The role of potassium in high temperature corrosion of superheater steels

In our previous studies, we found out that in addition to potassium chloride (KCl), which is known to be corrosive at elevated temperatures, also potassium carbonate (K2CO3), a chloride-free potassium salt found occasionally in fly ashes, was found to increase the oxidation rate of several types of steels. This suggested that potassium has, along with chloride, an active role in the corrosion reactions. In addition, when the influence of KCl and K2CO3 was compared under both dry and humid conditions, it differed depending on salt.In this paper, the role of potassium in high temperature corrosion of steels is addressed by studying and comparing the reactions of two potassium salts, KCl and K2CO3 at elevated temperatures relevant to biomass combustion. Two different methods were used: 1) Simultaneous Differential Thermal and Thermogravimetric Analysis (DTA/TG), and 2) Scanning Electron Microscope with Energy Dispersive X-Ray Analysis (SEM/EDXA) for the cross section and elemental distribution analyses. Three different commercial steels; a ferritic, an austenitic, and a nickel-based, were used in the tube furnace exposures to mimic the genuine superheater tube materials, whereas pure chromium and chromium oxide were used in the DTA/TG tests to study the separate reactions.Both potassium salts were found to be corrosive. In the exposure studies with the genuine tube materials, the nickel-based alloy had the best corrosion resistivity, and the ferritic steel the worst. The presence of water vapor affected the oxidation behavior of the steels in the cases where no salt or KCl was present. However, the oxidation behavior of the steels in the presence of potassium carbonate was not affected remarkably by water vapor. Both salts were found to produce potassium chromate (K2CrO4) as an intermediate.