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<p>[QUOTE=&quot;Richard M. Renneboog, post: 2852235, member: 89693&quot;]The formation of that &#039;protective&#039; layer is called &#039;passivation&#039;, and it is different for different metals. Aluminum forms a layer of aluminum oxide that effectively seals the metal from contact with air. Otherwise, aluminum would burn as well and as bright;y as magnesium. Copper forms a green-blue oxide coating called verdigris, which is the stuff that makes copper clad roofs turn green. But the extent to which such a layer is able to protect the metal underneath from contact with atmospheric gases and liquids depends very much on the crystal lattice structure of the oxide layer. Iron and steel readily form an oxide layer (rust) but it offers no protection whatsoever to the metal underneath because the crystal lattice of the material is wide open and easily allows oxygen and water to go through it. Copper and aluminum oxide passivation layers, on the other hand, have very compact crystal lattice structures that do not allow oxygen, water, etc. to pass through. A little coinage metal science for you...[/QUOTE]</p><p><br /></p>

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