Metal Finishing Guide Book


Issue link:

Contents of this Issue


Page 425 of 843

Temperature, 68-72OF As the anodizing temperature is increased, the oxide becomes more porous and improves in its ability to absorb color; however, it also loses its hardness and its luster, due to the dissolution action of the acid on the oxide surface. As the pore size increases, sealing becomes more difficult and a greater amount of color is bled (leached) out into the sealing bath. The ideal anodizing temperature, except where a special effect is desired, is 70OF. Oxides produced by anodizing in chromic acid solutions may also be dyed. The opaque nature of the oxide film produced in this manner has a dulling effect upon the appearance of the dyed work. Consequently, some dyes, notably the reds, which produce pleasing shades on sulfuric acid anodized metal, are unsuitable for use with a chromic acid coating. Fade resistance of this type of dyed oxide is extremely poor, possibly because the oxide is not thick enough to contain the amount of dye needed for good lightfastness. The best chromic acid coatings for dyeing are produced with a 6 to 10% by weight solution operated at 120OF. A potential of 40 to 60 V is used, depending upon alloy, copper- and silicon-bearing materials requiring the lower voltage. The usual time is from 40 to 60 minutes. 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 electrolyte 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, 60-80OF 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 produce 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 treatment 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. 418

Articles in this issue

view archives of Metal Finishing Guide Book - 2013