Aluminum, like other metals, is available in several different alloys, whose qualities are indicated by numbers. As the numbers change, the alloys' strength, corrosion resistance, and other qualities change. Finding the alloy you want is really a matter of mixing and matching, looking for the number that offers the best for all of what you need. Aluminum alloys are widely available, and if you don't find an alloy that meets your needs, you can discuss custom alloys with many metal manufacturers.

Malleability and Strength 

Alloys will have different amounts of strength and malleability. Generally, the more malleable the aluminum, the lower the strength, and the lower the alloy number. Malleability doesn't completely disappear from stronger alloys, but it does become harder to form the aluminum into the pieces you want. If you need very malleable aluminum, you'll want a lower alloy number. Stronger aluminum alloys have higher numbers.

When You Need Corrosion Resistance

Aluminum doesn't rust in the same sense that iron and steel do. The reddish, and sometimes greenish, rust that you see on iron is not one you'd see on aluminum unless there were bits of iron residue on the aluminum's surface. This can happen if the tools used to form the aluminum were made of iron.

What does happen to aluminum is that it corrodes, with a grayish to a whitish layer of aluminum oxide forming all along the surface. You can remove the corrosion, but it will just occur again. This is because some alloys are less corrosion resistant. Remove the corrosion from an alloy that doesn't have much resistance, and you'll get the same result eventually because the alloy will just keep corroding, especially if exposed to salty environments. As you can guess, a higher alloy number indicates a better resistance to corrosion.

Anodizing and Coloring

You can fend off corrosion in one way when you have an alloy that isn't very resistant. This is done through anodizing, a special coating process that actually blends a protective layer with the aluminum. (In other words, it's not an exterior coating.) Anodizing also makes it easy to color aluminum. Some alloys lend themselves better to anodizing than others. It's a little harder to base the ability to anodize on the alloy numbers, though; what you'll have to do is mention to the company you're buying the aluminum from that you plan to anodize the metal. Generally, a higher alloy number indicates that anodizing might be easier, but you'll have to ensure that the specific alloy you choose will actually do well when anodized.