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Al–Cr–N multilayer coatings for stainless steel bipolar plate in polymer electrolyte membrane fuel cells
Surface & Coatings Technology 258 (2014) 1068–1074
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Investigation of C/Al–Cr–N multilayer coatings for stainless steel bipolar plate in polymer electrolyte membrane fuel cells Zhiyuan Wang a, Yibo Wang a, Zhuguo Li a,c,⁎, Kai Feng a,c,⁎, Jian Huang a, Fenggui Lu a, Chengwu Yao a, Xun Cai a,b, Yixiong Wu a,b,c a b c
Shanghai Key Laboratory of Materials Laser Processing and Modification, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China State Key Laboratory of Metal Matrix Composites, Shanghai Jiaotong University, Shanghai 200240, China Academician Expert Office Workstation (Jiansheng Pan), Lin'an, Zhejiang Province, China
a r t i c l e
i n f o
Article history: Received 26 March 2014 Accepted in revised form 8 July 2014 Available online 15 July 2014 Keywords: Polymer electrolyte membrane fuel cell Bipolar plates Surface conductivity Corrosion resistance Physical vapor deposition
a b s t r a c t To improve the corrosion resistance and surface conductivity of stainless steel bipolar plate, C/Al–Cr–N multilayer coatings with varying Al:Cr target current are deposited on stainless steel 316 L (SS316L) by magnetron sputtering. Scanning electron microscope (SEM) results show that the coating is about 2.3 μm in total thickness, consisting of 0.9 μm carbon layer and 1.4 μm Al–Cr–N sub-layer. The interfacial contact resistance (ICR) result indicates that the surface resistance decreases with the increasing content of chromium, and achieves the minimum value of 6.17–4.88 mΩ-cm2 at the compaction force between 120 and 150 N cm−2 for the Al2Cr4 sample. The electrochemical and inductively coupled plasma atomic emission spectrometry (ICP-AES) results disclose that C/Al–Cr–N multilayer coatings with Cr:N atomic ratio of 1:1 and lower aluminum content have higher chemical inertness and better corrosion resistance. Al6Cr0 and Al5Cr1 samples exhibit more negative corrosion potential and higher passive current density with galvanic corrosion on the surface due to the existence of AlN phase. Therefore, C/Al–Cr–N multilayer coatings with lower aluminum content are preferred for bipolar plate application. Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.
1. Introduction Currently, numbers of coatings prepared by various surface modification methods have been investigated as alternative bipolar plate materials in polymer electrolyte membrane fuel cells (PEMFCs) to improve the corrosion resistance and surface conductivity. Among these surface modification methods, physical vapor deposition (PVD) could be a promising technique because it allows the deposition of coatings with designed chemical composition and properties on a wide range of substrates. Metal nitrides, such as TiN [1–4], CrN [5–8] and carbon-based coatings [9–13] have been deposited on metallic bipolar plates by PVD technique due to their excellent electrical conductivity and corrosion resistance. However, these single layer coatings are prone to local corrosion and the corrosion current density will increase dramatically after long time immersion in the simulated PEMFC environment [2,14] mainly due to the intrinsic defects such as pinholes and macroparticles in the coating created during PVD process [15]. Because of the establishment of corrosion potential difference between the coating material and the less
⁎ Corresponding authors at: Shanghai Key Laboratory of Materials Laser Processing and Modification, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. Tel.: +86 21 54745878; fax: +86 21 34203024. E-mail addresses: [email protected] (Z. Li), [email protected] (K. Feng).
http://dx.doi.org/10.1016/j.surfcoat.2014.07.028 0257-8972/Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.
noble steel substrate, aqueous corrosion of single layer coatings on steel usually takes place in a localized form [16]. Localized corrosion, especially through-coating corrosion, will greatly decrease the corrosion resistance of the coatings. Therefore, multilayer structures of PVD coatings are developed to improve the corrosion resistance by interruption of through-coating pinholes. Nam et al. [17] compared the corrosion resistance of TiN/Ti, TiN/Cr, CrN/Ti, and CrN/Cr films prepared by reactive RF magnetron sputtering. The results indicated that the CrN/Cr film had the best corrosion resistance and the highest charge transfer resistance. Tian [18] prepared a CrN/Cr composite film on stainless steel by PVD technique and found an improved interfacial conductivity and corrosion resistance. Sun et al. [19] investigated the corrosion resistance and interfacial contact resistance (ICR) of TiN film, carbon film and C/TiN multilayer coating in simulated PEMFC environment and concluded that C/TiN multilayer coatings are excellent candidates for further research and long-term in-situ fuel cell testing. In a previous study, we fabricated a new multilayer coating composed of amorphous carbon as the outer layer and CrN as the sub-layer (C/CrN multilayer) [20]. The results showed that the current density of C/CrN multilayer coated SS316L at 0.6 V is decreased to 0.5 μA cm−2 and ICR is minimized to 2.6 mΩ-cm2 at compaction force of 150 N cm−2. CrN and CrN-based multicomponent nitrides are a well-established coating system. They have been reported exhibiting high oxidation resistance and corrosion resistance. Ding et al. [21] have pointed out that the coating structure becomes finer with the incorporation of Al
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into CrN coating, as revealed by the XRD patterns. This will result in a discontinuous crystallite boundary in the columnar structure, which decreases the opportunity for the through coating defects. Additionally, the addition of Al in the coating is expected to improve the corrosion resistance due to the formation of a dense and passive Al2O3 layer on the surface of the coating in the corrosive solution [22,23]. In this study, Al–Cr–N ternary sub-layer with carbon coating on the top (hereafter nominated C/Al–Cr–N multilayer coatings) is deposited by close field unbalanced magnetron sputter ion plating (CFUBMSIP). The chemical composition of the Al–Cr–N sub-layer is varied by changing the current density of Cr and Al targets to optimize the composition. The cross-sectional SEM, EDS line scan and the crystallographic structure of C/Al–Cr–N multilayer coatings are studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electrochemical behavior in simulated PEMFC environment and ICR are also systematically investigated and discussed. 2. Experimental procedure Stainless steel 316 L (SS316L), purchased from Trinity Brand Industries, Inc., was chosen as the substrate material. The chemical composition of SS316L is shown in Table 1. SS316L samples were cut into 12 mm × 12 mm pieces with 4 mm thickness, polished up using No. 2000 SiC waterproof abrasive paper, cleaned with acetone and distilled water in an ultrasonic bath, and dried before subjecting to coating process. The surface roughness (Ra) of the as-polished SS316L sample is observed by using an atomic force microscope (AFM) and calculated to be 2.6 nm. The coatings were deposited by Teer UDP 650 CFUBMSIP coating system consisting of two graphite targets, one Cr target and one Al target. The experimental set-up scheme of the coating system was shown in Fig. 1. The sample stage rotated clockwise with speed of two circles per minute. High purity argon (99.99%) was used as the sputtering gas. The chamber was depressurized at a base pressure below 3.0 × 10−3 Pa using a diffusion pump and a rotary pump. During the coating deposition process, the gas flow of Ar was kept at 20 standard cubic centimeter per minute (sccm). Prior to the deposition, a small current of 0.5 A was applied to Cr and Al targets and the substrates were sputtered for 30 min at a bias voltage of −500 V to clean and remove the native passive film on the surface. Then, a Cr transition layer was deposited at an applied current of 5 A on the Cr target for 10 min to enhance the adhesion, followed by deposition of the multilayer coatings. Al–Cr–N layer was deposited by reactive sputtering with nitrogen flow rate of 20 sccm, for 45 min. In this process, Cr and Al target current was kept at (5 A, 1 A), (4 A, 2 A), (3 A, 3 A), (2 A, 4 A), (1 A, 5 A) and (0 A, A 6) (hereafter nominated as Al1Cr5, Al2Cr4, Al3Cr3, Al4Cr2, Al5Cr1 and Al6Cr0). Afterwards, a thin intermediate MCx layer was deposited as transition layer by reducing the current supplied to Cr or Al targets, and increasing simultaneously the current supplied to the carbon targets from 0.5 A to 5 A. This process was kept for 30 min. Lastly, the outside carbon layer was deposited at C target current of 5 A for 1.5 h. The surface morphology and cross section of C/Al–Cr–N multilayer coatings were examined by field-emission scanning electron microscopy (FE-SEM) of HITACHI S-4800. Energy-dispersive X-ray spectroscopy (EDS) was carried out to determine the cross-sectional chemical distribution. In order to explore the crystallographic structure of C/Al–Cr–N multilayer coatings, grazing incidence X-ray diffraction (GIXRD) was carried out on a RIGAKU D/MAX 2550 diffractometer with Cu Kα radiation (λ = 0.15406) operated at 40 kV and 60 mA. The incident angles were kept at 1° and the step size was 0.02°.
