A Comparison of the Behavior in Molten Sulfate of Oxidized and Carburized Type-304 Austenitic Steel over 2000 hours and the Quantitative Analysis of Corrosion Products in the Melt by a Novel Treatment of their Absorption Spectra

摘要

Our aim was to compare and quantify thin film hot corrosion by sulfate at 650℃ on oxidized and carburized type 304 austenitic steel. Our objective was to determine if superheater and reheater steel tubes that had become carburized as a consequence of an aborted start-up were more prone to hot corrosion, and by how much, and the effect upon maintenance intervals. Previous laboratory studies had not satisfactorily replicated the thin film of molten sulfate that existed in generating plant. We also wanted to test an alternative and potentially cheaper method for measuring corrosion rates as a function of time by determining the concentration of corrosion products within the molten sulfate layer using a novel computer treatment of their electronic spectra. These results would be compared with those obtained, on the same samples, using ICP-AES, (inductively coupled plasma atomic emission spectroscopy). A ceramic crucible was designed and machined such that a 1 mm molten salt layer around a steel coupon could be maintained for up to 2000h, while simultaneously allowing proper interaction, and monitoring, of synthetic flue gas within the melt. Coupons of pre-oxidized steel and of pre-carburized 304 steel were placed in a silica carousel in the hot zone of a furnace, under flowing flue gas, and one sample of oxidized and one of carburized were removed every 200 h. After cooling, the salt layer was removed and part analyzed by ICP-EAS and the remainder by absorption spectroscopy, for independent comparison and to provide a severe test for our attempted new approach to a novel technique we considered possible for the analysis of dilute multi-component solutions. Using a fiber optic spectrometer absorption spectra were recorded digitally and were repeatable to the fifth decimal place. Absorbance values were low but still showed in the charge transfer edge of Fe(III) the overlapping bands characteristic of Cr(III) and Ni(II). By mathematical smoothing, derivative analysis procedures and band "stripping" the known individual transition metal spectra plus their concentrations were reliably obtained, and confirmed the repeated start-stop mechanism for superheater hot corrosion. Previous research studies have not replicated the 1 mm molten salt layers as closely as this study, nor confirmed their results with an independent analysis technique. Carburized specimens were attacked approximately twice as fast as pre-oxidized specimens. Our new technique is thus viable but needs an experienced operator, and time, to obtain data, and is much cheaper than purchasing an ICP-EAS instrument.