High Temperature Corrosion of Stainless Steels - The Effects of Chloride Salts and SO2(g)
The high corrosion rate of superheaters in waste- and biomass- fired plants is often linked to the presence of alkali chlorides. One way to mitigate the corrosion is to replace the superheater material with more corrosion- resistant materials. Another way is to change the corrosive environment by introducing fuel additives. In both cases it is important to know as much as possible about the corrosion process.
This study, investigates the corrosive effect of alkali chlorides and the beneficial effect of using sulphur-containing additives by laboratory and field exposures. In the laboratory study, the influence of three chloride salts, KCl, NaCl and CaCl2, on the oxidation of 304L type (Fe18Cr10Ni) austenitic stainless at 600°C in O2 + H2O is investigated. The effect of sulphur is tested by adding 300 ppm SO2 to the KCl exposure. In the field study the effect of sulphur is investigated by adding sulfur-rich municipal sewage sludge to the fuel which is rich in alkali and chlorine.
The laboratory results show that KCl and NaCl strongly accelerate the high temperature corrosion of 304L. The corrosion attack is initiated by the formation of alkali chromates through the reaction of alkali with the protective oxide. Chromate formation is a sink for chromium in the oxide; this leads to a loss of its protective properties. In contrast to NaCl and KCl, CaCl2 is not very corrosive under the exposure conditions tested. CaCl2 is rapidly converted to CaO and only small amounts of CaCrO4 were detected in areas where CaO was in direct contact with the scale. The addition of SO2 to the gas results in a drastic reduction of the corrosion rate of the KCl treated samples. The corrosion mitigating effect of SO2 in this environment is attributed mainly to the rapid conversion of KCl to K2SO4. Unlike KCl, K2SO4 does not deplete the protective oxide in chromium by forming K2CrO4. It is also suggested that the formation of a thin sulphate film on the oxide surface impedes chromium volatilization in O2 + H2O environment and that it may decrease the rate of oxygen reduction on the oxide surface. The field study shows that the addition of sulphur rich material to the fuel mitigates corrosion in the superheater region of the power plant. Without additives, a thick corrosion product layer formed and the deposit was dominated by alkali chlorides. With additives, the amount of alkali chlorides in the deposit layer was very low and the steel was protected by a thin oxide. It is concluded that the beneficial effects of sulphur observed in the field can be explained in analogy to the lab experiments.
Keywords: Alkali induced corrosion, Sulphur additives, Biomass, Waste, Stainless Steels