Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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concern is thermal exposure to the boiling water. Therefore, this process was not a focus of this study. SEALING PROCESS AND ACCEPTANCE CRITERIA OO-ALC personnel reported that OO-ALC has two sodium dichromate sealer tanks for sealing anodized parts. Both of these tanks contain solutions with con- centration of 5 to 9 ounces per gallon (oz/gal) sodium dichromate. The pH is maintained between 5.0 and 6.3 with sodium hydroxide, and the temperature is maintained between 95–98 degrees Celsius (°C) (203–208 degrees Fahrenheit [°F]). Considering the requirements of MIL-A-8625F and the needs identified by OO- ALC, acceptance criteria for an alternative sealing process for Type II and Type IIB anodize were defined, as listed in Table 2 [Ref. 6]. OO-ALC personnel confirmed that the above tests would be sufficient to test alternatives for their acceptance as candidates to replace the sodium dichro- mate sealer. TECHNOLOGY ASSESSMENT: AVAILABLE ALTERNATIVES TO SODIUM DICHROMATE SEALERS To identify candidate technologies that meet the requirements described above (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 technolo- gies. Each of these categories is briefly described in the following sections. 280

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