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Passivating Stainless Steel with Citric Acid

adv.    koslow passivation test kit

(2007)

Q. 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, Massachusetts


(2007)

A. 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
- Navarre, Florida


(2007)

A. 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 prevent further oxidation (unless exposed to a reducing liquid or chloride).

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como



(2007)

A. 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 A967 [link by ed. 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.

McHenry, Illinois



(2007)

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, North Carolina


(2007)

Thanks Robert! You're clearly right to call attention to selective quotation, but after 12 years of countless postings on the subject here, I admit that I too am weary of this "debate" between proponents and opponents of citric acid passivation.

A decade ago we asked for references to independent studies showing that citric acid "worked", and we immediately received them. For me personally, I don't feel the shoe is on their foot anymore, but on the other foot; i.e., that those who remain opposed to citric acid passivation should quote some recent independent studies explaining in what cases or under what circumstances it isn't a satisfactory substitution.

For my own part I've had enough experience with presumably good parts being inexplicably destroyed by nitric acid, plant evacuations caused by steel parts accidentally put into nitric acid passivating tanks and releasing clouds of toxic poisonous NOx, harassment from regulators with their nonsense that a nitric acid passivation tank is a "nitrate factory" which must maintain "nitrate production stats", and pressure from the government against using nitric acid because of its ties to explosives to want to continue to use nitric acid for passivation unless there is clear and present evidence that in a specific case it remains absolutely necessary.

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


(2007)

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 ASTM A967.

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

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois


(2007)

A. Citric acid is the reducer and promote de-passivation of the surface. Passive film formed 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.

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

anna_berkovich
Anna Berkovich
Russamer Labs
 
Pittsburgh, Pennsylvania




(2007)

Q. 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" arguments.

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

Brian Terry
Aerospace - Yeovil, Somerset, UK


(2007)

A. 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 negligible. 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, California, USA


(2007)

A. 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 passivate, even with citric, but we have seen many successes in dealing with them. Citric Passivation is accepted by both the ASTM and ASM passivation standards.

adv.
Let us know if we can help you.

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois


(2007)

A. We understand why you are not satisfied with the 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
Anna Berkovich
Russamer Labs
Pittsburgh, Pennsylvania



"It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong."
- Richard Feynman

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


(2007)

A. 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 divulge. 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

Lee Kremer
Stellar Solutions, Inc.

McHenry, Illinois



(2007)

A. 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 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 solutions 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 Kremer's 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 solution.
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 solutions.
May be that is the reason that the beer now- a days tastes better as I presume these industries have already stated citric solutions (which restricts re-deposit) to passivate beer containers in place of nitric passivation solutions!
Regards,

t k mohan
T.K. Mohan
    plating process supplier 
Mumbai, India



(2007)

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 A967 [link by ed. to spec at TechStreet] or AMS2700 [link by ed. to spec at TechStreet] instead of those boring humidity or copper sulfate tests...

lee gearhart
Lee Gearhart
metallurgist



(2007)

A. 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.
McHenry, Illinois


April 9, 2008

Q. 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


June 14, 2009

A. Hi,
I'm just a newbie here.
Something I learned a long time ago: There is always a better way. Don't close your eyes on the future. Citrisurf makes perfect sense. Someone approved it so it must be an effective replacement. I bought some to use on prints calling out the ASTM standard.
However, when my major customer found out I was going to use orange juice on their parts all of a sudden all the ASTM points were changed to AMS-QQ-P-35 [link by ed. to spec at TechStreet] spec. This company obviously don't believe in the times are a changing.
tyvm
Hope I have a chance to put my 2 cents in again.
Bob

Bob Winslow
dry lube and passivate shop - Nevada, USA



December 28, 2010

Q. How important is it to prevent evaporation to dryness of the citric acid solution on 304SS, prior to washing it off, and subsequent peroxide passivation? I have sometimes observed a "sticky" residue if the citric solution dries on the SS surface prematurely. The deposit appears to be removed with warm water washing.
I guess my question is, "What is downside of intermittent evaporation of the citric acid on the 304 SS surface if subsequent water wash appears to remove the "sticky film" prior to peroxide passivation?
Jack

Jack Bland
- Richmond, Virginia, USA


February 14, 2011

A. Jack,
Yes, is it important with passivation to have a complete rinsing of the surface. This ensures full access of the surface to oxygen in the air which creates the chrome oxide layer.

