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Characteristics and properties of CrN compound layer produced by plasma nitriding of Cr-electroplated of AISI 304 stainless steel
Surface & Coatings Technology 385 (2020) 125450
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Characteristics and properties of CreN compound layer produced by plasma nitriding of Cr-electroplated of AISI 304 stainless steel
T
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Hongyu Shen, Liang Wang , Juncai Sun Department of Materials Science and Engineering, Dalian Maritime University, Dalian 116026, China
A R T I C LE I N FO
A B S T R A C T
Keywords: CreN compound Cr coating Corrosion Plasma nitriding Electrical resistance
In this work, plasma nitriding of the Cr-electroplated coating on a 304 stainless steel (304 SS) was carried out at 700 °C in pure ammonia for different times (2 h, 3 h and 4 h). After plasma nitriding, a thin (1–2 μm) and compact chromium nitride layer mainly composed of Cr2N phase was formed on the surface region of Crelectroplated coating. The corrosion resistance of CreN layers was examined in the proton exchange membrane fuel cell working environment to evaluate the possible application of the duplex-treated 304 stainless steel for bipolar plates. Before and after corrosion test the interfacial contact resistance was measured to determine the effect of nitriding on the electrical conductivity of all samples. The results show that the CreN compound layer has a very low passive current density 0.13 μA cm−2 and interfacial contact resistance 4.5 mΩ cm2.
1. Introduction Bipolar plates play crucial roles in determining the performance of proton exchange membrane fuel cell system (PEMFCs). Their cost and weight account a great proportion of the whole PEMFCs [1–2]. Compared with graphite material, metallic bipolar plates (BPs) have many advantages such as low cost and high electrical conductivity. However, the conductivity of metals is always reduced if they are corroded with the corrosion products or a passive film adhering to the surface. Hence, the contact electrical resistance of metallic bipolar plates will be increased as a result of corrosion or passive film formation in PEMFCs working environment. Sometimes, metals may be dissolved into solution as metallic ions to contaminate PEMFCs. These two cases can greatly degrade the performance of PEMFCs. Austenitic stainless steel is considered one of many promising candidate materials for bipolar plates due to its high corrosion resistance, good electrical and thermal conductivity with lower manufacturing cost. But in PEMFC environment, its corrosion resistance is not enough to resist the aggressive attack of corrosive medium [3–4]. However, the corrosion behavior and contact resistance of bipolar plates are closely related to their surface properties. So, the bipolar plates may work well if a coating or layer with both high corrosion resistance and electrical conductivity is generated on their surfaces. Some of transition metal nitrides are highly chemical stability and electrical conductivity because the chemical bonds holding metal and nitrogen atoms together are composed of covalent and metallic bindings [5]. Thus, these compounds are
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commonly used to modify the surface of metallic bipolar plates. Considering that they are easily produced and good performance, chromium nitride coatings formed on the surface of bipolar plates or other alloys have been extensively investigated [6–13]. It is known that there are two different kinds of CreN compounds, Cr2N with hexagonal structure and CrN with face-centered cubic structure. Compared with CrN, the electrical resistance of Cr2N is much lower [14]. Therefore, Cr2N layers have been used to modify the surface characteristics of austenitic stainless steels and other alloys by different ways including physical vapour deposition (PVD), thermal nitridation of NieCr or high Cr-containing alloys and plasma or gas nitridiation of Cr-electroplated coating [15–19]. Recently, the research showed that the corrosion resistance and conductivity of 304 stainless steel was greatly improved by combining physical deposition of a very thin (5–200 nm) Cr coating followed by plasma nitriding at low temperature of 450 °C [20]. Although these compounds are highly chemical stable in nature, their resistance to corrosion mainly depends on the quality of coatings, since various defects often exist in coatings especially deposited by physical vapour deposition. Corrosion problems during application these coatings generally related to pinholes or other defects. Local corrosion may occur in those sites and cause an accelerated degradation of the metallic substrate [21]. Their very good resistance to corrosion is most often obtained by preventing or reducing defects in the coatings. Therefore, a dense CreN layer without or with very few defects on the surface of 304 SS will help it to find application as bipolar plate. In this paper, a thin CreN layer mainly composed of Cr2N was prepared by
Corresponding author. E-mail address: [email protected] (L. Wang).
