Microstructural understanding of the oxidation of an austenitic stainless steel in high-temperature steam through advanced characterization

Abstract

It is well-known that steels always oxidize faster in the environments containing water vapour than in dry oxygen. Due to the difficulties in obtaining necessary experimental scale of observations, the mechanisms responsible for the steam-accelerated oxidation are still unclear. Through a combination of multiscale characterization techniques, the surface oxide film formed on an Fe-17Cr-9Ni stainless steel after exposure to high-temperature steam has been studied in detail. The characterization results obtained in this study reveal that the formation of the inner oxide layer, which is critical in protecting the base metal, is due to internal oxidation instead of external oxidation. The classic internal oxidation model underestimates the thickness of the inner oxide layer by one order of magnitude. This difference can be explained by the existence of fast diffusion channels in the inner oxide layer. This study provides direct evidence of a high density of nanopores in the oxide phase of the internal oxide layer, which can act as fast-diffusion channels if interconnected, and proposes their mechanisms of formation, a consequence of water dissociation-induced protons promoting the formation, migration, and clustering of both cation and anion vacancies.

The Morphology and Microstructure of Oxide Scale Grown on Austenitic Steel during Steam Oxidation at 700 °C for 500 h

“…The oxidation resistance of the currently reported steel at 700 °C was superior in comparison to the 17%Cr-9%Ni steel at lower temperature. The latter one, after 1500 h of steam oxidation developed a two layered, thick (~110 µm) oxide scale, consisting of outer Fe oxide and an inner FeCr 2 O 4 spinel [ 35 ], which was not foreseen for the currently tested material, since a well protecting Cr 2 O 3 layer was formed. Fine grained, Cu-modified 304 steel at 700 °C developed the thin and protective Cr 2 O 3 oxide scale, and the process underwent logarithmic kinetics [ 27 ] (despite the Authors’ claim that it is parabolic), whereas the coarse grain material oxidizes according to parabolic law and it developed a two-layered Fe 2 O 3 /FeCr 2 O 4 oxide scale.…”

Microstructure understanding of high Cr-Ni austenitic steel corrosion in high-temperature steam

“…The corrosion behaviours of these steels in high-temperature steam are very similar to that in carbon dioxide [55][56][57][58][59] and liquid metals [60][61][62][63] where low partial pressure of oxygen is ensued. Hence, the corrosion mechanisms of Fe-based steels in these different oxidation mediums are generally believed to be similar [64][65][66][67][68][69][70][71][72]. This multi-layered oxide scale normally contains both outer oxides above the original metal surface and inner oxides below this layer.…”

“…steam condition, are still not clear. It becomes even more challenging because of very fine oxide structures obtained in the steam environment which cannot be identified properly by conventional analysis methods [44,[70][71][72]. In the screening and development of proper corrosion-resistant structural materials for the application in high-temperature environments, understanding the fundamental mechanisms that govern material corrosion is crucial.…”

On the role of intergranular nanocavities in long-term stress corrosion cracking of Alloy 690

“…As a result, the mechanisms controlling long-term IGSCC crack growth at constant load remain unclear and require further study. The high-resolution microscopy techniques have achieved remarkable improvements recently, which have significantly advanced our understanding of a range of previously unresolved issues [15][16][17][18][19] . Here, we use a combination of microscale secondary electron microscopy (SEM), energy dispersive X-ray (EDX), electron back scattered diffraction (EBSD), nanoscale on-axis transmission Kikuchi diffraction (TKD), transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS) to study Alloy 690 before and after a long-term IGSCC testing at constant load.…”