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

Preferential thermal nitridation to form pin-hole free Cr-nitrides to
protect proton exchange membrane fuel cell metallic bipolar plates
Abstract
M.P. Brady a,* , K. Weisbrod b , I. Paulauskas c , R.A. Buchanan c , K.L. More a ,
H. Wang d , M. Wilson b , F. Garzon b , L.R. Walker a
a
Oak Ridge National Laboratory, Metals and Ceramics, ms 6115, Oak Ridge, TN 37831-6115, USA
b
Los Alamos National Laboratory, Los Alamos, NM 87545, USA
c
University of Tennessee, Knoxville, TN 37996, USA
d
National Renewable Energy Laboratory, Golden, CO 80401, USA
Received 18 September 2003; received in revised form 15 December 2003; accepted 19 December 2003
Preferential thermal nitridation was used to form a pin-hole defect free CrN/Cr2N surface on a model Ni–Cr alloy. Excellent
corrosion resistance and negligible contact resistance increase was observed over a 4100 h exposure in 80 °C sulfuric acid and when
used as a metallic bipolar plate in a 1000 h proton exchange membrane (PEM) fuel cell test.
Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Keywords: Nitrides; Corrosion; Electrical properties; Electrochemistry; Coating
1. Introduction
Scripta Materialia 50 (2004) 1017–1022
Transition metal nitrides are of interest for a wide
variety of functional coating applications due to their
unique combinations of properties (e.g. high hardness,
electrical conductivity (usually), chemical stability, etc.)
[1]. However, conventional deposition methods typically
leave pin-hole through-thickness defects, which severely
limits their usefulness in applications also requiring
protection from corrosion [2]. Here we show that preferential
thermal (gas) nitridation of a nitride-forming
element in a metallic alloy can be used to inexpensively
and controllably grow discrete, pin-hole defect free nitride
surface layers on complex-shaped components. The
approach is demonstrated for the highly coating-defect
sensitive problem of protecting metallic bipolar plates
for proton exchange membrane fuel cells (PEMFCs).
PEMFCs are of interest for power generation due to
their high efficiency and near-zero emissions [3,7,8]. Cost
remains a key barrier to their widespread use. One of the
most expensive components in PEMFCs are the bipolar
plates, which serve to electrically connect the anode of
*
Corresponding author. Tel.: +1-865-574-5153; fax: +1-865-241-
0215.
E-mail address: bradymp@ornl.gov (M.P. Brady).
1359-6462/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.scriptamat.2003.12.028
www.actamat-journals.com
one cell to the cathode of the next into a stack to achieve
a useful voltage. They also separate and distribute
reactant and product streams; to accomplish this flowfield
grooves are manufactured into the faces of the
plates. Solid graphite is used for bipolar plates, but is
brittle and expensive to machine. Polymer/carbon fiber
[9] and carbon or graphite composite bipolar plates
[10,11] have shown promise; however, issues remain
regarding their amenability to high-volume manufacturing
techniques, performance, and the power densities
achievable. Metallic alloys (e.g. stainless steels) would be
ideal as bipolar plates [12–14] because they are amenable
to low-cost/high-volume manufacturing, offer high
thermal and electrical conductivities, and can be made in
thin sheet or foil form (0.1–1 mm thick) to achieve high
power densities. However, the inadequate corrosion
behavior of most metals in PEMFC environments [4–6]
has prevented their use.
Coatings for metallic bipolar plates have thus far not
proven sufficiently viable due to pin-hole defects, which
result in local corrosion and metallic ion contamination
of the membrane; levels as low as 5–10 ppm can degrade
membrane performance [4]. Measures to mitigate the
presence of pin-hole defects (i.e. the use of interlayers)
are being pursued [2,15,16], but can significantly increase
cost to an unacceptable level.

1018 M.P. Brady et al. / Scripta Materialia 50 (2004) 1017–1022
Pin-hole defects are not an issue for thermal nitridation
and related oxidation reactions because at elevated
temperatures thermodynamic and kinetic factors favor
reaction of all exposed metal surface. Rather, the key
issues are the extent to which the desired element(s) in
the alloy can be preferentially reacted to achieve the goal
surface layer composition, the morphology of the layer
(subsurface precipitates vs external continuous), and the
adherence/possible cracking of the layer on cooling.