Fig. 1. The top view of the experimental set-up scheme of Teer UDP 650 CFUBMSIP coating system.
The ICR measurement for all C/Al–Cr–N coated SS316L was carried out at room temperature by a method similar to that proposed by Davies et al. [24]. In this structure, two pieces of conductive carbon paper were sandwiched between the sample and two copper plates. A constant current of 0.1 A was applied through the two copper plates and the total voltage drop was measured with increasing compaction force. The total voltage drop is the sum of all the interfacial contact resistances and the intrinsic resistance of each one of the materials contained in this system. The ICR value between the samples and the carbon paper can be calculated by subtracting the separately measured values of the rest of the resistances at each compaction force. At least three samples were tested for each deposition parameter and the average value is used. The deviation of ICR value is calculated within 5%. The electrochemical behavior of the bare and coated SS316L was measured in acid media containing 0.5 M sulfuric + 0.2 ppm F− at 70 °C by potentiodynamic tests and potentiostatic tests on the Zahner Zennium electrochemical workstation. A three-electrode system including platinum sheet as the counter electrode, mercury sulfate electrode (MSE) as the reference electrode and sample as the working electrode was used. The MSE was separated from the solution by a Luggin capillary to avoid anion contamination. Before the electrochemical test, the open circuit potential (OCP) of the samples was recorded for 1 h to ensure electrochemical stability. Potentiodynamic polarization was performed with a potential scanning rate of 1 mV s−1. The solution was bubbled with either air (to simulate cathode environment) or hydrogen gas (to simulate anode environment) prior to and during the electrochemical tests. The potentiostatic test was conducted for 10 h at a potential of 0.23 V vs MSE (equal to 0.6 V vs SCE) with purging air and of −0.47 V vs MSE (equal to −0.1 V vs SCE) with purging H2 to simulate the cathode and anode operating environments, respectively. To ensure reproducibility, at least two samples are measured for each deposition parameter in the electrochemical tests. After the potentiostatic test, the solution of about 100 ml was analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES) to determine the amounts of Fe, Al, Cr and Ni ions released to the solutions. 3. Results and discussion 3.1. SEM and XRD
Table 1 Chemical composition of the bare SS316L. Cr
Ni
Mo
C
Mn
Si
P
S
Fe
16.0–18.0
10.0–14.0
2.0–3.0
0.03
2.0
1.0
0.04
0.03
Balance
Fig. 2 depicts the surface morphology and cross section view as well as the EDS line scan of a typical C/Al–Cr–N multilayer coated SS316L, Al5Cr1 sample. As observed in Fig. 2, the carbon layer is continuous
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process. Thus, increased Cr ion density allows higher probability for nitrogen ions to react and deposit on the surface. For the Al1Cr5 sample, very low Al content of 3.31 at.% is incorporated into the Al–Cr–N coating due to its lower sputtering yield compared to Cr. The XRD patterns of the C/Al–Cr–N multilayer coatings with varying Al and Cr target current are shown in Fig. 3. The peak of carbon can be observed for all the samples. A broad halo peak at about 50° corresponding to AlN (102) is found only in the Al6Cr0 sample, indicating that the grain size of AlN is small and there is no AlN phase in other samples. With the increase of Cr content in the Al–Cr–N sub-layer, CrN (100) peak shows increased intensity in Al3Cr3, Al2Cr4 and AlCr5 samples, which reveals an increase of the content of CrN phase. The metallic carbides, which are formed during transition process from metallic nitrides to carbon coating, are identified in all the samples with MeC (111) and (200) peaks.
(a)
3.2. Interfacial contact resistance (ICR)
(b)
Al
Fe
Cr
C
N
Fig. 2. SEM images of surface morphology (a) and cross section view and EDS line-scan (b) of the Al5Cr1 sample.
and dense with spherical grains. Obvious defects such as pinholes and micropores are not observed within the observed area. It can be seen from the EDS line scan in Fig. 2(b) that the C/Al–Cr–N multilayer coating is about 2.3 μm in total thickness, consisting of 0.9 μm carbon layer and 1.4 μm Al–Cr–N sub-layer. The C/Al–Cr–N multilayer coating shows a dense and non-column microstructure, suggesting that this multilayer coating has great potential for use as a protective layer. In order to investigate the chemical composition of Al–Cr–N layer in the multilayer, Al–Cr–N single-layer coatings were deposited with the same deposition parameters in the C/Al–Cr–N coating process and EDS analysis was carried out. In EDS experiment, each sample was analyzed with three randomly selected areas and the average values of Al, Cr, and N content are listed in Table 2. It can be seen that the increase of Al target current remarkably increases Al content in Al–Cr–N coatings. However, the nitrogen content in Al–Cr–N coatings increases with the increasing Cr content and the atomic ratio of Cr:N is kept approximately 1:1 for Al3Cr3, Al2Cr4 and Al1Cr5 samples, despite the increasing Al content. It is probably due to the reason that Cr ions around the substrate surface are more prone to react with nitrogen ions during coating
ICR, originating from the interface between the carbon paper and the bipolar plate, is a key parameter in gauging the efficiency of the PEMFCs. It has been reported that increases in ICR will result in power losses on the order of 2–5% per 25 mV (per cm2) compared with graphite plates [25]. The ICR values of C/Al–Cr–N multilayer coated SS316L samples are investigated as a function of compaction force and shown in Fig. 4. In general, ICR decreases logarithmically with the compaction forces, which drops dramatically under lower compaction force while keeps relatively stable at higher compaction force. It can be seen from Fig. 4 that the ICR value decreases with the increasing content of chromium and reaches the minimum value for the Al2Cr4 sample. At the compaction force between 120 and 150 N cm−2, the ICR value of the Al2Cr4 sample is between 6.17 and 4.88 mΩ-cm2, which is much lower than the bare SS316L of 477–370 mΩ-cm2 at the same compaction force [10]. Moreover, the Al2Cr4 sample shows the ICR value lower than the DOE 2020 target (≤ 10 mΩ-cm2 ) for bipolar plate application. It is also observed that C/Al–Cr–N multilayer coatings with Cr:N atomic ratio of 1:1 have much better surface conductivity than Al5Cr1 and Al6Cr0 samples. Since all the samples have the same carbon top coating, the difference in ICR is mainly due to the presence of CrN phase in the Al–Cr–N sub-layer, because CrN has high electrical conductivity [26]. 3.3. Potentiodynamic test The corrosion behavior of the bare and C/Al–Cr–N multilayer coated SS316L samples is investigated using potentiodynamic polarization. The
Table 2 Chemical composition of the Al–Cr–N sub-layer with varying Al and Cr target current density. Sample
Al6Cr0
Al5Cr1
Al4Cr2
Al3Cr3
Al2Cr4
Al1Cr5
Al Cr N
68.55 – 31.45
49.20 17.24 33.56
32.24 31.09 36.67
18.72 41.89 39.39
9.33 47.56 43.11
3.31 50.14 46.55
Fig. 3. XRD pattern of C/Al-Cr-N multilayer coated SS316L.
