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Simple Passivation Test for Type 420 / 4xx / 400 Series Stainless Steel?

adv.    koslow passivation test kit


Q. Can anyone suggest a simple and quick test which can distinguish between a piece of nitric acid passivated 420 stainless steel and a non-passivated one?

Could anyone give some comment on my question? Many thanks.

Kwok-wai Chook
- Hong Kong, China


Q. Hi Surface Finishing Professionals, After my 2nd inquiry without any feedback, I need to conclude that there is no known simple method to distinguish between a piece of nitric acid passivated 420 stainless steel and a non-passivated one. Regards.

Kwok-wai Chook [returning]
- Hong Kong, China


A. No, Mr. Chook, sometimes people are away or busy, or have failed to reply for some reason. Lack of response doesn't imply anything about the ultimate answer to your question. But you may find the following letters helpful: 3451, 3571, and 4878. Good luck.

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

Passivation of Metals and Semiconductors
from Abe Books



A. Because of the way nitric acid passivation works there is no easy way to test whether it's been done or not. Nitric acid simply dissolves the oxides from the surface and forms a near complete layer of oxide, which protects the underlying metal from further oxidation. Since no chemical is deposited, and it's a relatively weak method of passivation, it would be very hard to test.

You can compare panels before and after in a corrosion chamber to see the results, but I'm not sure you could test individual panels and determine whether they had been passivated conclusively.

Sorry no-one replied, there's not much of an answer to your question...

Jeff Watson
Jeff Watson
- Pearland, Texas


Q. Dear Mr. Jeff Watson, Thank you very much for your comment. I appreciate your sincere reply.

From what you've told me, I understand that there may not be a simple test. When you mentioned a test in corrosion chamber, I would like to have your suggestion on what type of corrosion test (humidity, salt spray or sulphur dioxide) could be done and how?

I am looking forward to hearing from you very soon. Regards.

Kwok-wai Chook [returning]
- Hong Kong, China


A. It was my understanding that you could easily check the passivation status of a part using a solution of copper sulfate and dilute sulfuric acid. An unpassivated part will show copper immersion.

Megan Pellenz
- Syracuse, New York


A. Hi Megan,

the copper sulfate test is still rather subjective on 420 steel because it has so darn much iron vs the 300 series. Temperature of the part is enough to cause it to pass or fail from one day to another.

A friend of mine switched from nitric acid to Stellar Solutions [a supporting advertiser] citric acid passivation for 400 series and swears that he has not had a failed passivation or a damaged part since the switch. I would surely recommend a lab test by anyone that does 400 series SS. You only have to have one lot damaged in 30 seconds to hate 400 SS.

James Watts
- Navarre, Florida


A. The simple tests for passivation are the acidified copper sulfate test (referred to above) and a ferric cyanide test - both specified in ASTM ASTM A380 [link is to the practice at TechStreet] . These tests are primarily checking for free iron on the surface and cannot confirm that nitric acid passivation has specifically been performed. There is a commercial passivation test available from Koslow [a supporting advertiser] that works on some type of electrochemical basis. This might be more effective, but I don't know for sure.

larry hanke
Larry Hanke
Minneapolis, Minnesota


A. You may want to refer to page 552 of the "ASM Metals Handbook Vol. 13 "CORROSION" [link is to info about book at Amazon]. The only approved test for the 400 series passivation test is 100% humidity for 24 hours and no rust or corrosion shall be observed. I am working on 444 SS passivation check up.

- Dallas, Texas


Q. I need to know the mixture formula for copper sulfate immersion test on stainless steel.

Tony Santos
Engineer - East Hanover, New Jersey

December 12, 2011

A. I work in a stainless steel company. Since I need to check the passivation layer of sanitary grade pipe, I purchased some equipment from Japan. It cost around USD $1000 for 350 times testing.

The way it works is you drop diluted acid on a filter paper, stick it to the sample and use voltmeter reading (one probe touching the filter paper, another is to the metal sample). This test will basically determine how fast the diluted acid attacks the Chromium-oxide layer. The initial reading will jump to 0.6V in 2 seconds. Then you observe how fast it decays to 0.1 V. This equipment has two kind of probes: one is for 300 series, and another for 400 series stainless.

Luckily I have not found defective passivation layer in all my Chinese goods. But these Chinese goods perform badly compared to Japanese. Japanese stainless steel jumps to 0.6 V initially and decays to 0.4 after 50 seconds. I did not continue reading until 0.1 as this Japanese stainless has definitely a good passivation layer.

Chinese product: jumps to 0.4 V and decays to 0.1 V only within 20 seconds. But according to the passivation kit seller, this is still not considered as defective, although they said it is not a good passivation layer.

Teddy G.
- Surabaya, East Java, Indonesia

February 2, 2012

Q. We are using a ferritic stainless steel product that is exhibiting a high degree of corrosion. This happens in the raw material stage, sub-component stage, and final assembly which includes grinding and welding.

The material is a high chromium, ferritic stainless steel that goes by the trademark 3CR12 or UNS41003. We'd like to be able to test for iron contamination at various stages of our manufacturing processes.