Does it hurt anything if you let some citric solution dry on there as long as you rinse it off later on? Probably not. You're certainly a lot safer with residual citric than you are with residual nitric acid, which can cause bad localized etching and pitting.

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois



sidebar April 30, 2011

This is a highly technical discussion. It seems that for the purpose that I intended to use a tested-n-tried solution that remove rust from stainless steel safely, I will stick with what I have been using safely and successfully, Bar Keepers Friend in powder.

Monty Betancour
- Coconut Creek, Florida


May 2, 2011

Thanks, Monty. You are right that this discussion has been about how manufacturers should process their materials to reduce or eliminate the likelihood of their stainless steel rusting; it is not about what a consumer should do when they encounter a rusty stainless steel surface. Readers in a similar suggestion will probably appreciate your advice!

Regards,

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



September 19, 2012

Q. We operate a set of chemical tankers and have recently carried out passivation onboard one of our ships using Citric Acid Solution.

Can this be disposed out at sea after suitably diluting it to around ph 6.5.
Is there any regulation prohibiting discharge in open seas?
Is the product harmful to marine life in any way?

thanks for your quick response.

gurcharan_sachdeva
Gurcharan Sachdeva
shipping - Singapore


October 24, 2012

A. Gurcharan,
Citric acid is most certainly safe for marine disposal and not harmful to wildlife. As for the regulations, we've been trying to figure that out lately. There seem to be a lot of regulations from several different organizations and it's difficult to figure out which takes precedence.

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois


March 12, 2013

RFQ: G'day
I would like to give some Stellar Solutions or equivalent a test. I make products out of 304 and 316 stainless. It sounds like I will still have to use another method to remove weld scale but I want to be able to safely and effectively submerse my parts to remove possible steel contamination. I currently use hydrofluoric paste on weld scale but I would need far too much to submerse whole parts. Can you get citric acid for this purpose in Australia?

Any Advice would be great

Regards

Timothy Stewart
owner operator - Charters Towers, QLD, Australia
^- Sorry, this RFQ is outdated
     View Current RFQs



March 13, 2013

A. adv.
Timothy, please contact us directly regarding distributors of our products in your area of the world. Our contact information is available in the supplier directory here on finishing.com, or we are easy to find with your favorite internet search engine.

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois



May 20, 2013

Q. We recently switched our stainless steel passivate solution from nitric to citric. The citric solution grows an algae or bacteria, which we have to keep filtering out. Is there any way of preventing the growth, that doesn't affect the solution?

Dave Foot
- Weymouth, Dorset, UK


May 30, 2013

A. Dave,

A small amount of hydrogen peroxide should be ok. I know some folks passivate with citric and kill bacteria with hydrogen peroxide. I would think 1/2% of a 35% solution would be more than enough. This however will have to be trial and error like all metal finishing applications. You may want to do a beaker test to verify and you could get some at the drugstore, but note that it's already diluted for the most part. Also note that hydrogen peroxide doesn't stay around. If it works, you might only need to do a visual to see the need for more H2O2.

bob adams
Bob Adams
Tijuana, Mexico


June 5, 2013

A. Hello Bob
Exactly my thoughts, Right now I investigate this substitution, and the only reason I can find not to do this substitution, is that the NO released from the nitric acid is known to kill bacteria.
This effect should be possible to obtain by say 1% hydrogen peroxide in mild sulfuric acid, as a byproduct you ensure that nickel/chrome is oxidized.
Any reason this might not work?

Bo Koenig
- Aalborg, Denmark


June 10, 2013

A. That's not a bad idea Bob, and peroxide can be helpful for passivation as well.

A commercially available biocide should also be fine to prevent growth in your bath. Citric is an organic molecule, so this can happen. However I've also noticed that it seems to happen mostly when the bath sits stagnant. If the bath is in daily use or agitated daily, it may also help prevent growth.

