New insights into the oxidation mechanisms of a Ferritic-Martensitic steel in high-temperature steam

Abstract

The microstructure of the surface oxide film formed on an Fe-9Cr ferritic-martensitic (F-M) steel after exposure to deaerated high-temperature steam at 600 °C for 100 h has been analyzed in detail by advanced characterization techniques. The surface oxide film has been revealed to have a triplex structure, including an outer oxide layer, an inner oxide layer, and an internal oxide layer. Although the outer and inner oxide layers are continuous, the internal oxide layer has been proved to consist of interconnected metallic and chromite phases, which is a typical feature of internal oxidation. The formation mechanisms of each layer have been discussed, finding that, contrary to what the available space model suggests, an external oxidation is not the controlling oxidation mechanism of F-M steels in high-temperature steam. The higher resolution used in this study confirms that the controlling mechanism is internal oxidation.

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

“…The well-polished coupons were then exposed to the deaerated high-temperature steam at 600 C in 25 MPa for 1500 h. The content of dissolved oxygen in the steam was controlled at lower than 10 ppb by bubbling high-purity Ar through the water. More details about the high-temperature testing can be found in [44,[70][71][72]. In a Zeiss Merlin Scanning Electron Microscope (SEM) equipped with an Oxford Instruments Energy Dispersive X-ray (EDX) detector system, the microstructure and chemical composition of the surface oxide scale developed on 800H alloy were studied.…”

“…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.…”

Magnetic Evaluation of Heat-Resistant Martensitic Steel Subjected to Microstructure Degradation

“…Heat-resistant martensitic (HRM) steels are widely employed in thermal and nuclear power plants, owing to their good creep and oxidation resistance, together with excellent thermal properties and low cost as compared to bainitic steels and austenitic steels [ 1 , 2 , 3 , 4 , 5 , 6 ]. The microstructural degradation of HRM steels during operation at elevated temperatures impairs the performance of these steels in thermal and nuclear power plants.…”

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.…”