aherrumbrar

To rust or turn rust-colored — used for metal literally corroding (el clavo se aherrumbró) and, less often, for anything taking on a reddish-brown, rust-like tint. Slightly more descriptive/technical than the everyday oxidarse, which is the more common word for rusting in casual speech.

• Las rejas se aherrumbraron con la humedad del mar. — The iron gates rusted from the sea humidity.
• El agua salió aherrumbrada por la vieja cañería. — The water came out rust-colored from the old pipe.
• Sin pintura, el portón se aherrumbrará en poco tiempo. — Without paint, the gate will rust in no time.

Reaching for aherrumbrar in everyday conversation about a rusty bike or tool — oxidarse is what most speakers actually say; aherrumbrar reads as more literary or technical.