[pilsetnieks]

Rot is decomposition of organic matter; rust is a chemical reaction, oxidation, in fact.

[hobofan]

That may have been a pun on rust the fungus: https://en.wikipedia.org/wiki/Rust_(fungus)

[TeMPOraL]

From practical point of view, rusting is just inorganic version of rotting via oxidizing agents. Both happen naturally over time, given appropriate environment, and both result in interesting/useful stuff turning into less interesting/useful stuff.