Control of such phenomena is the basis for protective
oxide scale formation by heat-resistant alloys during
high temperature corrosion e.g. [17,18], but have not
been well explored as a synthesis method to form protective
functional nitride surface layers.
2. Experimental methods
2.1. Nitridation
Nitrides have been identified as candidate coatings
for metallic bipolar plates due to their combination of
high electrical conductivity and good corrosion resistance
[6]. A nitrided model Ni–50Cr alloy was selected
for study, based on screening polarization studies of a
series of nitrided Ni–X base alloys (X ¼ Cr, Nb, Ti, V) in
PEMFC environments [19]. Test coupons of the model
Ni–50Cr (wt.%) alloy were manufactured from arc-cast
and heat treated (1150 °C, 8 h) Ni–50Cr. The coupons
were ground to a 240 grit surface finish. Nitridation was
accomplished in an alumina vacuum furnace backfilled
with high-purity nitrogen to 1 atm; the nitrogen flow
was stopped and the coupon was heated to 1100 °C, held
for 1–2 h, and furnace cooled to room temperature. The
mass gain due to nitrogen uptake was 1.9–2.3 mg/cm 2 .
Nitridation for the anode and cathode plates for the fuel
cell test were conducted in a graphite hot press. They
were heated to 1100 °C in slowly flowing, high-purity
nitrogen, held for 2.25 h, and furnace cooled. Mass
gains were 2.25 and 1.75 mg/cm 2 , respectively, for the
anode and cathode plates (nitrided in two separate
runs).
2.2. Corrosion test cell
Long-term exposure in PEMFC anode (reducing)
and cathode (oxidizing) bipolar plate environments was
simulated using the corrosion test cell shown in Fig. 1a
[20,21]. In this test, a pH 3 solution of H2SO4 containing
2 ppm F at 80 °C is used in lieu of the polymer
membrane. The catalyst layer, an ELAT â electrode (0.5
mg Pt/cm 2 , 20% Pt on C), was used to establish the
electrochemical potential in the electrolyte. Hydrogen
and air at 1 atm were supplied to the anode and cathode
faces, respectively, of the flat (no flow-field features) test
coupon (nitrided Ni–50Cr, 2.5 cm diameter, 1 mm thick,
Fig. 1. Corrosion test cell: (a) cell schematic and (b) cell potential
transients for nitrided Ni–50Cr.
1100 °C, 1 h treatment) to simulate bipolar plate anode
and cathode environment conditions. Platinum screens
passed 1 A/cm 2 electrical current through the backing
layers and the bipolar test coupon, and the electrical
potential was recorded in situ to monitor resistive surface
phase (e.g. oxide) growth (measured voltage change
also includes losses through the platinum screen,
ELAT â , and carbon backing layer).
2.3. Fuel cell test
Cast and hot-rolled (nominal 1150 °C) Ni–50Cr was
manufactured into 50 cm 2 active area anode and cathode
plates, with simple serpentine, continuous flow-field
grooves ( 1.1 mm wide and 1.1 mm deep milled
grooves) according to standard graphite plate hardware
design (standard hardware commercially available from
Fuel Cell Technologies, Albuquerque, NM). A membrane
electrode assembly (MEA) was prepared for fuelcell
testing [22] with anode and cathode loadings of
0.20 mg Pt/cm 2 on Nafion â 112. The gas diffusion
layers (backings) were uncatalyzed ELAT â . Doublesided
ELAT â was used on the anode sides and singlesided
on the cathodes. Glass-reinforced Teflon â and
silicone gaskets of appropriate thicknesses were used to
seal the periphery and provide the desired level of
compression on the assembly. The cells were operated at
3 atm (absolute) and at 80 °C. The humidifiers on the
anode and cathode sides were heated to 100 and 80 °C,

espectively. Purified hydrogen was introduced at 0.3
standard liters per minute (SLPM). Compressed room
air was provided to the cell at 1.8 SLPM. A disruption
of the flow of hyrdrogen to the fuel cell occurred during
the first 500 h of the test, which damaged the MEA. A
second MEA was prepared with anode/cathode loadings
of 0.23/0.37 mg Pt/cm 2 on a thicker membrane, Nafion
1135, and used for an additional 500 h of testing, for a
total of 1000 h using the nitrided plates.
