Looking up explanations of galvanic corrosion on the 'interwebz' seems to support the 'inert layer' theory offered by jimmer, but in an unusal way... In that copper is relatively inert, but not entirely. Let me try and explain my theory...
It seems that galvanic corrosion would need an electrolyte present to encourage the migration of ions. The electrolyte we would encounter, as cyclists, would probably be rain water, or water used for cleaning, mixed with road salt or other salts (pollutants?). Certainly the presence of the grease in copper grease would initially inhibit the penetration of the electrolyte into the threads. After the grease has washed away, though, the naked metal to metal contact of copper to aluminium or copper to steel WOULD produce an electrochemical reaction. But this reaction would serve to BREAK DOWN any potential bond between the metals. This is the wikipedia entry I am basing all this 'stuff' on:
"Copper should not be in only mechanical contact with metals of different electropotential (for example, a copper pipe joined to an iron pipe), especially in the presence of moisture, as the completion of an electrical circuit (as through the common earth ground) will cause the juncture to act as an electrochemical cell (as is a single cell of a battery). The weak electrical currents themselves are harmless but the electrochemical reaction will cause the conversion of the iron to other compounds, eventually destroying the functionality of the union."
This is why, in plumbing, steel and copper pipes are not joined directly as they would react with each other and fall apart. This is also why the copper outer layer of the Statue of Liberty threatened to fall away from its iron inner structure until an intervening layer of teflon was introduced. Okay, these examples involve two metals that want to move away from each other, as opposed to being held together by force (as in the continually tightening thread that you get with pedals) but hear me out. So there IS a corrosive reaction, once the grease has gone, but its only ever between either aluminum and copper or steel and copper and not between aluminium and steel. The copper is in loose particle form. So, in an interface between aluminium and steel where copper particles sit inbetween the two, the particulate nature of the copper prevents any STABLE bond from taking place. Within the copper particles, that sit inbetween the aluminium and the steel, I imagine three layers where some copper particles are adhering to the steel, some are adhering to the aluminium, and some loose copper particles sit inbetween the two which prevent the steel and aluminium from coming together and bonding. Copper compunds that have formed due to bonds to aluminium and steel will not want to bond with each other, so, even if there are only a few copper particles present, bonding between steel and aluminium is prevented.
There are other advantages of copper in this situation. Copper is highly malleable yet resistant to pressure at a molecular level, so it will 'bed in' the threads nicely as the pedals are tightened on, without altering in any significant way. Relating to jimmer's previous point, copper is also the most inert metal out of copper, aluminium and steel and therefore would not become 'used up' quickly, as with a more active 'sacrificial anode', such as magnesium. The inertness of copper means that, although electrochemical reactions would take place, these reactions would happen only slowly and the main main body of pure copper particles would be around for a long time (which is what you want). I am not sure how long copper particles resist being 'washed off' by the physical ingress of water, though. Maybe someone has an idea of how long copper particles stick around in sufficent quantities? Certainly, if enough copper particles are washed away then aluminium and steel will touch and the pedal and crank will bond together... If my ramblings prove in any way to have a relation to fact, it is preferable to lavish the copper in the threads before application, rather than use 'sparing' quantities. And, as has been mentioned before, reapplication of any corrosion inhibitor is ALWAYS going to be needed.
As far as other 'metal greases' are concerned, I doubt aluminium grease would work as well as aluminium is one of the materials you are trying to keep from corroding together. This, to me, would neither significantly speed up or slow down the bonding of the steel part to the aluminium part... A more active metal, such as magnesium, might be a good idea, as it would behave as a 'sacrficial anode' and corrode first, although I dont know how long it would be before the magnesium would be used up and you would be back to steel on aluminium with nothing of note inbetween... A more inert metal, such as silver or gold (imagine GOLD grease - bling! bling!) might be good because it would be more likely to just sit there with hardly any galvanic reactions taking place. But is some 'sacrificial' reaction preferred between the corrosion inhibitor and that which it lies between, to prevent the bonding of the main structures? It seems to me that copper would in part react with the other metals and yet remain partly inert, which is perhaps a nice combination of sacrifice and inertness... Oh I don't know, I've just read all this on the internet. No doubt someone will rubbish my theories shortly. But gold grease does sound so utterly cool...
