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Simple Passivation Test for Type 420 / 4xx / 400 Series Stainless Steel?.
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, P.E. RET
Pine Beach, New Jersey
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...
- 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.
- 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'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
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 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.
materials testing laboratory
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.Dianatkhah
- 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.
- 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
A. Copper sulfate and ferroxyl spot testing are accepted methods. Good luck - you'll need it to get ferritic stainless to pass.
Jeffrey Holmes, CEF
- Spartanburg, 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.
Stellar Solutions, Inc.
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.
- 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.
Koslow Scientific Company
Englewood, New Jersey
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.
- 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.
- 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
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.
Stellar Solutions, Inc.