Mitigation of Fireside Corrosion of Stainless Steel in Power Plants: A Laboratory Study of the Influences of SO2 and KCl on Initial Stages of Corrosion
Journal article, 2014
The effect of SO2(g) on the initial oxidation of the stainless-steel 304L, sprayed with 0.1 mg/cm(2) KCl and exposed in 5% O-2 and 40% H2O at 600 degrees C, was investigated. In the absence of SO2(g), KCl accelerates the corrosion attack by the formation of K2CrO4. The reaction with KCl depletes the oxide in chromium and converts it into an iron-rich, poorly protective oxide. When SO2(g) was introduced to the gas flow, KCl rapidly transformed into K2SO4. In contrast to KCl, K2SO4 does not form K2CrO4. Hence, it does not accelerate the corrosion rate. Although the conversion of KCl to K2SO4 is fast, the corrosion rate of KCl samples exposed in the presence of SO2(g) is higher than samples exposed in the presence of K2SO4. It is therefore suggested that small amounts of unreacted KCl react initially with the protective oxide, forming K2CrO4, which depletes the oxide in chromium. However, because of the presence of SO2(g), K2CrO4 immediately reacts with SO2(g) to form K2SO4. This study shows that the initial stages of the corrosion attack are of great importance. The initial complex interactions between the flue gas, deposit, and oxide scale affect the future corrosion resistance of the steel.