Metal Finishing Guide Book

2012-2013

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and production costs. Because boiling DI water is approved as a sealer in MIL-A-8625F and is already used at OO-ALC for Type III anodizing operations, it was a viable sealer for Types II and IIB anodizing. As such, it was considered to be a baseline process for future testing. CHROMATE SEALING Sodium dichromate solutions have been used as seals for anodized coatings for many years. The abilities of chromates to inhibit corrosion are well recognized and have been reported by Brooman [Ref. 14], Klingenberg [Ref. 15], and numerous others. Unlike water-based sealing technologies, chromate sealing technologies form either aluminum oxydichromate (lower pH ranges) or aluminum oxychromate (higher pH ranges) in the coating micropores [Ref. 11]. Cr+6 is used in most chromate sealing systems, especially those used for military applications, due to the enhanced corrosion protection that they impart. The Cr+6-containing sodium dichromate solution (5���9 oz/gal) being used at OO-ALC was identified as the baseline being targeted for replacement. The bath temperature for this seal is maintained at 90���100��C (194���212��F) and the pH is maintained between 5.0 and 6.0; immersion time is 15 minutes [Refs. 6, 11]. It is noted that, because OO-ALC is actively trying to reduce the use of or replace chromium-containing processes, even sealers based on trivalent chromium, which have been developed in recent years and are available and being used commercially, were not considered under this pollution prevention effort. However, it was determined that a sealing solution with a reduced sodium dichromate content (50 parts per million [ppm] as chromium) could be evaluated as a benchmark because successful results had been achieved in past work [Ref. 16] and reducing the Cr+6 content in the existing solution would be an interim contribution to OO-ALC���s goal for chromium reduction. SODIUM SILICATE SEALING TECHNOLOGIES Sodium silicate sealing is a mature, effective sealing technique that is commonly 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 applications, 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 recommended 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 technologies, 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 usually based on either nickel acetate (approved for use under MIL-A-8625F) or nickel fluoride. Nickel acetate sealing is one of the most predominant sealing 378

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