Sulfuric Acid Anodizing
Some sulfuric acid anodizing processes require post-process sealants such as
nickel acetate of sodium dichromate in order to provide optimum corrosion resistance. Sulfuric acid anodizing can produce coatings with thickness ranging from 1.8µm to 25µm and while it is most commonly performed on
aluminum alloys, it can also be done on materials including
titanium, zinc, magnesium, niobium and tantalum. This anodizing process produces
clear anodization, although color can also be added during the process. Due to the wide range of thicknesses as well as material capabilities, sulfuric acid anodizing is utilized in diverse industries including: military, for protective tank and aircraft bodies as well as housings for varied equipment; industrial manufacturing, for durable housings of broad range of assembly and processing machinery; electronics, for protective casings of equipment such as cameras, satellites and mp3 players; and residential, for resilient cookware such as pots and pans and for aesthetic purposes as decorative trim on many household appliances.
There are two main types of sulfuric acid anodizing: Type II sulfuric acid anodizing and Type III sulfuric acid hardcoat anodizing, also known as
hard anodizing. Type II sulfuric acid anodizing is a much more frequently used process than Type III and involves immersing the material into sulfuric acid, which is a highly corrosive and oily acid formed from sulfur dioxide. While the material is immersed an electric current, typically direct current (DC), is sent through the sulfuric acid and reacts with anodes that are attached to the surface of the material. This creates the hard outer layer of oxidation. Type III hard anodizing creates a much thicker layer of oxidation on the material's surface than Type II does, and is consequently more often used in heavy-duty applications such as in the military and aviation industries. Besides the two main types of sulfuric acid anodizing, there are two other types that are less well known: boric/sulfuric acid anodizing (BSAA) and thin film sulfuric acid anodizing (TFSAA). Boric acid is a water-soluble, slightly acidic white or colorless crystalline compound that is often utilized as a comparable alternative chromic acid. BSAA uses immersion in a boric/sulfuric acid bath then a second immersion in a chromate sealing bath. BSAA is commonly used in military and industrial applications. Instead of trying for a very thick, hard protective coating, TFSAA produces a thin oxide film on the material and is able to be performed on more materials than other types of anodizing processes.
