Letter 36031

Need a coating/plating on a steel part to prevent interaction with aluminum  

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We are a contract manufacturer in Michigan. I am quoting a small (2" by 1") automotive part for a customer. The material will be steel. It will be fastened to a couple of aluminum pieces (grade unknown at this time). They are concerned that there will be an unfavorable interaction between the steel and the aluminum at the joint. Two questions:
1) Is this likely?
2) What plating could I apply to the steel part to prevent this?

Ed Curry
- Spring Lake, MI, USA

Bare steel in contact with aluminum will rapidly rust in the presence of moisture. Either anodize the aluminum, which will slightly delay the rust, or nickel plate the steel, which will prevent rust for a long time.

Jeffrey Holmes, CEF - Spartanburg, SC, USA

Yes. Cadmium is the best commonly available plate that is used. Tin-cadmium is the best performer, but it definitely is not common or easy or cheap.

James Watts
- Navarre, Florida

Yes, there will be interaction between the metals, especially if ANY moisture is present.
Then it becomes a crude battery.
There are very few metals that do not interact with each other.
You can use chrome, paint, powder coat, any number of finishes to prevent or reduce interaction between the two metals.
You have to decide which system suits your application best.

Steve Clark
polishing - Belfast, Maine, U.S.A.

Jeffrey, why do say the steel will rust faster in contact with aluminium? Surely the aluminium, being less noble than the steel, will corrode preferentially to the steel? See discussions #35405.

Of course, there may well still be a corrosion problem for Ed Curry to consider, but will it not be a problem of aluminium corrosion, not steel corrosion?

Bill Reynolds    consultant metallurgist Ballarat, Victoria, Australia

Hi, Bill,

I agree, it would seem that aluminum in contact with steel will corrode preferentially, and that the steel will to some extent be protected from corrosion. As a practical matter, just the opposite happens; corrosion of the steel is accelerated. I have seen this phenomenon many times, both inland and in salt air locations.

I believe the explanation is this: If you look at Standard Electrode potential charts, you will see that the Al/Al+3 potential is -1.66 v, and the Fe/Fe+2 potential is -0.44 v, thus the aluminum should corrode preferentially.

However, in real life the Al surface is covered with a mostly continuous layer of aluminum oxide, an insulator if you will, and thus the effective electrode potential is much lower than the theoretical value, and is probably near to 0.0 v. Against this value, iron is the less noble surface, and the iron corrodes.

I can't offer you any proof of this theory, but if you look around, you may find some Al/Fe things, maybe an Al sign tacked up with bare steel nails, or something similar. I think you'll see that the steel is very, very rusty, and the Al barely damaged.

Jeffrey Holmes, CEF - Spartanburg, SC, USA

I think Jeffrey's observation of real world facts is accurate, but that the explanation is that not all corrosion is instigated by galvanic pressures between dissimilar metals. To put it more simply, steel rusts, and being in contact with aluminum doesn't stop it.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

Thanks to both of you for your explanations. I will indeed look around more carefully, and maybe also nail some stuff to the back fence!

Bill Reynolds    consultant metallurgist Ballarat, Victoria, Australia

An actual test by someone who knows what they are doing like yourself would be super, Bill--especially if that person were indisposed to believe it. I have only anecdotal observations but, like Jeffrey, I have observed numerous instances of rusted steel pins and hinges on aluminum folding chairs, rusted steel bolts on aluminum railing systems, etc.

Ted Mooney, P.E. 
finishing.com
Brick, New Jersey

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