Rust is the result of an electrolytic reaction. That means there are two different metals, and water, and of course, oxygen from the air.
But you say, there is only one piece of steel here. This is the problem. Steel is iron, and carbon, alloyed together. The rust (oxidation) actually starts on a molecular level, between an iron atom, and a carbon atom. there is enough of a difference in the metals that one atom starts the rust on the next atom, with the carbon atom. Just like sticking a piece of zinc, and another metal into a lemon, and making a battery. An electric current will flow, because of the electrolytic reaction.
Different types of metal have different voltages, if used in the same acid type solution. Once steel starts to rust, the rust is also electrolytically active, and even more so that non rusted steel. There is a higher electrolytic voltage between rusty steel, and plain steel, than there is between the iron molecule and the carbon molecule, used to make steel from iron. So, once rust starts, the rusting is accelerated by the presence of rust.
If you want to read some geeky stuff about iron, and rust, read this.