Stainless Steel

The important exception to the rule that metals form protective surface coatings is iron, which rusts in the presence of air and moisture. The orange complex of ferric oxide and water (Fe₂O₃.H₂O) is a loose powder rather than a continuous film, and so does not protect the metal surface from further contact with the air. Unless it’s protected by some other means, iron metal will corrode continuously (this is why pure iron is not found in nature). Efforts to make this cheap and abundant element more resistant to rusting resulted in the 19th century in the development of stainless steel, an iron-carbon alloy that—in cookware—is formulated with about 18% chromium and 8–10% nickel. Chrome is synonymous with bright and permanent shininess because chromium is extremely prone to oxidation and naturally forms a thick protective oxide coat. In the stainless steel mixture, oxygen reacts preferentially with the chromium atoms at the surface, and the iron never gets the opportunity to rust.