Alkali Induced Corrosion of 304L, The Influence of Potassium Salts and Water Vapour

This work investigates the influence of alkali salts on the oxidation of 304-type (Fe18Cr10Ni) austenitic stainless steel in laboratory and field exposures. Stainless steel coupons were exposed isothermally in laboratory furnaces at 400-600 °C in 5% O2 and 5% O2 + 40% H2O. Thermobalance exposures were also performed. The samples were coated with KCl, K2CO3 or K2SO4 prior to exposure. The amount of salt was equivalent to 0.1 mg/cm2 KCl, based on potassium. The exposure time was 1-168 hours. Uncoated samples were exposed for reference. In the field exposures, sample rings mounted on a cooled probe were exposed in the 75MW waste fired boiler at Händelö, Norrköping, Sweden. The fuel was a mix of household and industrial waste. The exposure time was set to 10 hr and the material temperature was 450 °C. The flue gas temperature was 650°C. Field exposures were performed with and without adding sulphur to the fuel. The samples exposed in the field and in the laboratory were analysed by SEM/EDX, XRD, FIB, TEM, IC and AES. The results from the laboratory exposures show that KCl and K2CO3 strongly accelerate the high temperature corrosion of 304L. Corrosion starts by the formation of potassium chromate(VI) through the reaction of potassium with the protective oxide, i.e. the initiation step. Chromate(VI) formation is a sink for chromium in the oxide and leads to a loss of its protective properties. This results in rapid oxide growth. In the propagation stage there is a thick scale consisting of an outer porous hematite layer with K2CrO4 particles and an inner layer consisting of spinel oxide, (Fe,Cr,Ni)3O4. The duplex scale is relatively poorly protective and is permeable to, e.g., chloride ions. In contrast to KCl and K2CO3, K2SO4 does not react with chromium in the oxide to form chromate. Therefore, potassium sulphate does not destroy the protective oxide and is only slightly corrosive towards 304L. The field exposures focused on the effect of adding sulphur to the fuel on corrosion. The results showed that adding sulphur to the fuel reduced the amount of deposits and corrosion products formed. Chromate(VI) was detected only on samples exposed in the absence of sulphur additions