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I keep seeing mention of aluminum oxide being soft. what is the most popular abrasive used to make sandpaper? Aluminum Oxide, because it is extremely hard! Hard anodized aluminum is very resistant to abrasion. However if the load is high enough to move the aluminum underneath then it goes away in a big hurry. Aluminum is one of the best electrical conductors. Guess what high voltage power line insulators are made from? Aluminum Oxide, because it has among the highest electrical resistance of all materials, and is very strong. So the reason that aluminum oxide protects aluminum so well is that it is a very hard insulator.
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