View single post by Zeb
 Posted: Sat Jul 1st, 2006 01:32 pm
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Zeb



Joined: Sun Jun 11th, 2006
Location: Madison Heights (Detroit Metro), Michigan USA
Posts: 237
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 In metallurgy, stainless steel (inox) is defined as a ferrous alloy with a minimum of 10.5% chromium content. The name comes, obviously, from the fact that stainless steel does not rust or corrode easily, as regular steel does.  

The mechanism that makes it corrosion resistant is called passivation, which is the spontaneous formation of a hard, non-reactive surface film. This layer is usually an oxide or nitride that is a few atoms thick.  

In the case of stainless steel, the chromium atoms in the alloy form chromium(III) oxide (Cr2O3) when exposed to oxygen.  The layer of chromium oxide is impervious to water and air, and is too thin to be invisible, so the steel stays shiny.

Other materials exibit passivation as well.  We're all familiar with the blue-green patina of copper that's been exposed to the elements.  The patina is the very thick and not-so-invisible passivation layer formed by copper in reaction with water and oxygen. 
Aluminum forms a powdery white coating of aluminum oxide when exposed to air and moisture, and this coating is very hard, harder than the aluminum itself.  Think about it: Aluminum oxide occurs as corundum, which is used as an abrasive, and in a crystaline form as the gemstone sapphire.  Synthetic sapphire is used to make jewels and crystals for watches.

So, from stainless steel to jewels and crystals.  Pretty cool!

Source: Wikepedia

Dan:)