Though, while I am usually not one to comment about the science of a power, I do think this is important as it effects the spirit of the power. Rusting and decaying are two different processes. Decaying is basically just something being broken down to its compenent parts, whereas rust is where metal and water mix into a new material. 

I think it would help if you just had a list of items and say "When see touches biological stuff it rots, When she touches metal it rusts" and if you have any other ideas say them. 

Though, she would need something that she could touch with it breaking apart, or else she would  become a naked zombie, and I think there is afew rules against that. 

Just my two cents. I figured that as a co-gm I should say my piece, though this still Cat's RP so what he says goes.