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois


June 28, 2013

thumbs up signSorry for the slow reply, but I've been away. Thanks to all for your input, very helpful. I've been asked to enquire about the best method to check the strength of the citric acid in the bath.
Thanks

Dave Foot
- Weymouth, Dorset, UK


July 30, 2013

A. Dave,
The traditional methods are usually best. For a straight citric acid bath you can track the pH. If the citric is pH buffered at all, monitoring that way won't work, so instead something like specific gravity, conductivity, or refractive index. Most of these presume you filter the bath to keep it clean of other materials.

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois



Citric acid passivation of an assembly of two different types of stainless steel

August 13, 2014

Q. Can you citric passivate an assembly that consists of 17-4SS and 304SS parts? I thought that different materials would react off of each other -- or is that with the Nitric only?

dawn hoke
- dayton, ohio


August 25, 2014

Dawn,
In theory, any pair of dissimilar metals or alloys that are touching each other while immersed in water or acid can undergo a galvanic reaction. However, I've never heard of this actually being a problem for multiple grades of stainless during passivation. I suspect that different grades of stainless are similar enough to each other that the effect is negligible. Always run some trials before going into production, but if things look okay you should be fine.

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois


August 13, 2014

Q. When using a hand applied passivation of citric acid, I have not seen any mention of rinsing, thus risking mineral deposits from the water.
Please advise.

Karen Schultz
Tool & Engineering - Romeoville, Illinois USA


August 2014

A. Hi Karen. Demineralized water can be purchased if only a small quantity is needed, or easily and cheaply made on site ("Call your Culligan man") ... perhaps I don't quite understand the question?

Regards,

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


August 13, 2014

Q. So after the hand application of citric acid is applied, I am guessing it needs to be rinsed? My engineer is stating the product he is using in a hand application for passivation is some type of paste consistency. He states if he rinses it off, iron may be left on the surface since our water is hard and has a lot of iron in it.

I want to know if there is a product (citric acid is nothing more than fruit juice) that does not have to be rinsed and the part still be passivated?
Karen

Karen Schultz [returning]
- Wesley Chapel, Illinois USA


August 25, 2014

A. Karen,
All passivation, be it parts dipped in a bath, a spray-on product, or a gel, must be rinsed clean with water afterwards. All that the acid (nitric or citric) does is remove iron from the surface. The passivation process is not complete until the chromium in that iron-depleted surface is exposed to air to enable chromium oxide to form. I am sure that the manufacturer's directions for the passivation product you are using must state that rinsing is required.

You are correct in saying that water with unusually high levels of iron can leave some of that iron on the surface, which can lead to minor surface rusting later on. The surface is only as clean as the water used to rinse it. If the only water available to you is high enough in iron to be a problem, you should do as Ted suggested and invest in a water filtration system or bring in clean water from elsewhere.

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois



Can I passivate stainless steel with orange juice?

September 3, 2014

Q. So what's the difference between citric acid and using orange juice to passivate 316SS for my sailboat chainplates (one time, not in production)? I am going to polish the production finish for looks on the exposed portion of the chainplate. Thanks

Thomas Erk
- Oakland, California USA


September 10, 2014

A. Because orange juice is a complex mixture that contains many things besides citric acid. Citric acid is cheap - why reinvent the wheel?

dave wichern
Dave Wichern
Consultant - The Bronx, New York


September 19, 2014

A. Thomas,
Freshly squeezed or something from the grocery store? Just kidding :-)

Ready-to-use citric acid based passivation products are readily available in small quantities. Save the orange juice for drinking. :)

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois



October 16, 2014

Q. The A967 has a table to describe the nitric acid passivation treatments for different grades of stainless steel. Does anyone has any smilar table with recommended citric acid passivation treatments (per ASTM967) for different grades of stainless steel?

Carlos Lopez
- Caguas, Puerto Rico


October 17, 2014

A. Citric Acid passivation is allowed by ASTM 380 and ASTM 967.

You can get a copy here:

http://www.astm.org/Standards/A967.htm

However, note the parts must pass the relevant testing.

