Evaluation of Alternatives to Electrodeposited Cadmium for Threaded Fastener Applications 2006-01-3169
Cadmium (Cd) plating offers good corrosion resistance, lubricity, solderability, adhesion, and ductility, as well as consistent torque-tension and uniform thickness on components with complex geometries. However, the intrinsic Environmental, Health, and Safety issues associated with Cd have driven many users to seek alternatives. Currently, various Cd replacement teams/programs such as the Joint Group on Pollution Prevention (JG-PP)/Boeing, the Joint Cadmium Alternatives Team (JCAT), the Canadian Cadmium Replacement Program (CCRP), and the REFOCUS Program (AEA Technology, European) have been investigating alternatives to Cd plating. Some of the current coatings being considered in the aerospace industry include Zn-Ni alloy plating, electrodeposited aluminum, electroless nickel, nickel composite, and molten salt aluminum manganese (Al-Mn). Each option has its own particular characteristics; however, most experiments have been conducted on aerospace structural panels or components. There is a need to evaluate various alternative coatings for complex geometries such as threaded fasteners.
To this end, Lockheed Martin Aeronautics and Alcoa Fastening Systems have teamed to evaluate Cd alternatives for threaded fasteners used in the inlet of F-16 aircraft. One F-16 manufacturing process generates Cd dust from the fastener heads during sanding. The United States Air Force has funded Lockheed Martin Aeronautics to qualify a replacement for the Cd. The primary fasteners in question are NAS 1580 and NAS 4452 fasteners. Four candidate coatings were selected for evaluation: electroless nickel (EN), electroless nickel composite (EN-PTFE), electrodeposited surface mineralization based zinc-nickel (Zn-Ni), and electroplated aluminum (Al). Stress corrosion, salt spray (fog) tests, torque-tension relationship, locking and breakaway torque measurements, and push-in and interference tests were used to evaluate alternatives to Cd coatings used for corrosion protection and lubricity on threaded parts. For comparison, Cd plated fasteners were tested at equivalent conditions as the controls.
Testing results showed that the surface mineralization based Zn-Ni coating best seems to mimic the overall characteristics of fasteners coated with cadmium. However, no one coating appears to offer the same broad range of properties as Cd plating. Salt spray tests indicated that EN and EN-PTFE exhibited red rust after 96 hours exposure. However, EN-PTFE demonstrated the best coefficient of friction feature, having the least overall torque to achieve the required tension. During torque tension tests, Zn-Ni performed nearly as well as Cd plating. Locking and break-away tests showed that Zn-Ni, EN-PTFE, and Cd nut/bolt sets meet the specification requirements. Al, Zn-Ni, and Cd showed acceptable resistance to stress corrosion up to 400 hours. However, only Al passed the 587 hours consistent load stress corrosion tests. Push-in tests revealed Cd plating required the minimum load to insert fasteners into aluminum structure with interference, followed by Zn-Ni. Furthermore, cross-sectional metallurgical examinations and interference tests were conducted to evaluate coating characteristics and adhesion strength between substrate and coating.