Mitigation of Fireside Corrosion in Power Plants: The Combined Effect of Sulfur Dioxide and Potassium Chloride on the Corrosion of a FeCrAl Alloy

Abstract

The corrosion behavior of a FeCrAl alloy (Kanthal APMT) was investigated in 5% O2 with 40% H2O plus 300 ppm of SO2 at 600 °C in the presence or absence of KCl, and the results were also compared to exposures performed without SO2 and KCl. The influence of preoxidation was also examined. The kinetics was followed using mass gain measurements, and the formed corrosion products were examined using XRD, SEM/EDX, AES, IC, and SIMS. The oxidation rate of Kanthal APMT was very low in O2/N2/H2O + 300 ppm of SO2, and the outward alumina growth appeared to be suppressed. Interestingly, no sulfur was detected at the scale/metal interface. KCl strongly accelerated the corrosion of Kanthal APMT in O2/H2O/N2 at 600 °C, forming K2CrO4 and gaseous HCl. Chromate formation depletes the protective scale in Cr, triggering the formation of a fast growing iron-rich scale. Adding SO2 suppressed the corrosion due to the conversion of the corrosive KCl to the stable K2SO4. If any K2CrO4 was formed on the surface of the material initially, it was also rapidly converted to K2SO4. Preoxidation of Kanthal APMT had a strong beneficial effect on the subsequent exposure at 600 °C in the presence of KCl and SO2, resulting in the formation of K2SO4 and the evaporation of HCl and KCl. In summary, the alumina-forming FeCrAl material Kanthal APMT is not completely inert to KCl in an oxidizing SO2-containing atmosphere at 600 °C. However, the corrosion rate is significantly lower than that of the commonly used chromia-forming alloy, 304L. Preoxidation decreases the corrosion rate even further, making Kanthal APMT a promising candidate material for combustion plant components, particularly from a corrosion point of view.