Gareth, just putting my 2 cents in here, and that may be all that it's worth, but although I agree with the posts above regarding electrolytic action I also think that the amount of corrosion which is likely IF the assembly is not in a MARINE environment is not great. The aluminum becomes the "sacrificial" anode in the reaction and it is that which will corrode (oxidize). The unfortunate upshot of this is some loss of electrical conductivity over time, not so much structural weakening or nasty looking corrosion. Passivating the aluminum should help, of course. Cadmium plating is another method, but that's expensive because of cadmium's high toxicity. Your sketch (with o-rings) appears as if you're trying to seal something, but I'm not sure why you're using aluminum in the first place. If you need both thermal and electrical conductivity, why not use copper, or a copper alloy?