Metal Finishing Guide Book


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Page 428 of 843

COLORFASTNESS OF THE DYED COATING Of the many dyes that color anodized aluminum, possibly several hundred, it should be understood that only a few possess sufficient inherent resistance to fading to be considered for applications where exposure to direct sunlight is intended. Where items of long life expectancy are involved, for example, architectural components, even greater selectivity must be imposed, since all organic colorants now known will exhibit some fading when subjected to sunlight of sufficient intensity and duration. Also, the parameters of application as well as the colorant are involved in the resistance to premature loss or change of color. The following additional factors are considered by most authorities as affecting the lightfastness of the dyed coating. Coating Thickness and Penetration of the Dyestuff Accelerated and long-term exposure tests and practical experience both here and abroad verify that an anodic oxide thickness in the order of 0.8 mil (20 microns) and its complete penetration by the colorant is required for optimum resistance to fading and weathering. This means that, in some applications, the dye time may be extended to 30 minutes for complete dye saturation. Intensity of Shade Usually, the greater the amount of dye absorbed, the better its resistance to fading. Also, whatever fading may occur will be less apparent to the observer. Pastel shades may, therefore, be expected to exhibit inferior light and weather fastness as compared to full strength dyeing. Type and Degree of Sealing Those dyes that are reactive with the nickel or cobalt salts present in the sealing bath usually require this treatment for optimum performance. It is reported that certain selected dyestuffs benefit from after-treatment with other heavy metals; for example, lead, copper, zinc, or chromium. Generally, such treatments are not utilized because of the requirement of an individual sealing tank for each dye. In the case of extremely porous anodic oxides, for example, those formed on alloys of high copper content, effective sealing is particularly important with certain dyes to prevent color loss from sublimation of the dye or by chemical reaction in oxidizing or reducing environments. ELECTROLYTIC COLORING (2-STEP) This electrolytic coloring process consists of conventional sulfuric acid anodizing followed by an AC treatment in a bath containing tin, nickel, cobalt, or other metal salts to produce a series of bronze to black colors as well as blues, greens, burgundies, and golds. The most common bath is one containing tin. The colors produced are not alloy or thickness dependent and are easier to control. The process is not as energy intensive as the integral color process. It is for this reason that this process has almost entirely replaced the integral color process in recent years. Unlike sulfuric acid anodizing, the coloring process is controlled by voltage and time, rather than by current density. Depending upon the bath used, the coloring time can range from 20 sec for champagne to 10 min for black. The use of specially built AC power supplies, using electronic timing and voltage control, helps produce a finish that is reproducible time after time. 421

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