rust
n. C / Un. the reddish-brown substance that forms on iron or steel when it gets wet. It slowly eats away at the metal until it becomes weak and breaks.
n. a reddish-brown coating of iron oxide formed on iron or steel by oxidation, especially in the presence of moisture. Uncountable when referring to the substance; countable when referring to specific fungal diseases in plants.
The old bicycle was covered in rust after being left in the rain.
The mechanic warned that the rust on the car's frame was becoming a serious safety concern.
Centuries of exposure to the salty sea air had allowed rust to compromise the structural integrity of the pier's iron supports, necessitating a complete reconstruction.
Uncountable in its primary sense as a chemical byproduct; often paired with the verb 'to form' or 'to accumulate'.