rust

Any of various powdery or scaly reddish-brown or reddish-yellow hydrated ferric oxides and hydroxides formed on iron and iron-containing materials by low-temperature oxidation in the presence of water.

Any of various metallic coatings, especially oxides, formed by corrosion.

A stain or coating resembling iron rust.

Deterioration, as of ability, resulting from inactivity or neglect.

Rust fungus.