I have often wondered about that. Wouldn't the zinc actually work like a sacrificial annode?
Absolutely if in direct contact. Galvanic corrosion will occur to a greater of lesser extent were there are different metals with a different anodic index and an electrolyte is present. When the localised zinc on the steel was fully sacrificed I suspect that the ali would then start to rot. Not a big problem as it can be mitigated through good design. Are you listening Morgan!
It occurs all over the place in our Morgans. The pic below shows my engine block rotting away owing to the galvanic action between the Ali block and Cu/brass radiator (the rot is very slow though).
