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Galvanic Problems - Zinc substituted for Cadmium

2002

I have an assembly, primarily carbon steel/stainless steel, that has 2 parts (HR steel) currently cadmium plated (.0002"). My plan is to substitute the Cadmium plate with a Zinc plate (ASTM B633 SC-2, Type 2 yellow chromate). However, the issue was raised about Galvanic reaction concerns.

Everything else being the same (no galvanic cell problems to date), would switching to zinc possibly introduce previously unseen galvanic corrosion problems?

Zinc is more anodic than Cadmium but I want to be sure that I won't actually make things worse by introducing this change. I'm just looking for some expert opinion on the matter.

Thanks for the help!

Brian Bukoski
- Erie, Pennsylvania

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Hello Brian!

If you take a look at MIL-STD-889 (available from the DoD at astimage.daps.dla.mil/quicksearch/) you'll notice that there is no difference between rows B (zinc) and C (cadmium). This means that at least as far as military aerospace is concerned, there's no real difference between the two coatings from a galvanic standpoint. If you didn't have a problem before, you won't have a problem now. If you had to take some mitigating measures before, you got to take them now.

Trouble is so much depends on the specific application that these charts can only guide you. For instance, if you have a lot of zinc coating touching a little area of steel, then there won't be much in the way of galvanic corrosion. If you have a lot of steel touching only a bit of zinc, in the presence of seawater, then you'll quickly corrode the zinc.

Good luck!

Lee Gearhart
metallurgist - E. Aurora, New York
2002

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First the standard disclaimer: you may get food for thought here, but you don't get expert opinion free of charge over the Internet from a stranger who has never even seen the details of the situation first hand. You won't introduce any galvanic problems elsewhere, just that the plating may corrode faster since the zinc is more active. You may wish to use a heavier zinc plating, or an alloy plating like zinc-nickel from which you can obtain an activity level similar to cadmium.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2002

Thanks for the help everyone. I appreciate the responses. Mr. Mooney: You should edit the standard disclaimer. From my experience, opinions are always free, it's the facts that can cost you. Thanks for everything!

Brian Bukoski
- Erie, Pennsylvania
 

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Galvanically similar, however, the corrosion products are quite different. Cad product is far less voluminous and used to be favored over zinc in electronics for this reason.

Milt Stevenson, Jr.
Syracuse, New York