Experimente essa abordagem, como descrevo no meu blog :
1) Fill the pot or pan or other aluminium utensil with water.
2) Add a tablespoon of vinegar or two teaspoons of cream of tartar for every quart of water.
3) Boil this solution for 10 to 15 minutes and scour the vessel with a soapy steel wool pad.
4) Rinse and dry.