btw.. as far as iron oxide goes (common rust), it is not actually stuck tight to the base metal. The rust is just laying there on the surface and is all cracked and loose. This is why iron rust spreads. It does NOT coat the metal and prevent water and oxygen from getting through the rust to corrode fresh, clean metal underneath.
Aluminum oxide does stick tight to the metal and prevents air/water from contacting clean base metal.. So a thin coating of aluminum oxide (aluminum "rust") is a hard, smooth protective surface. Further oxidation is discouraged.
What i'm getting at is the bike chain was able to be cleaned because vibrations disloged the iron rust.. but ultrasonics have a tough time cleaning aluminum.