Corrosion of an industrial alloy: experimental and computational study

S. SIMONETTI; C. LANZ; G. BRIZUELA; A. JUAN

Conferencia; International Conference for Academic Disciplines; 2014

High-temperature corrosion is generally known as a material degradation process that occurs at the surface of engineering components. In the case of internal corrosion, the corrosive species penetrates into the material by solid-state diffusion leading to the formation of internal precipitates, for instance, oxides, nitrides and carbides (carburization) and resulting in a strong deterioration of the properties of a material. This investigation combines microstructural characterization analyses and atomistic modeling to better elucidate the mechanism of stress corrosion cracking of an iron-nickel-chromium alloy exposed to high temperature. We compared two samples of an austenitic cast stainless steel (Fe-25Cr-20Ni-0.40C), the as-cast material with the same one obtained of the tubes of the cracking furnace from a petrochemical plant after 25,000 h of service at 900-1200 ºC. In the service-exposed material, we observed bigger carbides in the austenitic matrix comparing with the as-cast material, cavities between the grains and micro fissures in grain edges, indicating that the material present damage caused by creep. The theoretical calculations help us to interpret the changes in the alloy electronic structure and the chemical bonding after the corrosion phenomena. The atomic orbital occupations of the metallic bonds close to the carbon atoms are affected. The mainly changes are presented in Ni 4s and Fe, Cr 4p orbitals. An electron transfer to the carbon atoms from the chromium, nickel and iron nearest neighbor atoms is observed. The strengths of the, Cr-Ni, Fe-Cr, Fe-Ni and Ni-Ni bonds nearest neighbors to carbon atoms are the most affected. The metallic bond weakening is mainly a consequence of the new C-Ni, C-Fe and C-Cr interactions formed after carburization.