Passivation Test for Stainless Steel series 400

A. Hi, Mr. Chook. Sometimes people are away or busy, or fail to reply for other reasons. Lack of response doesn't imply anything about the answer to your question. Patience please :-)
Good luck.

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...

A. It was my understanding that you could easily check the passivation status of a part using a solution of copper sulphate ⇦ this on eBay or Amazon [affil links] and dilute sulfuric acid. An unpassivated part will show copper immersion.

A. Hi Megan,

the copper sulphate 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 finishing.com 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.

A. The simple tests for passivation are the acidified copper sulphate test (referred to above) and a ferric cyanide test - both specified inASTM A380 . 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 Scientific Testing Instruments [a finishing.com supporting advertiser] that works on some type of electrochemical basis. This might be more effective, but I don't know for sure.

A. You may want to refer to page 552 of the ASM Metals Handbook Vol. 13 'CORROSION' ⇦[this on Amazon affil links]. 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.

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.

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

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 sulphate 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.

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

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.

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.

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.

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 sulphate 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.

Regards,

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.

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.