Rust; the reddish (or sometimes greenish) corrosion that forms on metal due to oxidation.
庭のフェンスに錆が出てきたので、塗り直すことにした。
Rust appeared on the garden fence, so I decided to repaint it.
自転車のチェーンが錆で固まって回らなかった。
The bicycle chain was seized up with rust and wouldn't turn.
海沿いは湿気が多く、金属製品に錆がつきやすい。
Coastal areas are humid, and metal items tend to rust easily.