Metal Finishing Guide Book

2013

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Room Temperature (Cold) Seals (70-90°F) A significant modification in the sealing of anodized aluminum was the development of "room temperature sealing" (70-90OF). Unlike the high temperature and mid-temperature seals, which depend on hydration for sealing, the cold seals rely on a chemical reaction between the aluminum oxide and the nickel fluoride contained in the seal solution. Unfortunately, this reaction is slow at ambient temperatures and the sealing process can proceed up to 24 hours; however, it has been found that a warm water rinse (160OF) after the cold seal immersion will accelerate the sealing process, allowing for handling and packing of the sealed parts. The sealing of organically dyed parts in cold seals has been found to be advantageous. Light stability testing (fade resistance) has shown that parts sealed in cold seals gain additional lightfastness. OTHER ELECTROLYTES A number of other electrolytes are used for specialized applications. Chromic acid is used in marine environments, on aircraft as a prepaint treatment, and in some cases when finishing assemblies where acid may be entrapped. Although the film produced is extremely thin, it has excellent corrosion resistance and can be colored if desired. A typical bath might contain from 50 to 100 g/L of chromic acid, and be run at about 95 to 105OF. There are two main processes, one using 40 V and a newer process using 20 V. The equipment needed is similar to that used in sulfuric acid processes. Oxalic acid is sometimes used as an anodizing electrolyte using similar equipment. This bath will produce films as thick as 2 mils without the use of very low temperatures and usually gives a gold or golden bronze color on most alloys. The typical concentration is from 3 to 10% oxalic acid at about 80 to 90OF, using a DC voltage of about 50 V. Phosphoric acid baths are used in the aircraft industry as a pretreatment for adhesive bonding. They are also very good treatments before plating onto aluminum. A typical bath might contain from 3 to 20% of phosphoric acid at about 90OF, with voltages as high as 60 V. SUMMARY Aluminum is a most versatile metal. It can be finished in a variety of ways. It can be made to resemble other metals, or can be finished to have a colorful as well as a hard, durable finish unique unto itself. Only the imagination limits the finish and colors possible with anodized aluminum. 424

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