01/11/2018
Scanning electron micrograph micrograph of a mixed oxide/sulfide layer grown on a Fe-Cr model alloy

Scanning electron micrograph micrograph of a mixed oxide/sulfide layer grown on a Fe-Cr model alloy during an in situ EDXRD experiment.

Source: BAM, Division Materialography, Fractography and Ageing of Engineered Materials

Real time investigations into surface changes of high temperature alloys exposed to hot, reactive and gaseous atmospheres provide important aspects for the prediction of the long term stability of the material applied in these environments. Ferritic-martensitic alloys with 2 % - 13 % of weight chromium are applied as super heater or boiler tube material in coal, biomass or co-fired powerplants. All these technologies have in common, that the materials are frequently exposed to SO2 containing gases. The reaction with the gases causes a mixed oxide/sulfide layers at the gas interface, accompanied by the loss of the former alloy surface. The material loss and the precipitation of sulfides within the base alloys at the grain boundaries are changing the intended material properties and may act as potential risk for the operation of the plant. The current work presents a method to study the combined oxidation and sulfidation of ferritic model alloys in a hot SO2 containing atmosphere in real time using energy dispersive X-ray diffraction. This method provides detailed insights into the early surface degradation mechanism under the aggressive atmosphere. The applied high temperature reactor allows the transmittance of high energetic X-rays in order to collect X-ray diffraction pattern of the sample surface in situ during the reaction with the reactive environment. The results revealed that the first phases crystallizing on iron with 2 wt% chromium and with 9 wt% chromium are oxides. The exposure of iron with 2 % chromium at T=650°C with 0.5% SO2 and 99.5% Ar were followed in situ and caused an external mixed oxide-sulfide layers. On the higher alloyed material the external layer consists of iron oxides and the inner layer of mixed (Fe,Cr)-oxides and chromium-sulfides. The oxide content continuously increases parallel to an increase of the sulfide amount. Thus, the initially formed (Fe,Cr)-oxides do not have a protective character and support the transport of sulfur through the growing oxide scale.

Real time observation of high temperature oxidation and sulfidation of Fe-Cr model alloys
Christiane Stephan-Scherb, Kathrin Nützmann, Axel Kranzmann, M. Klaus, C. Genzel
Materials and Corrosion, 2018, Volume 69, Issue 6
BAM Department Materials Engineering, Division Materialography, Fractography and Ageing of Engineered Materials