Oxidized metal is a metal that has an oxide film formed on its surface as a result of oxidation.

Material Corona layered mtl
The color is set procedurally using a gradient. You can change the colors and noise in the gradient parameters to achieve the desired shade.

Version Corona 10
Materials CoronaLegacyMtl
In versions CoronaRender 6 and below, CoronaLegacyMtl=CoronaMtl
CoronaLegacyMtl requires a version of Corona Renderer of at least 1.5