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of the coated samples varies with the chemical composition of the Al–Cr–N sub-layer dramatically, especially for Al6Cr0 and Al5Cr1 samples. In specific, the Al6Cr0 sample exhibits the worst corrosion resistance among all the tested samples. The current densities at operation potential of the Al6Cr0 sample are 17.25 μA cm−2 in anode environment and N 40 μA cm−2 in cathode environment, which are even higher than those of the bare SS316L. Associating with XRD and EDS analyses, the poor corrosion resistance of Al6Cr0 and Al5Cr1 samples is related to the existence of AlN phase and high Al content, while the samples with Cr:N atomic ratio of 1:1 have better corrosion resistance. The Al3Cr3 sample shows the best corrosion resistance with corrosion potential of about − 204 mV and current density of 0.20 μA cm− 2 at PEMFC cathodic working potential in cathode environment. From the potentiodynamic test results, it can be concluded that C/Al–Cr–N multilayer coatings with aluminum content lower than 32.24 at.% and Cr:N atomic ratio of 1:1 can provide higher chemical inertness and better corrosion resistance in both the cathode and anode environments. Fig. 4. ICR values of C/Al–Cr–N multilayer coated SS316L versus compaction force, with ICR of the bare SS316L sample for comparison.
potentiodynamic curves at anodic and cathodic PEMFC environments are shown in Fig. 5(a) and (b), respectively. Based on the potentiodynamic curves, the corrosion potential, corrosion current density, current density at anodic and cathodic operation potential are calculated and listed in Table 3. In general, it can be noticed that the corrosion behavior
3.4. Potentiostatic test Potentiostatic test results in PEMFC anode and cathode environments are presented in Fig. 6(a) and (b), respectively. The inset in Fig. 6(a) shows the current density of C/Al–Cr–N multilayer coated SS316L at the last hour of the experiment. The current density in the beginning is closely related to the relationship between the open circuit potential (OCP) of sample and applied potential. It can be seen that the current density obtained from the bare SS316L and Al6Cr0 samples decreases with a positive current in the beginning because the applied potential of − 0.47 V vs MSE is nobler than their OCP. The current density of the bare SS316L gradually stabilizes to a low negative current, indicating that a stable passive film with a different composition is formed on the entire surface, while the current density of the Al6Cr0 sample keeps at a relative high level of 15 μA cm− 2, indicating that the corrosion reaction is highly active. The current density of the Al5Cr1 sample starts from about zero and slowly increases with time. For other multilayer coated samples, the current density increases from negative direction in the beginning and gradually stabilizes at a lower current density due to nobler corrosion potential. The cathodic current indicates that the samples are cathodically protected because the corrosion potential of these samples is nobler than the anodic operation potential. It is noted that the current density of the Al4Cr2 sample is more negative than that of the other samples, being about −0.4 μA cm−2, as shown in the inset figure in Fig. 6(a). In the simulated PEMFC cathode environment as shown in Fig. 6(b), the current density of the bare SS316L decreases gradually and then stabilizes at around 1.3 μA cm−2, indicating that a stable passive film is formed. For Al6Cr0 and Al5Cr1 samples, the current density increases dramatically with Table 3 Corrosion potential (Ecorr), corrosion current density (Icorr), current density at anodic operation potential of −0.47 V vs MSE (Ia) and current density at cathodic operation potential of 0.23 V vs MSE (Ic) obtained from potentiodynamic curves in Fig. 5 (N represents that the current density is negative).
Anode
Cathode
Fig. 5. Potentiodynamic behavior of the bare and C/Al–Cr–N multilayer coated SS316L in the simulated PEMFC environment: (a) bubbled with H2 (anode environment) and (b) bubbled with air (cathode environment).
Samples
Ecorr (mV)
Icorr (μA cm-2)
Ia (μA cm-2)
Ic (μA cm-2)
SS316L Al6Cr0 Al5Cr1 Al4Cr2 Al3Cr3 Al2Cr4 Al1Cr5 SS316L Al6Cr0 Al5Cr1 Al4Cr2 Al3Cr3 Al2Cr4 Al1Cr5
−700 −864 −438 −186 −162 −186 −238 −669 −888 −584 −201 −204 −222 −220
34.63 4.04 0.10 0.03 0.01 0.02 0.06 6.64 10.43 0.25 0.02 0.01 0.02 0.03
2.68 17.25 N N N N N – – – – – – –
– – – – – – – 8.48 – 2.48 0.39 0.20 0.27 0.41
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Z. Wang et al. / Surface & Coatings Technology 258 (2014) 1068–1074 Table 4 Fe, Al, Cr, and Ni ion concentrations in the simulated PEMFC solutions after 10 h potentiostatic test.
(a)
Sample
SS316L Al1Cr5 Al2Cr4 Al3Cr3 Al4Cr2 Al5Cr1 Al6Cr0
Ion concentration in PEMFC cathode environment after 10 h (ppm)
Ion concentration in PEMFC anode environment after 10 h (ppm)
Fe
Cr
Ni
Al
Total
Fe
Cr
Ni
Al
Total
7.62 0.55 0.45 0.54 0.68 0.76 1.26
0.88 0.01 0.01 0.01 0.01 0.11 0.15
– 0.02 0.02 0.03 0.01 0.03 0.10
0.69 1.18 0.85 1.10 1.60 1.24 3.49
9.19 1.76 1.33 1.68 2.30 2.14 5.00
6.28 0.54 0.63 0.38 0.38 0.41 0.78
0.90 0.01 0.01 0.01 0.01 0.04 0.02
– 0.02 0.03 0.02 0.02 0.03 0.01
0.61 1.19 0.84 0.81 0.64 1.09 2.41
7.79 1.76 1.50 1.22 1.05 1.57 3.22
3.6. SEM observation after potentiostatic test
(b)
Fig. 6. Potentiostatic behavior of the bare and C/Al–Cr–N multilayer coated SS316L in the simulated (a) anode (−0.47 V vs MSE bubbled with H2) and (b) cathode (0.23 V vs MSE bubbled with air) environment.
test time, which discloses that sub-layer with high Al content will not effectively protect SS316L from corrosion. While for other C/Al–Cr–N multilayer coating samples, the stabilized current density is much lower than that of the bare SS316L, which discloses that less electrochemical reaction occurs on the interface between the carbon coating surface and adjacent electrolyte. It is reported that addition of lower Al content can effectively refine the microstructure of CrN coating [21]. The Al–Cr–N sub-layer with fine microstructure will decrease the opportunity for the through coating defects. Consequently, the diffusion of corrosive anions though the finer-grained coating is slow and difficult. Whereas for C/Al–Cr–N multilayer coatings with higher Al content, the coating roughness increases resulting in an increase of defects in the coating [27]. 3.5. ICP-AES measurement The metal ion concentrations in the solution after 10 h potentiostatic test are analysed by ICP-AES and the results are listed in Table 4. The metal ion concentration of the bare SS316L is much higher than that of the C/Al–Cr–N multilayer coated SS316L in both PEMFC cathode and anode environments. The Fe ion shows a selective dissolution due to its higher mobility in the passive film [28]. By contrast, the metal ion concentrations released from the C/Al–Cr–N multilayer coated SS316L are greatly reduced due to the higher chemical inertness and better corrosion resistance. However, Al ion exhibits higher concentration than Fe, Cr, Ni ion concentrations for all C/Al–Cr–N multilayer coated samples, indicating that Al in the sub-layer is prone to be corroded.
