Performance of Zinc and Zinc Alloy Platings on Grade 8 Fasteners 960815
Cadmium has been identified by the United States Army's Tank and Automotive Command as a threat to worker health and the environment. Based on already completed cadmium substitute testing, an evaluation program was conducted to quantitatively analyze the resistance to vibrational loosening, torque-tension properties, environmentally assisted cracking susceptibility, and corrosion properties of zinc-based alternatives to cadmium, with and without supplemental coatings. The primary performance issues studied were fastener behavior after vibrational joint motion, mechanical performance in corrosive environments, and realistic corrosion performance. Data was generated from various laboratory evaluations and natural marine exposure of fasteners under loaded conditions. Test specimens were prepared by applying electrochemically sacrificial plating layers and supplemental coatings to 1/2-20 UNC Fine, Grade 8 fasteners and 4340 steel tensile specimens.
Program findings indicate that some zinc and zinc/cobalt plated fasteners experience a significant drop in breaking torque after cyclic joint motion that does not cause a similar change in cadmium fasteners. Zinc and zinc/cobalt alloy plating systems were also found to significantly lower the ductility of tensile test specimens when tested in a 3.5% salt solution environment at a slow, constant extension rate. Tin/zinc systems had less of an effect on the ductility of the constant extension rate samples. None of the coatings contributed to environmentally assisted corrosion cracking on test fasteners, or experienced a significant change in breaking torques during four months of marine atmospheric exposure under a nominal, dynamic load.