Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 416 of 707

the same bath without danger of contamination. Mid-Temperature Sealing (160-190OF) Due to the higher energy costs inherent in hydrothermal sealing, chemical man- ufacturers have developed "mid-temperature" seals (160-190O F). These seals, which contain metal salts such as nickel, magnesium, lithium, and others, have become very popular due to the lower energy costs and their ease of operation. One disadvantage of the lower temperature is the tendency of organically dyed parts to leach during sealing. This can be compensated for by a slight increase in the bath concentration and by operating the solution at the upper tem- perature limits (190O F). "Nickel-free" seals (or more "environmentally friendly" seals, as they are called) are fast becoming the seal of choice where clear or electrolytically colored parts are concerned. Because there is nothing to leach, these mid-temperature seals accom- plish hydration of the oxide without the use of the heavy metal ions. When the seals become contaminated or are no longer effective, they can be discharged to the sewer without subsequent treatment (except possible pH adjustment). This offers the finisher a safer alternative to the effluent treating necessary with heavy metal con- taining seals. Room Temperature (Cold) Seals (70-90OF) A significant modification in the sealing of anodized aluminum was the development of "room temperature sealing" (70-90O F). Unlike the high temperature and mid-tem- perature 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 (160O F) 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 sta- bility 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 105O F. 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 equip- ment. 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 90O F, using a DC voltage of about 50 V. Phosphoric acid baths are used in the aircraft industry as a pretreatment for adhe- sive 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, 415

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