Passivating 316 SS

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We recently got an order in from one of our regular customers and the paper work called out zinc barrel plate w/ yellow chromate. Which we did in an alkaline zinc and to my amazement I later found out that these parts were 316 SS and should have been passivated instead of plated.

My first question is why did these 316 SS parts zinc plate as well or better than the same parts do in steel? And my real problem is when I stripped the zinc off and go to passivate them they form a brownish green smut on the parts that will not come off in the nitric passivation solution. Any Ideas?

Paul P

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Paul: Here is a reply to your question by Max Stein, who has been doing alkaline zinc plating for many years:

"Zinc oxide: Alkaline zinc plating solutions are famous for co-depositing a matrix of Zinc oxide within the metallic Zinc. Most probably, that is what it is. Right, it must normally be white, but when talking about very small particles... (As plated, coating from alkaline zinc are often yellowish to brownish-green: due to the same phenomena). It is not easily dissolved neither in hydrochloric nor nitric acid. Reprocessing through alkaline clean - hydrochloric acid - scale conditioner (hot caustic with potassium permanganate: often used in passivation pre-cycle on heat treated CRES) will definitely help.

Another option - concentrated chromic/nitric acid mix. It must be interesting to see if you have obtained a reasonable degree of adhesion: bend or file test can be done. If adhesion is very good (which I doubt very much), you may have developed a new method of activating CRES. Cheers, Max"

"PlaterB" Berl Stein
NiCoForm, Inc.

Rochester, NY

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Sometimes SST will plate just fine without a nickel strike. I never could figure out why. As for the smut after stripping zinc, try soaking for 3 minutes in a spent (or new) cyanide based nickel stripper (room temp).

Ken Rosenblum
finishing shop - Minneapolis, Minnesota

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Paul, you are really putting your plow into new ground, I have never heard of anything like this before, and I am fascinated. I would like to take a look at the problem, and do some research work to try to determine what is going on, and how to resolve the issue. If you send me parts, I will do the work, and report to you on what results we got. And of course, since this likely must be done yesterday, we will move on it.

Rudy Sedlak
- Mountainview, California