MadSci Network: Chemistry

Re: What is the oxidation state of chromium found in steels/metal alloys?

Date: Fri Oct 29 01:27:42 1999
Posted By: Joseph Weeks, President, Thermal Products, Inc.
Area of science: Chemistry
ID: 941139354.Ch

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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