Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 315 of 707

plated tin-zinc to repair this coating.1,5 ELECTROLESS NICKEL TECHNOLOGIES Figure 2. AlumiPlate-coated electrical connector, After 2,000 hours B117exposure.12 A number of new EN-based coating systems continue to be considered for electrical connector shells. However, as mentioned previously, the corrosion properties of nickel—and subsequent electrical properties—are considerably different than those of cadmium.2 Further testing would be required to fully assess this candidate for military electrical connector shells. Despite these concerns, at least one leading manufacturer of electrical connectors is investigating the use of EN with occluded particles (polytetrafluoroethylene, or PTFE) as a cad- mium replacement.16 While the inclusion of these particles will provide lubric- ity, the corrosion characteristics and electrical properties imparted to the con- nector shell must be considered and are being evaluated. ELECTROPLATED TIN ALLOYS Among the most mature and promising tin alloy coatings for electrical connec- tor shells are tin-zinc coatings. Tin-zinc electroplating processes are mature, commercially available systems that can deposit alloys of 20–30% zinc (balance tin) from an aqueous solution. Tin-zinc coatings have been considered promis- ing for cadmium replacement 2,7,17, and this finish was found to be a top per- former in past studies. 18 However, more recent studies have derived less positive results. An extensive study on potential cadmium replacements conducted by the NDCEE19 found that a proprietary tin-zinc coating failed both cyclic corrosion and wet notch environmentally influenced cracking (EIC) tests yet passed hydro- gen embrittlement and cooked EIC. A summary of test results from this effort can be found in Table 1.19 In this study, it was noted that the deposited tin-zinc coating was found to have an insufficient amount of zinc in the deposit to provide adequate corrosion protection (less than 1% zinc, versus the anticipated >20% zinc concentration found in more corrosion-resistant coatings that had been tested under related projects). This implies that, while tin-zinc does show promise for some appli- cations, some bath chemistries may not be robust enough to provide a consis- tent coating composition (and, hence, sufficient corrosion resistance) for the harsh environments to which military electrical connectors are routinely sub- mitted. Promising results under past studies imply that this candidate could pro- vide comparable performance to cadmium if the deposit composition could be made more consistent. It is noted that other tin alloys, specifically tin-indium coatings, are being con- sidered for both commercial and military applications, but these would take con- siderable development to be considered for electrical connector shells. ELECTROPLATED ZINC-COBALT Zinc-cobalt plating is typically used to finish relatively inexpensive parts that 314

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