Curated with aloha by
Ted Mooney, P.E. RET
The authoritative public forum
for Metal Finishing 1989-2025
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Galvanic Problems - Zinc substituted for Cadmium
Quickstart (no readers left behind):
All metals except the precious metals (gold, platinum, etc.) react with oxygen in the air over the years and form rust, tarnish, and corrosion. It's basic chemistry -- oxygen is hungry for electrons to fill its electron shells and metals have those extra electrons, so they combine into a metal oxide.
But different metals have a different strength of grip on those electrons and what happens with galvanic corrosion is that if two different metals are touching, when oxygen tries to grab those electrons from the metal with a stronger grip, that metal will in turn steal electrons from the metal with the weaker grip so that metal will rust, tarnish, or corrode instead.
Sometimes this is used to advantage. Steel ships use zinc anodes, most steel bolts & nuts are zinc plated, and electrical towers are galvanized (zinc coated) to protect the steel by sacrificing the zinc.
Sometimes it's a problem rather than an advantage. Aluminum has only a weak grip on its electrons, for example, so there are few materials that can be used for fasteners on airplanes because the fastener can grab electrons from the aluminum and cause it to corrode.
Q. I am looking for information on galvanic corrosion.Can you help me, please?
Thank you
1996
A. Hello, Carl.
Different metals have different EMFs, i.e., tendency to dissolve into ionic form. Those with a strong tendency are called active metals or base metals; those with a weaker tendency are called noble metals.
If two different metals are in an environment capable of conveying ions (basically if they are wet) they comprise a battery. If the two metals are touching, the circuit is complete, and the battery can discharge - that is, the more active metal can dissolve into ionic form. Carbon-zinc batteries, nickel-cadmium batteries, etc. are examples where the electricity generated by galvanic corrosion is harnessed as useful power.
The way to minimize galvanic corrosion is to pick two metals that are close to each other in activity (close to each other on a chart of galvanic activity), or to break the metallic circuit (don't let different metals touch), or to break the ionic circuit. Breaking the ionic circuit usually involves keeping the parts dry and salt-free.
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
Q. 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
2002
A. 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
A. Standard disclaimer: you can 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
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!
- Erie, Pennsylvania
Ed. note: Touché
A. 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
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