Behaviour of Metallic Materials in Simulated Service Environments of CO2/H2O Co-electrolysis Systems for Power-to-X Application

Abstract

In the present study, the ferritic steel Crofer 22 H as potentially suitable interconnect material for SOEC stacks as well as joints between the steel and Ni- and CuNi contact materials was investigated with respect to the behaviour in simulated service environments of an SOEC system for CO2/H2O co-electrolysis. Exposures up to 1000 h at temperatures between 600 and 800 °C were carried out in CO2/H2O- and CO/H2-rich gases, thus simulating conditions at the stack inlet and outlet, respectively. It was found that the steel formed protective surface oxide scales consisting of chromia and/or Cr/Mn spinel in all studied test conditions. No indication of carbon transfer from the gas atmosphere into the steel was found even in the high carbon activity CO/H2-rich gas simulating stack outlet conditions. However, in the latter gas substantial carbon transfer from the gas to the steel via the Ni- or CuNi-wires resulted in the formation of a carburized zone with substantial M23C6 and/or M7C3 precipitate formation. This effect was more pronounced for the joints of the steel with the Ni-wire than with the CuNi-wire. In the gas simulating the service environment at the stack inlet, only minor carbon transfer was found in case of the Ni/steel joint at 600 °C but not at 800 °C. In case of the CuNi-wires, partial loss of contact between wire and interconnect steel and formation of Kirkendall voids as a consequence of interdiffusion between wire and steel were observed. The experimental results are discussed using thermodynamic considerations involving gas equilibria and stability of possible external and/or internal formation of oxide and carbide phases.