Metal Finishing Guide Book

2012-2013

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(technologies that may be promising for future study), a thorough technology assessment was conducted. The following activities were undertaken: Review of past efforts that evaluated sealers for anodized coatings, specifically those focused on sodium dichromate replacement Internet searches Discussions with vendors Literature search(es) Processes undergoing research and development (R&D), as well as commercially available processes, were considered. For processes that were evaluated under past efforts, vendors were contacted to obtain the most recent information regarding their products. This assessment also identified recently developed or improved inorganic sealers that would require technology verification testing if found to be promising. As expected, a number of technologies were identified for sealing anodized Al alloys. These alternatives are based on a variety of materials, such as nickel acetate, silicon, cobalt and molybdenum fluorides, and trivalent chromium, among others. Prior work in this field [Ref. 11] has classified the more prominent sealing processes into general categories that include: Water-based sealing processes, such as boiling DI water and steam Chromate sealing Sodium silicate sealing Nickel-based sealing technologies, such as nickel acetate and cold nickel fluoride The additional category of cobalt-based sealers was identified during the current effort. A special assessment also was performed for emerging technologies. Each of these categories is briefly described in the following sections. WATER-BASED SEALING TECHNOLOGIES Water-based technologies have been a viable method for sealing anodized coatings for many years. The performance of the seal is dependent upon the pH, temperature, and purity of the water, as well as immersion time and current density used during anodizing [Refs. 4, 11]. DI water is commonly used for this reason. Past work has shown that hot water sealing may be the best sealing process for enhancing the dielectric strength of anodized coatings on Al [Ref. 12], which is essential for applications requiring electrical conductivity [Ref. 13]. Water-based sealing technologies work by converting aluminum oxide (anodic coating) to boehmite, which fills the micropores of the anodic coating. The mechanism is presented in Equation 1 [Ref. 11]: Al2O3 + H2O 2AlO(OH) (1) Steam sealing is a variation of boiling DI water sealing. This technique is used frequently in Japan and Europe and is generally more efficient than boiling DI water immersion, with a reported reaction rate increase of two to four times that of DI immersion [Ref. 11]. However, steam sealing requires specialized equipment, and it is rarely employed in the United States due to the high capital investment 377

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