In order to explore the mechanism and extent of corrosion, the samples are observed after the potentiostatic test using SEM and the micrographs are shown in Figs. 7 and 8. Large round area of corrosion can be easily found as shown in Figs. 7(a) and 8(a). EDS element analysis corresponding to the spot pointed out in Figs. 7(a) and 8(a) is listed in Table 5. From these results, we can see that the round corrosion spot is the SS316L substrate and the white ribbon around the corrosion spot is the AlN coating. Therefore, it can be concluded that a large area of Al6Cr0 coating is peeled off by galvanic corrosion leaving the substrate exposed to the corrosion medium in both PEMFC cathode and anode environments. Al5Cr1 coating has a similar corrosion behavior as observed in Figs. 7(b) and 8(b). This phenomenon can well explain the worse corrosion resistance and much higher metal ion concentration of Al6Cr0 and Al5Cr1 samples. For other C/Al–Cr–N multilayer coatings, they exhibit a similar corrosion behavior in either PEMFC cathode or anode environment. Thus, a typical surface morphology, Al2Cr4, is presented in Figs. 7(c) and 8(c). There are some corrosion by-products distributing around the micro-defect between adjacent granules after potentiostatic test in PEMFC cathode environment as shown in Fig. 7(c). It is probably because much nobler applied potential in cathode environment makes corrosive anions easy to penetrate through C/Al–Cr–N multilayer coatings and causes active corrosion site. Nevertheless, Al2Cr4 is still continuous and dense after potentiostatic test in the anode environment, indicating better corrosion resistance in this corrosion condition. Combining the results of electrochemical test, ICP-AES, and SEM, it is demonstrated that C/Al–Cr–N multilayer coatings with Cr:N atomic ratio of 1:1 and low Al content can provide excellent corrosion resistance for the stainless steel bipolar plates. 3.7. ICR after polarization In real PEMFC operating condition, the bipolar plate will suffer from corrosion and passivation caused by corrosive electrolyte and the ICR will change subsequently. Hence, the ICR values of the bare and C/Al–Cr–N multilayer coated SS316L after 10 h potentiostatic test in the simulated cathode and anode environments are measured and plotted in Fig. 9. The bare SS316L exhibits an extremely high ICR value of 957.3 and 755.5 mΩ-cm2 after being polarized in cathode and anode environments, respectively. It is because a thicker passive film with higher oxygen content compared to air-formed passive film is formed after polarization [29]. By contrast, the ICR of multilayer coated SS316L is quite low due to excellent electrical conductivity of carbon coating as well as sub-layer coatings. In general, the ICR increases with increasing Al content in the Al–Cr–N sub-layer. Since the carbon coating on the surface will not change in bonding type as verified in Ref. [30], the change in ICR value is mainly due to passivation of the Al–Cr–N sub-layer. The ICR values of Al1Cr5 and Al2Cr4 samples are the lowest of being around 8–9 mΩ-cm2 after potentiostatic test in both cathode and anode environments, which are lower than the DOE
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(a)
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(a)
A
C
B
(b)
(b)
(c)
(c)
Fig. 7. SEM observation of (a) Al6Cr0, (b) Al5Cr1 and (c) Al2Cr4 samples after 10 h potentiostatic test in PEMFC simulated cathode environment.
Fig. 8. SEM observation of (a) Al6Cr0, (b) Al5Cr1 and (c) Al2Cr4 samples after 10 h potentiostatic test in PEMFC simulated anode environment.
2020 technical target of ≤ 10 mΩ-cm2. However, Al6Cr0 and Al5Cr1 samples have relative higher ICR due to a large area of coating peeling off as shown in Figs. 7 and 8, as well as passivation of Al during polarization [22].
keeps around 1:1 and CrN phase exists in the Al–Cr–N sub-layer for the C/Al–Cr–N multilayer coating with higher Cr content. AlN phase exists in the Al6Cr0 sample. The ICR value decreases with the increasing
4. Conclusions
Table 5 EDS element analysis of the Al6Cr0 sample after 10 h potentiostatic test in PEMFC cathode and anode environments (at.%).
C/Al–Cr–N multilayer coatings with varying Al:Cr target current ratio are deposited on SS316L by CFUBMSIP. The SEM and EDS line scan results show that the C/Al–Cr–N multilayer coating, consisting of 0.9 μm carbon layer and 1.4 μm Al–Cr–N sub-layer, is dense and continuous. EDS and XRD analysis results disclose that the Cr:N atomic ratio
Analysis spot
Fe
Cr
Ni
Al
N
O
C
A B C
56.78 21.31 12.09
32.64 14.35 11.18
7.98 2.84 1.68
1.55 15.70 40.07
– 22.09 20.11
– 4.24 14.86
1.06 19.46 –
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Acknowledgments Financial support provided by the National Natural Science Foundation of China under Grant Number 51201106, “Chen Guang” project supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation (Grant Number 13CG07), “Chenxing” young scholar project of Shanghai Jiao Tong University (Grant Number 14X100010017), start-up fund for junior young teacher of Shanghai Jiao Tong University (Grant Number 13X100040022), and the Ministry of Science and Technology of the People's Republic of China (Grant Number 2009DFB50350) are acknowledged.
References
Fig. 9. ICR values of the bare and C/Al–Cr–N multilayer coated SS316L at compaction force of 150 N cm−2 after potentiostatic test in PEMFC cathode and anode environments.
content of Cr and reaches the minimum value of 6.17–4.88 mΩ-cm2 at the compaction force between 120 and 150 N cm−2 for the Al2Cr4 sample. The electrochemical and ICP-AES test results indicate that the corrosion resistance of the C/Al–Cr–N multilayer coating changes significantly with Al content. In specific, the Al6Cr0 sample exhibits the worst corrosion resistance with corrosion potential of − 0.9 V and much higher passive current density than the bare SS316L, due to the presence of AlN phase and increased surface roughness. On the other hand, C/Al–Cr–N multilayer coatings with Cr:N atomic ratio of 1:1 and lower aluminum content have higher chemical inertness and better corrosion resistance. The surface morphology of the C/Al–Cr–N multilayer coating after potentiostatic test indicates that galvanic corrosion takes place on the surface of Al6Cr0 and Al5Cr1 samples and a large area of coating is peeled off in both PEMFC cathode and anode environments. For other C/Al–Cr–N multilayer coated samples, they show some corrosion by-products distributing around micro-defect between adjacent granules in cathode environment, while no obvious corrosion is found in anode environment. The ICR result after potentiostatic tests shows that Al1Cr5 and Al2Cr4 samples have the lowest ICR value due to good electrical conductivity of CrN phase in the Al–Cr–N sub-layer and low Al content. Therefore, it is concluded that the C/Al–Cr–N multilayer coating, which contains CrN phase and low Al doping, has good surface conductivity and corrosion resistance. Conflict of interest None.
