Preferential thermal nitridation to form pin-hole free - Oak Ridge ...

1020 M.P. Brady et al. / Scripta Materialia 50 (2004) 1017–1022
Fig. 3. Cross-sections of nitrided Ni–50Cr: (a) BSE images; (b) TEM
of corrosion test cell air (cathode) face and (c) TEM of corrosion test
cell H2 (anode) face.
properties over the course of the 4100 h corrosion test
cell exposure.
Based on this promising behavior, a single-cell fuel
cell test was conducted using separate anode and cathode
plates of nitrided Ni–50Cr. The plates were too
large to accommodate in the alumina-tube vacuum
furnace used to nitride the corrosion test cell coupon;
therefore, a graphite hot-press furnace was modified to
accomplish the nitridation treatment. Excellent coverage
of plate flow-field groove features was obtained with no
evidence of nitride cracking (Fig. 4a). Compositional
analysis of the nitrided surface indicated the incorporation
of at least 1–5 at.% C resulting from the presence
Fig. 4. Ni–50Cr plate nitrided in the graphite hot press: (a) as-nitrided
cathode plate and (b) BSE cross-section.
of graphite in the nitriding environment. Cross-section
back scatter electron (BSE) imaging (Fig. 4b) also revealed
the presence of a high volume fraction of Croxide
inclusions at the p phase/Cr2N interface and
intermixed within the CrN/Cr2N layers, indicative of the
presence of O impurities during nitriding. Because such
O impurities may be encountered on processing scaleup,
and therefore reflect on its robustness, these plates
were used for in-cell evaluation.
The fuel cell was operated continuously at 0.7 V. An
inadvertent cut-off in the supply of hydrogen to the cell
during the test resulted in membrane damage and performance
degradation such that the test was halted after
500 h and the damaged membrane electrode assembly
(MEA) replaced. The second MEA was also used for
500 h, for a total of 1000 h of fuel-cell testing of the
nitrided plates. Cell resistance, an indicator of membrane
contamination, did not increase a discernable
amount over the second 500 h (resistances were measured
with the initial MEA but were untrustworthy due
to the damaged MEA). The baseline interfacial resistance
contributions between the nitrided plates and the