Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 283 of 707

SODIUM SILICATE SEALING TECHNOLOGIES Sodium silicate sealing is a mature, effective sealing technique that is com- monly used for Type III anodizing. The bath temperature is maintained at 85–95°C (185–203°F) and the pH is maintained at around 11.0; immersion time is 10-15 minutes [Ref. 11]. Like hot water sealing, silicate sealing converts aluminum oxide to boehmite (Eq. 1) [Ref. 11]. While promising for some appli- cations, these processes are reported to be inferior with respect to corrosion resistance [Ref. 11] to other types of sealing processes, specifically hot water, dichromate, and nickel-based sealers. In addition, silicate sealers are not rec- ommended for applications that require dielectric strength [Ref. 12]. Furthermore, silicate is recognized as being detrimental to conventional anodizing electrolytes, as contamination of the anodizing bath with silicates reduces the corrosion resistance of the resulting coating [Ref. 17]. Due to these limitations, as well as a lack of available information to justify the consideration of silicate sealing tech- nologies, these types of sealers were not considered for further study. NICKEL-BASED SEALING TECHNOLOGIES A number of nickel-based sealing technologies are available. These are usual- ly based on either nickel acetate (approved for use under MIL-A-8625F) or nick- el fluoride. Nickel acetate sealing is one of the most predominant sealing processes in U.S. anodizing operations due to the superior corrosion resistance it imparts [Refs. 4, 11]. Nickel fluoride sealing technologies have been adopt- ed 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 clas- sifiable 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 promis- ing based on the review of the technical literature. Based on the maturity and avail- ability of nickel-based sealers, as well as their approval under MIL-A-8625F, a com- mercial-off-the-shelf (COTS) nickel-based sealer product was selected as a 282

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