From FinnegansWiki
Rust: Rust is the substance formed when iron compounds corrode in the presence of oxygen and water. It is a mixture of iron oxides and hydroxides. Rusting is a common term for corrosion, and usually corrosion of steel.
Other forms of corrosion appear in this passage: "mouldaw" (mould, mildew), "gangres" (gangrene). These corrosions and "stains" relate directly to "sin" (another corrosion).
