Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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uids are salts with a low melting point, which originates in their chemical struc- ture (a mix of anions and large organic cations). These liquid salts have unique properties that allow easy dissolution of normally insoluble chemicals, such as cellulose. Ionic liquids enable electrochemical plating of metals like aluminum; deposition rates of one micron per minute at low temperatures (60 to 100°C) have been reported.11 These deposition rates are significantly superior to other low-tem- perature aluminum coating methods. While this process is not yet mature enough to enable the plating of commodity items such as electrical connector shells, work is progressing rapidly and promising results will be forthcoming. ELECTRODEPOSITED ALUMINUM AND ALLOYS AlumiPlate® is a proprietary process in which a pure aluminum coating is elec- trolytically deposited onto a substrate that has been immersed into a non-aque- ous, fully enclosed solution in an inert atmosphere. The resulting coating is high- ly versatile. It can be anodized or topcoated with the standard CCC post treatment, trivalent chromium post-treatments (TCPs), or non-chrome post-treat- ments (NCPs). TCPs are much less hazardous than CCCs and meet require- ments under the European Union's Reduction of Hazardous Substances (RoHS) Directive—although this substance is still regulated under U.S. requirements. Additionally, the AlumiPlate® process does not appear to impart hydrogen embrittlement—a concern with cadmium plating.5 AlumiPlate® is one of the more promising new processes for cadmium replacement on electrical connectors. Researchers at the Naval Air Systems Command (NAVAIR) conducted 2,000 hours of salt spray corrosion testing on electroplated aluminum electrical connectors with TCP12,13 ASTM B117.14 , in accordance with . NAVAIR found that all connectors performed equal to or better than the cadmium-plated controls with respect to visual appearance of corrosion. A plated connector is shown after 2,000 hours of B117 exposure in Figure 2.12 . From a functionality standpoint, all tested connectors met the requirement for shell-to-shell conductivity, with the exception of the AA6061 AlumiPlate® coat- ing with TCP at 25% concentration (the most dilute). The AA6061 AlumiPlate® coating with Class III post-treatment was the top performer. Other projects involving this process include a partnership between Lockheed, Alcoa, and the U.S. Air Force, which is evaluating several coatings, including AlumiPlate®, to replace cadmium for military and commercial fasteners.15 Based on the results from both the NAVAIR testing and this partnership, the AlumiPlate® coating is currently being qualified for electrical connectors under MIL-DTL-38999L as well as relevant internal manufacturers' specifica- tions. Specifically, qualification and approval of the AlumiPlate® coating is anticipated for Model 38999 electrical connectors with spring fingers, which will be used on the Lockheed Martin F-35 Lightning II (also known as the Joint Strike Fighter) program. Despite the good performance of this candidate and its recent qualification, several drawbacks remain with the use of AlumiPlate®. Due to the use of the non- aqueous electrolyte, it is unlikely that this process could meet the environmen- tal requirements that would allow its use in a DoD facility.1 Furthermore, the process requires the use of highly specialized equipment (e.g. high start-up cost). Finally, there are questions regarding whether the plated coating can be repaired, although initial work has found that it may be possible to use brush- 312

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