Left metal radical — five-stroke side-form of 金 gold/metal. As the indexing radical it carries the literal meaning: rust is metal eaten by oxidation. Same radical places 锈 with 铁 iron, 铜 copper, 银 silver, 钢 steel — rust is what happens to all of them eventually.
Right 秀 supplies the full sound — xiù read identically in the parent. There is also a wry counter-image: 秀 means flowering or elegant, while 锈 is its opposite — the slow decay overtaking shining metal. Same component appears in 诱 to entice.
