Anodizing is an electro-chemical coating process applied to metal parts and components to increase the thickness, density, and regularity of the naturally forming oxide layer on their surfaces.
The term “anodizing” refers to the fact that the part being treated forms the anode (or positive electrode) of an electrical circuit during the process. The oxide coating is grown by passing a direct current through an electrolytic bath, most commonly based on sulfuric acid. This current releases hydrogen at the cathode (negative electrode), and oxygen on the surface of the treated part, creating a buildup of oxide. The voltage required can vary greatly. For thicker and more dense coatings formed in sulfuric and organic acid solutions, higher voltages are typically required.
The electrolytic bath used in anodizing is usually an acid solution that slowly dissolves the oxide. This is counteracted by the oxidation rate of the anodizing process, which forms a coating of 10-150 nanometer pores in the metal’s surface. These pores allow the electrolyte solution and electrical current to reach the metal substrate under its oxide layer and enable it to grow the coating to a greater thickness than that created by auto-passivation. While the pore structure is highly regular, very minor defects in the stitching between pores allows water and corrosive ions to reach the substrate if not sealed.
The concentration of the electrolyte bath, its acidity and temperature, and the voltage applied must all be carefully controlled to create a consistent oxide layer on the metal surface. Oxide film thicknesses can range from less than 0.5 micrometers to more than 150 micrometers, depending on the application.
The first industrial use of anodizing was in 1923, when seaplane parts were treated to help prevent corrosion. Called the Benough-Stuart process, it used a chromic acid-based process that is still used today, despite its requirements for a complicated voltage cycle that is now known to be unnecessary. Many variations of the process were soon developed. The first “modern” sulfuric acid anodizing process (now the most common method) was patented in 1927 by Gower and O’Brien.