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?