Self-piercing rivets (SPR) are commonly used on aluminum body panels because they duplicate the performance of spot welds without heating the aluminum. They are specially-coated steel rivets that pierce the outer body panel and mushroom into the inner body panel without piercing it. A special coating, usually tin/zinc, prevents galvanic corrosion. SPR joints are achieved by pressing the self-piercing rivet, using a specialty SPR gun and die, through the metals to be joined. The two pieces of metal are joined by piercing the first and embedding into the second to form something that looks like a mushroom (see below).