Impact of Forming, Welding, and Electropolishing on Pitting and the Surface Finish of SRF Cavity Niobium

A broad range of coupon electropolishing experiments are described to ascertain the mechanism(s) by which large defects are formed near superconducting radio-frequency (SRF) cavity welds. Cold-worked vs. annealed metal, the presence of a weld, and several variations of electropolishing (EP) parameters were considered. Pitting is strongly promoted by cold work and agitation of the EP solution. Welding also promotes pitting, but less so compared with the other factors above. Temperature increase during EP did not strongly affect glossiness or pitting, but the reduced viscosity made the electrolyte more susceptible to agitation. The experiments suggest that several factors that are rather benign alone are combined by the cavity forming, welding, and processing sequence to promote the formation of defects such as pits. Process changes to mitigate these risks are discussed.