Some say citric passivation is marginal to pass, others disagree.

Your money, your choice.

jeffrey holmes
Jeffrey Holmes, CEF
Spartanburg,
      South Carolina



November 10, 2014

A. Carlos,
The chart is important for the nitric baths because there are several distinct nitric formulations listed in the standards. However, on the citric side there is only one formulation given. You'll note the various options on the citric side relate only to time vs. temperature choices. If A967 had a grade per bath type table for citric, every column in every row would be checked, so there's really no point to it.

However, if you are using a commercially available citric acid passivation blend, the manufacturer may have such a table to choose between the different variations of their product.

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois



December 19, 2014

Q. Can Alloy 88 be passivated along with 316SS in a food grade piping and equipment system? Thanks.
Here is its makeup:

Alloy 88 corrosion resistance is equal to AISI 300 series Stainless Steel and has the following chemical composition by percent:

(Nickel Balance)
Tin 3-5, Manganese < 1.5, Phosphorous < .03
Chromium 11-14, Molybdenum 2-3.5, Silicon < .5 Sulfur < .03
Bismuth 3-5, Iron < 2.0, Carbon < .05

Jeremy Steingraeber
- Howes Cave, New York USA


January 6, 2015

A. Jeremy,
Should be fine, but you may want to run a bench test first to make sure the 88 won't have any etching from the process you are planning.

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois



March 13, 2015

Q. How is the bath life of a citric passivation bath determined -- what criteria is used to determine the activity of the bath? We have noticed very little chemical additions in a 5-week period based on holding concentration levels; however, I find it hard to believe that no additions of "fresh" chemical is needed. Please help! Thanks.

Gary Raihl
- Wernersville, Pennsylvania


March 13, 2015

Q. I'd like to piggy back my own questions on to this post. We use citric acid recirculation for the cleaning/passivation of 316/316L SST. Are there any guidelines for the reuse of the weak acid cleaning vs that of a strong acid such as nitric acid? Are there any chemical methods to essentially "clean" the recirculation solution for reuse?

Shawn Way, PE
- Stafford, Texas USA


March 16, 2015

A. Gary & Shawn,
If you are purchasing a citric acid based product, the manufacturer should be able to provide you with a bath maintenance procedure with recommendations on this, no doubt based on a few common-sense things that I shall describe here.

Running a filter on the tank will help keep the bath clean in general.

As for the acid concentration, you can check it the same way you do any other acid, typically pH or titration. To be compliant with industry standards you are looking for a range of 4-10 wt% acid.

The key exception to this is if the acid has been buffered to an elevated pH, in which case the easy relationship between the acidity of the solution and the wt% of citric present is severed. In which case alternative measuring methods such as specific gravity are often employed.

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois


March 17, 2015

Q. Thank you very much. We are using a citric acid solution with an ammoniated EDTA complex, so I think we're going to need to come up with something along the lines of the specific gravity.

Shawn Way, PE [returning]
- Stafford, Texas USA


March 26, 2015

A. Shawn,
For 316 I doubt that's necessary. EDTA as a complexing agent is redundant to the citric acid itself, and 316 holds up fine in a low-pH citric bath so the ammonia isn't needed either. But yes, if that's what you are being required to use, then monitor with specific gravity.

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois



January 7, 2016

Q. I am inspecting parts and noticed that our supplier did not passivate the parts (Stainless Steel - 17-4) according to the callout in our drawing "PASSIVATE IN ACCORDANCE WITH ASTM A967 [link by ed. to spec at TechStreet] , CITRIC ACID ONLY".

The supplier performed Acid Pickling (3X type) by means of i)Hydrochloric Acid (1-2 mins), ii)Camm SUS CP118 (2-3 mins), and then iii)Nitric Acid (1-2 mins).

Our customer wanted these parts to be passivated specifically using citric acid. Before I reject these parts, I have the following questions:

1) Is it good for a single part to be passivated using more than one method (e.g., passivate using an acid pickling method with the use of Hydrochloric Acid and Nitric Acid, then wanting to go back and passivate using Citric Acid)?