[1] E.A. Cho, U.S. Jeon, S.A. Hong, I.H. Oh, S.G. Kang, J. Power Sources 142 (2005) 177–183. [2] M. Li, S. Luo, C. Zeng, J. Shen, H. Lin, C. Cao, Corros. Sci. 46 (2004) 1369–1380. [3] P.L. Hentall, J.B. Lakeman, G.O. Mepsted, P.L. Adcock, J.M. Moore, J. Power Sources 80 (1999) 235–241. [4] Y. Wang, D.O. Northwood, Int. J. Hydrog. Energy 32 (2007) 895–902. [5] Y. Fu, M. Hou, G. Lin, J. Hou, Z. Shao, B. Yi, J. Power Sources 176 (2008) 282–286. [6] E. Dur, Ö.N. Cora, M. Koc, J. Power Sources 246 (2014) 788–799. [7] L. Wang, D.O. Northwood, X. Nie, J. Housden, E. Spain, A. Leyland, A. Matthews, J. Power Sources 195 (2010) 3814–3821. [8] Y. Fu, G. Lin, M. Hou, B. Wu, H. Li, L. Hao, Z. Shao, B. Yi, Int. J. Hydrog. Energy 34 (2009) 453–458. [9] K. Feng, Y. Shen, H. Sun, D. Liu, Q. An, X. Cai, P.K. Chu, Int. J. Hydrog. Energy 34 (2009) 6771–6777. [10] K. Feng, X. Cai, H. Sun, Z. Li, P.K. Chu, Diam. Relat. Mater. 19 (2010) 1354–1361. [11] S.J. Lee, C.H. Huang, Y.P. Chen, J. Mater. Process. Technol. 140 (2003) 688–693. [12] P. Yi, L. Peng, L. Feng, P. Gan, X. Lai, J. Power Sources 195 (2010) 7061–7066. [13] Y. Mori, M. Ueda, M. Hashimoto, Y. Aoi, S. Tanase, T. Sakai, Surf. Coat. Technol. 202 (2008) 4094–4101. [14] Y. Wang, D.O. Northwood, J. Power Sources 165 (2007) 293–298. [15] B. Elsener, A. Rota, H. Böhni, Mater. Sci. Forum 44 (1989) 29–38. [16] C. Liu, A. Leyland, Q. Bi, A. Matthews, Surf. Coat. Technol. 141 (2001) 164–173. [17] N.D. Nam, M.J. Kim, D.S. Jo, J.G. Kim, D.H. Yoon, Thin Solid Films 545 (2013) 380–384. [18] R. Tian, J. Power Sources 196 (2011) 1258–1263. [19] H. Sun, K. Cooke, G. Eitzinger, P. Hamilton, B. Pollet, Thin Solid Films 528 (2013) 199–204. [20] K. Feng, Z. Li, H. Sun, L. Yu, X. Cai, Y.X. Wu, P.K. Chu, J. Power Sources 222 (2013) 351–358. [21] X. Ding, A.L.K. Tan, X.T. Zeng, C. Wang, T. Yue, C.Q. Sun, Thin Solid Films 516 (2008) 5716–5720. [22] Y. Lv, L. Ji, X. Liu, H. Li, H. Zhou, J. Chen, Appl. Surf. Sci. 258 (2012) 3864–3870. [23] L. Chunha, M. Andritschky, L. Rebouta, K. Pischow, Surf. Coat. Technol. 116 (1999) 1152–1160. [24] D.P. Davies, P.L. Adcok, M. Turpin, S.J. Rowen, J. Appl. Electrochem. 30 (2000) 101–105. [25] A. Heinzel, F. Mahlendorf, O. Niemzig, C. Kreuz, J. Power Sources 131 (2004) 35–40. [26] A. Tarniowy, R. Mania, M. Rekas, Thin Solid Films 311 (1997) 93–100. [27] M.A.M. Ibrahim, S.F. Korablov, M. Yoshimura, Corros. Sci. 44 (2002) 815–828. [28] R. Kirchheim, B. Heine, H. Fischmeister, S. Hoffman, H. Knote, U. Stolz, Corros. Sci. 29 (1989) 899–917. [29] K. Feng, G. Wu, Z. Li, X. Cai, P.K. Chu, Int. J. Hydrog. Energy 36 (2011) 13032–13042. [30] K. Feng, Z. Li, F. Lu, J. Huang, X. Cai, Y. Wu, J. Power Sources 249 (2014) 299–305.
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Investigation of C/Al–Cr–N multilayer coatings for stainless steel bipolar plate in polymer electrolyte membrane fuel cells Zhiyuan Wang a, Yibo Wang a, Zhuguo Li a,c,⁎, Kai Feng a,c,⁎, Jian Huang a, Fenggui Lu a, Chengwu Yao a, Xun Cai a,b, Yixiong Wu a,b,c a b c
Shanghai Key Laboratory of Materials Laser Processing and Modification, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China State Key Laboratory of Metal Matrix Composites, Shanghai Jiaotong University, Shanghai 200240, China Academician Expert Office Workstation (Jiansheng Pan), Lin'an, Zhejiang Province, China
a r t i c l e
i n f o
Article history: Received 26 March 2014 Accepted in revised form 8 July 2014 Available online 15 July 2014 Keywords: Polymer electrolyte membrane fuel cell Bipolar plates Surface conductivity Corrosion resistance Physical vapor deposition
a b s t r a c t To improve the corrosion resistance and surface conductivity of stainless steel bipolar plate, C/Al–Cr–N multilayer coatings with varying Al:Cr target current are deposited on stainless steel 316 L (SS316L) by magnetron sputtering. Scanning electron microscope (SEM) results show that the coating is about 2.3 μm in total thickness, consisting of 0.9 μm carbon layer and 1.4 μm Al–Cr–N sub-layer. The interfacial contact resistance (ICR) result indicates that the surface resistance decreases with the increasing content of chromium, and achieves the minimum value of 6.17–4.88 mΩ-cm2 at the compaction force between 120 and 150 N cm−2 for the Al2Cr4 sample. The electrochemical and inductively coupled plasma atomic emission spectrometry (ICP-AES) results disclose that C/Al–Cr–N multilayer coatings with Cr:N atomic ratio of 1:1 and lower aluminum content have higher chemical inertness and better corrosion resistance. Al6Cr0 and Al5Cr1 samples exhibit more negative corrosion potential and higher passive current density with galvanic corrosion on the surface due to the existence of AlN phase. Therefore, C/Al–Cr–N multilayer coatings with lower aluminum content are preferred for bipolar plate application. Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.
1. Introduction Currently, numbers of coatings prepared by various surface modification methods have been investigated as alternative bipolar plate materials in polymer electrolyte membrane fuel cells (PEMFCs) to improve the corrosion resistance and surface conductivity. Among these surface modification methods, physical vapor deposition (PVD) could be a promising technique because it allows the deposition of coatings with designed chemical composition and properties on a wide range of substrates. Metal nitrides, such as TiN [1–4], CrN [5–8] and carbon-based coatings [9–13] have been deposited on metallic bipolar plates by PVD technique due to their excellent electrical conductivity and corrosion resistance. However, these single layer coatings are prone to local corrosion and the corrosion current density will increase dramatically after long time immersion in the simulated PEMFC environment [2,14] mainly due to the intrinsic defects such as pinholes and macroparticles in the coating created during PVD process [15]. Because of the establishment of corrosion potential difference between the coating material and the less
⁎ Corresponding authors at: Shanghai Key Laboratory of Materials Laser Processing and Modification, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. Tel.: +86 21 54745878; fax: +86 21 34203024. E-mail addresses: [email protected] (Z. Li), [email protected] (K. Feng).
http://dx.doi.org/10.1016/j.surfcoat.2014.07.028 0257-8972/Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.
noble steel substrate, aqueous corrosion of single layer coatings on steel usually takes place in a localized form [16]. Localized corrosion, especially through-coating corrosion, will greatly decrease the corrosion resistance of the coatings. Therefore, multilayer structures of PVD coatings are developed to improve the corrosion resistance by interruption of through-coating pinholes. Nam et al. [17] compared the corrosion resistance of TiN/Ti, TiN/Cr, CrN/Ti, and CrN/Cr films prepared by reactive RF magnetron sputtering. The results indicated that the CrN/Cr film had the best corrosion resistance and the highest charge transfer resistance. Tian [18] prepared a CrN/Cr composite film on stainless steel by PVD technique and found an improved interfacial conductivity and corrosion resistance. Sun et al. [19] investigated the corrosion resistance and interfacial contact resistance (ICR) of TiN film, carbon film and C/TiN multilayer coating in simulated PEMFC environment and concluded that C/TiN multilayer coatings are excellent candidates for further research and long-term in-situ fuel cell testing. In a previous study, we fabricated a new multilayer coating composed of amorphous carbon as the outer layer and CrN as the sub-layer (C/CrN multilayer) [20]. The results showed that the current density of C/CrN multilayer coated SS316L at 0.6 V is decreased to 0.5 μA cm−2 and ICR is minimized to 2.6 mΩ-cm2 at compaction force of 150 N cm−2. CrN and CrN-based multicomponent nitrides are a well-established coating system. They have been reported exhibiting high oxidation resistance and corrosion resistance. Ding et al. [21] have pointed out that the coating structure becomes finer with the incorporation of Al
