Ingenieurwissenschaften und zugeordnete Tätigkeiten
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T92 steel was oxidized at 650 °C for 1000 h in dry and wet oxyfuel gases. The microstructure of inner oxide layer was investigated using scanning transmission electron microscopy and energy dispersive spectroscopy on thin lamellas of oxide cross-sections. The oxides were composed of fine equiaxed grains and separated into Fe-rich and Cr-rich regions. Fe-rich regions were wustite and iron sulphide while Cr-rich regions consisted of Fe-Cr spinel with different stoichiometries. Precipitates of (W,Mo)-rich oxides were formed within the oxide scale and beneath the oxide/alloy interface. Often iron sulphide and (W,Mo)-rich oxide were surrounded by Cr-rich spinel.
Comprehensive insights into the early stages of corrosion mechanisms provide fundamental knowledge to further understand and model long time material behaviour. The present work studies the early stages of combined oxidation and sulphidation of ferritic model alloys for time scales up to 250 h at 650 °C to observe the influence of chromium during the corrosion under SO2. Model alloys were used to focus on the reaction of the intended elements: Fe, Cr, S, and O. Pure iron simultaneously forms magnetite and iron sulphide in an early stage of corrosion, covered by a pure oxide layer after 100 h. Iron with 13 wt% Cr shows hematite and mixed Fe-Cr-oxides first, before sulphides nucleate in the inner corrosion zone. With increasing ageing time a magnetite layer is observed below the hematite layer. Quantitative phase fractions of all corrosion products observed were determined from cross section images. Characterization of the Fe13Cr corrosion scale by FIB revealed a highly porous structure in the inner corrosion zone where Cr-rich (Fe, Cr)-sulphides are present, and caused the scale to spall easily.
This work presents the microstructure and chemical composition of oxide scales created on Ni – base coating after corrosion experiments in aggressive gases and ashes. The Inconel 686 coating applied on the low carbon steel 13CrMo4-5 was performed by a CO2 laser cladding process. The experiments were carried out in oxidizing gas atmospheres containing O2, COx, SOx. The second Variation of corrosion experiments were performed in a reducing atmosphere with ashes containing elements such as: Na, Cl, Ca, Si, C, Fe, Al etc. After 240 h and 1.000 h corrosion experiments the oxide scales on the substrate and overlay were created in both cases.
The sulfur compounds were found on the top of the coating surface (EPMA) and also higher contents of silica compounds were revealed on specimens covered by ashes during the experiments. The microstructure and chemical composition of the clad and scales were investigated by means of a light microscope and an electron microscope (SEM)equipped with an EDS detector.
Insights into early damage mechanisms during high-temperature gas corrosion provide important aspects for the prediction of the long-term stability of hightemperature materials exposed to a hot and corrosive environment. The current work presents a real time study concerning the combined oxidation and sulfidation of ferritic model alloys in a hot SO2 containing atmosphere using energy dispersive X-ray diffraction. 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 aging experiments at T = 650° C with 0.5% SO2 and 99.5% Ar were followed in situ and caused an external mixed oxide-sulfide layer on top of iron with 2 wt% chromium. On the higher alloyed material the external scale consists of iron oxides and the inner scale 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.
The Ni-Cr-Mo-W alloy is characterized by the excellent high-temperature corrosion resistance, good strength and ability to work in aggressive environments. To protect the surface of the substrate 13CrMo4-5 steel against aggressive environments the Inconel 686 as clad layer was used. The corrosion process was performed in oxidizing mixture of gases like O2, COx, SOx. The second part of corrosion Experiment concerned the corrosion test of the coating in reducing atmosphere of the specified gases with ashes, which contained e.g. Na, Cl, Ca, Si, C, Fe, Al.