I would think this wouldn't be desirable due to the thin oxide layer that is formed after the part is initially passivated.

2) Would "re-passivation" remove the thin oxide layer or would it add another thin layer on top - resulting in a finished part that has slightly larger overall dimensions?

Thanks for your input!

Jeff Bone
- Denver, Colorado, USA


January 8, 2016

A. Jeff,
Hydrochloric acid is bad, bad, bad for stainless steel. It's not something to use for pickling even if you wanted pickling. Thank goodness it was only 2 minutes.

I have no idea what Camm SUS CP118 is.

In the major industry passivation standards, every listed nitric acid passivation treatment requires a minimum of 20 or 30 minutes.

Not only did they not passivate your parts they way you asked, they used nothing resembling any known accepted pickling or passivation treatment and probably did the parts more harm than good. Find a different supplier, and as a parting shot you can ask them if they were TRYING to ruin your parts.

As to your questions:
All the acid bath is doing in passivation is removing iron. (Well, nitric passivation picks away at the other metals too, but more slowly.) Extra time in the passivation bath/additional treatments generally will just have no effect, since all the iron at the surface is gone already. In your case I would definitely passivate again (if the parts still look okay), since there is probably a lot of iron there after the abuse your parts went through. Since you already are locked into using citric acid, you could even do it yourself if you wanted to. Run them an extra long time just to make sure.

Note that "oxide layer" does not mean passivation is a coating. Iron is removed, forming a thin iron-depleted surface layer in the metal itself (mostly chromium at that point), and the oxide layer is formed from that via addition of oxygen atoms from the O2 gas in air. The chromium oxide that's there (there's always at least a little of it, even on an unpassivated part) is unaffected by the acid bath (totally unaffected by citric, mostly unaffected by nitric). The acid just picks away the iron from around the chromium oxide.

Let me know if I can help you further with this.

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois


January 11, 2016

A. Hi Jeff,

I am with Ray on this one, the process used could potentially damage your parts such that they may fail in service.

17-4PH steels typically have Ultimate Tensile Strengths (UTS) above 140ksi, meaning that if they have been anywhere near hydrochloric acid they need de-embrittlement, a minimum of 6 hours at 375 °F within 4 hours of treatment (if lower end of the strength range), 24 hours at 375 °F within an hour of treatment (if upper end of strength range).

If you are using the 17-4PH steel in the H1150 state (UTS 135ksi) you may just about get away with it, otherwise I would be really concerned about these parts.

Brian Terry
Aerospace - Yeovil, Somerset, UK



September 6, 2016

Q. I am using Citric Acid for passivating SS surgical instruments. As for the usual process, the chemical bath will have contamination of detergent (Potassium hydroxide solution) and some oil stains which is caused the chemical bath becoming opaque color.
My intention/plan is to purify the chemical bath instead of changing to new acid so that the acid returns back to clear condition. What method would be possible to clear off the chemical bath?

Thank you.

Ooi Cherng Wee
medical industries - Penang, Malaysia
  ^- Privately contact this inquirer -^


September 13, 2016

A. Ooi Cherng Wee,
You really shouldn't be dragging potassium hydroxide precleaning solution on your parts into the acid bath. All it does is neutralize the acid and waste good acid. Use a good clean water rinse between your preclean stage and your acid dip stage.

If you are dragging oil into your acid bath, you also need a better precleaning stage than you have currently.

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois



September 16, 2016

Q. How do you determine the proper specific gravity and pH levels based on Citric 3 and Nitric 3 passivation per ASTM A967?

Soogie Machamer
- Frazer, Pennsylvania, USA



September 29, 2016

Q. Hi, I'm a newbie here! and I read all the comments and suggestions here. We are passivating our water system which is made up of SS 316L with citric acid followed by nitric acid. Can citric acid be used alone for passivation and omit the use of nitric acid? If ever we are omitting the latter what is the best explanation for that?

Thank you!

Jemarie Perez
- San Pablo City, Philippines


September 29, 2016

A. Jemarie,
Yes, passivation uses either nitric acid or citric acid, not both together. Refer to the industry standards (e.g. ASTM A967) on this.

