New chemical mechanism explaining the breakdown of protective oxides on high temperature steels in biomass combustion and gasification plants
Biomass is considered a replacement fuel over fossil fuels to mitigate climate change. The switch to biomass in the combustors changes the inorganic chemistry of the flue gases and leads to more severe corrosion of the construction materials of the combustors. The integrity of most high temperature steels relies on the formation of a protective Cr2O3 layer on the steel surface at a high temperature environment. The ash compound found on the heavily corroded steel in biomass combustion and gasification plants is KCl, but the mechanism, which triggers the breakdown of the protective Cr2O3 layer under the KCl salt is not known. We studied the chemical reactions involved with furnace exposure of KCl and KOH with Cr2O3 and identified the formed reaction products with XRD analysis. The amount of reaction products was analyzed from the leachates of the salt-oxide mixtures by UV/VIS spectroscopy. We also used thermodynamic Gibbs energy minimization calculations to evaluate the evolution of reactions as a function of temperature. The results suggests that the reaction of KCl with Cr2O3 involves a KOH reaction intermediate that forms before K2CrO4 is formed. The amount of reacted potassium as a function of temperature follows the trend of KCl decomposition to KOH and HCl(g) as predicted by thermodynamics calculations. Therefore, we argue that the suggested overall reaction of KCl with Cr2O3 as found in the corrosion literature: , starting with the initiation step: KCl + H2O(g) ⇒ KOH + HCl(g) and then the formed KOH reacts with Cr2O3 to form K2CrO4. This explains the initial breakdown of the protective Cr2O3 under KCl salt in water containing high temperature atmospheres. The result is essential for the development of new alloys for biomass fired combustors.