|MadSci Network: Chemistry|
An interesting question. When you pick up a chunk of metal, the metallic elements are in their elemental form. When you have a metal in anything other than its elemental state, it becomes a chemical compound, such as a metal oxide, sulfide, carbide, etc. A good rule of thumb is that if an object is shiny and reflects light, then it is a metal in its elemental state. So when the government regulates chromium VI, implicit in the definition is that the chromium is combined with some anion to form, in this case, a soluble compound. Metallic chromium is present in a variety of everyday objects. Stainless steel, whether it is the fork you are eating lunch with or the clip which holds your pen in its pocket, is an alloy of chromium, nickel, and iron. It is the very thin layer of chromium oxide which forms on the surface of the stainless steel object when exposed to air which gives stainless steel its protection from corrosion and oxidation. Without the chromium, you would be surrounded by a lot of rusty metal. Since room temperature air is only mildly oxidizing to a piece of stainless steel, you would expect only the lower, more stable oxides would be formed. Finally, you can see chromium in its almost pure form every time you look at that shiny faucet in the kitchen or bathroom. Electroplated chromium is frequently used as a decorative coating on brass and other metals. It is usually deposited on top of an electroplated nickel coating. Thicker chrome coatings are used on hydraulic cylinders and on the compression rings around pistons in cars and trucks. The hardness and wear resistance of the chromium accounts for its extensive use. During the plating process, the metal object is placed in a bath which contains chromium VI. When the metal part is made cathodic by the application of an external voltage, some of the chromium reduces to its elemental form and forms the decorative coating. It is these plating processes most affected by the regulations on hexavalent chromium. We wouldn't even use chromium plating processes if they weren't so useful.
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