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into CrN coating, as revealed by the XRD patterns. This will result in a discontinuous crystallite boundary in the columnar structure, which decreases the opportunity for the through coating defects. Additionally, the addition of Al in the coating is expected to improve the corrosion resistance due to the formation of a dense and passive Al2O3 layer on the surface of the coating in the corrosive solution [22,23]. In this study, Al–Cr–N ternary sub-layer with carbon coating on the top (hereafter nominated C/Al–Cr–N multilayer coatings) is deposited by close field unbalanced magnetron sputter ion plating (CFUBMSIP). The chemical composition of the Al–Cr–N sub-layer is varied by changing the current density of Cr and Al targets to optimize the composition. The cross-sectional SEM, EDS line scan and the crystallographic structure of C/Al–Cr–N multilayer coatings are studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electrochemical behavior in simulated PEMFC environment and ICR are also systematically investigated and discussed. 2. Experimental procedure Stainless steel 316 L (SS316L), purchased from Trinity Brand Industries, Inc., was chosen as the substrate material. The chemical composition of SS316L is shown in Table 1. SS316L samples were cut into 12 mm × 12 mm pieces with 4 mm thickness, polished up using No. 2000 SiC waterproof abrasive paper, cleaned with acetone and distilled water in an ultrasonic bath, and dried before subjecting to coating process. The surface roughness (Ra) of the as-polished SS316L sample is observed by using an atomic force microscope (AFM) and calculated to be 2.6 nm. The coatings were deposited by Teer UDP 650 CFUBMSIP coating system consisting of two graphite targets, one Cr target and one Al target. The experimental set-up scheme of the coating system was shown in Fig. 1. The sample stage rotated clockwise with speed of two circles per minute. High purity argon (99.99%) was used as the sputtering gas. The chamber was depressurized at a base pressure below 3.0 × 10−3 Pa using a diffusion pump and a rotary pump. During the coating deposition process, the gas flow of Ar was kept at 20 standard cubic centimeter per minute (sccm). Prior to the deposition, a small current of 0.5 A was applied to Cr and Al targets and the substrates were sputtered for 30 min at a bias voltage of −500 V to clean and remove the native passive film on the surface. Then, a Cr transition layer was deposited at an applied current of 5 A on the Cr target for 10 min to enhance the adhesion, followed by deposition of the multilayer coatings. Al–Cr–N layer was deposited by reactive sputtering with nitrogen flow rate of 20 sccm, for 45 min. In this process, Cr and Al target current was kept at (5 A, 1 A), (4 A, 2 A), (3 A, 3 A), (2 A, 4 A), (1 A, 5 A) and (0 A, A 6) (hereafter nominated as Al1Cr5, Al2Cr4, Al3Cr3, Al4Cr2, Al5Cr1 and Al6Cr0). Afterwards, a thin intermediate MCx layer was deposited as transition layer by reducing the current supplied to Cr or Al targets, and increasing simultaneously the current supplied to the carbon targets from 0.5 A to 5 A. This process was kept for 30 min. Lastly, the outside carbon layer was deposited at C target current of 5 A for 1.5 h. The surface morphology and cross section of C/Al–Cr–N multilayer coatings were examined by field-emission scanning electron microscopy (FE-SEM) of HITACHI S-4800. Energy-dispersive X-ray spectroscopy (EDS) was carried out to determine the cross-sectional chemical distribution. In order to explore the crystallographic structure of C/Al–Cr–N multilayer coatings, grazing incidence X-ray diffraction (GIXRD) was carried out on a RIGAKU D/MAX 2550 diffractometer with Cu Kα radiation (λ = 0.15406) operated at 40 kV and 60 mA. The incident angles were kept at 1° and the step size was 0.02°.
Fig. 1. The top view of the experimental set-up scheme of Teer UDP 650 CFUBMSIP coating system.
The ICR measurement for all C/Al–Cr–N coated SS316L was carried out at room temperature by a method similar to that proposed by Davies et al. [24]. In this structure, two pieces of conductive carbon paper were sandwiched between the sample and two copper plates. A constant current of 0.1 A was applied through the two copper plates and the total voltage drop was measured with increasing compaction force. The total voltage drop is the sum of all the interfacial contact resistances and the intrinsic resistance of each one of the materials contained in this system. The ICR value between the samples and the carbon paper can be calculated by subtracting the separately measured values of the rest of the resistances at each compaction force. At least three samples were tested for each deposition parameter and the average value is used. The deviation of ICR value is calculated within 5%. The electrochemical behavior of the bare and coated SS316L was measured in acid media containing 0.5 M sulfuric + 0.2 ppm F− at 70 °C by potentiodynamic tests and potentiostatic tests on the Zahner Zennium electrochemical workstation. A three-electrode system including platinum sheet as the counter electrode, mercury sulfate electrode (MSE) as the reference electrode and sample as the working electrode was used. The MSE was separated from the solution by a Luggin capillary to avoid anion contamination. Before the electrochemical test, the open circuit potential (OCP) of the samples was recorded for 1 h to ensure electrochemical stability. Potentiodynamic polarization was performed with a potential scanning rate of 1 mV s−1. The solution was bubbled with either air (to simulate cathode environment) or hydrogen gas (to simulate anode environment) prior to and during the electrochemical tests. The potentiostatic test was conducted for 10 h at a potential of 0.23 V vs MSE (equal to 0.6 V vs SCE) with purging air and of −0.47 V vs MSE (equal to −0.1 V vs SCE) with purging H2 to simulate the cathode and anode operating environments, respectively. To ensure reproducibility, at least two samples are measured for each deposition parameter in the electrochemical tests. After the potentiostatic test, the solution of about 100 ml was analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES) to determine the amounts of Fe, Al, Cr and Ni ions released to the solutions. 3. Results and discussion 3.1. SEM and XRD
Table 1 Chemical composition of the bare SS316L. Cr
Ni
Mo
C
Mn
Si
P
S
Fe
16.0–18.0
10.0–14.0
2.0–3.0
0.03
2.0
1.0
0.04
0.03
Balance
Fig. 2 depicts the surface morphology and cross section view as well as the EDS line scan of a typical C/Al–Cr–N multilayer coated SS316L, Al5Cr1 sample. As observed in Fig. 2, the carbon layer is continuous
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process. Thus, increased Cr ion density allows higher probability for nitrogen ions to react and deposit on the surface. For the Al1Cr5 sample, very low Al content of 3.31 at.% is incorporated into the Al–Cr–N coating due to its lower sputtering yield compared to Cr. The XRD patterns of the C/Al–Cr–N multilayer coatings with varying Al and Cr target current are shown in Fig. 3. The peak of carbon can be observed for all the samples. A broad halo peak at about 50° corresponding to AlN (102) is found only in the Al6Cr0 sample, indicating that the grain size of AlN is small and there is no AlN phase in other samples. With the increase of Cr content in the Al–Cr–N sub-layer, CrN (100) peak shows increased intensity in Al3Cr3, Al2Cr4 and AlCr5 samples, which reveals an increase of the content of CrN phase. The metallic carbides, which are formed during transition process from metallic nitrides to carbon coating, are identified in all the samples with MeC (111) and (200) peaks.
(a)
3.2. Interfacial contact resistance (ICR)
(b)
Al
Fe
Cr
C
N
Fig. 2. SEM images of surface morphology (a) and cross section view and EDS line-scan (b) of the Al5Cr1 sample.