https://doi.org/10.1016/j.surfcoat.2020.125450 Received 12 November 2019; Received in revised form 4 February 2020; Accepted 6 February 2020 Available online 07 February 2020 0257-8972/ © 2020 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 385 (2020) 125450
H. Shen, et al.
(EDS). The phase composition of nitride layer was revealed by X-ray diffraction (XRD) on a Rigaku D/Max-Ultima diffractometer in θ-2θ Bragg–Brentano geometry with Co Kα radiation (λ = 1.79 nm). The corrosion resistance of samples was evaluated by potentiodynamic and potentiostatic polarization curves measured on a CHI660C electrochemical workstation in PEMFCs working conditions (70 °C 0.05 M H2SO4 + 2 ppm F−) at anode (bubbled hydrogen) and cathode (bubbled air). The potentiostatic test was carried out at anode potential of −0.1 V (versus saturated calomel electrode (SCE)) and cathode potential of 0.6 V for 4 h. One 4 h nitrided and an untreated 304 stainless steel samples were tested at 0.6 V for 100 h. Interfacial contact resistance (ICR) of all samples before and after corrosion tests was measured using the method proposed by Wang et al. [22]. In this method, the sample was sandwiched by two pieces of conductive carbon papers (Toray, Inc.) and two copper plates. By recording the voltage, the electrical resistance of the sample can be calculated.
Table 1 Chemical composition of 304 stainless steel (wt%). C
Cr
Ni
S
Si
Mn
P
Fe
0.038
18.16
8.40
0.001
0.356
1.37
0.022
Balance
plasma nitriding of Cr-electroplated 304 stainless steel plate. The properties of corrosion and electrical conductivity were measured in PEMFCs environment.
2. Experimental The Cr coating with the thickness of 25 μm was deposited on a 5 mm thick AISI304 stainless steel plate in an electrolyte solution (chromic acid 250 g/L, sulfuric acid 2.5 g/L, 40 °C, 20 A/dm2) for 2 h and then cut into the samples with a size of 10 mm × 10 mm. Chemical composition (wt%) of 304 stainless steel substrate measured by a QSN750 (OBLF) optical emission spectroscopy is given in Table 1. Plasma nitriding was carried out in an ion nitriding furnace in an ammonia (NH3) atmosphere with a pressure of 300–400 Pa. A 600 V voltage and 5.5 A current were applied to maintain the temperature at 700 °C for nitriding 2–4 h. The surface morphology and cross-sections of the chromium nitride layer were analyed using a SUPRA 55 SAPPHIRE (ZEISS) scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry
3. Results and discussion Fig. 1 shows the surface morphologies of the Cr-electroplated 304 SS samples before and after plasma nitriding. It is clearly seen that some of cracks distributed on the surface of original Cr-electroplated coating (Fig. 1(a)). The reason for appearing these cracks was the high tension stress developed in the coating deposition on the substrate. These cracks may provide easy ways for aggressive ions in the solution to
Fig. 1. SEM surface morphology of samples (a) Cr-electroplated coating on 304 stainless steel, (b) nitrided for 2 h, (c) nitrided for 3 h and (d) nitrided for 4 h. 2
Surface & Coatings Technology 385 (2020) 125450
H. Shen, et al.
Fig. 2. Cross-sectional morphology of samples (a) Cr-electroplated coating on 304 stainless steel, (b) nitrided for 2 h, (c) nitrided for 3 h and (d) nitrided for 4 h.
increasing to 4 h, all the micro-cracks were fully closed. Many reports in the previous publications had shown that plasma nitriding could cause the cracks in the Cr coating disappear or close and ascribed this result to the volume expansion accompanied by transforming Cr-coating into Cr2N or CrN layer during nitriding [27,28]. In fact, according to lattice parameters of Cr (bcc, a = 0.2895 nm) and Cr2N (hcp, a = 0.4805 nm and c = 0.4479 nm), the same amount of Cr is transformed into Cr2N, the relative volume expansion will be 250% [15]. The cross-sections of Cr-electroplated and nitrided samples are shown in Fig. 2. The thickness of Cr-electroplated coating was 25 μm (Fig. 2(a)). Several microcracks almost perpendicular to the surface of substrate are observed in the coating. After plasma nitriding for 2 h, 3 h and 4 h, a thin and dense CreN layer was formed on the surface region of Cr-electroplated coatings, which is 1.2 μm, 1.6 μm and 2.3 μm thick respectively (Fig. 2(b), (c) and (d)). The thickness of nitride layer was raised slightly with the nitriding time. There were no microcracks or other else evident defects in the chromium nitride layer. Such chromium nitride layers are thick enough From above results, plasma nitriding of Cr-electroplated coatings at 700 °C for 4 h could produce a chromium nitride layer which was thick enough for improving the corrosion resistance of substrate. The previous research showed that a 1.8 μm thick CreN layer was formed after gas nitriding of 40–60 μm Crelectroplated coating in a gas mixture of NH3 and Ar at 750 °C for 2 h [29]. Another study reported that a CreN layer about 8.5 μm was formed on the Cr-electroplated coating by plasma nitriding at 720 °C for 20 h in a gas with ratio of 5 CH4 + 95 NH3 [30].
Fig. 3. XRD patterns of samples (a) Cr-electroplated coating on 304 stainless steel, (b) nitrided for 2 h, (c) nitrided for 3 h and (d) nitrided for 4 h.
reach the substrate and cause the Cr-coating lose its protective function [23–26]. In contrast, a large amount of micro-particles appeared on the nitrided sample surface, and some cracks was fully closed (Fig. 1(b)) after plasma nitriding at 700 °C for 2 h. With the nitriding times 3
Surface & Coatings Technology 385 (2020) 125450
H. Shen, et al.
Fig. 4. EDS spectra and chemical composition of sample nitrided for 4 h measured on (a) the surface and (b) the cross section of CreN layer.
conditions used in this work. The chemical composition of CreN layer measured by EDS on the surface and cross-section of CreN layer was shown in Fig. 4. The ratio of chromium to nitrogen on the cross-section is near to that of Cr2N compound. While, the ratio obtained on the surface is near to that of CrN. This difference indicates that a very thin CrN layer was formed on the outer surface and the CreN layer is composed of a mixture of Cr2N and CrN in accordance with the results of XRD patterns. It is free energy that determines whether or not one phase is stable relative to another phase. The free energy of formation is almost the same for CrN and Cr2N phase at the nitriding temperature used in this study [35]. In addition to, the structure of phases changed from Cr to Cr2N and CrN with increasing nitrogen content in a working atmosphere for the deposition of CreN coating by PVD [36–39]. The contact angle of deionized water on the surfaces of 304 SS and CreN layer is shown in Fig. 5. The contact angle increased from 59° on 304 substrate to 102° on the sample nitrided for 2 h. This value was further increased to 109° as nitriding time increased to 4 h. In general, the material is hydrophobic if the contact angle with water is larger than 90°, otherwise it is hydrophilic. In the PEMFCs working environment, bipolar plates with hydrophobic surface not only favor the water removed quickly, but also enhance the reaction of electrode [40]. The corrosion rate of materials is proportional to the corrosion current density recorded by polarization test [41–43]. The potentiodynamic polarization curves of the bare 304 SS substrate and nitrided layer in cathode side (air purge) and anode side (hydrogen purge) of PEMFCs environment are shown in Fig. 6. The data of the corrosion
Fig. 3 shows the XRD patterns of the Cr-electroplated 304 SS sample and nitrided for 2 h, 3 h and 4 h, respectively. Among these diffraction peaks from nitrided samples, except a very weak peak at 2θ = 77.5° from Cr coating, another three weak peaks at angles of 44.1°, 51.1° and 74.2° may be ascribed to CrN phase. All other peaks correspond with different planes of Cr2N phase indicating that the CreN layer was mainly composed of Cr2N phase. A very strong preferred orientation of (300) plane of Cr2N appeared at 80.6°. With the nitriding time increase, the intensity of (300) diffraction peak of Cr2N phase obviously became stronger. The preferred orientation of crystal plane of coating/layer is affected by many factors. Several researchers had reported the preferred orientation of (300) plane of Cr2N phase in both CreN coatings deposited by physical vapour deposition and layers produced by nitriding of Cr-electroplated coatings. A CreN coating with strongly (300) preferred orientation of Cr2N phase was deposited by vacuum ARC evaporation in a nitrogen partial pressure of 0.5 Pa [31]. An extraordinarily strong (300) texture of Cr2N phase was appeared when the arc-deposited CreN coating containing 34 at.% nitrogen [32]. Similar preferred orientation of CreN layer was reported after plasma nitriding of electroplated chromium at 700 °C for 1 h [33]. After gas nitridation of electroplated chromium on AISI 316 stainless steel at temperatures of 700–900 °C for 10 h, a strong (300) preferred orientation of Cr2N phase in CreN layer was observed [34]. The reasons for this (300) preferred orientation of Cr2N are not completely clear. It can be concluded from these results that Cr2N phase accounts the principal part of CreN layer formed by plasma nitriding under 4
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Fig. 5. Contact angle of the samples with deionized water for (a) untreated 304 stainless steel, (b) nitrided for 2 h, (c) nitrided for 3 h and (d) nitrided for 4 h.
Fig. 6. Potentiodynamic polarization curves of samples measured in 0.05 M H2SO4 + 2ppmF+ solution at 70 °C with (a) a cathode condition and (b) an anode condition.
continuing to shift towards the more positive direction to 186 mV with a further decrease in corrosion current density for 3 h nitrided sample. In anodic environment the polarization curves of all samples were similar to those in the cathodic environment confirming that the CreN layer was highly corrosion resistant in both cathodic and anodic conditions. On the other hand, a small active peak appeared at a potential just above corrosion potential and an evident pitting potential about 800 mV was observed from the curve of bare 304 SS substrate. For nitrided samples, there were no active-passive transition regions. They entered the passive region after the potential was immediately above their corrosion potentials and remained passivation in whole measuring scope up to 1600 mV. These results may be attributed to the CreN layer more stable and without defects. Furthermore, the corrosion performance of the layer with thickness of 1.6 and 2.3 μm was slightly better than that of 1.2 μm thick, which is confirmed by having a lower current density in the passive region as well as the higher corrosion potential. In order to meet the requirement of practical application, the metallic bipolar plates must have a stable corrosion resistance for a long term in PEMFCs working condition. Potentiostatic corrosion test can be
Table 2 Data of potentiodynamic and potentiostatic polarization tests in cathodic and anodic environments. Sample
Bare 304 SS 2 h nitrided 3 h nitrided 4 h nitrided
Cathodic environment
Anodic environment
Ecorr (mV)
Icorr (μA/ cm2)
I (0.6v) (μA/ cm2)
Ecorr (mV)
Icorr (μA/ cm2)
I (−0.1v) (μA/cm2)
−335 −69 186 97
43.4 0.15 0.11 0.06
36.7 0.52 0.24 0.13
−242 155 230 155
29.5 0.13 0.09 0.05
9.3 −0.92 −0.89 −1.85
tests is listed in Table 2. In cathodic environment, the corrosion potential (Ecorr) of the 2 h nitrided sample was −69 mV which was more positive than that of untreated 304 SS substrate (−335 mV vs. SCE), and corrosion current density (Icorr) was decreased by almost two orders of magnitude. With the nitriding time increasing, the Ecorr was 5
Surface & Coatings Technology 385 (2020) 125450
H. Shen, et al.
Fig. 7. Potentiostatic polarization curves of samples measured at 70 °C in 0.05 M H2 SO4 + 2ppmF− solution at (a) a cathode condition (0.6 V) for 4 h and (b) an anode condition (−0.1 V) for 4 h. (c) a cathode condition (0.6 V) for 100 h.
Fig. 8. SEM surface micrographs of samples after potentiostatic polarization test at 0.6 V for 100 h (a) untreated 304 SS substrate and (b) nitrided for 4 h.
SS substrate rapidly decreased from about 45 μA cm−2 at the initial stage within 20 min showing a change from active to passive state, then gradually reduced to 38 μA cm−2 after 2 h. Beyond this scope, it remained almost constant until the end of test. However, for nitrided samples, the current density was very small and almost unchanged with testing times. With the thickness increase, it decreased from 0.52 μA cm−2 to 0.13 μA cm−2. Such low current density indicated that
utilized to assess this performance. Fig. 7 shows the time-current density curves of potentiostatic corrosion test of samples for 4 h in PEMFCs working environment. The samples of nitrided for 4 h and untreated 304 SS substrate were test at 0.6 V for 100 h. The untreated 304 SS substrate presented a very different corrosion behavior from that of nitrided samples at 0.6 V (in cathodic environment). From Fig. 7(a), it was clearly shown that the corrosion current density of untreated 304 6
Surface & Coatings Technology 385 (2020) 125450
H. Shen, et al.
Fig. 9. ICR values of untreated and nitrided samples of (a) before corrosion test, (b) after corrosion test at a cathode condition for 4 h and (c) after corrosion test at an anode condition for 4 h.
