Those of you who have been detecting for some years know that not all aluminum you dig up is the same when it comes to corrosion. Aluminum ranges from extremely corrosion resistant like aluminum cans to extremely corrosion prone like cheap aluminum castings. The difference is in the alloy. While aluminum cans and structural aluminum require tightly controlled alloy, cast aluminum can contain lots of impurities depending on the use. Copper and magnesium are the two main additions for high quality structural aluminum these metals don't cause much of a corrosion problem but in castings, zinc, iron and minor amounts of tin and lead may be mixed in. These elements cause the aluminum to corrode rapidly in the ground.
As a young man I worked in an aluminum foundry and everything went into the melting pot. zinc auto parts, cans, aluminum pots and pans, vacuum cleaners, and occasionally a chunk of lead would sneak into the mix. Needless to say the products from this foundry varied widely in corrosion resistance. Some would start corroding before they could be painted. It didn't matter much because most were household ornamental items that didn't see a corrosive environment until they reached the dump.
The reason I mention this is occasionally I have come across large areas of ground that give falseing. I dig and dig and come up with coin size scraps of highly corroded aluminum or "pot metal" as it often is called. One field where I detect regularly is like this. The plowing has spread the bits of aluminum over an area about an acre. I keep bringing up these irregular shaped pieces of aluminum that detect as a dime or quarter or nickel. I finally found out what made the mess. I came up with a 4 inch piece of highly corroded aluminum with a stainless steel tag with the name MAYTAG. the model and serial number. Best bet is this was a washing machine tub made from cast aluminum. gambol
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