rust away
1. To become increasingly covered in rust. Why don't we finally sell that old car? It's just been rusting away in the street for the last two years.
2. To dissolve or disintegrate due to corrosion. The access panel completely has rusted away, exposing the sensitive electronics inside to the elements.
rust away
to dissolve away into rust. In a few years, this car will rust away if you don't take care of it. The bridge is rusting away, little by little.