A Model for High-Temperature Pitting Corrosion in Nickel-Based Alloys Involving Internal Precipitation of Carbides, Oxides, and Graphite

The present investigation is concerned with fundamental studies of the mechanisms of pitting corrosion in the Ni-based alloys 602 and 693, following long-term exposure to syngas at 540 °C and a 35-bar total pressure. The 4-years’ plant-exposed alloys were examined using synchrotron X-ray diffraction (XRD) in combination with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It is concluded that the pitting corrosion attacks start when carbon diffuses into the bulk of the alloys following the breakdown of the protective Cr₂O₃-Al₂O₃ surface oxide layer. During the incubation period, this oxide layer provides an effective barrier against carbon intrusion by virtue of its ability to restore cracks and flaws through diffusion. The corrosion pits then grow by a process of internal carburization and oxidation, in which carbides, oxides, and graphite form separately within an approximately 30-μm-thick belt in front of the pits (referred to as the white zone). In particular, the oxidation of the internal Cr₃C₂ carbides occurring close to the white zone/pit interface is associated with large volume changes. This volume expansion results in the buildup of high mechanical stresses within the white zone and, eventually, to the complete disintegration of the original alloy matrices into a layered pit microstructure consisting of Ni + Fe and Cr₂O₃ + Al₂O₃ + graphite. The observed microstructural changes have been rationalized through detailed modeling of the physical reactions involved, leading to the development of new and comprehensive models for high-temperature pitting corrosion in Ni-based alloys.