|
||||
| Anodizing is a process generally associated with aluminum but can be applied to titanium and magnesium as well. Here we will discus the properties of anodize on aluminum in a sulfuric acid electrolyte. An anodize coating is formed by applying an anodic coating to a part in an electrolyte. This process will oxidize the aluminum and form an aluminum oxide coating. The oxide film grows from or out of the base material. The coating is not formed on top of but is formed out of the parent material of the part. This property makes the coating very adherent. Anodic coatings can have a wide range of properties depending on the electrolyte used to create them as well as the operating conditions under which the coating if formed. The thickness, hardness and resulting ductility of the coating can be controlled. Sulfuric acid is the most common electrolyte used for anodizing. By controlling the amount of power and temperature of the solution a very hard anodic coating can be created known as hardcoat. Normal sulfuric acid anodize is a bit softer than hardcoat and generally much thinner. Sulfuric acid anodize if clear in color when it is thin and takes on a darker tint as it builds up. The alloy of the part will greatly affect the color of the anodize film as the coating develops thickness. A property of the coating that is of great importance is porosity. An anodize coating is filled with thousands upon thousands of microscopic pours. These pours can be filled with organic dyes and pigments to impart many colors to the coating. The appearance of a dyed aluminum anodized part cannot be duplicated by paint. The metallic luster of the finish is unique. The pours while being useful also present a liability that must be overcome by sealing. Dyed or un-dyed anodic coatings have a surface that will act like a sponge. This is a great attribute when properly sealed, but if the coating is not properly sealed oils, acids, and other environmental contaminants will become entrapped in the pours and cause a reduction in the corrosion resistant properties of the coating. To prevent corrosion the final step to anodizing is sealing. Three common seals are widely used; they are sodium or potassium dichromate, nickel or cobalt acetate, and hot water. Specialty seals that contain Teflon and resin seals to impart additional corrosion resistance are available. The seal will fill the pours and deterioration of the anodic film. Dichromate sealing will impart the best corrosion resistance because of the hexavalent chrome that is trapped in the pours will leach our and protect surfaces that have become mechanically damaged by nicks and scratches. Nickel acetate sealing is used on parts that are dyed. The nickel or cobalt will fix the dye and enhance its fade resistance. Nickel acetate is also useful when the anodize coating is being used as a mask for another anodize process such as hardcoat. Hot water sealing will provide the clearest anodize coating. A thin anodize with a hot water seal is very hard to detect visually. The properties of the coating will be present. The coated surface will be harder, abrasion and corrosion resistant. |