The corrosion of superheater tube material in straw-fired boilers was investigated in the
laboratory. Metal test elements of boiler tube steel (X20CRMV121 and AISI 347FG) were covered
with synthetic (KCl and/or K2SO4) and real deposits, and exposed to a synthetic flue gas (6 vol
% O2, 12 vol % CO2, 400 ppmv HCl, 60 ppmv SO2, balance N2) in 550 °C electrically heated
ovens. Exposure times from 1 week to 5 months were used. The corrosion of the metal test
elements was, in general, quite uniform, and the corrosion products consisted mainly of oxides
of iron and chromium. All test elements covered with KCl suffered from minor internal attack,
and some elements had severe pits with chlorine found in the pit. A dense layer of potassium
sulfate and iron oxide was found adjacent to the metal oxide layers on all the metal test elements
covered with a deposit containing KCl. The layer had a characteristic structure, with iron oxide
threads in a dense potassium sulfate matrix. A mechanism for chlorine corrosion is suggested.
The mechanism is based on gaseous chlorine attack coupled with a fast sulfation of KCl to K2SO4 in a melt of KCl, K2SO4, and iron compounds formed adjacent to the metal. Aspects of the
sulfation of potassium chloride to potassium sulfate are discussed in the paper.
Taenite fields from several meteorites have been investigated by TEM, Electron microprobe and Mössbauer spectroscopy. The investigations have disclosed a delicate pattern of antiphase domains in the tetrataenite and the presence of low-Ni taenite at the antiphase boundaries, in what was hitherto believed to be pure tetrataenite. The observations indicate that the "cloudy taenite" (cloudy zone II) was formed by a magnetically induced spinodal decomposition of the metastable taenite during slow cooling below 400°C.As recently demonstrated [18, 20], such a decomposition is likely to occur when the Curie-temperature of the alloy changes rapidly with composition, as it does in f.c.c. iron-nickel alloys containing approximately 28-43 % Ni. (All compositions in weight %). The large contribution to Gibbs free energy from magnetic ordering gives rise to inflections in the Gibbs free energy curve, making the alloy unstable towards decomposition, here into a magnetically and atomically ordered Ni-rich alloy plus a magnetically and atomically disordered Ni-poor alloy.The model accounts apparently well for the structure and composition of the two phases in the cloudy taenite, and also for the peculiar response of f.c.c. iron-nickel alloys to irradiation with energetic neutrons or electrons at temperatures below 400°C.
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