Metal Finishing Guide Book

2012-2013

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SEALER ADDITIONAL INFORMATION Sodium dichromate solution, 5-9 oz/gal Baseline Boiling deionized water Baseline Unsealed Baseline Sodium dichromate solution, 50 parts per mil- Benchmark lion (ppm) COTS permanganate solution Candidate COTS low nickel seal solution, with hot water Candidate seal COTS low nickel seal solution, without hot Candidate water seal Table 3. Test Matrix: Sodium Dichromate Sealers and Most Promising Seals to Evaluate for Replacement (typically 12 to 22 volts). The coatings become harder and denser as the purity of the aluminum in the substrate increases [Ref. 4]. Parts that have been anodized with the Type II and IIB processes are sealed with the sodium dichromate sealer; therefore, these processes were the primary focus of this study. The main Environmental Health and Safety (EHS) concerns expressed by OO-ALC personnel for these processes are the corrosive nature of the sodium dichromate solution and the fact that it contains Cr+6. HARD COAT (TYPE III) ANODIZING OO-ALC performs the hard coat (Type III) anodizing process, after conducting Type II anodizing on a part, as part of their aircraft refurbishment processes. Because Type III anodized coatings are sealed in boiling DI water, the primary EHS 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 concentration 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 OOALC, 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 dichromate sealer. TECHNOLOGY ASSESSMENT: AVAILABLE ALTERNATIVES TO SODIUM DICHROMATE SEALERS To identify candidate technologies that meet the requirements described above 376

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