel is treated in nitric acid solution or other more environmentally friendly solutions with high redox-potential in order to create thick passivating film. http://en.wikipedia.org/wiki/Reduction_potential Such film will increase corrosion resistance and reliability of corrosion protection.

We have developed such passivating solution for 300-steel for food and medical industry. We also have passivating solution for 400-steel which deploy both functions – removing of fee iron and passivating film formation.

Anna Berkovich Russamer Lab Pittsburgh, Pennsylvania

I have been following this thread with quite a bit of interest. As we have been having the perennial problems with 416 and 440C on a very intermittent basis we were advised that citric acid may well be the way to go as the potential for attack was greatly reduced (correct me if I'm wrong).

My problem is that when I read such polar views I am disinclined to investigate too much further.

So, in general, are we for or against passivation with citric acid? Please give me something to go on instead of the "it works/no it doesn't" arguements.

Looking forward to seeing some reasoned arguements, preferably backed up with some more literature sources.

Brian Terry
Aerospace - Yeovil, Somerset, UK

From my experience, Anna has presented the most accurate information. Let me just share some personal experience from a non-biased perspective. Citric acid indeed behaves differently than nitric acid in the way that it helps achieve passivation. The comparison of the resulting passivation layer thicknesses is key here. The thickness of the layer helps determine corrosion resistance. The one thing that many are overlooking is the application you are using the SS for. For many applications Citric is sufficient and the difference is negligable. However, I have first-hand experience using both citric and nitric for metal injection molded (MIM) parts, as well as some other medical device applications and nitric sometimes out-performs citric. This is especially important if cosmetic finish is a factor. It also depends on how smooth your surface is pre-passivation.

Dwayne Bell
- San Jose, CA, USA

Anna,
The current understanding of passivation here at Stellar Solutions is that iron removal takes place in the bath and oxidation of the non-ferrous metals takes place in the air, and that this is what happens in both the citric and nitric processes. I realize this view is not shared by many old-schoolers.

Stainless steel is etched by nitric acid given enough time, as I'm sure you are all aware. I.e. All the metals present in the alloy including chromium are potentially removed. This is mutually exclusive with the idea that a passive chrome oxide layer is formed in the nitric acid bath.

Testing data we have indicates a BETTER chrome oxide layer with citric passivation, not a thinner one. Citric does a better job of forming a chromium enriched surface because unlike nitric it only removes the iron. For this reason, the benefits of a citric passivation would be negated by following it with a nitric bath as you suggest. However, it is true that a post-passivation bath of oxidizer (peroxide, etc.) is likely beneficial, or at least faster than mere air exposure.

Brian:
416 and 440C are difficult to passivation, even with citric, but we have seen many successes in dealing with them. Let us know if we can help you.

Citric Passivation is accepted by both the ASTM and ASM passivation standards.

Ray Kremer Stellar Solutions, Inc. Algonquin, IL, USA

We understand why you are not satisfied with present state of passivation theory. Numerous opinions are caused by differences in theoretical explanations, and mostly by different real conditions of applications. For example:

1. Stainless steel parts have relatively clean surface with no free iron, no areas depleted by chrome, no signs of rust;
2. SS parts have iron on it surface left from previous mechanical treatment, there are visible signs of rust, areas depleted by chrome.

In the first case treatment in citric acid will not enhance passivation quality, moreover – it will significantly decrease corrosion resistance, decrease potential of dissolving in corrosion environment ( the only output of such treatment will be decreasing of the thickness of the passivating film and decreasing of Chrome and Fe (III) in such passivating layer).

In the second case it is opposite -citric acid will remove iron from the surface, remove areas depleted in chrome, remove rust. Thin uniform passivating film will be formatted on cleaned surface. Corrosion potential will be similar as in case 1, but significantly higher than if no citric treatment is conducted (since without treatment metal part will behave as steel, not as stainless steel).

