Metallurgical Investigation of Metal Dusting Corrosion in Plant Exposed Nickel-Based Alloy 602CA

In the present investigation metallurgical characterisation of the nickel-based superalloy Nicrofer 602CA has been carried out following field exposure at approximately 540˚C 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, extending about 20μm from the surface and into the bulk of the material is affected by carbon intrusion. The subsequent Field Emission Gun Scanning Electron Microscope (FEG SEM) and Synchrotron X-ray diffraction (S-XRD) analyses of the exposed Nicrofer 602CA alloy revealed the presence of various phases at the corroded surface, including nepheline (NaAlSiO4) and presumably also corundum (Al2-xCrxO3). 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.