The influence of SO2 on the high temperature corrosion of Fe and Fe-Cr alloys
Abstract: Sulfur dioxide is an important pollutant in combustion gases and plays a major role in the high temperature corrosion of many metals and alloys. This work investigates the influence of small amounts of SO2 on the high temperature corrosion of iron, chromium and several Fe-Cr alloys. The materials were exposed to carefully controlled environments consisting of pure O2 and O2 + 1-1000 ppm SO2 in the temperature range of 400-800 °C. Alloy 304L was exposed at 600 °C to O2, O2 + 40% H2O and O2 + 40% H2O + 100ppm SO2. Thermobalance and tube furnace exposures were carried out. The samples were analyzed by a number of surface sensitive techniques, including GI-XRD, SEM/EDX, AES, XPS, IC, GDOES, FIB and TEM. Under certain conditions, small amounts of SO2 reduce the rate of oxidation of iron and Fe-Cr steel in oxygen. The inhibitive effect of SO2 is attributed to adsorbed sulfate that forms on the surface. It is proposed that the adsorbed sulfate interferes with the reduction of oxygen molecules by blocking active sites on the oxide surface. When adsorbed sulfate does not form, e.g., because of high temperature, SO2 has no effect on the oxidation rate. SO2 also influences the structure of the scale. At 500 °C in oxygen, iron forms a scale consisting of an outer hematite layer and a duplex magnetite layer. It is shown that the inner magnetite layer became thinner in the presence of SO2, indicating that SO2 diminishes the inward diffusion of oxygen through the scale. Iron sulfide crystallites form at the metal/oxide interface in the presence of SO2. SO2 also inhibits the oxidation of alloy 304L in a mixture of water vapor and oxygen. Also in this case, sulfate formed on the scale surface. The rapid corrosion of alloy 304L in H2O/O2 mixtures is caused by the vaporization of chromic acid and results in the formation of a characteristic oxide island morphology. It is shown that the formation of surface sulfate slows chromium evaporation and it is argued that this reduces the rate of corrosion. The slowing of chromium evaporation is suggested to be caused by surface sulfate occupying the same surface sites as chromium (VI) oxide, which is an intermediary in the evaporation of chromic acid. In addition, SO2 reduces the growth in thickness of the oxide islands on alloy 304L. This effect is attributed to the inhibition of oxygen reduction on the surface.
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