Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 284 of 707

candidate. This product operates at a lower temperature than the sodium dichro- mate sealer (29–35°C [85–95°F]), contains no chromates, and has low nickel con- tent (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 slow- er 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, based on a review of past work [Refs. 20–23] as well as vendor surveys. The Non-Chrome Post Treatment (NCP) process was found to be promising as a non-chromium seal for anodizing. NCP is a proprietary product of the United States Naval Air Systems Command (NAVAIR). Testing was conducted by NAVAIR under past work [Ref. 20], and the results demonstrated exception- al performance in corrosion-resistance testing and adhesion testing. While these results demonstrate that the NCP sealer might be viable for the needs of OO-ALC, the process was unavailable for demonstration at the time of testing; therefore, it could not be considered. A COTS chromium- and nickel-free anodize sealer based on permanganate was identified that reportedly provides similar characteristics to those of the dichro- mate sealer. This solution was evaluated by the AFRL and CTC under past work [Ref. 20] and showed exceptional performance in corrosion resistance. This sealer also passed the dry tape adhesion test, but failed the wet tape adhe- sion test. Sealing anodized coatings by using a liquid colloidal dispersion that solidi- fies (e.g., sol-gel) has been demonstrated. Specimens were anodized and then immersed in a working solution at 270°C for 15 minutes; the resulting specimens reportedly demonstrated comparable corrosion resistance to dichromate sealed specimens [Ref. 22]. However, this process is more expensive and decreases the abrasion resistance and hardness of the overall coating system. The incorporation of additional materials within the sealer, such as polytetrafluoroetheylene (PTFE) has also been explored [Ref. 23]. These technologies are typically used for low fric- tion and/or "non-stick" applications; sufficient data that these sealers could meet the stringent needs of military specifications could not be obtained, and so they were not considered. 283

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