Cr2N coated martensitic stainless steels by pack cementation process as materials for bipolar plates of proton exchange membrane fuel cells

Bipolar plates in proton exchange membrane fuel cells must have corrosion resistance in an acidic environment and also high electrical conductivity. For steels to be used as bipolar plate materials, surface treatment process suitable for volume production must be developed so that both of these two key property requirements can be met within cost limit. Cr2N is both corrosion resistant in acidic environment and electrically conductive, but it is technically challenging to produce Cr2N-coated steels by thermochemical process such as gas-nitriding, which is suitable for volume production purpose. Here, it is shown that the pack cementation process, which is another form of thermochemical process, can be applied to form a pinhole free Cr2N surface layer on martensitic stainless steels, providing a novel method for producing Cr2N-coated steels for use as bipolar plate materials for proton exchange membrane fuel cells. The corrosion resistance and the pinhole free feature of such Cr2N-coated steel are both confirmed by immersion test in H2SO4 solutions at 60 degrees C in which the Cr2N-coated steel shows no mass change whereas the uncoated steel exhibits rapid mass loss.