What is Aluminum Anodizing?
Aluminum anodizing is an electro-chemical process that creates a protective layer on the aluminum surface of a product to offer extended resistance to corrosion, as well as aesthetic enhancement. This technology has been in existence for over 100 years, but has only been applied in mass production for the past 60 years.
Preparing Products for Aluminum Anodizing
The process of aluminum anodizing involves reinforcing the naturally occurring process of oxidization, which can utilize one of several possible methods. It is an electrolytic process where the most commonly used electrolyte is sulfuric acid, although chromic acid, oxalic acid, organic acid or phosphorous acid can also be employed. In the process, the metal to be oxidized becomes the anode electrode in the electrical circuit, and thus the term anodizing.
The aluminum product is placed on an electrically conductive rack, which is also made of aluminum or other exotic and electrically conductive materials such as titanium. A cleaning process is performed and the product is treated with either an etched or bright finish. An etched finish is a flat or dull finish that is effected through an alkaline bath, whereas a bright finish is achieved through a more acidic bath for a shiny or glossy surface.
The Process of Aluminum Anodizing
Once the surface has been prepared, a current is run through the product at about 15 to 25 >Volts, depending on the size of the item. This is done in a bath of sulfuric acid and water with opposing charges. There are cathodes that are a permanent part of the bath, while the charged product itself becomes the anode. The exchange of positive and negative particles builds a coating of aluminum oxide on the surface of the aluminum.
This creates a “clear” finish, also known as an anodic film. The longer the product remains in the bath, the thicker the coating becomes. One of the major features of aluminum anodizing is that it brings about a permanent change in the surface and dimensions of the metal object. At a microscopic level, the anodic film appears as a structure of straws, which is advantageous when moving on to add colour to the product.
Colouring Products in Aluminum Anodizing
An anodized aluminum product can be used once the clear finish is in place, or colour can be added. For colourizing, the product is moved to another bath that contains organic dyes, similar to those used in the food industry. The dye fills in the microscopic “straws” that were created in the anodizing process. The length of exposure to the dyes in the bath determines the depth or intensity of the colour. For example, blue will eventually become black if left for the appropriate amount of time.
After colourization, the product’s new finish is sealed in a hot water and chemical process, passing through multiple steps between at 70 to 97 degrees Celsius. This closes in the honeycomb of straws created during the anodization process to lock in the colour for long life.