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Simple Test for Passivated Stainless Steel
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
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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- Hong Kong, China
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No, Mr. Chook, sometimes people are away or busy, or have failed to reply for some reason. Lack of response does not imply anything about the ultimate answer to your question. But you may find the following letters helpful: 3451, 3571, and 4878. Good luck.
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 Ted Mooney, P.E. RET finishing.com Brick, New Jersey |
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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...
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Jeff Watson - Pearland, Texas |
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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- Hong Kong, China
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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
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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's 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
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The simple tests for passivation are the acidified copper sulfate test (referred to above) and a ferric cyanide test - both specfified in ASTM ASTM A380 [link is to spec at TechStreet]. These tests are primarily checking for free iron on the surface and cannot confirm that nitric acid passivation has specfically been performed. There is a commercial passivation test available from Koslow that works on some type of electrochemical basis. This might be more effective, but I don't know for sure.
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Larry Hanke materials testing laboratory Minneapolis, Minnesota |
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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.
Dianatkhah- Dallas, Texas
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I need to know the mixture formula for copper sulfate immersion test on stainless steel.
Tony SantosEngineer - East Hanover, New Jersey
December 12, 2011
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.
- 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?
- Rainier, Oregon, USA
February 8, 2012
Copper sulfate and ferroxyl spot testing are accepted methods. Good luck - you'll need it to get ferritic stainless to pass.
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Jeffrey Holmes, CEF - Spartanburg, South Carolina |
March 8, 2012
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.
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Ray Kremer Stellar Solutions, Inc.  McHenry, Illinois |
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