Anomalous Fatigue Failure of Nickel 201 Capacitor Electrical Leads

The presence of hydrogen can profoundly affect the fracture behavior of engineering alloys. Alloys such as commercially pure nickel (Nickel 201) which are regarded as comparatively less susceptible to environmentally assisted cracking than classically susceptible high-strength ferritic alloys can exhibit brittle fracture surface morphology under both cyclic and monotonic loading. This presentation details a unique failure analysis of an electrolytic tin plated Nickel 201 (Ni201) capacitor wire lead that failed in a macroscopically brittle manner during routine electrical function testing with no intentional mechanical loading. Scanning electron microscopy revealed the lead wire fracture surfaces contained predominantly smooth intergranular fracture with interspersed transgranular fracture with striations. Bend testing, microstructural, and bulk composition analysis of the lead wire indicated no operative embrittlement despite the observation of intergranular fracture features on broken leads. Further analysis of the capacitor testing apparatus revealed unanticipated cyclic flexural loading of the wire due to operational vibration from a compressor within the test chamber cooling system resulting in high-cycle fatigue failure of the lead wires. Hydrogen present in the Ni201 wire from cathodic charging during the electrolytic tin plating process contributed to the unique intergranular fracture surface morphology. Changes to the test apparatus fixturing to mitigate cyclic flexural load eliminated subsequent wire lead fatigue failures.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525.