How anodizing builds color into aluminum
Anodizing uses electricity to grow a thicker layer of aluminum oxide from the metal itself. The aluminum part is connected as the anode—the positively charged electrode—in an acidic liquid bath. Current drives a reaction at its surface, converting aluminum into oxide rather than laying a separate coat of paint on top.
The resulting oxide is hard, but it is not initially a smooth, solid skin. Under common anodizing conditions, it forms a tightly packed field of microscopic pores. These tiny channels are the key to coloring the surface: dye can enter them and lodge below the outermost face of the metal.
Sealing closes the pores
After coloring, the part is sealed, often with hot water. The oxide reacts with water and swells slightly, narrowing or closing the pores. That helps hold the dye in place and makes the surface less open to contaminants. The color is therefore within the oxide layer, not merely a film resting on the aluminum.
Anodizing can also be done without dye, leaving a clear or metallic-looking finish. In either case, the oxide layer helps protect the underlying metal from corrosion and wear. Its useful combination of color and protection comes from the same sequence: electricity creates a porous oxide, the pores take up dye, and sealing locks the finished surface in place.
