Aluminum Anodizing process
In China aluminum industry, major two types of anodizing, long piece anoding and short piece anodizing.
In long piece anodizing, the aluminum part is hung on stripped aluminum or titanium racks ensuring good electrical contact. Throughout the anodizing process, parts are affixed and the racks are suspended in a series of tanks.
- Aluminum part is dipped in a hot tank containing a soak cleaning agent to remove all surface dirt.
- The part is rinsed to avoid contaminating the solution in subsequent tanks.
- The next tank de-oxidizes the part with an acid solution (chromic, sulfuric, nitric or phosphoric) removing the thin, non-uniform aluminum oxide surface.
- Again the part is rinsed to avoid tank contamination.
- Etching is performed by suspending the part in a tank containing a sodium hydroxide solution. Etching removes the natural shine of the aluminum and provide a soft, matte, textured appearance.
- The part is suspended in the anodizing tank, which contains a diluted acid and water mixture that is capable of permitting electrical current flow. The type of acid, percent solution and temperature are all critical parameters and dependent on the desired finish and color. The negative leg of the electrical circuit is connected to the rack of parts and the positive side of the circuit is connected to one or more “cathodes” that introduce electricity into the tank. Quantity and placement of the cathodes varies with the size and shape of the part as well as the total square footage of aluminum surface to be treated. Surfaces closest to the cathode will receive a thicker anodic coating. For normal sulfuric anodizing a DC power source capable of producing up to 24 volts is used, with the voltage held generally between 18 and 24 volts. The amount of current applied to the anodizing tank will vary depending on the amount of surface to be treated, as a rule between 12 and 16 amps are required for each square foot of coverage. The electrolyte solution is agitated during the anodizing process to provide uniform solution temperature. The anodizing tank process, under normal conditions, takes less than an hour.
- To add color (dye), the part is dipped in a tank with a diluted, water soluble, organic dye. Each dye varies in the length of time and temperature for this immersion.
- The final consideration in the anodizing process is sealing the now dyed outer surface so it doesn’t sunlight bleach or stain. Unsealed, the porous outer surface has a lowered corrosion resistance. For non-dyed coatings, the anodized aluminum part is put into boiling de-ionized water for 20 to 30 minutes. This converts the unstructured pores of the aluminum oxide to a more solid crystalline hydrate form. If the anodized parts are dyed, the sealing process is performed for 3 to 5 minutes in a tank with a nickel acetate solution.
- Hard anodizing, depending on the process, uses a mixture of sulfuric and oxalic acid. Relatively low temperatures are used along with a higher current and a much higher voltage. The “gray” oxide layer produced is typically 2 to 3 mils and is very dense, wear and corrosive resistant.
Short piece anodizng is an alternative to racking, bulk anodizing which is more ideal for anodizing small, irregularly shaped parts such as rivets, ferrules and medical hubs. Instead of racks, parts are processed in perforated aluminum, plastic or titanium baskets. Whether you need coil or batch production, anodizing provides one of the best aluminum finishing options in the industry.
Another alternative is coil anodizing. Coiled aluminum is pre-anodized to lower finishing costs, save production time, and reduce material handling. Advantages of pre-anodized aluminum can be applied to most products that are fabricated from sheet or coil stock. Products that are made from extrusions, castings, rod bar, or plate are limited to piece anodizing processes such as racking or bulk.
Though most aluminum alloys will build aluminum oxide in an anodizing tank, they tend to anodize differently. Some alloys are more difficult to anodize while others anodize to slightly different shades of color. When anodized, various alloys provide different levels of workability (machining, grinding, polishing), environmental resistance properties, and dimensional stability.