Cool pic! The only qualm I have about it is that the top of the bottom (does that make sense?) seems a little soft. There isn't much detail there - I don't know whether it's just detail-less or it really is soft. Love the colors though, and the DOF is right on.
This works well for the challenge, because I can't really tell what level the rust is on. Visually, it's hard to place the depth. The colors compliment each other nicely. Well framed, I really like this.