Effects of interdiffusion in nickel coated AISI 441 steel

Ferritic steel AISI 441, electroplated with nickel, is explored for the use as interconnects in solid oxide cell stacks. High-temperature oxidation tests are performed in air, and samples are investigated using scanning electron microscopy, energy-dispersive x-ray spectroscopy, and electron backscatter diffraction. Results revealed in-depth diffusion of nickel that promotes austenite formation during typical solid oxide cell operating temperatures accompanied by chromium depletion, grain refinement, and an enhanced depletion of Laves phase in the surface-near region of the steel. Effects of a formed interdiffusion zone on the protective oxide scale are discussed, considering the role of diffusion along grain boundaries and across the scale/alloy interface. The interdiffusion zone alters the morphology and continuity of Si-rich oxide that forms in this steel, and an underlying mechanism is suggested. Associated implications on the expected performance of nickel coated AISI 441 as interconnects for solid oxide cell stacks are postulated: Although no corrosion or scale spallation is observed and a good contact to the simulated oxygen electrode is maintained, there are signs of limited oxidation resistance and scale adhesion, which potentially affect lifetime and robustness of solid oxide cell stacks during (cyclic) oxidation of longer exposure than tested here.