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 Cr 2 O 3 -Al 2 O 3 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-lm-thick belt in front of the pits (referred to as the white zone). In particular, the oxidation of the internal Cr 3 C 2 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 Cr 2 O 3 + Al 2 O 3 + 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.
In the present investigation, metallurgical characterisation of the nickel-based superalloy Nicrofer 602CA has been carried out following field exposure at approximately 540uC for 2 years. In general, the Nicrofer 602CA alloy is found to exhibit a high intrinsic resistance to metal dusting corrosion under the prevailing circumstances. Only a thin zone is affected by carbon intrusion, extending about 20 mm from the surface and into the bulk of the material. The subsequent field emission gun scanning electron microscope and synchrotron X-ray diffraction analyses of the exposed Nicrofer 602CA alloy revealed the presence of various phases at the corroded surface, including nepheline (NaAlSiO 4 ) and presumably also corundum (Al 22x Cr x O 3 ). None of these oxide phases are commonly observed in connection with metal dusting corrosion. It is believed that the nepheline phase forms as a result of reactions between corundum and the substances Na and SiO(g) transported to the surface by the syngas.
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