2.4. Characterization
Composition data was obtained on nitrided surfaces
and metallographically prepared cross-sections by electron
probe microanalysis using pure element standards
for Cr and Ni; a BN standard for N; and an Al2O3
standard for O. TEM cross-sections were prepared by
focused ion beam milling.
3. Results and discussion
The nitrided model Ni–50Cr alloy exhibited a voltage
change of only 2 mV/1000 h in the anode environment
and 2.7 mV/1000 h in the cathode environment over the
course of 4100 h in the corrosion test cell (Fig. 1b).
The anode face of the coupon effectively experienced
)0.51 V vs Ag/AgCl and the cathode face experienced
+0.36 V vs Ag/AgCl ()0.31 and +0.56 vs standard
hydrogen electrode, respectively). Ni levels of 0.85, 0.56,
2.4 ppm were measured in the anode-face exposed
solutions and 0.034, 0.019, 0.027 ppm Ni in the cathodeface
exposed solutions for 0–1500, 1500–3400, and
3400–4100 h segments of exposure, respectively. Cr was
not detected. Visual analysis of the test coupon showed
no evidence of corrosive attack.
Interfacial contact resistance (ICR) data as a function
of compaction pressure are shown in Fig. 2 (method
described in [13,14]) The Ni–50Cr alloy (no nitridation
treatment) had a lower contact resistance than 316L
stainless steel, shown for comparative purposes (Fig.
2a). Subsequent nitridation of Ni–50Cr significantly
lowered contact resistance, especially at the low compaction
forces relevant to PEMFC stacks ( 100–150 N/
cm 2 range) (Fig. 2a). ICR measurements of the 4100 h
exposed bipolar test coupon (Fig. 2b) showed no increase
in ICR relative to as-nitrided Ni–50Cr.
The as-nitrided Ni–50Cr microstructure (Fig. 3a)
consisted of a continuous external nitride scale overlying
an internally-nitrided zone. The external nitride scale
consisted of three layers: a 1 lm thick semi-continuous
outer layer of CrN of composition Cr–(40–50)N–(0.5–
1)Ni atomic percent (at.%) (the large range of measured
N content resulted from scatter associated with nitride
surface roughness), an intermediate, dense 3–5 lm thick
M.P. Brady et al. / Scripta Materialia 50 (2004) 1017–1022 1019
ICR, mOhm-cm 2
(a)
2X ICR, mOhm-cm 2
(b)
800
700
600
500
400
300
200
100
0
0
350
300
250
200
150
100
50
316L Stainless Steel
Ni-50Cr Metal
Nitrided Ni-50Cr
(1100˚C, 2h, N2)
Compaction Force, N/cm2 50 100 150 200
As-Nitrided(2 faces)
4100 h in corrosion test cell
(pH3 H2SO4 , 2ppm F- , 80˚C)
0
0 50 100 150 200
Compaction force, N/cm 2
Fig. 2. Interfacial contact resistance (ICR) data: (a) as-nitrided and (b)
after 4100 h in corrosion test cell. (Note that the ICR measurements of
the bipolar test coupon include both the anode- and cathode-exposed
faces; for comparative purposes the ICR data for as-nitrided Ni–50Cr
shown in Fig. 2a was doubled in Fig. 2b.)
layer of Cr2N of composition Cr–(31–33)N–(0.5–1)Ni
at.%, and an inner layer of composition Cr–(11–14)N–
(31–34)Ni at.%, consistent with the p [23,24] phase. The
internally nitrided zone consisted of p phase (light gray
in Fig. 3a) dispersed in Cr-depleted Ni(Cr) metal containing
25–35 wt.% Cr in the vicinity of the surface nitride.
