Why does steel in concrete erode?

One way to understand why steel corrodes is to look at how it is made. Steel starts out as iron ore. Iron ore is in a stable, natural state. When we process iron ore into steel, we add lots of energy in the form of heat.

The freshly manufactured steel holds onto that added energy. It is in an elevated state of energy. Left to nature, the steel will give off this added energy and return to a natural stable state, which is iron ore.

The thing to remember here is that on a basic level, steel is in an elevated state of energy. Corrosion happens when steel loses that extra energy.