Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 282 of 707

WATER-BASED SEALING TECHNOLOGIES Water-based technologies have been a viable method for sealing anodized coat- ings for many years. The performance of the seal is dependent upon the pH, tem- perature, and purity of the water, as well as immersion time and current densi- ty used during anodizing [Refs. 4, 11]. DI water is commonly used for this reason. Past work has shown that hot water sealing may be the best sealing process for enhancing the dielectric strength of anodized coatings on Al [Ref. 12], which is essential for applications requiring electrical conductivity [Ref. 13]. Water-based sealing technologies work by converting aluminum oxide (anod- ic coating) to boehmite, which fills the micropores of the anodic coating. The mechanism is presented in Equation 1 [Ref. 11]: Al2O3 + H2O 2AlO(OH) (1) Steam sealing is a variation of boiling DI water sealing. This technique is used frequently in Japan and Europe and is generally more efficient than boiling DI water immersion, with a reported reaction rate increase of two to four times that of DI immersion [Ref. 11]. However, steam sealing requires specialized equipment, and it is rarely employed in the United States due to the high capital investment 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 numer- ous others. Unlike water-based sealing technologies, chromate sealing tech- nologies 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 mil- itary 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 com- mercially, 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 con- tribution to OO-ALC's goal for chromium reduction. 281

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