1. Does anyone know of the best passivity test for this material?
2. Any recommendations for cleaning or passivation chemicals for ferritic stainless?

Jack Johnston
- Rainier, Oregon, USA

February 8, 2012

A. Copper sulfate and ferroxyl spot testing are accepted methods. Good luck - you'll need it to get ferritic stainless to pass.

jeffrey holmes
Jeffrey Holmes, CEF
      South Carolina

March 8, 2012

A. Oh, indeed, those iron-indicator solution tests aren't recommended for use with stainless grades containing less than 16% Cr, like the alloy Jack is using. They are prone to false failures.

For stuff that copper sulfate doesn't do well with, I generally fall back on water immersion testing. There are many variations depending on what document you look at, but I tend to do it heated to reduce the time needed to a few hours.

Please let me know if we can assist you with your passivation needs.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
stellar solutions banner

June 15, 2012

A. I recently bought some 316 S/S bolts to use as liquid level conductivity probes. I pickled them after hand polishing using a 5% hydrofluoric acid and a 22% nitric acid mixture, which is commercially available. I then passivated them with a passivating paste containing a 22% nitric acid solution.
I immersed the probes into a salt water solution and used a multimeter on the millivolt scale to test the voltage between the stainless piece and the multimeter probe. The voltage began rising and reached about 135 mV. This probe is well and truly pickled and passivated.

Brian Gray
- Cape Town, Western Province, South Africa

August 10, 2012

A. Electronic Testers have also been found effective for the identification of passive layers on stainless steel. Far quicker then chemical methods of marking the stainless surface.

Wolf Koslow
Koslow Scientific Company
supporting advertiser  
Englewood, New Jersey
koslow banner

July 23, 2012

Q. I want to make stainless steel testing chemical for sorting 316 and 304. If anyone knows about this, how to make this chemical, then please write here.

Shabbir Sheikh
- Faisalabad Pakistan

April 26, 2013

Q. I am attempting a passivation validation using the boil test and 420 stainless. My problem is that I cannot get the boil test to generate rust at all, even after using a file to remove the surface passivation layer (and then some) and immediately immersing in boiling water. I need to generate rust to compare against a passivated sample to show that the Citrisurf product (2310) will generate a passive layer. Can anyone explain why my 420 seems to be totally immune to rust? I am following ASTM F1089 Standard Test Method for Corrosion of Surgical Instruments (30 minute boil, 3 hr rest, 2 hr ambient dry).

Mike Grissom
- Honolulu, Hawaii, USA

September 4, 2013

A. We share the same experience. If you are looking for puffy rust after the boil test, it will be rare to find this on 420. (I have not try the boil test on 420F.) You may find rust spots on 440C. The unclear part of the boil test is "shall show no signs of corrosion" (from ASTM F1089 [link by ed. to spec at TechStreet] ). If there is material removal or surface deterioration, it is definitely corrosion. What about discoloration with brown films or staining?

Copper sulfate test gives much easier results to interpret but is prone to false positives.

Kai Lorcharoensery
- Warsaw, Indiana, USA

December 6, 2013

A. It may be that there is just not enough free iron on the surface to generate red rust. We have often taken fresh machined parts (usually 303, 304, and 17-7) and subjected them to testing (24 hour Humidity and Copper Sulfate) with no failures, without any passivation, only degreasing.

The most common source of failure is ferric metal smear or contamination from Metal tools/cutters and sanding/grinding contamination. Using carbide tools and being careful with abrasives can greatly reduce failures. While passivation will remove ferric compounds on the surface that exist as part of the alloy, the material will usually self passivate with time.

To get a failure, you may need to expose the material to iron in processing.

Paul Slater
- Cedar Rapids, Iowa, USA

XRF Passivation Test?

December 9, 2013

Q. I've recently heard references to using XRF for passivation testing. While I've never heard of it, and have no idea how it would work, there are other things I've not heard of so I want to check with the community of experts ... any of you ever heard of using XRF to verify acceptable passivation?

Bob Denney
- Loudon, Tennessee

December 12, 2013

A. Bob,
It's not overly common, and certainly not listed in the industry passivation standards, but years ago some customers we had in the semiconductor industry did indeed use techniques such as ESCA and AES to evaluate the passive layer of stainless steel in terms of the depth of the layer and the chromium to iron ratio. (If anybody would like to see this data, let me know.) If you cut a cross section of a piece of stainless and can get good enough resolution from your instrument, I could see getting data via EDS or WDS as well, though the passive layer is only 20-30 angstroms deep at best, so that's probably unlikely.

