锈
xiù · HSK 7
to corrode
🔩 锈 = 钅 metal radical + 秀 (xiù, gives the sound) — what eats old metal: rust
The 钅 metal radical tells you it is about metal; the 秀 part is phonetic, carrying the xiù sound. Rust and corrosion are what attack metal over time.
锈 — metal radical 釒 + 秀 (xiù, elegant/phonetic)
A semantic-phonetic compound. The metal radical 釒 (金) signals the meaning—something happening to metal—while 秀 (xiù) supplies the sound. It originally referred to the reddish-brown coating that forms on iron and other metals, i.e. rust, and by extension the verb 'to rust.'