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois



October 6, 2016

Q. Hi,
We sent our part to supplier to do passivation for B8-CL2 material. But the result showed two types of color as per picture. This is not same lot but different order and different day. Please see attached file. Right side is too dull for us we rejected this part.

46618

How do we know if the part is a reject or not ? Please help.

Nora Noraini
F- Johor Bahru, Johor, Malaysia



October 12, 2016

Q. Hi there,

I got some 316 SS parts citric passivated for use in a product for medical/surgical use. The parts seem greasy or waxy.

What do you think this is? Is it that the citric acid solution dried prior to washing it off?

How can I fix it? Warm water bath, or citric passivate again, or nitric passivate, or something else? Is this harmful for use in our product?

Do you have any testing suggestions to avoid this in the future? I preferred citric passivation, rather than nitric, because it is environmentally friendly, but I worry this may be a problem in the future. Please help!

Thank you in advance!

Jane King
- Baltimore, Maryland, USA


November 10, 2016

A. Jane,
Somehow I missed this question earlier.

My best guess is bad rinsing with rinse water that was not being refreshed enough to stay clean. We have seen that happen before. If the rinse bath is good, check the source and age of the material in the acid bath. Usually there is no shelf life for this sort of thing but odd things have been known to happen from years in storage in a hot warehouse or whatnot.

Depending on what the residue actually is, probably a warm water soak will clean them off, but failing that I would try some solvents (acetone, alcohol, etc.)

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois



November 3, 2016

Q. Hello,

I read all the interesting conversation about using nitric/citric to passivate stainless steel. I'm currently a student working in a company who wants to change its bath of nitric acid to a citric one; I am in charge of looking to THE solution, in order to have a less-toxic treatment. I also have to find how to use hydrogen peroxide (as an oxidant I suppose, in post-passivation as I read).
I have currently no idea about the concentration of citric I have to use, or what I have to put in the bath.
If you can forgive my english, i would like a little precious help about this project.

Thank you for reading!

Clément POIRÉ
- Lyon, Rhône, FRANCE
  ^- Privately contact this inquirer -^


November 7, 2016

A. Clément,
As has been mentioned many times above, ASTM A967 [link by ed. to spec at TechStreet] is a good reference document for the passivation process.

A peroxide post-treatment isn't used very often. Usually letting the surface dry in air is enough to form the chromium oxide layer. Most people aren't in such a hurry that they have to use peroxide to speed up that process.

(adv. If you like, I can put you in touch with the distributor for my products in Germany.)

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois


November 7, 2016

Citric Acid does not passivate (oxidize) stainless steel. It activates (reduces) the surface so that, then, the air can oxidize (passivate) it. The types that contain nitric acid actually oxidize (passivate) the surface for better protection.

Robert H Probert
Robert H Probert Technical Services

Garner, North Carolina

Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como



November 2016

Here we go again :-)

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


November 10, 2016

A. Robert,
No claim is made as to the formation of chromium oxide within the citric acid passivation bath. It's certainly not reducing the surface, though. The acid (H+ ions) oxidizes (removes electrons from) the iron atoms, which converts them to ions and removes them from the surface. The chromium remains zero-valence (electron count matching the proton count) until oxidized (formation of metal oxide via giving up electrons to an oxygen atom resulting in a covalent bond) by oxygen gas, peroxide, or a similar oxygen source.

It is a long-standing belief by many in the industry that the formation of chromium oxide takes place within the nitric acid bath, but chromium oxide measurements comparing nitric-passivated and citric-passivated parts have called this into doubt. Certainly nitrate is an oxidizing agent in the sense that it can takes electrons away from other things, but that would result in chromium atoms removed from the metal surface and released into solution as ions (which indisputably does happen in nitric acid) rather than the formation of chromium oxide.

Regardless of when or how the chromium oxide layer is formed from nitric acid passivation, test data shows the results with citric acid to be equal or superior.

Ted,
;-P

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois


November 16, 2016

thumbs up signGood day Ray.

