Yep; as stated above, that residue is a product of a chemical reaction between the dish-washing detergent and aluminum cookware.

In the chemical reaction between soap and aluminum ATOMIC hydrogen is formed. Atomic hydrogen has a very weird property of actually being able to move completely through solid aluminum......because it is such a small particle. Atomic hydrogen cannot be contained in an aluminum vessel.....it moves right through it as though it weren't there. It remains in the atomic state only until it meets another atomic hydrogen atom which then produces the hydrogen molecule H2. Molecular H2, which is a gas, cannot pass through solid aluminum. This process of hydrogen going from the atomic form to the molecular form causes the metal to actually flake (sometimes even crack) helping produce the powdery residue that you saw. The hot water in the dishwasher greatly accelerated this reaction.

Soak is OK to use on properly seasoned cast iron, but must be used with CAUTION on cast, or coated aluminum cookware. Soaking SCRATCHED Teflon coated aluminum cookware in soapy water will cause the Teflon to rapidly separate from the aluminum........but this another whole new thread onto itself too.

What I have just revealed is based on solid chemistry and science. I have dealt with this type corrosion mechanism on major industrial processing units in my former career.

Bottom line......don't wash aluminum cookware in a dishwasher. Don't soak your scratched coated aluminum cookware for too long either......leaving it soaking in soapy water will accelerate the deterioration of the coating. Don't soak cast aluminum either as it will cause pitting.

Dan