Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 310 of 707

• Environmental resistance, particularly corrosion resistance • Galvanic coupling • Inhibition of algae growth • Low cost • Lubricity (meets established torque/tension requirements) • Shock resistance • Solderability • Temperature resistance • Vibration resistance CCCs are applied over cadmium coatings to provide additional properties, the most important of which are: • Enhanced corrosion resistance • Paintability (meets requirements for paint adhesion to coating) • Color As seen above, the synergistic benefits provided by this coating system have made its replacement challenging. One example of the unique protective prop- erties that are imparted by cadmium and hexavalent chromium is corrosion resistance. Cadmium coatings provide galvanic corrosion protection to electri- cal connector shells, and very few metals can provide a similar level of corrosion protection in this application. This is demonstrated by the position of cadmium in the galvanic series, shown in Figure 1. 1 Figure 1 demonstrates that zinc and zinc alloys, beryllium, magnesium, and aluminum alloys are generally the most active metals in corrosive environments and, therefore, are the only materials that can provide sacrificial corrosion pro- tection similar to cadmium in this application. However, beryllium is more haz- ardous than cadmium, and magnesium and pure zinc both corrode too rapidly for many engineering applications. Other examples of the unique properties that are imparted by cadmium to con- nector shells involve electrical properties, specifically EMC and EMI effectiveness. Connector mating resistances must be kept to a minimum (e.g. less than 2.5 mil- liohms) for EMC, because greater mating resistances can lead to high voltages (and subsequent failures) when induced by sudden surge currents (such as a lightning strike). Cadmium-plated connectors meet this requirement throughout the life of the connector (i.e., the corrosion products of cadmium are generally non-insu- lating). Other plated coatings, such as electroless nickel (EN), meet this require- ment initially, but lose effectiveness over time due to the resistances that are gen- erated by corrosion products. VIABLE ALTERNATIVES TO CADMIUM The DoD has been interested in cadmium replacement for many years, and numerous potential replacements have been identified and explored in past work conducted by Brooman2,3,4 , Gaydos5 , Klingenberg3, 4, 6 , Legg1 ; and Shahin7 , among many others. It is the intent of this paper to summarize past work that has been accomplished in this area, with the intent of providing a rationale for the 309

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