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Ferritic-martensitic high temperature alloys are used as building components for different power plant technologies. Depending on the type of fuel, the used power plant materials are exposed to different temperatures and reactive atmospheres containing e.g. CO2, O2, or SO2. Despite the sulfur chemistry is commonly present as an impurity in fossil or bio fuels; its role in high temperature corrosion is not entirely understood. During high temperature corrosion, high-alloyed steels often show sulfur precipitates with the ignoble alloy component(s) along grain boundaries within the base material. Sulfur precipitates are known to seriously influence the mechanical properties of the building component. In the case of VM12 and T92 steels, sulfur phases penetrate the base material along grain boundaries during the corrosion under oxyfuel atmosphere up to 20 µm within the first 960h (Fig. 1a). Figure 1a shows the oxide scale and (Cr, Mn, Fe)xSy grain boundary precipitates in the base material for a T92 steel aged for 960h under oxyfuel atmosphere. Figure 1b shows a thin oxide scale with nodules and also sulfur precipitates of (Fe, Cr)xSy along grain boundaries of the base material for a Fe13Cr model alloy aged for 24h under SO2 atmospheres. After 24h, sulfur precipitates already reached a depth of ca. 15 µm.
The present work shows the corrosion behavior of Fe-Cr model alloys with Cr-contents similar to technical steels up to 13 wt%, aged under oxyfuel (27H2O/60CO2/1SO2/10N2/2O2) and SO2 atmospheres in the temperature range of 550 °C < T < 700 °C and for different time scales between 24 h < t < 960 h. During aging, the reactive gases were added when the experimental temperature was reached. To focus on the reaction of the intended elements Fe, Cr, S, and O, model alloys of high purity are used. Transport depths of sulfur and the nucleation of the precipitates are discussed for both, model alloys and technical steels.
Carbon Capture, Utilization and Storage (CCUS) has been proposed as a promising technology for the mitigation of CO2 emissions into the atmosphere from fossil-fuel- operated power generation plants. As the reliability and cost effectiveness of the Pipeline transport network is crucial to the Overall operability and resilience of the CCUS system, it is vital to realize the possible corrosion risks of the employed Pipeline steels corresponding to the impurity Level of the gas source. Recent studies have shown that even the high alloyed materials might be susceptible to General and/or localized corrosion by the condensates forming from the impurities such as SOx, NOx, CO, O2 and water.
Up to now, however, there is no regulation procedure which defines the maximum acceptable level of impurities and the combination of them for each employed pipeline steels. Herein, systematic experiment series were conducted by mixing pure CO2 gas with varying concentration of each impurity and with the varying combination of them. Each time, the mixture was then fed (1 L/min) into the reactor containing 12 specimens for 120-600 h at 5°C (to simulate the sub-level Pipeline transport). The resulted condensate was collected and analyzed by ionic chromatography and atomic Absorption spectroscopy to determine the chemical composition. In this study, the “worstcase scenario” gas mixture, containing 2.5 % H2O, 1.8 % O2, 1000 ppm NO2, and 220 ppm SO2 as impurities, resulted in the condensate containing H2SO4 0.114 M and HNO3 0.0184 M (pH 2.13).
This “original” condensate was then reproduced to carry out exposure tests and electrochemical characterization including corrosion potentials and impedance spectroscopy in CO2 saturated condition for 7-14 days at the same temperature. The corrosion rate was also measured by mass loss method.
We can conclude that, at the initial stage, HNO3 plays the dominant role in Fe dissolution process, while H2SO4 is responsible for the pit initiation followed by pitting corrosion. Future studies will be focused on the combination effect from the impurities and the exposure test under the regularly changing condensate to mimic the real CO2 pipeline system.
In Kraftwerkstechnologien verwendete ferritische Fe-Cr-Stähle sind verschiedensten Temperaturen, Prozessdrücken und aggressiven, zum Teil schwefelhaltigen Verbrennungsgasen ausgesetzt, die zu Korrosion führen. Noch ungeklärt ist die Rolle der entstehenden Sulfide in der schützenden Oxidschicht und deren Auswirkung auf die Lebensdauer der Bauteile. Unsere Arbeit zeigt Korrosionsmechanismen ausgewählter Fe-Cr-Modelllegierungen unter Ar-SO2 Atmosphäre für verschiedene Zeitskalen. Der Focus dabei liegt auf Kurzzeitexperimenten (12h ≤ t ≤ 250h), um die initialen Stadien der Schwefelkorrosion zu untersuchen.