Science: Anodizing

In today’s world, automotive components are largely moving towards aluminum as a means of saving weight while still being strong. Manufacturers, like Ford, have gone to incredibly great lengths to drop as much weight from their trucks as possible and increase their utility by totally manufacturing their vehicle bodies from the universe’s greatest creation: aluminum.

But, aluminum is not perfect, it still can oxidize, much like steel, when exposed to the elements. For steel and its ferrous derivatives, we know its oxidation as rust. Rust is pesky and troublesome. It’s almost impossible to avoid unless constantly cared for, and it destroys some of our most precious things. The same thing happens to aluminum—its own version of rust.

Aluminum when exposed to water or air, will react and start oxidizing. Aluminum oxide forms on the surface and is a stable passive layer that protects the metal from further oxidation, almost like a skin. The process of anodizing is using an electrolytic process to increase the thickness of that oxidation layer further. By increasing the thickness, it can increase the aluminum components resistance to corrosion and wear, and provides an excellent surface for paint and primer to adhere to. Further, the anodizing process can be dyed to create a colored surface. The anodizing process hardens the surface of the aluminum and is particularly useful in threaded portions of aluminum to prevent thread galling as bolts are screwed in.

In 1923, an early method for anodizing was discovered by the British as a means to protect Duralumin Seaplane parts from salt water corrosion. That same year, the process was patented in Japan and was widely used throughout Europe.

The process of anodization entails passing DC electricity at a range of 1 to 300 V through an electrolytic solution, or acid, and submersing the positively charged aluminum item. The electric charge causes some water particles to split into positively charged hydrogen and negatively charged oxygen, where the oxygen atoms are then attracted to the positively charged aluminum and create the compound, aluminum oxide. As the anodized film builds, the electrolytic solution also dissolves it, creating nanopores 10-150nm (nanometer) in diameter. These pores allow the current to reach the raw aluminum below the oxide layer, to help further the thickness of the protective layer.