Investigation of Pitting in Large-grain Niobium Samples of Different Purity

Niobium samples cut from large-grain discs with residual resistivity ratio ranging between 60 and 300 were mechanically polished and subjected to chemical treatments similar to those applied during the fabrication of superconducting radio-frequency cavities for particle accelerators. Pitting occurred during the chemical treatments and the pit density was higher in the samples of lower purity. Electropolishing resulted in the lowest density of pits, regardless of sample purity, compared to buffered chemical polishing. A correlation between high pit density and grains with 100 orientation of the exposed surface was found, however, no such correlation emerged with respect to the dislocation density.