Metal Finishing Guide Book


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oxide formed. The use of low temperatures and acid concentration will yield less porous, harder films (hardcoating). Higher temperatures, acid contents, and longer times will produce softer, more porous, and even powdery coatings. It must be remembered that changing one parameter will change the others, since they are all interrelated. It should also be pointed out that the alloy being processed may significantly alter the relationship between the voltage and current density, often leading to poor quality coatings. This is particularly true when finishing assembled components, which may contain more than one alloy. Factors Influencing Shade In order to obtain reproducible results from batch to batch, a large number of variables must be kept under close control. First to be considered are those that affect the nature of the oxide. Alloy The particular aluminum alloy being used has a pronounced effect on shade, especially with certain dyes. The brightest and clearest anodic oxides are produced on the purest form of aluminum, the oxides becoming duller as the amount of alloying constituents are increased. Super-purity aluminum (99.99% Al) and its alloys with small amounts of magnesium produce an extremely bright oxide, which does not become cloudy upon being anodized for extended periods. Alloys containing copper, such as 2011, 2017, 2024, and 2219, although forming a thinner and less durable oxide than the purer forms, produce a heavier and duller shade. Magnesium in excess of 2% has a similar effect although not as pronounced. The presence of silicon imparts a gray color to the coating; alloys containing more than 5% silicon are not recommended for use with bright colors. Iron in the alloy can lead to very cloudy or ���foggy��� oxides. The majority of casting alloys contain appreciable amounts of silicon, ranging 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 value 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 current density), and the time of anodizing. These factors must be rigidly controlled 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 color is bled (leached) out into the sealing bath. The ideal anodizing temperature, 471

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