Degradation and protection phenomena of high-alloy Fe-Cr alloys in hot, multi-component gas systems
Physical Chemistry of Solids and Surfaces, Material Characterisation
Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
Final Report Abstract
The project aimed in a fundamental understanding of fireside degradation of boiler and heat exchanger materials, which are relevant for biomass and co-fired power plants. Especially with the energy-turnaround in Germany, changing from coal or lignite firing in power plants to the use of biomass like wood or straw causes serious problems in terms of corrosion and limits the operation temperature of such plants. The main reason is the different chemistry of biomass. Besides the main constituent’s carbon, oxygen, and hydrogen, the ashes of these fuels are rich in Ca, Si, Mg and K and other inorganic elements such as Cl, P, S. The alkali metals are mainly present as salts and chlorine and are immediately transferred to the gas phase, forming KCl and HCl, which causes an aggressive corrosion of the applied materials. The damage processes of the used alloys are driven by complex physicochemical interactions at the interfaces and surfaces between the materials, the deposits (ash caused by combustion of biomass), and the hot gas phase. Many individual processes for understanding these complex damage reactions are already known, such as active chlorine corrosion. However, specific details for formations of protective layers or condition which fasten the corrosion process are unkown. The project here mainly addresed the study of initial stages of degradation observed in the environment where they occur (in-situ). The project followed the aims: Identification of thermodynamic and kinetic boundary conditions for the formation of protective and damaging corrosion products. - Kinetics of nucleation and growth of reaction products in an early stage of corrosion. - Identification of reactive transport paths within thin films via lattice defects (grain boundaries, point defects). To gain insights into the corrosion mechanisms model alloys (Fe-13Cr; Fe-18Cr-12-Ni; Fe- 25Cr-20Ni) were used, as analogues to commercial steels for boiler tubes: X20, TP 347H and H3RC. Ni in these alloys is mainly included to stabilize an austenitic structure. Thus, all Ni containing alloys exhibited two phases (ferritic – austenitic). The project was structured into three workpackages covering the fundamental analysis of the alloys without KCl in SO2 containing atmosphere, with KCl and following the initial reactions and the impact of humidity by combining the corrosion experiment with X-ray diffraction at the synchrotron. The results showed, that all alloys formed protective oxide layers in reactive high temperature environment when the surfaces were not covered with KCl. While with KCl, strong signs of corrosion were observed. Moreover, it was demonstrated, that the Ni content in the alloy lead to Ni-enrichment and a large internal corrosion zone. It was successful to follow the initial stages of corrosion of the model alloys, covered with KCl in a dry and a humid atmosphere, at high temperature by applying X-ray diffraction during the corrosion experiments. In all cases, the alloys reacted within the first 100min of the experiment with the KCl and metal oxides were formed. Overall the presence of KCl has a strong impact on the corrosion resistance, whereby higher alloyed (high Cr and Ni content) performed worse.
Publications
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Corrosion of high alloyed steels in hot, multicomponent gas atmospheres. Poster bei der virtuellen Tagung der Deutschen Mineralogischen Gesellschaft (DMG)
Kingsbery, P. & Stephan-Scherb, C.
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Korrosions – und Schutzmechanismen von hochlegierten Fe-Cr Stählen in heißen, mehrkomponentigen Gasatmosphären. Vortrag beim Sektionstreffen der Sektionen „Angewandte Mineralogie“ und „Kristallographie“ der „Deutschen Mineralogischen Gesellschaft“ in Bad Windsheim vom 11.03.- 13.03.2020
Kingsbery, P. & Stephan-Scherb, C.
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Effect of KCl deposits in high‐temperature corrosion on chromium‐rich steels in SO2‐containing atmosphere. Materials and Corrosion, 73(5), 758-770.
Kingsbery, Phillip & Stephan‐Scherb, Christiane
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High‐temperature KCl‐induced corrosion of high Cr and Ni alloys investigated by in‐situ diffraction. Materials and Corrosion, 75(10), 1272-1281.
Kingsbery, Phillip; Manzoni, Anna M.; Ocaño, P. Suarez; Többens, D. M. & Stephan‐Scherb, C.
