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

Abstract

The effect of SO 2 (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% H 2 O at 600 °C, was investigated. In the absence of SO 2 (g), KCl accelerates the corrosion attack by the formation of K 2 CrO 4 . The reaction with KCl depletes the oxide in chromium and converts it into an iron-rich, poorly protective oxide. When SO 2 (g) was introduced to the gas flow, KCl rapidly transformed into K 2 SO 4 . In contrast to KCl, K 2 SO 4 does not form K 2 CrO 4 . Hence, it does not accelerate the corrosion rate. Although the conversion of KCl to K 2 SO 4 is fast, the corrosion rate of KCl samples exposed in the presence of SO 2 (g) is higher than samples exposed in the presence of K 2 SO 4 . It is therefore suggested that small amounts of unreacted KCl react initially with the protective oxide, forming K 2 CrO 4, which depletes the oxide in chromium. However, because of the presence of SO 2 (g), K 2 CrO 4 immediately reacts with SO 2 (g) to form K 2 SO 4 . 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.