Lori ~

What a great question. I got curious and found some information on the internet that might help you with the answer. Here is what I found:
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ALUMIMUM CHARACTERISTICS

Aluminum is a light-weight metal with a bright silvery luster. Small amounts of other metals are added to aluminum to make harder alloys for most uses. Its affinity for oxygen makes it resistant to corrosion and attack by most chemicals. Most aluminum used in visible parts of appliances is lacquered or otherwise coated, anodized or painted.

Aluminum reacts with air to grow its own thin oxide coating very fast. This hard, dark gray coating protects the metal. It's found on all bare aluminum surfaces, including utensils which, if rubbed on a counter or range top, or other material, makes a dark gray mark. If washed off the outside of the pan, it quickly forms again. A commercial process, called "anodizing", thickens this coat and often colors it. Anodizing does not rub off. A special anodizing process produces a very hard, dark gray finish on professional type cookware.
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CLEANING ALUMIMUM

Brighten aluminum utensils by cooking acid foods such as tomatoes, apples, or rhubarb or by boiling 1 to 2 teaspoons cream of tartar per quart of water OR 2 tablespoons vinegar per quart of water for 10 minutes in the pan.

Prevent discoloration in the bottoms of double boilers or egg poachers by adding 1 teaspoon vinegar or 1/4 teaspoon cream of tartar to the water in the bottom pan.

Remove stains from outside of aluminum pans with silver polish, or mild, nonabrasive cleaner. Soap-filled steel wool pads scratch the outside surface, so use only when removing burned-on food or grease is more important to you than the scratched pan.

Remove hard water mineral deposits (lime scale) from tea kettles where they have become crusted, boiling equal parts of vinegar and water for several minutes and letting stand an hour or so. The process may have to be repeated in severe cases. Rinse with plain water before using tea kettle.