Metal Finishing Guide Book


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Page 426 of 843

INTEGRAL COLOR ANODIZING This process, used mainly for architectural applications, requires the use of specially formulated electrolytes, usually containing organic sulfo acids with low contents of sulfuric acid and aluminum content, to produce a series of bronze to black shades. The color produced is dependent upon the time of treatment and the final voltage used. Specially formulated alloys are also required. Large amounts of heat are generated in the process due to the high current densities employed (up to 45 A/ft2), so efficient heat exchange equipment is needed to keep the bath cool. HARDCOATING Hardcoating (Type III) is a name used to describe a special form of anodizing. The process, which usually employs higher acid concentrations, lower temperatures, and higher voltages and current densities is sometimes referred to as an "engineering hardcoat." This is due to the fact that hardcoating imparts a very hard, dense, abrasion-resistant oxide on the surface of the aluminum. A dense oxide is formed due to the cooling effect of the cold electrolyte (usually 30-40OF). At these temperatures, the sulfuric acid does not attack the oxide as fast as at elevated temperatures. Because of the lower temperature, the voltages needed to maintain the higher current densities also help form smaller, more dense pores, thus accounting for the hardness and excellent abrasion resistance. Normal low temperature hardcoating is carried out under the following conditions: Acid concentration, 180-225 g/L Aluminum content, 4-15 g/L Temperature, 28-32OF There have been a number of organic additives developed in the past few years that allow the anodizer to hardcoat at elevated temperatures (50-70OF). These additives, by virtue of their chemical reaction in the oxide pores, help cool the material being anodized and retard acid dissolution of the coating. COLORING OF ANODIC COATINGS The coloring of anodic oxides is accomplished by using organic and inorganic dyes, electrolytic coloring, precipitation pigmentation, or combinations of organic dyeing and electrolytic coloring. After the anodizing step, the parts are simply immersed in the subject bath for coloring. The thickness of the anodic oxide can range from 0.1 mil for pastel shades up to 1.0 mil for very dark shades and blacks. Application of electrolytic coloring will be discussed below. Suffice it to say, the combination of organic dyeing and electrolytic coloring gives a more complete palette of colors from which to choose. Organic Dyes The actual process of dyeing the aluminum oxide is very simple. A water solution of 0.025 to 1.0% of dyestuff at a temperature of 140OF composes the dyebath. The aluminum, previously anodized, is simply immersed in this bath for a short period of time, usually 10 to 30 minutes, The work is then sealed and is resistant to further dyeing or staining. 419

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