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

More documents

Recommendations

Info

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
Page 1: Preferential thermal nitridation to
Page 5 and 6: adjacent gold-coated current collec

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

Create successful ePaper yourself

Delete template?

Save as template?