Nuts, bolts, brackets, steel girders these a few of my favorite things.
To emphasize the mechanical/metal/modernness of something. Put a few more bolts on frankenstein and I'd call him a robot.
If you can't sell the metal by itself, which can be tricky, just put in things that will help people identify it. Though if you can sell it as metal I'd still add a few extra bits. Metal is usually higher in contrast than a matte object like tree bark or paper. It reflects more like than it diffuses since it's a smooth material. Usually.