Corner bead is manufactured out of two different materials. Both are widely available. The older, more traditional corner bead is made of light-gauge, galvanized steel. Why galvanized? As you might expect, it's to inhibit rust. It stands a good chance of rusting in the long run, and the drywall compound (or mud) is water based. Without a galvanized coating, it would start rusting right out of the starting gate. I might add here that some steel corner bead has paper on both sides, so installation is a matter of just bedding it on a mudded corner.