Metal Finishing Guide Book


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Page 423 of 843

Etching Etching is the removal of some of the aluminum surface from a part using chemical solutions. There are a number of reasons for etching aluminum: 1. To impart a matte finish to the material (lower the specularity or gloss). 2. To remove surface contaminants. 3. To hide surface imperfections (scratches, die lines, etc.) 4. To produce an overall uniform finish. Chemical etching is accomplished using both alkaline and acid solutions. The most frequently used etch media is sodium hydroxide. Time, temperature, concentration, and contaminant level will affect the type of finish possible in an etch bath. Many proprietary solutions are available from the chemical suppliers. Close attention to the technical information included with the chemicals is important. Rinsing Probably one of the most abused steps in the finishing of aluminum is rinsing. Most anodizers practice some form of "water management," usually to the detriment of the other process tanks. Improper rinsing causes poor surface finish due to cross reactions of chemicals left on the surface from previous processing tanks reacting with the chemicals in further processing tanks. Cross contamination of expensive solutions is another fallacy of "water management." Cascading rinses, spray rings, or just cleaner rinse tanks with adequate overflow will go a long way in reducing poor finish and cross contamination. Deoxidizing/Desmutting After etching, a "smut" of residual metallic alloying materials is left on the aluminum surface. This must be removed before further processing. The use of deoxidizer/desmutters will accomplish this, leaving the treated surface clean for subsequent finishing steps. 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 deoxidizer 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 electropolishing 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. 416

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