Metal Finishing Guide Book

2013

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Figure 2. AlumiPlate-coated electrical connector, After 2,000 hours B117exposure.12 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 cadmium replacement.16 While the inclusion of these particles will provide lubricity, the corrosion characteristics and electrical properties imparted to the connector shell must be considered and are being evaluated. ELECTROPLATED TIN ALLOYS Among the most mature and promising tin alloy coatings for electrical connector 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 promising for cadmium replacement 2,7,17, and this finish was found to be a top performer 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 hydrogen 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 applications, some bath chemistries may not be robust enough to provide a consistent coating composition (and, hence, sufficient corrosion resistance) for the harsh environments to which military electrical connectors are routinely submitted. Promising results under past studies imply that this candidate could provide 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 considered for both commercial and military applications, but these would take considerable development to be considered for electrical connector shells. ELECTROPLATED ZINC-COBALT Zinc-cobalt plating is typically used to finish relatively inexpensive parts that require a high level of abrasion and corrosion resistance. This coating is reported to demonstrate particularly high resistance to corrosion in sulfur dioxide environments. Several suppliers of commercial electrical connectors offer connector shells coated with zinc-cobalt as a replacement for cadmium to meet RoHS criteria. Zinc-cobalt alloys are not commonly used in applications requiring heat treatment because these alloys have been reported to demonstrate reduced corrosion resistance when exposed to high temperatures. In one study20, after salt spray corrosion testing in accordance with ASTM B11714, zinc-cobalt-plated 356

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