Metal Finishing Guide Book

2012-2013

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processes in U.S. anodizing operations due to the superior corrosion resistance it imparts [Refs. 4, 11]. Nickel fluoride sealing technologies have been adopted for some applications, but, reportedly, do not perform as well as nickel acetate sealers [Ref. 11]. The mechanism for nickel acetate sealing appears to be more complex than that of hot water or silicate sealing. Along with the aluminum oxide conversion to boehmite (Equation 1), the precipitation of nickel hydroxide also occurs, as described in Equation 2 [Ref. 11]: Ni2+ + 2OH Ni(OH)2 (2) These concurrent precipitation reactions (e.g., Equations 1 and 2) fill the micropores. While nickel-based sealing processes are technically promising, nickel has also come under increased scrutiny from an EHS standpoint in recent years. Nickel is considered a carcinogen by the International Agency for Research on Cancer (IARC) and the National Toxicology Program (NTP), although it is not yet classifiable as a human carcinogen by the American Conference of Governmental Industrial Hygienists (ACGIH) [Ref. 18]. Nevertheless, the Occupational Safety and Health Administration (OSHA) maintains a long-standing permissible exposure limit (PEL) on the use of the soluble salts of nickel at one milligram per cubic meter (mg/m). Perhaps most importantly, nickel is on the OSD Emerging Contaminants Watch List [Ref. 19]. Despite these concerns, numerous nickel-based sealing technologies were identified under this activity, and at least three of these were found to be promising based on the review of the technical literature. Based on the maturity and availability of nickel-based sealers, as well as their approval under MIL-A-8625F, a commercial-off-the-shelf (COTS) nickel-based sealer product was selected as a candidate. This product operates at a lower temperature than the sodium dichromate sealer (29���35��C [85���95��F]), contains no chromates, and has low nickel content (5-10% by weight). Recommended immersion (sealing) time is between 4 and 15 minutes. It is noted that, as expected, the sealing reaction occurs much slower in lower temperature seals than in higher temperature seals. It has been found that a warm water rinse (71��C [160��F]) accelerates the sealing process [Ref. 4]. With this in mind, the COTS nickel-based seal was considered in two scenarios: with warm water rinse afterwards, and without the rinse. COBALT-BASED SEALING TECHNOLOGIES Cobalt-based sealers are also approved as a dichromate sealer replacement under MIL-A-8625F; however, these sealers appear to be a relatively new development, and sufficient data to support using cobalt as a sealer were not available. Even some process data, such as optimum immersion time and temperature, were not readily available. In addition, cobalt is on the OSD Emerging Contaminants Watch List [Ref. 19]. Based on these considerations, cobalt-based sealing technologies were not considered to be viable candidates for OO-ALC sealing requirements at this time. Therefore, they were not considered for further study. EMERGING TECHNOLOGIES Several additional technologies were found to be promising under this effort, 379

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