These are of course rather expensive tests, which is why you'll generally only see the semiconductor industry using them for this, and even then only for process evaluation, not as day-to-day production tests. The common test methods, as seen in the industry standards, are along the lines of iron-indicator test solutions and accelerated corrosion tests. These methods generally give a pass/fail only, or at best a number of hours before corrosion appears. The spectroscopy methods give hard numbers in terms of how much iron has been removed from the surface and how far down that removal extends, which is expected to correlate to corrosion resistance.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois

420SS Surface rust cleaning

October 22, 2015

Q. Hi, I have small cylinders made from 420SS that are showing surface rust. The parts are 420SS, vacuum heat treated to 40 HRc then polished in a mild vinegar/soap mixture to shine them. The parts are then processed with a soy based grinding oil, fixtured in aluminum trays and degreased with NPB (N-Propyl Bromide) in a vapor degreaser. A few hours/days later, spot surface rust develops. 2 questions:

1. Is there any way to clean the rust once it has formed and prevent it from coming back?
2. Is there something about the process that stands out as a major issue that would be causing this?

Any help or ideas is greatly appreciated!

Mike Quigley
Engineer - Lakewood, New Jersey USA

October 23, 2015

A. Good day Mike.

This seems very unusual, given that there is 12% chromium in the alloy. I have seen anomolies in salt spray with SS, as dark spots, but after proceeding with the copper sulfate spot test (as per passivation) there was no indication of iron as a red deposit.
Are you certain it is rust?
Personally, I would use SS for degreasing.
Just a thought.


Eric Bogner
Aerotek Mfg. Ltd. - Whitby, Ont. Canada.

simultaneous October 23, 2015

Q. Hi Eric,
Thanks for looking into this. We have confirmed with an outside lab that the stains are consistent with rust. The lab reports:
Sample B showed a band of rust-colored material on the central shaft. The contaminant is primarily composed of iron, oxygen, carbon, aluminum, and chlorine. Low levels of potassium, sulfur, sodium, and calcium were also detected. The material is generally consistent with iron oxide or ?rust.?

I am not sure where the Chlorine is coming from except maybe our rinse water during the polish process. Could that be an issue?

What do you mean by using SS for degreasing?


Mike Quigley [returning]
- Lakewood, New Jersey USA

October 23, 2015

A. 420 Will rust, but it is normally caused by ultra thin smudges of steel from fixturing as in your grinding operation and possibly the attachment to the aluminum part.
Salt and chlorine products will cause 420 to show rust in a few days, so it is possible.
Sharp knives are typically 400 series and normally will not show rust, but in the wrong use, they will show rust.
Have you checked the acid acceptance of your degreaser?

James Watts
- Navarre, Florida

October 23, 2015

A. Mike,
In all of that, you don't mention that you are passivating these parts. It would certainly help if you did that, though 420 still remains on the lower end of the scale of corrosion resistance as far as stainless steels go.

Rust can always be removed, but unless the parts are kept in a pristine cool and dry environment, without passivation the corrosion will likely come back sooner rather than later.

You should be able to take care of this easily with a citric acid based product.

Let me know if you would like help with this.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois

simultaneous October 26, 2015

James: Yes, we check acid weekly in the degreaser, it has been steady with minimal need for booster.

Ray: We don't currently passivate our parts. In the past, we have not had any rust issues. Recently, we changed the hardness value of the parts, decreasing from 50 HRc to 40HRc. After that change, the rusting started to show up sporadically. We cannot change back though due to other processes that now require the lower hardness. What is involved in a batch passivation process?

Thank you all for the responses.

Mike Quigley [returning]
- Lakewood, New Jersey USA

October 24, 2015

A. Mike,

If you want your cylinders be rust free, you need to electropolish them. No other methods will save from rusting in a long run. The surface looks fine and shiny. But if you look under microscope, you will see small holes underneath "smeared" particles. These holes can be filled with water, acid, etc.. Sooner or later such holes begin to rust.

Electropolishing 400 or 420 stainless steel, or similar alloy types will remove all impurities on the surface.

Electropolishing in Universal Electrolyte will provide thick layer of protective against the corrosion film.

Anna Berkovich
Russamer Labs
supporting advertiser 
Pittsburgh, Pennsylvania
russamer labs banner

October 29, 2015

A. Mike,
Okay, that makes some sense. Our recommendation with 400 series stainless is to heat treat to full hardness prior to passivation. Parts that are not heat treated become severely etched by the acid, clearly the microstructure that contributes to hardness also is a factor in holding up to corrosive exposure. All of which means you should to run some passivation trials here and see how things hold up.

What we recommend for 420 is an alkaline pretreatment, a rinse, citric acid product passivation acid bath, two or three rinses, and immediate drying.

ray kremer
Ray Kremer
Stellar Solutions, Inc.
McHenry, Illinois

Electropolishing, Anodizing and Electrolytic Pickling of Metals

November 2, 2015

thumbs up signAnna, Thanks for the advise. Are you aware of a good method of bulk electropolish? Our typical batches are 3,000 pieces and racking is not really feasible. Or is there some other method that could be better with a bulk process?

Ray, Thank you, we will look into some passivation trials.

Mike Quigley [returning]
- Lakewood, New Jersey USA

November 2, 2015

Hi Mike,

Universal electrolyte allows to electropolish in a wheel (if parts are not flat, do not stick to each other and move during wheeling. Bulk electropolishing is one of the advantages of UE. We can run some tests for you.

Anna Berkovich
Russamer Labs
Pittsburgh, Pennsylvania

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