Effect of Nb Addition on Oxidation Mechanisms of High Cr Ferritic Steel in Ar-H2-H2O

High chromium ferritic steels are being used as construction materials for interconnects in solid oxide electrolysis cells (SOEC). Addition of niobium in the range of a few tenths of a percent is suitable for increasing the high-temperature creep strength of this type of ferritic steel. In the present work, the high-temperature isothermal oxidation behavior of a niobium containing ferritic steel at 800 degrees C was investigated in Ar-4%H-2-4%H2O gas simulating the service environment in an SOEC (cathode side) and compared with that of a Nb-free counterpart alloy. Gravimetric data were correlated with the results from microstructural analyses using, among others, scanning and transmission electron microscopy as well as glow discharge optical emission spectroscopy. Atom probe tomography was used for obtaining atomic-scale insight into the segregation processes in external oxides and their interfaces. The oxidation rate was substantially higher for the Nb-containing than for the Nb-free alloy. Both alloys formed double-layered oxide scales consisting of inner chromia and outer MnCr2O4 spinel. Additionally, a thin layer of rutile-type Nb(Ti,Cr)O-2 oxide of 200-300 nm thickness was observed at the scale-alloy interface in the Nb-containing steel. Nb addition to the alloy led to its segregation at chromia grain boundaries which affected the diffusion of Cr and other solute species such as Ti, Mn and Si.