Holy crap!
I thought I knew about passivation and waste treatment.
What an eye opener! Ions, electrons, great explanation!
I do agree with the chelators (my gold drag outs are heavy!) and without using DTC, waste treatment was a headache.
Personally, I would send the citric acid rinse water STRAIGHT to sanitary, as iron limits here in Ontario are very lenient.
I do enjoy your posts!

Regards,

Eric Bogner, Lab. Tech
Aerotek Mfg. Ltd. - Whitby, Ontario, Canada


November 28, 2016

? Hopefully I do not offend, but surely there must be some method for the objective measurement of oxide film thickness.

dave wichern
Dave Wichern
Consultant - The Bronx, New York


December 2, 2016

A. Dave,
In fact there are methods for measurement of oxide film thickness. They are just expensive and most people in the general industry wouldn't want to use them.

We do have some of this data, however, that was gathered a number of years ago by customers in the semiconductor industry. They do show the thickness from citric passivation to be slightly higher than that of nitric passivation.

Ray Kremer
Stellar Solutions, Inc.

McHenry, Illinois



December 6, 2016

thumbs up signRay,
First, thank you for your answer.
I was at university for a month so I hadn't seen your post. I checked A967/A967M, it looks pretty interesting and useful for the rest of my research.
It looks like there are many things to learn about passivation of stainless steels; maybe I will come back to you for some advice! Thank you again for your help.

Clément POIRÉ [returning]
- Lyon, Rhône, FRANCE



Wastewater treatment for citric acid passivation

November 8, 2016

Q. I have a customer who I'm trying to sell on setting up a SS passivating line.

Question: does the rinse following the citric acid step pick up enough Ni and/or Cr to require waste treatment? I'm pretty sure that the post HNO3 passivate rinses would.

Is there a good IX method for treating such rinses? NO3- is a lousy complex former; citrate is something of a chelator. The former would likely respond to plain vanilla hydroxide precipitation; the latter, perhaps not.

dave wichern
Dave Wichern
Consultant - The Bronx, New York


November 10, 2016

thumbs up signActually, Mr Kremer on his website answered my questions. Apparently, the citric doesn't pick up Ni and/or Cr. I should have looked there before posting.

One thing that citric rinses will have that nitric rinses don't is biochemical oxygen demand (BOD) That might be cause for concern, in some places.

dave wichern
Dave Wichern
Consultant - The Bronx, New York


November 10, 2016

Dave,
With citric acid passivation, zero Ni and Cr is removed from the surface of the steel and these elements will not be found in the acid bath or the rinse water.

Correct, with nitric passivation you will find these elements in your waste stream.

The wastewater from a citric line can't be sent through a metals precipitation system due to the chelating nature of the citric, but then, you don't have to either since iron is the only metal in there and isn't considered hazarous.

(Some users must anyway because they perform other processes producing heavy metal waste and are mandated to treat all waste streams though the precipitation system, in which case the citric acid must be treated and removed from the solution first.)

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois


November 23, 2016

A. Dave,
Citric passivation does not pick up any Cr, Ni, or other heavy metals from the stainless. Only iron, for which no waste treatment is required.

If the company's total waste stream must be treated for heavy metals because they are doing other things in addition to citric passivation, then they do need to pretreat the citric waste stream to remove the citric due to it being a chelating agent.

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois



December 7, 2016

Q. Hello again,
My company asked me to find some solutions using hydrogen peroxide. By that, I mean using H2O2 as oxidant (yes, replace acid by peroxide). I've don't know if it is even possible, without heavily heating or under high pressure. If you have any idea, thank you !
Clément

Clément POIRÉ [returning]
- Lyon, Rhône, FRANCE


December 2016

A. Hi Clément. It is true that peroxide is an oxidizer. I am familiar with its use for etching and bright dipping copper, where it is mixed with sulphuric acid and used as an alternative to nitric acid, persulfates, etc.

I am personally not familiar with attempts to use it for passivation of stainless steel and cannot say whether it would have any usefulness.

