Plating has a number of applications such as to harden, to improve wear, to improve paint adhesion ect., however corrosion resistance is probably the most common application. Standard steel can be plated in a variety of substances ranging from zinc, to cadmium, to nickel, to chrome. Zinc plating provides excellent resistance against oxidation on steel by forming a barrier around the part and acting as a sacrificial anode if the barrier is damaged.
Cadmium plating is still used in aerospace applications because of its excellent corrosion resistance and it is even compatible with aluminum. Nickel plating offers a much shinier and harder finish, but is generally more expensive than zinc and has a lower corrosion resistance. Chrome plating gives one of the brightest finishes, but a part often has to be polished and buffed before plating because the plating process does not cover up any of the impurities in the metal.