Anodize Color Variation
Causes of color variation in the anodize process
"All Aluminum can be anodized." This is a valid statement but only to the extent it is understood that only the aluminum gets anodized, while the other metal constituents (zinc, magnesium, silicon) present in the alloy do not respond in the same manner. The variance of metal constituents in an alloy is the major reason for color variation—especially when using secondary, recycled aluminum billet. Other variables affecting color variation in the anodize process include temper, anodize tank chemistry, shape geometry and material load size.
Anodizers have control over their tank chemistry, including temperatures, solution concentrations and the time material spends in each tank, but they have no control over the alloy, temper, or shape of the aluminum parts. These variables can make it extremely difficult to achieve an exact color from run-to-run and load-to-load.
To ensure the most consistent anodize color:
Maintain metal consistency. The easiest way to ensure metal consistency is to work with one metal source/extruder
per project and request that all metal come from one "lot" of material.
Do not mix aluminum alloys. Mixed alloys, or even mixed tempers, will not produce uniform results. For best results use 6063 alloys for extrusions and 5005 for flat sheet stock and fabricated parts. (6061 and 5052 can be used if structural alloy is required but will not give similarly acceptable results.)
Perform as much bending and forming as possible prior to finishing. Anodic films are very hard, and as a result, most post-production bending causes the film to "craze," which produces a series of small cracks in the finish, giving it a spider-weblike appearance.
Be aware of anodizings effect on welds. The heat developed from the welding process will change the metallurgy on nearby metal or heat-affected zone, or HAZ, and cause localized discoloration, often referred to as a "halo" effect, after anodizing. Use the proper 5356 alloy welding wire and the lowest heat possible.
Select an anodizer who utilizes automation to reduce inconsistencies in the anodize process.