Grain Boundary Assisted Crevice Corrosion in CoCrMo Alloys

Cobalt chromium molybdenum alloys have been extensively used for biomedical implants, but are susceptible to grain boundary corrosion resulting from local chromium depletion, which is called sensitization. This work extended the understanding of chromium depleted zones in CoCrMo alloys and their role in corrosion to the nanoscale. Selected boundaries were analyzed from the millimeter to the nanometer scale in order to link the chemical composition and crystallographic structure to the observed local corrosion properties. The shape and severity of grain boundary corrosion crevices were measured, linked with the coincidence site lattice geometry. Additionally, direct high-resolution energy dispersive x-ray spectroscopy maps of chromium depleted zones at the grain boundaries were measured to completely characterize the grain boundary properties. Chromium depleted zones were found in 100% of corroded grain boundaries, yet were too small to follow classical models of sensitization. Nanoscale regions of chromium depletion were found to have significant effects on corrosion initiation. This led to a grain boundary crevice corrosion model connecting the chemical composition with electrochemical driving forces that control crevice corrosion propagation. The conclusions and model presented can be used to better develop processing techniques for CoCrMo and other alloys.