Frequently Asked Questions
A coating of aluminum oxide is grown from the aluminum by passing an electrical current through an acid electrolyte bath in which the aluminum is immersed. The coating thickness and surface characteristics are tightly controlled to meet end product specifications.
It only takes a few hours to process and pack a part. Most anodizers need anywhere from a few days to a few weeks to plan, process, and invoice for projects.
There are four ways to color aluminum:
1. Dye - The freshly anodized part is immersed in a liquid solution that contains dissolved dye. The porous anodic coating absorbs the dye. The intensity of the color is related to the thickness of the anodic film, the dye concentration, immersion time, and temperature, among other things.
2. Electrolytic Coloring (a.k.a. "two-step") - After anodizing, the metal is immersed in a bath containing an inorganic metal salt. Current is applied, which deposits the metal salt in the base of the pores. The resulting color is dependent on the metal used and the processing conditions (the range of colors can be expanded by overdyeing the organic dyes). Commonly used metals include tin, cobalt, nickel, and copper.
3. Integral Coloring - This so-called one-step process combines anodizing and coloring to simultaneously form and color the oxide cell wall in bronze and black shades, while more abrasion resistant than conventional anodizing. Two-step coloring has generally replaced this process.
4. Interference Coloring - An additional coloring procedure, recently introduced, involves modification of the pore structure produced in sulfuric acid. Pore enlargement occurs at the base of the pore. Metal deposition at this location produces light-fast colors ranging from blue, green, and yellow to red. The colors are caused by optical-interference effects, rather than by light scattering as with the basic electrolytic coloring process.
The dielectric strength of an anodic coating should be approximately the same as that of Alumina. The Handbook of Chemistry and Physics, 43rd Edition, (Published by The Chemical Rubber Publishing Co, Cleveland, Ohio) gives the dielectric strength of Alumina as between 40 - 160 volts/mil. In metric units, that is 1.6 - 6.4 V/uM
Almost all appearance features of the anodic coatings are somewhat variable. That means, “it depends”. There will be some variation in the appearance from batch to batch and even piece to piece. MPP strives to minimize these variations. If appearance uniformity is critical, agreed-upon “range samples” should be approved before the parts are processed.
Electrical contact must be made to each part that is anodized. The more electrical current required, the bigger the electrical contact must be. The size of the contact, therefore, depends on the anodizing process and the size of the part being anodized
Most aluminum alloys will build aluminum oxide in an anodizing tank. 2000 series alloys are generally the most difficult to anodize, and 5000 or 6000 series are the easiest. For the best appearance and performance, 5005 AQ alloy for sheet and 6063 alloy for extrusion are recommended. 5052 alloy for sheet and 6061 alloy for extrusions are popular, but they may have more appearance variations.
Castings are challenging to anodize because they are often porous. The alloy preferred for anodizing castings is 518. C443 is also good, but it is not inherently corrosion-resistant.