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Ted Mooney, P.E. RET
Pine Beach, NJ

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Yellow passivation on Stainless Steel

Q. I have recently received parts from a vendor which are supposed to be Corrosion resistant steel (17-4 PH CRES or 15-7 PH MO) which is passivated. Of the six parts I have received one looks "correct" and five of them look as though they are zinc plated with yellow dichromate. I am not aware of a stainless passivation process which would look like this ... is there something out there that I don't know about? The supplier hasn't certifified that these parts are indeed passivated stainless steel.

Ruth Zelaya
maintenances services company - LIC, New York
2001

A. You posted a similar question in letter 33644. As Lee Gearhart said in his response, I would suspect that your supplier has given you lower-grade steel parts with a zinc-yellow plating.

Tim Neveau
Rochester Hills, Michigan
2001

A. I have seen photos of steel in the process of electropolishing at a number of e.p. company websites. some of the steel parts do indeed turn to a light gold, depending on the bath, voltage, type of steel etc.. If your supplier is electropolishing the parts, passivation usually won't change the color unless they are using some type of "dichromate" to enhance the action of the solution, which is not uncommon.

However, if the company isn't willing to certify their finish, perhaps you can perform a simple, non-destructive corrosion test yourself. The simplest way is to cover a portion of your part with a good clear tape (packing tape works fine), immerse part in ethyl alcohol, then either boil (at least 30 mins.) or autoclave it. Then allow to air dry for 24 hours, peel off the tape and compare the surfaces. If any corrosion has appeared on the unmasked part, it should easily wipe away with a few good rubs on a soft cloth.

Hope this helps, good luck!

Jeff Swayze
- Kelowna, B.C., Canada
2001

A. This yellow on 17-4 is very common to material that has been heat treated. Even heat treating in a vacuum furnace will yield a yellowish to blue-ish color. Strictly speaking this must be removed before passivation, as passivation is "a final treatment" and not intended to remove heat scale, oxide, weld scale, dirt, oil etc. ASTM A380 I believe addresses treatments prior to passivation. We don't like them nor do we use them but we get the scale off with other means prior to passivation.

Jon Quirt
- Fridley, Minnesota
2001

A. 303 stainless that has been passivated in citric acid will turn yellow due to the sulfur ⇦ this on eBay or Amazon [affil links] being drawn to the surface. You can stop this be pretreating in a 5% solution of sodium hydroxide to remove the sulfur and then passivate in citric acid or nitric.

Michael Harry
- Elkhorn, Wisconsin, USA
2001

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