Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 316 of 707

require a high level of abrasion and corrosion resistance. This coating is report- ed to demonstrate particularly high resistance to corrosion in sulfur dioxide envi- ronments. Several suppliers of commercial electrical connectors offer connector shells coated with zinc-cobalt as a replacement for cadmium to meet RoHS cri- teria. Zinc-cobalt alloys are not commonly used in applications requiring heat treatment because these alloys have been reported to demonstrate reduced cor- rosion resistance when exposed to high temperatures. In one study20 spray corrosion testing in accordance with ASTM B11714 , after salt , zinc-cobalt-plated sleeves showed considerably less corrosion resistance after one hour heat treat- ment at 250°F as compared to the as-plated condition. While this process was ini- tially considered as being a worthy cadmium replacement, the questionable characteristics under high-temperature environments excluded its consideration under further review. ELECTROPLATED ZINC-NICKEL Zinc-nickel electroplating processes are mature, commercially available sys- tems that can deposit alloys of 5–15% nickel (balance zinc) from an aqueous solu- tion. Zinc-nickel alloys can be deposited from both acid and alkaline processes. Boeing has found that the alkaline process is easier to maintain and provides a more consistent coating composition.5 From a performance standpoint, the NDCEE found that a proprietary acid zinc-nickel coating with CCC passed bend adhesion, paint adhesion, and hydrogen embrittlement tests, but dis- played only marginal EIC performance19 (see Table 1). The corrosion resistance was significantly less than the cadmium baselines, but increased coating thick- ness and selecting a suitable conversion coating may improve those results— although the implications of these changes to the form, fit, and function of the electrical connector would need to be identified. The proprietary alkaline zinc- nickel coating with a CCC performed similarly to the acid zinc-nickel in this study19 (see Table 1). Previous TARDEC work also found alkaline zinc-nickel coat- ings with a CCC to be promising for some electrical connector designs, particularly on MIL-C-83513 microminiature D-subminiature connectors, but less promis- ing on other connector designs. Based on these promising results, zinc-nickel has seen implementation as a cad- mium replacement process in several areas. The NDCEE work19 provided infor- mation that assisted Rolls Royce Defense Aerospace in qualifying zinc-nickel as an acceptable alternative to cadmium on the T56 engine system. Boeing also found that zinc-nickel plating is an acceptable coating to replace cadmium on com- ponent parts made of low strength steel (less than 200 ksi), stainless steel, alu- minum, and copper alloys.1 Other ongoing projects involving this process include the aforementioned part- nership between Lockheed-Martin, Alcoa, and the U.S. Air Force, which is eval- uating several coatings, including both acid and alkaline zinc-nickel, to replace cadmium for military and commercial fasteners.15 It is recognized that both acid and alkaline zinc-nickel processes may provide an acceptable alternative coating for cadmium in many applications. Acid zinc- nickel processes have traditionally been used; however, some embrittlement issues have been related to this process.1 For this reason, Boeing restricts the use of acid zinc-nickel to steels with ultimate tensile strength of 220 ksi or less. While these issues may not be relevant for electrical connectors, a post-process 315

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