PNNL

Abstract

High-resolution transmission electron microscopy and atom probe tomography are used to characterize the initial passivation and subsequent intergranular corrosion of degraded grain boundaries in a model Ni-30Cr alloy exposed to 360 °C hydrogenated water. Upon initial exposure for 1000 h, the alloy surface directly above the grain boundary forms a thin passivating film of Cr2O3, protecting the underlying grain boundary from intergranular corrosion. However, the metal grain boundary experienced severe Cr depletion and grain boundary migration during this initial exposure. After sputtering away the surface protective film with a glancing angle focused ion beam, this degraded, Cr-depleted grain boundary was exposed to 360 °C hydrogenated water for a further 1000 h. During this second exposure, the surface failed to repassivate, and intergranular corrosion was observed through the Cr-depleted region. These observations show that, although high-Cr alloys are resistant to intergranular attack and stress corrosion cracking due to passivating film formation, degradation-induced changes in the underlying metal at grain boundaries make the material more susceptible once the initial passive film is breached.