rust, corrode
鏽 centers on metal breaking down through oxidation: rusting or corroding. The meanings are tightly linked to the chemical decay of metal surfaces.
鏽 combines 金 (metal) with 肅, which likely contributes the sound and may relate to the idea of deterioration or solemn decay. The exact historical development is uncertain.
Metal (金) on the left, and 肅 on the right looks like a solemn, decaying structure. Picture a once-proud metal gate now rusted and corroded, its surface flaking away.
For シュウ, imagine a rusty metal shoe (シュウ) left out in the rain: the shoe rusts and corrodes, and シュウ sounds just like 'shoe'.