Sigma and Random Grain Boundaries and Their Effect on the Corrosion of the Ni-Cr-Mo Alloy 22

Abstract

The corrosion of the Ni-Cr-Mo alloy C22, has been studied in a simulated crevice corrosion environment (3 M NaCl + 1.5 M HCl at 75°C). Analyzing the corroded surface by electron backscatter diffraction showed that corrosion damage localized predominantly in random as opposed to Σ3 grain boundaries. Scanning transmission electron microscopy and electron energy loss spectroscopy showed that the susceptible random boundaries were decorated with needle-like inclusions enriched in O and depleted in Ni, suggesting a slight degree of internal oxidation during processing. The presence of line dislocations in the vicinity of the random boundaries may also have contributed to their corrosion susceptibility.