What coating/plating on steel to prevent galvanic interaction with aluminum?

A. 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.

A. 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.

A. 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.

 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!

! 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.