But it would be important to find out why it was suggested. If it's because a sister company manufactures it and you wish to support that effort, for example, that could be a justification. But if it's based on some general principles issue of wanting to use less toxic or dangerous materials, you should fully investigate the hazards of hydrogen peroxide before assuming it's safer than proven solutions like citric acid :-)

Luck and Regards,

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


simultaneous December 8, 2016

Hi Clement,

We use sometimes hydrogen peroxide for thin colored oxide removal after titanium electropolishing, and as passivation liquid for stainless steel. The passivation composition for SS is listed below. It was developed for food industry application to be used after regular cleaning procedure. The same electrolyte mildly removes rust from SS.

Composition: (pure chemicals are assumed, need to be recalculated according to percentage).


H3P04-5-10 g/l
H202-1-3%
NaCl - 1 g/l

Room temperature, duration - 20-40 min. Solution can be corrected by 1% H2O2 after each application and reused. Rinse by clean water.

anna_berkovich
Anna Berkovich
Russamer Labs
Pittsburgh, Pennsylvania


December 8, 2016

Clement,
There are two parts in the normal passivation process.

1) Acid removal of iron from the surface
2) Chromium and other non-ferrous metals remaining on the iron-depleted surface join with oxygen to form metal oxide

A peroxide bath will accomplish #2, but probably not much of #1, or too slowly to be as effective as the traditional acid treatments.

You also see folks claiming that a nitric acid bath does both #1 and #2 but there's more evidence now that #2 happens primarily in the air after rinsing and drying.

Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois


December 19, 2016

Thanks Ted, Anna and Kray.

So it is possible to passivate stainless steels using H2O2, according to Anna. But Ray seems to say it is too long, so does the short duration have a link with H3PO4? Does it help for the #1 part ?
Ted, my company want to reduce its impact on environment, I will look for H2O2 properties.
I also need to work at neutral pH, I guess it will not be the case, using some H3PO4 !

Regards,

Clément POIRÉ [returning]
- Lyon, Rhône, FRANCE


December 22, 2016

Q. Hello,

My name is Clément, and I am a french student.

I am currently working in a company which deals with stainless steel.

At the moment, they are using nitric acid to passivate, and they have some problems with bichromate. All of that is not that good for environment, and that is why I have many objectives :

- Eliminate dichromate (used for martensitic stainless steel).
- Find some issues on the side of the citric acid, maybe in order to replace the nitric.
- And develop a bath of passivation using hydrogen peroxide.

I have read about ASTM A967 [link by ed. to spec at TechStreet] on finishing, but i have not a lot of clue about the hydrogen peroxide.

If anyone know anything about that, it could be very useful for me !

Thank you for any help you can bring !

Clément

Clément POIRÉ [returning]
- Lyon, Rhône, FRANCE


December 2016

A. Hi Clément,

opinion!  If your company works hard to make your product last longer, they'll reduce their impact on the environment a hundred times as effectively as they will by trying to substitute "less toxic" passivating agents. I don't think it's coincidence that worldwide pressure to reduce the use of phosphatizing before painting coincides with the world now drowning in truly crappy steel patio furniture, lawn equipment, and other objects that last 1-3 years when we know perfectly well how to finish them to last 15-20 years. We eliminated a pound of phosphate at the cost of mining, smelting, rolling, fabricating, painting, packaging, selling and landfilling a ton of steel every 3 years instead of every 20 -- with all the transportation waste between each of those steps as well :-(

Let's hope your company doesn't move in the same direction when finishing stainless steel. The world cannot afford the assault on sustainability that today's pressure to reduce toxics in finishing seems to be wreaking. Proprietary citric acid formulations are a proven solution which eliminate chromic acid and nitric acid; you don't need to hunt for "less toxic" than that, although you should certainly endlessly hunt for "better at prolonging the life of stainless steel" :-)

Regards, and best of luck,

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


December 23, 2016

Q. Hi Ted,
I understand your point of view, and I only can agree. But I'm not really involved into the politic of my society, they just gave me this job, and here I am! I am a student, and this is my work's subject in the company (because I am one month at school, one month at company). So yeah, we should work on the life duration of produced steel, but this is not my job here. I am just a chemist working on bath's compositions :-)

Regards,

Clément POIRÉ [returning]
- Lyon, Rhône, FRANCE

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