Real time observation of high temperature oxidation and sulfidation of Fe‐Cr model alloys

Abstract

Insights into early damage mechanisms during high‐temperature gas corrosion provide important aspects for the prediction of the long‐term stability of high‐temperature 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.

A µ‐XANES study of the combined oxidation/sulfidation of Fe–Cr model alloys

“…In all studied materials, the sulfides occur at the alloy interface. The detailed mechanism for this observation was already reported in previous studies …”

“…All alloys are expected to possess a superficial very thin oxide layer before they react with the SO 2 gas in the furnace. As soon as the superficial oxide layer on the pure alloy reacts with SO 2 , a simultaneous formation of magnetite and troilite takes place …”

“…The detailed mechanism for this observation was already reported in previous studies. [8,20] The sulfidation proceeds solely through to the reaction with SO 2 . The formation of SO 3 can be neglected due to the very slow reaction [21] and the lack of free oxygen.…”

“…As soon as the superficial oxide layer on the pure alloy reacts with SO 2 , a simultaneous formation of magnetite and troilite takes place. [20] The overall reaction can be summarized as follows: With the increasing exposure time, iron cations from the alloy diffuse outwards through the scale into the gas direction and react with the incoming SO 2 molecules. In the initial stages of combined oxidation-sulfidation strong oxygen and iron gradient develops with increasing exposure time.…”

Identification and 3D Reconstruction of Cr5S6 Precipitates Along Grain Boundaries in Fe13Cr

“…At the surface, formation of a Cr-rich oxide in the beginning is thermodynamically preferred. According to a phase diagram calculated using FactSage 7.0 20 for the same system, 21 this Cr-rich oxide is most likely chromite FeCr 2 O 4 . Chromite consumes a high amount of Cr from the alloy, in this case 2 lm under the original surface, which agrees with the common oxidation model of Fe-Cr alloys.…”