Given sufficient time, oxygen, and water, any iron mass will eventually convert entirely to rust and disintegrate. Surface rust is flaky and friable, and provides no protection to the underlying iron, unlike the formation of patina on copper surfaces. Rusting is the common term for corrosion of iron and its alloys, such as steel. Many other metals undergo equivalent corrosion, but the resulting oxides are not commonly called rust.
The UV radiation from sunlight breaks bonds in molecules. These changes can result in a previously colored molecule either no longer giving color, or no longer giving the desired color.
Rusty, Crusty and Falling to Bits