Anodizing - Less Costly Than Plating
Based on an electrolytic process, anodizing puts a protective oxide film on a light metal. Through anodization the naturally occurring oxide on aluminum is removed. The properties of the coating can be engineered to meet end-use requirements. Titanium, magnesium and zinc can be anodized, but these metals represent a small part of the industry's activity.
In the process, the aluminum (or any other metal being anodized) serves as the anode in the chemical bath that will carry electricity. As for the cathode, a carbon rod is often used in the bath to create the electrolytic cell. An electric current is passed through the cell and the surface of the aluminum begins to change. The acid liberates oxygen at the anode and this combines with the aluminum to produce the oxide. It is also possible to add color during the process.
There are three types of anodizing: Type I uses chromic acid, Type II employs sulfuric acid and Type III is a sulfuric acid hard coat with thicknesses to 2 mil to resist wear and corrosion. A number of factors impact the effectiveness of the anodized hard coat. They include the coating thickness, the workpiece, chemicals used and the process itself. Regular and abrasion-type hard coatings can be generated. Also, Teflon can be impregnated into hard coatings in thicknesses from 0.0003" to 0.004" at tolerances of +0.0002".
Anodizing is reportedly less costly than plating and is environmentally friendly. The anodized surface is more durable than paint, integrally bonds to the substrate and is a ceramic-type coating. A couple of limitations of anodizing are the need for masking when treating dissimilar metals; also the process does not lend itself to multi-metal assemblies.
The size of the piece to be anodized is limited by the size of the tank. The usual shop will use tanks to accommodate 6 ft lengths, but tanks that can handle lengths to 50 ft are available. Poles and sailboat masts are examples of long lengths that have been anodized. Small parts can be anodized on a rack at production rates up to 80,000 pieces/hr in some applications.
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