Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 410 of 707

The majority of casting alloys contain appreciable amounts of silicon, rang- ing as high as 13%, and present difficulty in anodizing. Use of a mixed acid dip (normally containing hydrofluoric and nitric acids) prior to anodizing is of val- ue when high-silicon alloys are encountered. Since the various alloys produce different shades when anodized identically, the designer of an assembled part must use the same alloy throughout if the shades of the individual components are to match. Anodizing Conditions Other variables affecting the nature of the oxide i.e., its thickness, hardness, and porosity) are the acid concentration and temperature of the anodizing bath, the current density (or the applied voltage, which actually controls the cur- rent density), and the time of anodizing. These factors must be rigidly con- trolled in order to achieve consistent results. The "standard" sulfuric acid anodizing bath (Type II) produces the best oxides for coloring. The standard anodizing solution consists of: Sulfuric acid, 180-200 g/L Aluminum, 4-12 g/L 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 col- or is bled (leached) out into the sealing bath. The ideal anodizing temperature, except where a special effect is desired, is 70O F. 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 lightfast- ness. The best chromic acid coat- ings for dyeing are produced with a 6 to 10% by weight solution operated at 120O F. 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. SIC Processes for ELECTROPOLISHING Stainless Steel, Aluminum NON-NITRIC BRIGHT DIPPING Aluminum castings BD 8605 STRIPPING ANODIC COATING Stripper 5275 ANODIZING Cleaners, Etch Alkaline & Acid, Desmut, Anodizing Additives, Seals, Dyes and Electrolytic Coloring SIC The Innovators (800) 394-9770 • Fax: (404) 351-9887 409 T e c hnologies T e c hnologies

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