The microstructure of the nitride on the 4100 h
cathode-exposed face (Fig. 3b) and the 4100 h anodeexposed
face (Fig. 3c) appeared essentially the same as
the as-nitrided material, with no evidence of significant
attack or surface oxide formation in the transmission
electron microscopy (TEM) sections.
Some local voids were observed at the CrN-layer/
Cr2N-layer interface, generally along the Cr2N grain
boundaries (Fig. 3a–c), however they did not extend
through the Cr2N layer to the underlying substrate (i.e.
there were no through nitride thickness defects observed).
These voids were found in as-nitrided material
as well as the cathode- and anode-exposed faces; however,
qualitatively, there may have been a slightly higher
number of these voids in the anode-exposed face. The
shape of the voids is suggestive of formation during
elevated temperature nitridation; and it is not clear if
they resulted from limited local attack of the Cr-nitride
surface layer in the corrosion test cell or were nitridation
artifacts present prior to testing. Overall, the CrN/Cr2N
surface exhibited little to no corrosive attack, little
metal-ion dissolution, and no degradation in electrical

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

adjacent gold-coated current collector plates were 5
mX cm 2 at the cathode plate and 1 mX cm 2 at the anode
(the total-cell resistances were on the order of 130
mX cm 2 ).
X-ray fluorescence (XRF) was used to examine the
anode- and cathode-side membranes and ELAT
backings from the two, 500 h tests. Only trace levels of
Ni and Cr were found, in the range of 0.01–0.3 lg/
cm 2 , which is on the order of the detection limit of
this measurement. This very low level of contamination
indicates inert and protective behavior by the
CrN/Cr2N surface with few, if any, through thickness
pin-hole defects. To put this result in context, Wind
et al. [25], for example, reported that 316L stainless
steel tested for 100 h at 75 °C as a bipolar plate material
resulted in Ni contamination levels of 76 lg/cm 2
(see Ref. [25] for specifics of these fuel cell test conditions).
It should be noted that some stagnant Cr-rich
liquid was found in one of the alignment pin ports on
disassembly of the cell, however, no membrane contamination
was found in this area. A small Cr–O–C rich
surface region ( 0.5 · 1 mm) found at this location
was likely the source of the Cr-rich liquid. A major
casting flaw or inclusion may have led to local poor
nitridation, making this area vulnerable to attack, although
the stagnant liquid may also have led to more
corrosive local conditions against which the Cr-nitride
was not sufficiently resistant. No Ni from the underlying
substrate was detected in this region, which
suggests that the problem was not the result of a pinhole
type defect.
4. Conclusions
Collectively, these results establish the viability of
preferential thermal nitridation as a synthesis route to
effectively pin-hole defect free, discrete nitride surface
layers on complex-shaped components. Other functional
nitride, carbide, or boride compounds, as well as complex
(ternary) nitride (and related phases) are also
potentially accessible by this general approach, through
proper selection of alloy composition/microstructure
and reaction conditions [26,27]. They also establish
thermally grown Cr-nitrides from Cr-bearing alloys as a
promising new approach to protect metallic bipolar
plates for PEMFCs.
Future work for PEMFC metallic bipolar plates will
focus on alloys with lower levels of Cr and the use of less
expensive Ni(Fe)- or Fe-base substrates to achieve
bipolar plate cost goals. Conventional stainless steel
alloys have not been designed to form external Crnitride
scales on thermal nitridation. Rather they tend to
form internal or mixed Cr–N, Fe–N, and Cr–Fe–N
phases which improve wear resistance, but generally
degrade corrosion resistance. Therefore, compositional
M.P. Brady et al. / Scripta Materialia 50 (2004) 1017–1022 1021
manipulation and modification of nitridation parameters
(beyond the simple pure N2 conditions in the present
work) will be necessary to form exclusive Cr-nitride
surfaces on these alloys.
Acknowledgements
The authors thank P.F. Tortorelli, D.F. Wilson, and
I.G. Wright for helpful comments in reviewing this
manuscript. Funding from the United States Department
of Energy (US DOE) Hydrogen, Fuel Cells, and
Infrastructure program is gratefully acknowledged. Oak
Ridge National Laboratory is managed by UT-Battelle,
LLC for the US DOE under contract DE-AC05-
00OR22725.
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