Chromium is the chemical element with the symbol Cr that gives stainless steel much of its corrosion resistance.
Chromium is a hard, lustrous metal with atomic number 24. In stainless steel, chromium reacts with oxygen at the surface to form an extremely thin, adherent layer of chromium(III) oxide. This passive film helps shield the underlying iron from further oxidation and rusting, and it can reform when damaged if oxygen is available.
Steel is generally classified as stainless when it contains at least about 10.5% chromium by mass, although commercial grades often contain more. Nickel, molybdenum, carbon, and other elements may also be added to adjust strength, toughness, or resistance to particular environments.
Chromium compounds can have different colors and chemical behavior. The element’s name comes from the Greek word chroma, meaning color, because many chromium compounds are vividly colored. Chromium metal should not be confused with hexavalent chromium compounds, which present significant health hazards.