The High Temperature Corrosion of Alloy Sanicro 28

Licentiatavhandling, 2006

This thesis investigates the influence of potassium chloride in solid and gaseous form on the oxidation of alloy Sanicro 28 (35Fe27Cr31Ni). KCl induced corrosion of chromia forming alloys is important e.g. in fireside corrosion in biomass and waste fired power plants. Polished steel coupons were isothermally exposed in tube furnaces at 600°C in 5% O2 and 5% O2 + 40% H2O. Salt free samples were exposed for reference. Exposure time was 1-672 hours. Some samples were coated with 0.10 mg/cm2 of KCl(s) prior to exposure. Other samples were exposed to KCl(g) in situ using a crucible with KCl(s) positioned upstream of the samples. The dew point of KCl was 590°C, corresponding to PKCl = 2∙10-6 atm. The samples were investigated by gravimetry, XRD, SEM, EDX, FIB, TEM and AES. The chloride and chromate concentrations were analyzed quantitatively by ion chromatography after exposure. Alloy Sanicro 28 forms a protective, corundum type (Fe1-xCrx)2O3 oxide in the absence of KCl. In the presence of water vapour, chromic acid, CrO2(OH)2, is vaporized. However, the resulting chromia depletion of the oxide scale does not lead to breakaway corrosion due to the high chromium content of the alloy. KCl strongly accelerates the high temperature corrosion of Sanicro 28. Corrosion is initiated by the formation of potassium chromate(VI), K2CrO4, through the reaction of KCl with the protective, chromium rich oxide. Chromate formation is a sink for chromium in the oxide and leads to deterioration of its protective properties. Chlorine leaves the surface as HCl(g). In the O2 + H2O environment, there is a stoichiometric relationship between the chromate formed and the chloride consumed. Only traces of chlorine-containing species were found on the corroded samples by AES profiling or EDX. Once the protective chromium rich oxide has been depleted in chromium by chromate formation, the alloy becomes susceptible to direct attack by the remaining KCl. The KCl induced corrosion of alloy Sanicro 28 is in principle the same regardless of whether KCl is present in solid or in gaseous form.