Any metallurgists here?

[dragonfly]

My understanding is that steel in contact with aluminum causes some kind of deterioration to the aluminum due to metallurgic differences. What is the problem in simple terms, and if I needed to construct something using both metals, what would I need to do to avoid corrosion? Is there some type of thin barrier material I can use?

[Bodo]

It's called Galvanic Action or electrolysis . Basically, when you join two pieces of dissimilar metals you create a small battery. Electrons do what electrons will do and you get corrosion.

How are you joining them? I assuming you're bolting or riveting them together. If it is not going to be outside I wouldn't worry about it. If you are going to expose it to the weather, I'd maybe paint each surface first. But I wouldn't worry too much about it either. As long as it's not in saltwater you should be ok.

[dragonfly]

Thanks Bodo. So just the use of paint will prevent corrosion? Yes, it will be an outside application, and specifically will be steel plate as a support for an aluminum martin house. I was thinking about using a thin layer of rubber between the two surfaces, but if painting will do, I'd prefer that.

[drobbins]

between any 2 dissimilar metals there will be some galvanic corrosion when they are electrically connected. The most common example of this is stuff in salt water.
read about it here

http://en.wikipedia.org/wiki/Galvanic_corrosion

paint the steel and try to have the aluminum anodized
failing that, paint them both
what are you building?

Dave

[dragonfly]

what are you building?

Dave

A purple martin house. Thanks, Dave, for the information.

[Hillside]

Several of my greenhouses have both steel and aluminum components. The aluminum roof clamps are held in place with stainless steel bolts. The buildings are over 40 years old and there is no appreciable corrosion. Even in places where the stainless bolts have been replaced with regular steel bolts (sometimes you just drop one, ya know), there is only normal rusting of the steel bolts and no visible corrosion of the aluminum.

[dragonfly]

Hillside, I've used stainless bolts on aluminum before, but I was just worried that plate steel sitting against a similar flat aluminum surface would cause problems. Thanks for the input.

[drobbins]

what happens to aluminum in the open air is it quickly oxidizes (a form of corrosion) and forms a layer of aluminum oxide a couple thousandths of an inch thick
this layer is impervious to oxygen and stops further oxidation, it is also not very electrically conductive and discourages galvanic corrosion. that's why hillsides greenhouse hold up pretty well and your birdhouse would too
still, a can of spray paint is a good idea
overkill is good:)

Dave

[dragonfly]

overkill is good:)

Dave

Yeah, that's one of the axioms I live by. ;)

[Hobie]

Several of my greenhouses have both steel and aluminum components. The aluminum roof clamps are held in place with stainless steel bolts. The buildings are over 40 years old and there is no appreciable corrosion. Even in places where the stainless bolts have been replaced with regular steel bolts (sometimes you just drop one, ya know), there is only normal rusting of the steel bolts and no visible corrosion of the aluminum.

The reaction is accentuated by the presence of water. We have stainless rivets attaching aluminum castings on our sailboat, and you have to watch because the rivet hole gets bigger over time. Fresh water is not so bad, but salt water causes serious problems. Also, contrary to intuitive thought, your plain steel bolts may rust faster, but they are generally less reactive galvanically than stainless steel (depending on type of SS). Google "galvanic table." The farther apart the two metals, the stronger the reaction.

[iddee]

I think your biggest problem with the "bee martins", aka purple martins, will be your loss of bees, rather than corrosion.

[dragonfly]

I think your biggest problem with the "bee martins", aka purple martins, will be your loss of bees, rather than corrosion.

Well, purple martins hunt really high in the sky, and bees tend to fly pretty low, so there should be a minimal loss of bees. Now the cliff swallows that live under the bridge that borders my property could be another matter, but there's not much I can do about that ;)

[iddee]

I hope you are right.

[dragonfly]

I've been watching them for years now. They hunt about 500-1000 feet up. Really, the cliff swallows are a possible threat. They hunt over the field at times. I've never seen martins hunting low.

[Bizzybee]

Yeah they do hunt high and I'm looking for a anti-aircraft gun now to start blasting em out of the sky when I see em!! Ever try getting queens mated and back in a hive with those blasted things on the prowl?? :mad: