Annealing induced interfacial layers in niobium-clad stainless steel developed as a bipolar plate material for polymer electrolyte membrane fuel cell stacks

Roll-bonded niobium (Nb)-clad 304 L stainless steel (SS) is currently being developed as a metallic bipolar plate material for polymer electrolyte membrane fuel cell stacks. Prior work has shown that post-roll annealing significantly softens the constituent core and cladding materials, enhancing the ductility and formability of each. However under the vacuum annealing condition employed in the previous study (900 s, 982 °C (1800 F)), an interfacial layer was observed to form between the two bonded materials. Subsequent bending and flattening tests indicated brittle failure of this interfacial region under high strain conditions. The present study employs transmission electron microscopy to examine the composition, structure, and thickness of phases generated at the Nb/SS interface as functions of annealing temperature and time. Corresponding selected electron diffraction patterns indicate that above a threshold annealing temperature of ∼950 °C, the formation of (Fe1−xCrx)2Nb appears to control the failure behavior of the Nb/304 L SS material. Annealing treatments conducted below this temperature generally avoid the formation of this intermetallic layer.