Spontaneous Ceramic Coatings on Metals

It turns out that most of the metals commonly used in kitchen utensils naturally cover themselves with a very thin layer of ceramic material. Metallic electrons are mobile, and oxygen is electron-hungry. When metal is exposed to the air, the surface atoms undergo a spontaneous reaction with atmospheric oxygen to form a very stable metal oxide compound. (The discoloration on silver and copper that we call tarnish is a metal-sulfur compound; the sulfur comes mainly from air pollution.) These oxide films are both unreactive and fairly tough. Aluminum oxide, when it occurs in crystals rather than on pans, makes up the abrasive called corundum, and is also the principal material of rubies and sapphires (the gem colors come from chromium and titanium impurities). The problem is that these natural coatings are only a few molecules thick, and are easily scratched through or worn away during cooking.