That is why there are various opinions on citric acid passivation. Russamer Lab Group opinion: in each particular case decision on which passivation method to use should be based on corrosion tests before and after passivation (for example test in Copper Sulfate, or test on potential reduction).

Anna Berkovich Russamer Lab Pittsburgh, Pennsylvania

All of us are tired of the back and forth opinions of citric vs nitric passivation.
The simple fact is that dozens of independent laboratories, universities and Fortune 500 companies have run exhaustive studies on this and, in all cases that we know of, these have proven that citric acid when formulated properly will out-perform nitric acid in almost every situation.
We would be happy to post in a public forum those studies which we are permitted to devulge. Anyone desiring to contact the universities, testing agencies or companies can do so.
We can also supply public data directly to interested parties.
Regardless of which opinion one is of the metallurgical mechanism (lots of opinions worldwide) the fact is that citric formulations DO yield not only a higher chrome oxide to iron oxide ratio, but also a THICKER oxide layer when the proper process is used.
All of this is verified by the thousands of companies around the world who have tested and are using these formulations. We can all debate the theory-- that is good and educational for all of us. But the test data showing improvements in corrosion resistance are in the end the only thing that really matters, because that is WHY we passivate.
None of us would ever say that nitric acid does not work, because it has been the workhorse for many years. But there is even independent university data indicating that nitric acid is harmful to the surface. So you can find data to prove probably whatever you want to prove.

Lee Kremer Stellar Solutions, Inc. Algonquin, IL, USA

Ted,
sorry to come in late in this interesting debate though I am a regular in our "electroplating and forming" section.
I am of opinion that Ray and Lee Kremer's are talking about the latest in citric passivations and Anna is referring about her initial studies in the lab. Debatably I would go with the Kremers as it seems they are ahead in their studies.
As we all know, Stainless steel is comprised of a mixture of materials, primarily iron, chromium, nickel (in some cases molybdenum and manganese) along with other materials in small ratios. Nitric or citric formulated passivation solns cause the formation of an effective outer protective layer consists of only oxides of chromium, iron or nickel, in different ratios which are highly resistant to corrosion. Though both Nitric and citric removes free iron from the surface it is very important to to apply good surface cleaning technique to remove the entire free iron from the surface to allow formation of good passive layer.
Nitric method tends to "re-deposit" contaminant iron back on the surface and some times it leaves behind a reddish-yellow rusty look. it is considered rust. If the dip time is extended for any reason,there is a chance of acid attack on the surface used in surgical/electrical usage.
One thing I want to make clear is that I am not in the pay roll of the Kremers but I am confident that they have mastered the citric technology, may be by formulating an additive that immobilizes removed metal ions which will not redeposit back on the surface as in the case with nitric soln.
Nitric passivation was developed at a time when the chemistry of passivation was little understood.With current technologies available for the study of the surface chemistry of stainless steel, I think we should be ready to listen and try out Kremer's finding of "higher chrome oxide to iron oxide ratio,and a THICKER oxide layer" when using citric passivation solns.
May be that is the reason that the beer now- a days tastes better as I presume these industries have already stated citric solns(which restricts re-deposit) to passivate beer containers in place of nitric passivation solns!!
Regards,

T.K. Mohan     plating process supplier  Mumbai, India

You know, I'm liking TK's comment about using beer tasting as a means of testing passivation effectiveness. There's got to be some way we can work that into ASTM A-967 [link is to spec at TechStreet] or AMS 2700 instead of those boring humidity or copper sulfate tests...

Lee Gearhart
- East Aurora, NY

Beer taste is one of the original passivation tests! It was the work by Coors in Germany decades ago that originally identified citric acid as an alternative passivation chemical.

Ray Kremer Stellar Solutions, Inc. Algonquin, IL, USA

I was interested to read T.K Mohan's following comment "Nitric method tends to "re-deposit" contaminant iron back on the surface and some times it leaves behind a reddish-yellow rusty look". We have sometimes observed it. Is there a solution to it?

Jari Hyvarinen
- Sydney, Australia