Utilization of stable oxygen isotopes in high-temperature corrosion studies

Substitution of fossil fuels with renewables such as biomass and waste-derived fuels has become a part of the key strategy in the battle against global warming. From the environmental point of view, the main advantage of biomass is that it can be considered a CO₂ neutral fuel. In addition, large and reasonably stable supplies of biomass can be found in many countries, in addition to which several countries offer tax relieves for power plants using renewable fuels. However, from a chemical point of view, biomass is an inhomogeneous fuel, usually with a high concentration of water and considerable amounts of potassium and chlorine, all of which are known to affect the durability of superheater tubes. This study focuses on the role of water vapor in high-temperature corrosion by introducing the simultaneous use of oxygen isotopes ¹⁶O and ¹⁸O. The use of oxygen isotopes clarifies the reaction mechanism of high-temperature corrosion in cases where multiple oxygen sources are available. The main goals of the study were: 1) to evaluate the applicability of ToF-SIMS in high-temperature corrosion studies; 2) to shed more light on the corrosion behavior of various steels under conditions, where potassium chloride and multiple oxygen sources are present.

Three steels (a low alloy ferritic, an iron-based high alloy austenitic, and a nickel-based high alloy austenitic steel) currently utilized in biomass- and waste-fired boilers, were exposed to KCl at 540°C under different gas atmospheres for 120 minutes. Part of the exposures were carried out under dry conditions and part under humid conditions containing water vapor labeled with the oxygen isotope ¹⁸O. After each exposure, all the samples were analyzed by means of scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDX) with a focus on surface morphologies and chemical compositions of local structures. Furthermore, X-ray photoelectron spectroscopy (XPS) was used to measure the thicknesses and chemical compositions of the formed oxides. In addition, the samples exposed to atmospheres containing the oxygen isotope ¹⁸O were analyzed by means of Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) to distinguish ¹⁶O from ¹⁸O.

ToF-SIMS proved to be an applicable tool in high-temperature corrosion studies, in this case, providing detailed information about the distribution of the two oxygen isotopes at the sample surfaces. The oxygen atoms originating from the water molecules prefer reacting directly with the steel surface, whereas the oxygen molecules from the air are mainly involved in a reaction with potassium chloride, forming potassium chromate (K₂CrO₄) crystals on the sample surface. Regardless of the prevailing conditions, the nickel-based high alloy austenitic steel withstood corrosion best, then the iron-based high alloy austenitic steel, whereas the low alloy ferritic steel had the poorest corrosion resistance ability. Interestingly, thinner oxide layers were formed on all three steels under humid conditions. This originated most likely from the formation of HCl, a chlorine-containing gaseous species, which is assumed to transport the reactive chloride ions away from the steel surface. Under dry conditions, no such species can form, which gives the chloride ions more time to interact with the steel, resulting in thicker oxide layers.