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Citric vs. Nitric Acid Passivation of Stainless Steel

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Q. We have been using both Nitric acid with Sodium Dichromate and Citric Acid for our passivation process for 440C. It seems that Nitric acid passivation is better than Citric acid passivation. We did an EDS and XPS test on samples for both types of passivation. Chrome oxide which is 80% to 90% forms the passive layer is thicker by 25% on samples that undergone Nitric passivation than the samples that undergone Citric passivation. This may be the result of the oxidizing effect of Nitric acid and Sodium Dichromate while on the other hand Citric acid can only do removal of free iron on the surface and very minimal oxidizing effect. What we want to do is make Citric acid passivation as effective as Nitric acid passivation. Does anybody have any idea how we can improve Citric acid passivation?

Leo Monsada
- Chatsworth, California, USA


A. I agree that Citric does a fine job of removing the free irons. More than anything, I appreciate working with a safer chemical. However, it does not passivate in the sense of oxidizing the chromium like the Nitric does.

For this reason, I have added a second passivation step when using the Citric acid as an added measure. I also rinse with de-ionized water.

What we really need is an oxidizer to help build the chrome oxide film in the absence of the Nitric acid.

My work is field passivation for heavy industry. I frequently work as part of a major construction project. My fear has been the stainless will become contaminated by things in the air before the chromium can react with air to form the chrome oxide film. This contamination could be in the form of free iron from other grinding and welding operations or simply attacked by the corrosive environment.

I've considered adding some other oxidizers to the rinse water such as sodium perborate, sodium persulfate or even a mild hydrogen peroxide. At minimum, I rinse with DI water with the goal of not allowing any chlorides from city water chlorine.

Any suggestions along these lines?

Todd Turner
- Monroe, Louisiana


A. Interesting question! What about sulfuric acid with peroxide as an oxidizing agent? Alternately, it certainly seems that it should be possible to electrochemically oxidize the stainless steel with a brush plating unit, but I don't know what acid would be best.

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


A. Leo:

We routinely use a hydrogen peroxide solution to fully oxidize the stainless steel surface after passivation with citric chelant blends. The oxidation of the surface is the final step which nitric acid completes that the standard citric acid based system does not perform. Oxidation of the surface is important, especially after removal of iron oxide or other surface oxides present on the surface (derouging).


Daryl L. Roll
Costa Mesa, California, USA


A. You are correct that straight citric acid is not a total answer to passivation. You need other ingredients to help make the chrome oxide layer deeper and a higher CrO/FeO ratio. This can be accomplished in a number of ways, including adding peroxides to the rinse water, etc.
Typical data that we get with proprietary formulations gives us both higher ratios and DEEPER layers used in semiconductor industry parts. The process of treatment is VERY critical in obtaining the correct depth and content of the passive layer, including the atmosphere in the drying stages.
There is no reason why you should not be able to exceed the % and nitric depth of CrO layer if you use the correct formulation and process.

lee kremer
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Lee Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
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Q. How would an oxygen generator fit into the application to passivate stainless steel.

Jerry Butler
- Martinsville, Indiana


Q. We are a small job shop and supplier to the medical industry. We use nitric acid to passivate stainless steel and titanium parts. I have two questions first: According to ASTM A967 [link by ed. to spec at TechStreet] -05 there are several different nitric bath solution concentrations and temperatures. The steel supplier as well recommends different processes for different materials say 17-4 or 400 series steels. Being a small shop we do not have the equipment to have several different tanks for each of the process solution concentrations and temperatures. It is not feasible to empty the tank and change solutions every time a part of a different material is processed. Could we use the solution of highest concentration and temperature to cover all the different materials and if not why?
Second: It has been brought to my attention that we should change solutions and not passivate titanium in the same solution that has passivated stainless. What is the reasoning behind not passivating stainless and titanium in the same tank?

Sam Spieth
- Fort Wayne, Indiana, USA


A. Dear Sirs

Titanium and Stainless steel is not the same material.

Stainless steel 300 series and 400 series have different passivation steps.

