noun
Refers to the reddish-brown coating that forms on iron or steel due to oxidation. Can also be used metaphorically for something that has deteriorated or become outdated.
自転車のチェーンに錆が出てきた。
Rust has started to appear on the bicycle chain.
長い間使っていなかった包丁
The kitchen knife that hadn't been used for a long time was rusty.