Aluminum materials are anodized to improve corrosion and wear resistance, increase surface hardness, improve lubricity and adhesion, and to allow for dyeing. There are several types of aluminum anodizing, each of which results in a unique anodic coating.
Standard Anodizing: “Standard” anodizing, more commonly known as Type II anodizing, is the based off military specification MIL-A-8625. It is the standard specification worldwide for commercially anodized aluminum for a wide range of industries, from medical and aerospace to military and defense. Standard anodizing utilizes a sulfuric acid solution.
Hard Anodizing: Hard or hard coat anodizing, also known as Type III anodizing, uses a similar process to Type II anodizing, but results in a much thicker and denser coating that significantly enhances abrasion and corrosion resistnace. Hard anodizing creates a very thick, hard coating that penetrates the treated aluminum—fully half the protective oxide layer will penetrate the surface, with the other half building up on the surface.
Micro-Crystalline Anodizing: Micro-crystalline anodizing improves on other anodizing processes by creating a coating with molecules packed in a regularly ordered, repeating pattern (the molecules in other anodized coatings are arranged randomly). Micro-crystalline anodic aluminum coatings also provide greater thermodynamic stability than other anodized coatings, as well as lower solubility rate when exposed to harsh chemicals.
Chromic Acid Anodizing: Also known as Type I anodizing, chromic acid anodizing was the first anodizing process used on an industrial scale. It produces thinner, more opaque films that are softer and more ductile than those created by other anodizing methods.
Organic Acid Anodizing: Using weak acids, high voltages and current densities, and strong refrigeration, organic acid anodizing produces coatings with yellowish integral colors without the use of dyes. Colors from pale yellow to bronze to black are possible with variations to the process; white coatings with 80% reflectivity can also be created via organic acid anodizing.
Phosphoric Acid Anodizing: Phosphoric acid anodizing meets ASTM standard D3933. It is usually used as a surface preparation technique for adhesives.
Borate and Tartrate Anodizing: Borate and tartrate anodizing baths create coatings that are free of pores. Aluminum oxide is insoluble in these chemistries, and coating growth stops when the treated part is fully covered. Thickness is linearly related to the voltage applied. This method is widely used in the manufacture of electrolytic capacitors.