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

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.