Alkali Induced High Temperature Corrosion of Stainless Steel - Experiences from Laboratory and Field

Abstract: The high temperature corrosion of steam superheaters is a major obstacle that has to be overcome in order to increase the electricity generating efficiency of power boilers burning biomass or waste. This thesis addresses this issue by a combination of well-controlled model experiments in the laboratory and short and long term field exposures using cooled probes in full-scale biomass and waste fired boilers. The work primarily concerns the effect of alkali salts such as KCl, K2CO3 and K2SO4 on the high temperature corrosion of 304L type stainless steel (Fe18Cr10Ni). The laboratory experiments show that KCl and K2CO3 strongly accelerate the high temperature corrosion of 304L steel. The two salts react with the chromium rich oxide on the sample surface, forming K2CrO4. The formation of chromate(VI) is a sink for chromium in the oxide and leads to a loss of its protective properties. This result in rapid oxide growth and the formation of a thick duplex scale consisting of an outer hematite (Fe2O3) layer with scattered potassium chromate particles on top and an inner layer consisting of FeCrNi spinel-type oxide. This scale shows striking similarities with the duplex scale developed on 304L in O2 + H2O in the absence of salt. This corrosion morphology is referred to as “Type A”. The duplex scale is relatively poorly protective and can be penetrated by chloride ions, resulting in the formation of transition metal chlorides at the metal/oxide interface. In contrast to KCl and K2CO3, K2SO4 does not react with chromium in the oxide to form chromate. This explains why potassium sulphate does not cause accelerated corrosion of 304L. Exposure in the presence of KCl also resulted in another type of corrosion (“Type B”) with corrosion product agglomerates (mainly iron oxide) forming at the KCl crystallites. The oxide agglomerations are not associated with a localized attack of the steel. Hence, “Type B” corrosion is considered to be of secondary importance. Several field exposures were performed in order to investigate if corrosion in commercial boilers can be mitigated by sulphur-containing additives to the fuel. Using cooled probes, the addition of sulphur was shown to decrease corrosion rate by 50%. Adding sulphur to the fuel completely suppressed the formation of chromate(VI) and decreased the amount of transition metal chlorides on the samples. The mitigation of corrosion is attributed to the conversion of alkali chloride to alkali sulphate in the flue gas and in the deposits. The results imply that the “chromate formation” mechanism is important for superheater tubes in biomass- and waste- fired boilers.