and dense with spherical grains. Obvious defects such as pinholes and micropores are not observed within the observed area. It can be seen from the EDS line scan in Fig. 2(b) that the C/Al–Cr–N multilayer coating is about 2.3 μm in total thickness, consisting of 0.9 μm carbon layer and 1.4 μm Al–Cr–N sub-layer. The C/Al–Cr–N multilayer coating shows a dense and non-column microstructure, suggesting that this multilayer coating has great potential for use as a protective layer. In order to investigate the chemical composition of Al–Cr–N layer in the multilayer, Al–Cr–N single-layer coatings were deposited with the same deposition parameters in the C/Al–Cr–N coating process and EDS analysis was carried out. In EDS experiment, each sample was analyzed with three randomly selected areas and the average values of Al, Cr, and N content are listed in Table 2. It can be seen that the increase of Al target current remarkably increases Al content in Al–Cr–N coatings. However, the nitrogen content in Al–Cr–N coatings increases with the increasing Cr content and the atomic ratio of Cr:N is kept approximately 1:1 for Al3Cr3, Al2Cr4 and Al1Cr5 samples, despite the increasing Al content. It is probably due to the reason that Cr ions around the substrate surface are more prone to react with nitrogen ions during coating
ICR, originating from the interface between the carbon paper and the bipolar plate, is a key parameter in gauging the efficiency of the PEMFCs. It has been reported that increases in ICR will result in power losses on the order of 2–5% per 25 mV (per cm2) compared with graphite plates [25]. The ICR values of C/Al–Cr–N multilayer coated SS316L samples are investigated as a function of compaction force and shown in Fig. 4. In general, ICR decreases logarithmically with the compaction forces, which drops dramatically under lower compaction force while keeps relatively stable at higher compaction force. It can be seen from Fig. 4 that the ICR value decreases with the increasing content of chromium and reaches the minimum value for the Al2Cr4 sample. At the compaction force between 120 and 150 N cm−2, the ICR value of the Al2Cr4 sample is between 6.17 and 4.88 mΩ-cm2, which is much lower than the bare SS316L of 477–370 mΩ-cm2 at the same compaction force [10]. Moreover, the Al2Cr4 sample shows the ICR value lower than the DOE 2020 target (≤ 10 mΩ-cm2 ) for bipolar plate application. It is also observed that C/Al–Cr–N multilayer coatings with Cr:N atomic ratio of 1:1 have much better surface conductivity than Al5Cr1 and Al6Cr0 samples. Since all the samples have the same carbon top coating, the difference in ICR is mainly due to the presence of CrN phase in the Al–Cr–N sub-layer, because CrN has high electrical conductivity [26]. 3.3. Potentiodynamic test The corrosion behavior of the bare and C/Al–Cr–N multilayer coated SS316L samples is investigated using potentiodynamic polarization. The
Table 2 Chemical composition of the Al–Cr–N sub-layer with varying Al and Cr target current density. Sample
Al6Cr0
Al5Cr1
Al4Cr2
Al3Cr3
Al2Cr4
Al1Cr5
Al Cr N
68.55 – 31.45
49.20 17.24 33.56
32.24 31.09 36.67
18.72 41.89 39.39
9.33 47.56 43.11
3.31 50.14 46.55
Fig. 3. XRD pattern of C/Al-Cr-N multilayer coated SS316L.
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of the coated samples varies with the chemical composition of the Al–Cr–N sub-layer dramatically, especially for Al6Cr0 and Al5Cr1 samples. In specific, the Al6Cr0 sample exhibits the worst corrosion resistance among all the tested samples. The current densities at operation potential of the Al6Cr0 sample are 17.25 μA cm−2 in anode environment and N 40 μA cm−2 in cathode environment, which are even higher than those of the bare SS316L. Associating with XRD and EDS analyses, the poor corrosion resistance of Al6Cr0 and Al5Cr1 samples is related to the existence of AlN phase and high Al content, while the samples with Cr:N atomic ratio of 1:1 have better corrosion resistance. The Al3Cr3 sample shows the best corrosion resistance with corrosion potential of about − 204 mV and current density of 0.20 μA cm− 2 at PEMFC cathodic working potential in cathode environment. From the potentiodynamic test results, it can be concluded that C/Al–Cr–N multilayer coatings with aluminum content lower than 32.24 at.% and Cr:N atomic ratio of 1:1 can provide higher chemical inertness and better corrosion resistance in both the cathode and anode environments. Fig. 4. ICR values of C/Al–Cr–N multilayer coated SS316L versus compaction force, with ICR of the bare SS316L sample for comparison.
potentiodynamic curves at anodic and cathodic PEMFC environments are shown in Fig. 5(a) and (b), respectively. Based on the potentiodynamic curves, the corrosion potential, corrosion current density, current density at anodic and cathodic operation potential are calculated and listed in Table 3. In general, it can be noticed that the corrosion behavior
3.4. Potentiostatic test Potentiostatic test results in PEMFC anode and cathode environments are presented in Fig. 6(a) and (b), respectively. The inset in Fig. 6(a) shows the current density of C/Al–Cr–N multilayer coated SS316L at the last hour of the experiment. The current density in the beginning is closely related to the relationship between the open circuit potential (OCP) of sample and applied potential. It can be seen that the current density obtained from the bare SS316L and Al6Cr0 samples decreases with a positive current in the beginning because the applied potential of − 0.47 V vs MSE is nobler than their OCP. The current density of the bare SS316L gradually stabilizes to a low negative current, indicating that a stable passive film with a different composition is formed on the entire surface, while the current density of the Al6Cr0 sample keeps at a relative high level of 15 μA cm− 2, indicating that the corrosion reaction is highly active. The current density of the Al5Cr1 sample starts from about zero and slowly increases with time. For other multilayer coated samples, the current density increases from negative direction in the beginning and gradually stabilizes at a lower current density due to nobler corrosion potential. The cathodic current indicates that the samples are cathodically protected because the corrosion potential of these samples is nobler than the anodic operation potential. It is noted that the current density of the Al4Cr2 sample is more negative than that of the other samples, being about −0.4 μA cm−2, as shown in the inset figure in Fig. 6(a). In the simulated PEMFC cathode environment as shown in Fig. 6(b), the current density of the bare SS316L decreases gradually and then stabilizes at around 1.3 μA cm−2, indicating that a stable passive film is formed. For Al6Cr0 and Al5Cr1 samples, the current density increases dramatically with Table 3 Corrosion potential (Ecorr), corrosion current density (Icorr), current density at anodic operation potential of −0.47 V vs MSE (Ia) and current density at cathodic operation potential of 0.23 V vs MSE (Ic) obtained from potentiodynamic curves in Fig. 5 (N represents that the current density is negative).
Anode
Cathode
Fig. 5. Potentiodynamic behavior of the bare and C/Al–Cr–N multilayer coated SS316L in the simulated PEMFC environment: (a) bubbled with H2 (anode environment) and (b) bubbled with air (cathode environment).
Samples
Ecorr (mV)
Icorr (μA cm-2)
Ia (μA cm-2)
Ic (μA cm-2)
SS316L Al6Cr0 Al5Cr1 Al4Cr2 Al3Cr3 Al2Cr4 Al1Cr5 SS316L Al6Cr0 Al5Cr1 Al4Cr2 Al3Cr3 Al2Cr4 Al1Cr5
−700 −864 −438 −186 −162 −186 −238 −669 −888 −584 −201 −204 −222 −220
34.63 4.04 0.10 0.03 0.01 0.02 0.06 6.64 10.43 0.25 0.02 0.01 0.02 0.03
2.68 17.25 N N N N N – – – – – – –
– – – – – – – 8.48 – 2.48 0.39 0.20 0.27 0.41
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Z. Wang et al. / Surface & Coatings Technology 258 (2014) 1068–1074 Table 4 Fe, Al, Cr, and Ni ion concentrations in the simulated PEMFC solutions after 10 h potentiostatic test.
(a)
Sample
SS316L Al1Cr5 Al2Cr4 Al3Cr3 Al4Cr2 Al5Cr1 Al6Cr0
Ion concentration in PEMFC cathode environment after 10 h (ppm)
Ion concentration in PEMFC anode environment after 10 h (ppm)
Fe
Cr
Ni
Al
Total
Fe
Cr
Ni
Al
Total
7.62 0.55 0.45 0.54 0.68 0.76 1.26
0.88 0.01 0.01 0.01 0.01 0.11 0.15
– 0.02 0.02 0.03 0.01 0.03 0.10
0.69 1.18 0.85 1.10 1.60 1.24 3.49
9.19 1.76 1.33 1.68 2.30 2.14 5.00
6.28 0.54 0.63 0.38 0.38 0.41 0.78
0.90 0.01 0.01 0.01 0.01 0.04 0.02
– 0.02 0.03 0.02 0.02 0.03 0.01
0.61 1.19 0.84 0.81 0.64 1.09 2.41
7.79 1.76 1.50 1.22 1.05 1.57 3.22
3.6. SEM observation after potentiostatic test
(b)
Fig. 6. Potentiostatic behavior of the bare and C/Al–Cr–N multilayer coated SS316L in the simulated (a) anode (−0.47 V vs MSE bubbled with H2) and (b) cathode (0.23 V vs MSE bubbled with air) environment.
test time, which discloses that sub-layer with high Al content will not effectively protect SS316L from corrosion. While for other C/Al–Cr–N multilayer coating samples, the stabilized current density is much lower than that of the bare SS316L, which discloses that less electrochemical reaction occurs on the interface between the carbon coating surface and adjacent electrolyte. It is reported that addition of lower Al content can effectively refine the microstructure of CrN coating [21]. The Al–Cr–N sub-layer with fine microstructure will decrease the opportunity for the through coating defects. Consequently, the diffusion of corrosive anions though the finer-grained coating is slow and difficult. Whereas for C/Al–Cr–N multilayer coatings with higher Al content, the coating roughness increases resulting in an increase of defects in the coating [27]. 3.5. ICP-AES measurement The metal ion concentrations in the solution after 10 h potentiostatic test are analysed by ICP-AES and the results are listed in Table 4. The metal ion concentration of the bare SS316L is much higher than that of the C/Al–Cr–N multilayer coated SS316L in both PEMFC cathode and anode environments. The Fe ion shows a selective dissolution due to its higher mobility in the passive film [28]. By contrast, the metal ion concentrations released from the C/Al–Cr–N multilayer coated SS316L are greatly reduced due to the higher chemical inertness and better corrosion resistance. However, Al ion exhibits higher concentration than Fe, Cr, Ni ion concentrations for all C/Al–Cr–N multilayer coated samples, indicating that Al in the sub-layer is prone to be corroded.
