Evaluation of Alternatives to Electrodeposited Cadmium for Threaded Fastener Applications (II) 2008-01-2312
Cadmium electroplating is coming under increasing pressure due to both environmental and worker safety issues. Since 2005, Alcoa Fastening Systems (AFS) and Lockheed Martin have been conducting a collaborative research program to identify the most appropriate fastener coating materials for a Cadmium (Cd) plating replacement. Four candidate coatings were selected for the initial Phase I evaluation: electroless nickel (EN), electroless nickel composite (EN-PTFE), electrodeposited surface mineralization based zinc-nickel (Zn-Ni), and electroplated aluminum (Al). The Phase I testing results indicated that the Zn-Ni and Al coatings were the best of the four candidates for Cd replacement. However, it is hard to conduct direct comparisons with different coating thicknesses, surface treatments, and lubrication among various Cd alternatives. Thus, further evaluation with more careful control of these parameters would be necessary. Phases II and III focused on more detailed characteristics of Zn-Ni and Al, along with a reference Cd plating. A variation of Zn-Ni plating using a trivalent chrome treatment (from here on designated as Zn-Ni2) was also included in Phase II and Phase III testing at the request of AFS.
During this study, all testing was designed to simulate the typical use of threaded fasteners. Detailed, carefully controlled experiments and microscopy studies were used, including general coating and tribological characteristics, fastener mechanical properties, corrosion performance, galvanic corrosion resistance, lubricity and vibration resistance, and adhesion and push-in tests. The primary fasteners used in this evaluation were NAS1580A3T14 fasteners. In stress corrosion tests, two slightly different joint systems were adopted to determine susceptibility of fastener/structural materials/coating to stress corrosion cracking up to 1000 hours. First, the standard stress corrosion testing per NASM 1312 using stainless steel was conducted, followed by a similar test using an Al 7075 fixture and nuts to aid in determining galvanic compatibility. In both salt spray and stress corrosion tests, corrosion potential measurements were conducted.
Testing results showed that all three candidates seem to mimic the overall characteristics of fasteners coated with cadmium in many aspects. However, no one coating appears to offer the same broad range of properties as Cd plating. Fatigue tests indicated that both Zn-Ni and Zn-Ni2 coated fasteners exhibited a lower fatigue life relative to others. However, the Zn-Ni candidate demonstrated the best adhesion and push-in tests, requiring the least overall load to push plated fasteners into the interference hole, and had the least variability from part to part. Data for Al and Zn-Ni2 show a mean break-away torque value dropping below specification limit on multi-cycle breakaway torque tests. But both candidates also show very low static zero-load friction coefficient. In corrosion testing, no evidence of stress corrosion and “structural” salt spray corrosion was found on any of the coated systems tested. Based on the corrosion results and potential data measured, there appears to be no significant differences in performance observed. Furthermore, fastener tensile and double shear tests were performed to evaluate plating effects on fastener mechanical properties.