There are two type of lines in this image. On the bottom you see the lines that are created by the shape of the rotor. In addition there is a secondary, very soft round line on the right hand side. I simply dropped a water on the surface of the rotor and the line separates the wet and the dry part of the surface rust. Many times I sprayed water on fruit or flowers when I photographed them so I decided to try it out with surface rust.
Have great ideas when you photograph your subject and don’t forget to have fun!