Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 411 of 707

DECORATIVE ANODIZING Decorative anodic oxides are used in a great many applications, from lighting reflectors to automotive trim. The thickness of the oxide might range from 0.1 to 0.5 mil (2.5 to 12 microns). As mentioned above the most common elec- trolyte is sulfuric acid and typical conditions are listed below. Parts that are to be given bright specular finishes are usually produced from special alloys formulated for their bright finishing capabilities. Typical decorative anodizing conditions are: Sulfuric acid, 165-180 g/L Temperature, 60-80O F Current density, 10-15 A/ft2 Voltage, depends on current density, temperature, and electrolyte Time, 12-30 minutes depending on film thickness desired. Longer times pro- duce thicker coatings. ARCHITECTURAL ANODIZING The conditions used in architectural anodizing are not much different than those used for decorative applications, except the anodizing time is usually longer and the current density may be slightly higher. In general the thickness of the oxide will be greater than for decorative coatings, and this relates to the treat- ment time. Interior For interior applications the coating will be probably 0.4 mil thick (10 microns). This means an anodizing time of about 20 minutes at 15 A/ft2 . Exterior For exterior uses the coating will be a minimum of 0.7 mil thick (18 microns) and this means an anodizing time of about 39 minutes at 15 A/ft2 . INTEGRAL COLOR ANODIZING This process, used mainly for architectural applications, requires the use of spe- cially formulated electrolytes, usually containing organic sulfo acids with low con- tents 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 "engi- neering 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 30-40O F). At these temperatures, the sulfuric acid does not attack the oxide as fast as at elevated tem- peratures. Because of the lower temperature, the voltages needed to maintain the higher current densities also help form smaller, more dense pores, thus account- ing for the hardness and excellent abrasion resistance. 410

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