When I wet an aluminum pot or cookie sheet and then dry it with a towel, it creates dark gray marks on the towel. This happens more if the aluminum is a little rough. It seems that there's a fine black powder coming off the aluminum. I would guess that's aluminum oxide, but I was under the impression that aluminum oxide forms a hard coating that protects it from corrosion. So is that stuff really aluminum oxide? Why does it come off that way? Does it mean the aluminum isn't protected from corrosion?

  • $\begingroup$ If you try rub it with a lemon slice instead will it still leave a dark stain on your rag? If not then you may have unexpectedly alkaline water and the stain may be an aluminium salt. I would decline to eat such deposit in large doses or for extended periods as it may lead to some ailments (some research points to it quite strongly). Baking cookies would not concern me but boiling soup would. $\endgroup$ – KalleMP Mar 7 '16 at 13:29

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