Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 409 of 707

Many alloys, during their heat treatment steps, will form heat treat oxides. If these oxides are not removed prior to etching or bright dipping, a differential etch pattern can develop, which will cause rejection of the parts. In this instance a deox- idizer must be used. The deoxidizer is designed to remove oxides, but is also extremely good at removing smut. A desmutter, on the other hand, will not remove oxides. It is apparent that a deoxidizer would be the preferred solution to have in an aluminum finishing line. Remember, a deoxidizer will desmut but a desmutter will not deoxidize. Bright Dipping and Electrobrightening A chemical or electrobrightening treatment is required where an extremely high luster is to be obtained on the aluminum surface. The electrobrightening or elec- tropolishing treatment is particularly applicable to the super-purity aluminum now used extensively in the jewelry and optical field. Proprietary chemicals for these treatments are available from a number of suppliers. Chemical brightening is most commonly used for most applications because of it's ease of operation. A number of companies offer proprietary solutions, which will give you the bright finish you desire. Specifics on the makeup and use of these solutions is available from the chemical suppliers. ANODIZING Properties of the Oxide Film The anodizing process conditions have a great influence on the properties of the 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 qual- ity 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 vari- ables 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, espe- cially 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 alloy- ing 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 heav- ier 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. 408

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