Because the alloying is different.

All passivating is to change the corrosion potential for the specific elements in the alloy.

I don't think a citric passivating is the best method to get a high corrosion protection for stainless steel 400 series. I believe the passivating in dichromate and nitric acid because nitric acid will remove iron and other contamination in the alloy and a following step in dichromate passivating will show the best results.

anders sundman
Anders Sundman
3rd Generation in Plating
Consultant - Arvika, Sweden


Q. How about Citric Acid passivation on 15-5 PH Stainless? This would be a selective passivation process where part of an assembly requires passivation. I'm in the process of testing this right now.

Dan Mitchel
- New Haven


Let me be more clear.

Looking for help with a selective passivation issue. The parts are made of 15-5 PH steel and get machined to approximately 1"square in size. They then get sent out for passivation. After machining and passivating the parts get stocked and then assembled with various components as needed. After assembly, the (15-5) parts get laser marked with serial numbers. We require that the laser marking be passivated with a touch up kit. I've been experimenting with the Citric Gel with inconsistent results.

The current process is as follows. Clean the laser marking very well with Isopropyl Alcohol [linked by editor to product info at Amazon] and a foam swab. Heat assembly to 100° F to raise temp for passivation. Apply room temp Citric Gel to laser marking immediately after baking. Let dry on part for 1 hour. Remove dried gel with distilled water and foam swab. Let dry.

I've been sending treated flanges for a 2 hour salt spray test per AMS2700 [link by ed. to spec at TechStreet] and getting inconsistent results in terms of corrosion in the marking. I've also tested parts with no treatment to prove that the marking will corrode. Those results are also inconsistent, some corrode, and some do not.

Can this process be adjusted to work every time without the use of harsh chemicals?

Dan Mitchel
Engineer - New Haven, Connecticut, USA


A. I think sodium dichromate is a good selection for is better to use 20 g/lit solution of sodium dichromate and adding chromic acid to adjust PH of bath about 4.

Ali Noroozi
tinplate specialist - Esfahan, Iran


Q. We just reviewed Auger analysis of nitric and citric acid passivated 316SS test coupons from our own in-house AAA nitric and Citric 3 processes. We also tested identically prepared coupons from another vendor's nitric and Stellar-citric processes for comparison. The CrO/Fe ratios for all four samples were similar, 0.6-0.7, well below what is the acceptable norm of 1.0 for well passivated parts, as reported by the independent test lab. In looking at the general commentary made on this question regarding enhanced CrO layers with additional oxidizing rinses and careful drying, is what we've experienced without the additional CrO layer processing in agreement with other's findings? Is there a technical reference for CrO/Fe ratios based on the different passivation processes being used or is this information proprietary? I would like to hear from others that have experimented with the different processes and conducted their own Auger analysis.

John Buchanan
- Attleboro, Massachusetts


A. John,

From the first day I wrote the inquiry, we did a lot of R&D and run a few DOE (design of experiments) concerning citric acid passivation. We were able to come up with a citric acid passivation process that can match nitric and even exceeds nitric acid passivation in terms of CrO/FeO ratio and passive layer thickness measured in angstrom. We used independent test lab to the ESCA and AES analysis. There is no current standard that details passive layer thickness but normal stainless steel will have a natural passivation in air with about 10 to 15 angstrom thickness. Below are just some results we have.I'm sorry I can't give you the formula since it's proprietary and awaiting patent.

ESCA Analysis:

Analyses are based on methods outlined in XPS Sematech Standard 90120403B-STD. The procedure involves measurement of the relative abundance of Cr and Fe as determined from the quantification of high resolution spectra acquired on the Cr 2p3/2 and Fe 2p3/2 photoelectron lines.

