A microstructural investigation of Fe-oxides for developing high-temperature corrosion lifetime prediction models

Abstract

The aim of this study was to investigate the microstructure of oxide scales formed in environments relevant to biomass- and waste boilers used for superheater appli cations. It also investigated how KCl can a!ect both the corrosion kinetics and mi crostructure, specifically the characteristics of oxide grain boundaries. The obtained data provides insights into high-temperature corrosion and ultimately help improve high-temperature corrosion lifetime prediction models for superheater tubes. The study starts with a literature review about corrosion and analytical methods. To simulate the environment that exists for the superheater tubes of biomass- and waste power plants, the furnace’s environment was prepared to attain constant levels of the following: 20 % water vapor, 5 % O2 and 75 % N2 at 400 °C. Each exposure of the sample groups was performed according to four phases, namely sample prepara tion, exposure to simulation of the environment, preparation before SEM analysis, and SEM analysis. Results showed that pure Fe samples exposed longer to the simulated environment experienced the corrosion process to a greater extent, which results in a thicker ox ide scale on the Fe sample surface. In addition, KCl appears to have increased the corrosion rate, as demonstrated by the greater oxide scale thickness, the higher mass gain of the samples, and the observability of oxide grains after both exposure times. The combined use of SEM-EDS (EDX) made it possible to perform EDX mapping and EDX point analysis of the exposed samples. This facilitated the identification of the Fe sample surface, the Fe-rich oxide scales, and the KCl layer located on top of the oxide scales. The flat BIB milling technology implemented during sample preparation prior to SEM imaging enabled the identification and di!erentiation of oxide grains, BIB milling marks, and mechanical polishing scratches. This indicates that there is a need for improvement in the flat BIB milling technique, which could be achieved by testing variations in the accelerating voltage, the angle of incidence of the ion beam, and the flat BIB milling exposure time. In conclusion, the flat BIB milling method, when combined with SEM, has the potential to reveal oxide grains at the nanoscale, making the observation of these oxide grains in the oxide scale more accessible than with widely utilized conventional technologies. Nevertheless, this study also shows that the method requires further development. Therefore, future work should focus on optimizing the BIB parameters to achieve the best possible Fe surface finish. Lastly, exposure time and the presence of KCl appears to have a positive e!ect on the detection of oxide grains and the growth of the oxide scale on pure Fe samples.