Electrolytic 2-Step Anodizing Process

The purpose of anodizing is to form a layer of aluminum oxide that will protect the aluminum beneath it. The aluminum oxide layer has much higher corrosion and abrasion resistance than aluminum. The typical anodizing employed in the architectural industry is called “two-step electrolytic.” The actual anodizing and coloring of the aluminum occur in separate steps of the process. The anodizing step takes place in a tank that contains a solution of sulfuric acid and water. The tank is charged with electrical current, and aluminum oxide is formed on the surface of the aluminum.

After anodizing is complete, the parts can be immersed in an optional coloring tank, to achieve bronze or black tones instead of the standard clear or silver finish. In the coloring tank, the anodized aluminum is immersed in a bath containing an inorganic metal such as tin, cobalt or nickel, which are deposited in the anodic pores by means of electrolytic current. The amount of time the part is immersed will determine the color achieved. Darker colors are created by extending the immersion time and increasing metal deposition. The colors typically seen on architectural products range from champagne to dark bronze and black.