Somebody correct me if I'm wrong but to anodize a nut or bolt it has to be aluminum. Which is why you see primarily screws being anodized. They also scratch very easily while being installed. I had some on my bicycle and they scratched (had to use tape to install) and the nuts would back out all the time. Had to carry tools with me. They can also be powder coated but that makes them thicker which cause tool problems because now the socket doesn't fit.