Why don't alloys self-galvanically corrode? Total chemistry newbie here (so this may be a fairly obvious question). With that said, I've had a niggling question that I can't quite resolve myself;

Given that galvanic corrosion is driven by the different electronegativities of different elements, why don't alloys made of significantly different elements self-galvanically corrode very quickly?

Essentially, if I were to place two electrically connected lumps in a conducting bath, one of copper and one of tin (with an anodic difference of 0.3V), I would expect them to corrode relatively quickly. Yet if I make these lumps small enough (i.e. make an alloy of them) the resulting bronze mass is fairly resilient. At what size do the individual lumps of copper and tin change from being rapidly corroding to something stable?
They certainly can. Most cases where the alloying compound is more noble this is a hazard. Nickel in Iron is a popular example, you can get corrosion acceleration on the iron by reduction happening on the Ni-surfaces. 

Where the alloying element is less noble you usually have superficial corrosion to begin with but in the end the resulting surface is protected by the main element - which is common for bronze(s). If you just have lumps, then you do not have a bronze and slowly but certainly, nature will reclaim the tin. 

One has to care about the homogeneity of the alloy.