In order to explore the mechanism and extent of corrosion, the samples are observed after the potentiostatic test using SEM and the micrographs are shown in Figs. 7 and 8. Large round area of corrosion can be easily found as shown in Figs. 7(a) and 8(a). EDS element analysis corresponding to the spot pointed out in Figs. 7(a) and 8(a) is listed in Table 5. From these results, we can see that the round corrosion spot is the SS316L substrate and the white ribbon around the corrosion spot is the AlN coating. Therefore, it can be concluded that a large area of Al6Cr0 coating is peeled off by galvanic corrosion leaving the substrate exposed to the corrosion medium in both PEMFC cathode and anode environments. Al5Cr1 coating has a similar corrosion behavior as observed in Figs. 7(b) and 8(b). This phenomenon can well explain the worse corrosion resistance and much higher metal ion concentration of Al6Cr0 and Al5Cr1 samples. For other C/Al–Cr–N multilayer coatings, they exhibit a similar corrosion behavior in either PEMFC cathode or anode environment. Thus, a typical surface morphology, Al2Cr4, is presented in Figs. 7(c) and 8(c). There are some corrosion by-products distributing around the micro-defect between adjacent granules after potentiostatic test in PEMFC cathode environment as shown in Fig. 7(c). It is probably because much nobler applied potential in cathode environment makes corrosive anions easy to penetrate through C/Al–Cr–N multilayer coatings and causes active corrosion site. Nevertheless, Al2Cr4 is still continuous and dense after potentiostatic test in the anode environment, indicating better corrosion resistance in this corrosion condition. Combining the results of electrochemical test, ICP-AES, and SEM, it is demonstrated that C/Al–Cr–N multilayer coatings with Cr:N atomic ratio of 1:1 and low Al content can provide excellent corrosion resistance for the stainless steel bipolar plates. 3.7. ICR after polarization In real PEMFC operating condition, the bipolar plate will suffer from corrosion and passivation caused by corrosive electrolyte and the ICR will change subsequently. Hence, the ICR values of the bare and C/Al–Cr–N multilayer coated SS316L after 10 h potentiostatic test in the simulated cathode and anode environments are measured and plotted in Fig. 9. The bare SS316L exhibits an extremely high ICR value of 957.3 and 755.5 mΩ-cm2 after being polarized in cathode and anode environments, respectively. It is because a thicker passive film with higher oxygen content compared to air-formed passive film is formed after polarization [29]. By contrast, the ICR of multilayer coated SS316L is quite low due to excellent electrical conductivity of carbon coating as well as sub-layer coatings. In general, the ICR increases with increasing Al content in the Al–Cr–N sub-layer. Since the carbon coating on the surface will not change in bonding type as verified in Ref. [30], the change in ICR value is mainly due to passivation of the Al–Cr–N sub-layer. The ICR values of Al1Cr5 and Al2Cr4 samples are the lowest of being around 8–9 mΩ-cm2 after potentiostatic test in both cathode and anode environments, which are lower than the DOE
Z. Wang et al. / Surface & Coatings Technology 258 (2014) 1068–1074
(a)
1073
(a)
A
C
B
(b)
(b)
(c)
(c)
Fig. 7. SEM observation of (a) Al6Cr0, (b) Al5Cr1 and (c) Al2Cr4 samples after 10 h potentiostatic test in PEMFC simulated cathode environment.
Fig. 8. SEM observation of (a) Al6Cr0, (b) Al5Cr1 and (c) Al2Cr4 samples after 10 h potentiostatic test in PEMFC simulated anode environment.
2020 technical target of ≤ 10 mΩ-cm2. However, Al6Cr0 and Al5Cr1 samples have relative higher ICR due to a large area of coating peeling off as shown in Figs. 7 and 8, as well as passivation of Al during polarization [22].
keeps around 1:1 and CrN phase exists in the Al–Cr–N sub-layer for the C/Al–Cr–N multilayer coating with higher Cr content. AlN phase exists in the Al6Cr0 sample. The ICR value decreases with the increasing
4. Conclusions
Table 5 EDS element analysis of the Al6Cr0 sample after 10 h potentiostatic test in PEMFC cathode and anode environments (at.%).
C/Al–Cr–N multilayer coatings with varying Al:Cr target current ratio are deposited on SS316L by CFUBMSIP. The SEM and EDS line scan results show that the C/Al–Cr–N multilayer coating, consisting of 0.9 μm carbon layer and 1.4 μm Al–Cr–N sub-layer, is dense and continuous. EDS and XRD analysis results disclose that the Cr:N atomic ratio
Analysis spot
Fe
Cr
Ni
Al
N
O
C
A B C
56.78 21.31 12.09
32.64 14.35 11.18
7.98 2.84 1.68
1.55 15.70 40.07
– 22.09 20.11
– 4.24 14.86
1.06 19.46 –
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Z. Wang et al. / Surface & Coatings Technology 258 (2014) 1068–1074
Acknowledgments Financial support provided by the National Natural Science Foundation of China under Grant Number 51201106, “Chen Guang” project supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation (Grant Number 13CG07), “Chenxing” young scholar project of Shanghai Jiao Tong University (Grant Number 14X100010017), start-up fund for junior young teacher of Shanghai Jiao Tong University (Grant Number 13X100040022), and the Ministry of Science and Technology of the People's Republic of China (Grant Number 2009DFB50350) are acknowledged.
References
Fig. 9. ICR values of the bare and C/Al–Cr–N multilayer coated SS316L at compaction force of 150 N cm−2 after potentiostatic test in PEMFC cathode and anode environments.
content of Cr and reaches the minimum value of 6.17–4.88 mΩ-cm2 at the compaction force between 120 and 150 N cm−2 for the Al2Cr4 sample. The electrochemical and ICP-AES test results indicate that the corrosion resistance of the C/Al–Cr–N multilayer coating changes significantly with Al content. In specific, the Al6Cr0 sample exhibits the worst corrosion resistance with corrosion potential of − 0.9 V and much higher passive current density than the bare SS316L, due to the presence of AlN phase and increased surface roughness. On the other hand, C/Al–Cr–N multilayer coatings with Cr:N atomic ratio of 1:1 and lower aluminum content have higher chemical inertness and better corrosion resistance. The surface morphology of the C/Al–Cr–N multilayer coating after potentiostatic test indicates that galvanic corrosion takes place on the surface of Al6Cr0 and Al5Cr1 samples and a large area of coating is peeled off in both PEMFC cathode and anode environments. For other C/Al–Cr–N multilayer coated samples, they show some corrosion by-products distributing around micro-defect between adjacent granules in cathode environment, while no obvious corrosion is found in anode environment. The ICR result after potentiostatic tests shows that Al1Cr5 and Al2Cr4 samples have the lowest ICR value due to good electrical conductivity of CrN phase in the Al–Cr–N sub-layer and low Al content. Therefore, it is concluded that the C/Al–Cr–N multilayer coating, which contains CrN phase and low Al doping, has good surface conductivity and corrosion resistance. Conflict of interest None.
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