This material is an example of rusted metal (or alloy) with principled shader.
The “rust generator” node group generates a map and normals for the rust,the explanation of the parameters of this node can be found at link https://blenderartists.org/t/rust-generator/684547
To create rust color i’ve used the “multiple color ramp” node,that allows you to decide colors and position points ,using input parameters,increasing flexibility.
Up to 5 colors can be chosen. The node can be obviously modified to accept more colors
The image,an ancient bronze elm:
The multiple ramp node:
The blend file:
elm.blend (1.9 MB)