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Passivation or Chromium Coating

Hi,

I'm from Canada, and I work in a surgical instruments company which sub-contract everything. I have problems with my passivation sub-contractor and I'm looking for an other point of view. Our instruments are made in 440c, 440a, 17-4Ph, 316 stainless steel. For their passivation, I really have two opinions here. I looked in all the reference book (ASTM ASTM A380 & ASTM A967 ) and our instruments are ok to be passivated. The other opinion is that the 420 and 440S/S needs chromium coating. Hope someone can give a neutral opinion.

Thanks in advance.

Marc Tebo
- Canada
2003

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First of three simultaneous responses

Marc, we passivate a lot of 440C for our aircraft part applications. We use a type VIII nitric bath, and we do not stain test the 440C. We accept that there are times that it just won't pass the stain test, due to the chromium/carbon ratio of the surface- so we don't bother. Interestingly, skipping the testing is allowed by the soon-to-be-cancelled AMSQQP35 [canceled], and by AMS2700. We continue to passivate because we think it's a good thing to do, and our passivation bath is on a regular testing schedule.

I'm not sure if this answers your question: if not, write back!

Lee Gearhart
metallurgist - E. Aurora, New York
2003

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Second of three simultaneous responses2003

These alloys should not need chromium plating. I presume that the real issue is a loss of surface finish and corrosion resistance due to formation of heat treatment scale on the 440 & PH alloys. When heat treated in air, these alloys form a chromium oxide-rich scale, leaving the surface of the metal depleted in Cr. This scale must be etched away prior to passivation, and a grayish, slightly rough surface is left due to exposed carbides and intermetallics. Repolishing may be advisable before passivation.

Perhaps, the best solution is to avoid oxidation by heat treating in inert gas or vacuum or by encapsulating parts in stainless steel foil or baggies. See letters #5089, 17460, 18577.

There is also a paint-like coating to prevent oxidation during heat treatment, letter10948.

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.

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Third of three simultaneous responses

440c can be a particular problem to passivate if not done correctly. 440a and 17-4 PH can also be some problem if not done with care. 316, of course, is very easy to handle.

We assume that the 440 grades are hardened. This is necessary in order to properly passivate them. Secondly, the 440c needs to be pretreated to remove carbon from the surface before passivation. We have special products to do this best, but it can be done with pH 13 alkaline cleaner to some extent, with ultrasonics if possible.

440 and 17-4 work best with special products that we can give you if you call us. We cannot mention the trade names on this site.

Lee Kremer
Stellar Solutions, Inc.

McHenry, Illinois

2003

Thanks for your answers everyone.

Marc Tebo
- Canada
2003