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I understand galvanic corrosion and the chemistry which causes the reaction. But under what circumstances can they be in contact?

On first impression I would think they could in a dry environment... but that's a tough definition to follow. I'm thinking of two specific applications:
1) I want to reflash the bottom of some of my siding, but this will be in contact with steel strong-ties on the wall and steel nails into the sheathing.

2) I also want to patch a metal duct with some of the leftover aluminum, which will then be sealed with foil-tape.

Both of these are "dry" applications, but moisture in the air could affect both, no?

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When there's mutual consent and no children present. :) –  BMitch Aug 1 '12 at 16:11
@BMitch these liberal metals laws are corroding our periodic-family values.... –  Matthew Aug 1 '12 at 16:39

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up vote 1 down vote accepted

Unfortunately, you're correct: the moisture in the air can and will affect both.

In reality, both of the scenarious you describe happen pretty often. Sometimes it's terrible, sometimes not.

The ultimate choice for you is a balancing act based on practicality: How easy/cheap to install vs. how meaningful/terrible are the consequences (and after how much time).

When it comes to flashing, I'd do it right, first time, no exceptions. The consequences are just too dire for a poorly done job.

As for using an aluminum patch on a gavanized duct: yes it'll corrode after a while. But no big deal IMHO.

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In terms of the flashing that means taping over the steel ties and removing all the steel nails to drive aluminum ones. –  Matthew Aug 1 '12 at 15:35

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