How can leaking electrical current possibly damage steel reinforcements in a building?

Corrosion of iron is an electrical phenomenon. Very slight electrical currents are produced by contact of iron with salts in water. These currents cause the combination of iron with oxygen to make iron oxide, which is rust. Eventually the iron is eaten away.

Current leaking through steel from an outside source can actually accelerate the corrosion process. However, there has to be water present also.

The leaking current is DC voltage and in the right direction you get galvanic corrosion really fast.

You put two different types of metal in conductive solution you get a battery and will eat up one plate while building up on the other.

Well it is all electrolysis ,and can eat up most metals,and especially like galvanize and copper. If u need to connect u must put cadmium in between . Even a hole that is corroded into a pipe ,it is all electrolysis.

Extra electrons could accelerate a redox reaction. Redox is what causes metal to rust. The transfer of electons would make reduction occur in some places and oxidation in others. I can't rememberthe specifics of what happens to steel ( iron with a bit of carbon mixed in). but you should look up redox reactions.