Analytical Parameters


Physical Electronics 5500/5600

X-ray source

Monochromated Alk? 1486.6eV

Take-off angle


Analysis area

2000?m x 800?m


Low Energy Electron Source

Pass Energy


Calculation of the elemental Cr/Fe and oxidized Crox/Feox ratios are determined from high resolution data using integrated peak areas determined from nonlinear least squares curve fitting of the individual Cr 2p3/2 and Fe 2p3/2 spectra in conjunction with the appropriately applied sensitivity factors and background corrections. Sample Cr/Fe Crox/Feox

#1, Citric w/ enhancer formula 1 2.7 3.8
#2, Citric w/ enhancer formula 2 2.9 3.9
#3, Nitric with Sodium Dichromate 2.1 2.4

AES Analysis:

This analysis was performed in accordance with SEMATECH Semaspec 91060573B-STD, Feb. 22, 1993 revision. The Auger Electron Spectrometer used was a PHI 660. Each survey provides information from the outermost few molecular layers of the surface. The composition values listed in Table 1 and used in the Atomic Concentration Profiles (ACP) were calculated using sensitivity factors based on pure elements or selected compounds and are expressed as atom percentages for the elements that were detected.

Depth profiles of Fe, Ni, Cr, O and C concentrations were acquired to a depth of 150Å.

Sample Description





#1 Citric enhancer formula 1





#2 Citric enhancer formula 2





#3 Nitric with Na2Cr2O7





Sample Passive Layer Thickness(Å) FeO Thickness

#1 Citric enhancer formula 1 24
#2 Citric enhancer formula 2 26
#3 Nitric with Na2Cr2O7 25

Leo Monsada
- Chatsworth, California, USA

November 2, 2010

A. Wow. People working with dichromate and even chrome trioxide.
Bold guys, daring and tough. Better leave it aside. Ever tried caroate? A good oxidizer and acid at the same time.
Good luck

wilfried schuler
SES Bonn - Huenstetten Germany

Caro's Acid

November 3, 2010

Hi, Wilfried. The way you've addressed the previous writers, although probably meant to be gentle, can easily be misinterpreted as condescending. And caroate (caro's acid, peroxymonosulfuric acid, H2SO5, persulfuric acid, peroxysulfuric acid) is not milk & honey either; according to Wikipedia "... highly explosive. Explosions have been reported at Brown University and Sun Oil". We are all constantly in search of safer and more environmentally friendly approaches to surface finishing issues, but I used to sleep much better on airliners than I do in these days of relentless environmental pressures to make substitutions for time-proven metal finishing processes like cadmium and chrome plating :-)

Can you point us to any articles in respected metal finishing journals where caro's acid has been tested against citric acid and/or nitric acid for the passivation of any types of stainless steel? Have any standards been written by respected standards-writing organizations for the use of caroate as a passivation agent for stainless steel? Thanks!


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

Passivation of stainless with 10% dichromate

January 28, 2014

Q. I am updating a drawing of an aerospace part form the later 50's. It specifies an internal company spec for passivation with just a dichromate (10% solution by weight) solution for 20 to 30 minutes. The material is 440C type stainless steel hardened to Rockwell "C" 55-62.

I believe the reason is because of the +/- 0.0001 tolerance. Does anyone know of an industry spec (AMS, ASTM, etc.) that describes this method?

Tom Dalessio
- Somerset, New Jersey

January 30, 2014

A. Tom

See section 10 of ASTM A967/A967M-13 for some of the details. ASTM A967 [link by ed. to spec at TechStreet]

Willie Alexander
- Colorado Springs, Colorado

February 27, 2014

A. Tom,
Since your part is for aerospace, you may want to refer to AMS 2700, but overall either that or ASTM A967 will be fine for you. The SAE crowd decided that aerospace should have their own version rather than use ASTM's replacement for QQ-P-35, but both standards are pretty much the same in the end.

I'm not sure how successful the dichromate alone was in the past for that part, a nitric-dichromate mix would have been the normal treatment. I suppose the tight tolerance might have lead someone to think avoiding the nitric was a good idea. Fortunately these days you can avoid both nitric and dichromate by using citric acid, which if done correctly should give you no problems in maintaining the tolerance.

Let us know if we can help.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
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February 28, 2014

thumbs up signThanks for the information guys

Tom Dalessio
- Somerset NJ

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