Anodizing is an electrolytic passivation process that grows the natural oxide layer on aluminum parts for protection from wear and corrosion, as well as for cosmetic effects. It is a conversion coating, similar to Alodine, meaning that the surface of the aluminum recedes dimensionally before the protective oxide layer is built up. After the process is complete, the oxide layer is integral to the aluminum substrate below, which means it won’t chip or flake.
The name anodizing comes from the fact that the treated part forms an anode (positive electrode) in an electrical circuit. During this process, the part to be anodized is hung on a conductive rack and submerged in an electrolytic solution, where a direct current of electricity is introduced. While the acidity of the solution dissolves the oxide layer of the part, the electric current releases oxygen at its surface, which builds up a protective layer of aluminum oxide. By balancing dissolve rate with build-up rate, the oxide layer forms with nanopores, allowing continued growth of the coating beyond what is naturally possible.
The final steps of the anodizing process involve sealing the nanopores. Otherwise, they are the perfect passageways for corrosion initiation! Just before sealing, however, they are sometimes filled with other corrosion inhibitors or colored dyes for cosmetic purposes. After the process is complete, the coating will be 0.0002-0.0012” in thickness, in accordance with the common engineering spec MIL-A-8625 Type II.
Type III Anodizing (commonly specified with MIL-A-8625 Type III in North America) is the same general process as Type II and provides similar benefits of increased resistance to wear, corrosion, and other general environmental effects. However, this type is also known as hard or hardcoat anodizing, as the protective oxide layer must be thicker than 0.001”, with coatings up to 0.004” sometimes applied.
While the thicker coating of Type III offers greater physical protection to a part, it comes with some tradeoffs:
Growing the oxide layer to a consistent and specified thickness requires far more process control than with Type II. This, in addition to the lower temperatures and much higher voltages needed to run the process, causes the cost of this process to be higher than Type II Anodizing.
Impact on Tolerances
A second tradeoff of Type III is the increased care that must be taken to ensure parts stay within specified tolerance. Type II thickness is minimal enough that most dimensions should remain similar after the process, but even the thinnest Type III can impact a tight tolerance on a reamed bore, render a threaded hole non-functional, etc. Therefore, plugging holes and masking sensitive surfaces is almost always employed with this type of anodizing.
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