rust

noun

• The deteriorated state of iron or steel as a result of moisture and oxidation.

  "The rust on my bicycle chain made cycling to work very dangerous."

• A similar substance based on another metal (usually with qualification, such as "copper rust").

  "aerugo. Green or blue-green copper rust; verdigris. (American Heritage Dictionary, 1973)"

• A reddish-brown color.
• A disease of plants caused by a reddish-brown fungus.
• Damage caused to stamps and album pages by a fungal infection.