Preparation
The preparation for anodizing is basically the same for any item made from aluminum, regardless of size. Before anodizing, the aluminum must be thoroughly cleaned in a strong caustic solution such as sodium hydroxide that is heated to 145 degrees to remove residue and oils that may be on the surface. After cleaning, the aluminum must be thoroughly rinsed with deionized water.
General Process
The anodizing process begins when electrodes are attached to the aluminum, and the metal is placed in an acid solution. The electrodes are attached to an electrical source. It is possible to use alternating current, but direct current gives better results. Voltage for anodizing runs between 1 and 300 volts, with 15 to 21 volts being the most frequently used range.
Sulfuric Acid
One method for anodizing involves using a15 percent solution of sulfuric acid to produce a moderately thick anodized coat of about one mil. A cathode connected to a negative terminal is placed into a sulfuric acid solution. The positive terminal is then attached to the piece of aluminum that is to be anodized. The process uses five amps of current in the solution at seventy degrees F per square foot of aluminum. Turning on the current will release CO2 molecules which combine with the aluminum to form an aluminum oxide (Al2O3) coating.
Hardcoat
Hardcoat anodizing uses a 15 percent sulfuric acid solution lowered to 32 degrees F. Hardcoat anodizing uses 23 to 37 Amps for 20 to 120 minutes depending on the thickness of the coating. Hardcoating aluminum results in a tougher surface because of the thickness of the coating. Because hardcoat anodizing leaves a darker finish, it is not suitable for dyeing.
Chromic Acid
A third method for anodizing aluminum uses chromic acid. Because chromic acid is less corrosive than sulfuric acid it leaves less corrosion and is easier to rinse. The chromic acid method of anodizing also minimizes the possibility of metal fatigue that occurs when hardcoat and sulfuric acid methods are used. It is not possible to achieve thick coatings of over .1 mil using chromic acid for anodizing. The biggest benefit is that it is easier to rinse complex parts. Chromic acid should use 4.5 amps of current per square foot at a temperature of 60 to 75 degrees for 90 minutes.
