Evaluation of Alternatives to Electrodeposited Cadmium for Threaded Fasteners Applications (III) 2009-01-3228
Since 2005, Alcoa Fastening Systems (AFS) and Lockheed Martin have been partnering to identify a Cadmium (Cd) plating replacement for threaded fasteners. Previously reported Phase I, II and III studies resulted in alternative coatings that indicated promise as suitable plating materials. Phase IV and V studies continued the program by testing two different fasteners (NAS1580 and NAS4452) manufactured in AFS facilities. Testing included plating material characterization such as coating thickness, torque-tension relationships, locking and breakaway torque measurements, salt spray (fog) corrosion, stress corrosion, and push-in and interference properties. Additionally, mechanical properties of the plated fasteners were tested (tensile, double shear, durability, and fatigue). Candidates included two electroplated zinc-nickel coating systems (Zn-Ni and Zn-Ni2) and an electrodeposited aluminum coating (AI).
Testing results consistently showed that all three candidates were quite similar in the overall characteristics of fasteners coated with cadmium. Fatigue tests showed very large variations among different kinds of fasteners, even within the same group. Multi-cycle run-on and break-away tests showed a continuous dropping of torque values for Al as the cycle numbers increase. On the other hand, the Zn-Ni coating demonstrated increased values, especially for the NAS1580 fasteners. The Zn-Ni2 coating showed fairly stable values for both NAS1580 and NAS4452 fasteners, including consistency from cycle to cycle and within each cycle. Due to surface lubrication, all Cd alternatives have similar torque to achieve the required tension for both NAS1580 and NAS4452 parts. Furthermore, fastener tensile and double shear tests revealed all coated fasteners meet product specifications.