"The oxide on aluminum is naturally corrosion resistant, an insulator and very tenacious."
Mario S. Pennisi, consultant
A rusty automobile is a shame. A rusty piece of aluminum, however, is not only desirable, it is anodized. When exposed to oxygen, pure aluminum metal builds up a layer of aluminum oxide. The aluminum oxide has a significantly greater resistance to corrosion and abrasion and consequently serves as a sturdy shell to protect the rest of the aluminum.
Anodizing is a process right out of a mad scientist movie. In 1927 Charles Gbw.er and Stafford O'Brien patented a sulfuric acid anodizing process that is now the most common way to anodize aluminum. The aluminum is first immersed in electrified sulfuric acid. Electric charges cause oxygen to build up on the surface of the aluminum, creating a thick coat of aluminum oxide. Next, the aluminum can be easily colored and used in countless applications. The metal's new coat is porous and thus can have additives like coloring dyes or lubricity aids easily infused into it. Finally, the aluminum is sealed. Sealing closes up holes at the surface of the coat and helps to reduce any color loss or scratching.
Anodized aluminum's protective oxide finish is one of the hardest naturally occurring substances. Applications include MP3 players, appliances, satellites, computer hardware, and buildings like the Sears Tower.
