Corrosion Mechanism of Alloy 310 Austenitic Steel beneath NaCl Deposit under Varying SO₂ Concentrations in an Oxy-fuel Combustion Atmosphere

Alkali chlorides and sulfates are well-known corrosive species found on superheater and reheater boiler tube deposits. The corrosivity of alkali chlorides toward Fe–Cr alloys is connected to the formation of molecular chlorine at the oxide surface. Chlorine can accelerate metal oxidation, establishing a self-sustained cyclic process of metal chloride and metal oxide formation, a corrosion phenomenon referred to as “active oxidation”. In the convective zone of a boiler, chlorine can be released by either sulfation of alkali chlorides by SOx or reaction of alkali salts (NaCl/KCl) present in deposits on heat-exchanger surfaces with metal and/or their oxides or both. To minimize the possible risk of corrosion induced by alkali chlorides, conversion of these chlorides to sulfates is discussed as an attractive solution. The aim of this study is to evaluate the influence of higher SO2 concentrations, during oxy-fuel combustion, on in-deposit sulfation of chloride salts and related corrosion mechanisms. To signify the influence of SO2, one exposure test (OI) was performed without SO2, while the other exposure test (OII) was performed with high SO2 content (1.5%), possibly obtained during oxy-fuel combustion while using a sulfur-rich coal. Synthetic NaCl salt was used as a deposit layer over alloy 310 (Fe–25Cr–21Ni) to evaluate its corrosion behavior at 650 °C in both atmospheres. The exposure atmosphere represents a typical oxy-fuel flue gas composition (test OI, 77.5% CO2 + 4.5% O2 + 18% H2O; test OII, 76% CO2 + 4.5% O2 + 18% H2O + 1.5% SO2). The exposure duration in each test was equal to 350 h. To evaluate the influence of the temperature, FactSage calculations were performed. Electron microprobe analysis (EMPA) was used to observe the extent of corrosion and elemental distribution at sample cross-sections to identify corrosion products. The depth of attack and corrosion progress faced by alloy 310 beneath the NaCl deposit was significantly reduced in the presence of SO2 in the exposed atmosphere. In the absence of SO2, “active oxidation” was the dominant corrosion mechanism, along with strong evidence of internal attack, while in the presence of SO2, none of these corrosion mechanisms was observed. Instead, the local sulfidation attack observed in the presence of SO2 points toward the possibility of type II “hot corrosion”.