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Author: rorschach
Time: 04/21/06 07:06 PST
This is a reply to message no. 17500 by elohor
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metallurgy

Spike is right, Aluminum is known as a reactive/passive metal. Meaning it is very reactive with oxygen, but in reacting to oxygen, it forms a thin layer of aluminum oxide that is passive. Because that layer is very tough and adhereant and impermeable to oxygen, the reaction is self limiting, it stops reacting because it is prevented from accessing any additional oxygen. Stainless steels work similarly, they form a thin chromium oxide coating. that is important to remember because anything that actively damages that coating or prevents it's formation will undermine it's corrosion resistance. In the case of stainless steel, anything that ties up the chromium and prevents it's reaction with oxygen (carbon is the usual suspect here) will undermine it's corrosion resistance. the oxidation reaction in Aluminum can be made more protective by anodizing in which the aluminum is immersed in an acidic bath and the reaction leaves a much thicker coat of Al2O3.

Gold is known as a noble metal, in fact it is one of the MOST noble metals in that it reacts with almost nothing (Mercury being one of the few exceptions).  since it does not react with oxygen, it cannot oxidize.

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