The process of anodizing aluminum enhances the corrosion resistance and durability of the surface of the metal by adding an anodic oxide finish. This is done through an electrochemical process that is carefully controlled to provide a specific amount of oxidation to the surface aluminum.
Unlike paint or other types of surface applications, the aluminum anodizing process fuses the aluminum oxide with the aluminum in the part or component. With this fusing or integration within the surface aluminum, the anodizing is permanent and will never peel or chip. The anodizing process also creates a surface on the aluminum that is ideal for secondary processes such as painting, coloring, and sealing.
How Does It Work?
To anodize aluminum, the parts are completely submerged in a large tank that contains a specially formulated acid electrolyte bath. The aluminum parts in the bath act as an anode while the cathode is placed inside the tank. As the electric current passes through the bath, it causes the bath solution to release oxygen ions that react with the surface of the aluminum part in the tank, creating the anodized surface area.
This process is ideal for complex parts and components as the process is evenly distributed across all aluminum surfaces in the bath. This allows us to provide a quality anodization process for small or large batches of aluminum parts.
Preparing the Aluminum for Anodizing
It is critical to remove any surface contaminants from the aluminum prior to the anodizing process to get a consistent, attractive finish.
Your anodizer typically uses an alkaline detergent that removes surface grease, oil, or contaminants without causing any surface damage to the aluminum. The parts are then rinsed thoroughly to remove any detergent residue, leaving a clean, rinsed, and ready to anodize part.
Excessively dirty parts can consume the cleaning capacity of the chemicals in the clean process, and worse, drag oil into other tanks in the process and contaminate the line if not caught in receiving inspection. When this happens, it causes the anodizer to have to replace chemicals in multiple downstream tanks. You will likely be charged for special manual cleaning if your process repeatedly allows dirty parts to be delivered to the anodizer.
It is important that blind holes in the parts be reasonably clean, particularly the threaded blind holes. The aluminum anodizing process uses multiple tanks with a wide temperature range. If machining coolant has dried in the blind holes it will remain in the hole during the cooler operating tanks and it will be dissolved in the hot dye tanks. When the coolant dissolves, it flows out onto the exposed surface of the part and can prevent the area it coats from accepting dye. This is a commonly observed defect. We strongly recommend that machined parts are rinsed in water and all holes, as well as the surface, be blown off shortly after machining is finished.
Another common source of defects is water spots. If a part is not dried off well and is stacked against another part or on plastic, the aluminum in the moist area will begin to oxidize. In the aluminum anodizing process, all native oxides are chemically removed. The moist area that oxidized will be more matte than the rest of the part and will have a microscopic step at the edge that is very visible on the finished part. Using clean water in a post machining rinse, drying parts well, stacking them individually, and placing them on an absorbent material such as a towel can essentially eliminate this problem.
For more information on any aspect of aluminum anodizing, contact the team at Sapphire Metal Finishing by calling 208-614-4050, filling out our contact form, or requesting a free quote.