corrosion test, the ICR was 68.5 mΩ cm2 for untreated 304 SS substrate under compaction force 140 N cm−2. While, much smaller values were measured from the nitrided samples, 5.5 mΩ cm2, 4.3 mΩ cm2 and 4.1 mΩ cm2 was respectively corresponding to samples nitrided for 2 h, 3 h and 4 h. After 4 h potentiostatic corrosion test in PEMFCs cathodic and anodic working condition, the ICR of untreated 304 SS substrate was dramatically increased to 84.5 mΩ cm2 (cathode), 80.2 mΩ cm2 (anode), because a passive film or the corrosion product was formed on the surface. In contrast, the ICR of sample nitrided for 4 h was slightly increased to 4.5 mΩ cm2 and 4.3 mΩ cm2 respectively.
the corrosion resistance of 304 stainless steel was markedly improved by CreN layer formed on the Cr-electroplated coating. At −0.1 V (in anodic environment, Fig. 7(b)), the current density of untreated 304 SS substrate was 9.27 μA cm−2 (at cathodic polarization state) while that of nitrided samples remained negative ranging from −0.92 to −1.85 μA cm−2 (at the anode protection state) because −0.1 V was lower than their corrosion potentials. Fig. 7(c) shows the variation of current density tested at 0.6 V for 100 h. The current density of untreated 304 SS substrate was very unstable after testing for 40 h. During the test, a lot of corrosion products were accumulated on the surface of 304 SS and continuously dropped into the solution. The surface morphologies after corrosion test showed that a number of pits with different size and shape were left on the surface of 304 SS substrate indicating that severely corroded process was occurred during testing (Fig. 8(a)). On the contrary, the current density of sample nitrided for 4 h remained at a very value and almost unchanged from the begin to the end of the test. After 100 h corrosion test, no evident corrosion phenomena were observed on the surface sample nitrided for 4 h. The surface morphology remained almost unchanged (Fig. 8(b)). The results of potentiostatic corrosion tests in either cathodic or anodic condition revealed that the CreN layer produced in this study had a high and stable corrosion resistance in PEMFC working environment. The ICR curves of untreated 304 SS substrate and nitrided samples before and after potentiostatic polarization test in cathodic and anodic environments are shown in Fig. 9. The ICR was decreased with the compaction force for all the samples because increasing the compaction force will create a large contact surface area. Before potentiostatic
4. Conclusion Chromium nitride layer with about 2 μm thick was prepared on the surface region of Cr-electroplated coating on 304 stainless steel substrate by plasma nitriding at 700 °C for 4 h. This layer was mainly composed of Cr2N phase. Compared with the bare 304 substrate, the corrosion potential was raised from −335 mV to 97 mV as well as the corrosion current density was reduced from 43.38 to 0.06 μA cm−2. These results confirmed that plasma nitriding of Cr-electroplated coating could greatly improve the corrosion resistance of 304 stainless steel in PEMFC working environment. The ICR measurements showed that the CreN layer had a very low interface contact resistance of 4.0 mΩ cm2 (under 140 N cm−2) much lower than that of 304 stainless steel (68.5 mΩ cm2) and was slightly increased to 4.5 mΩ cm2 after corrosion test for 4 h nitrided sample. Both corrosion resistance and electrical conductivity of CreN layer provide a possibility for application of bipolar plates in PEMFCs. 7
Surface & Coatings Technology 385 (2020) 125450
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CRediT authorship contribution statement
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Hongyu Shen: Writing - original draft, Investigation.Liang Wang: Supervision.Juncai Sun:Supervision. Declaration of competing interest This research did not receive any specific grant from funding agencies in the public, commercial, or not -for- profit sectors. References [1] J. Scholta, B. Rohland, V. Trapp, U. Focken, Investigation on novel low-cost graphite composite bipolar plate, J. Power Sources 84 (1999) 231–237. [2] R.F. Silva, D. Franchi, A. Leone, L. Pilloni, A. Masci, A. Pozio, Surface conductivity and stability of metallic bipolar plate materials for polymer electrolyte fuel cells, Electrochim. Acta 51 (2006) 3592–3598. [3] J. Wind, R. Späh, W. Kaiser, G. Böhm, Metallic bipolar plates for PEM fuel cells, J. Power Sources 105 (2002) 256–260. [4] H.S. Choi, D.H. Han, W.H. Hong, J.J. Lee, (Titanium, chromium) nitride coatings for bipolar plate of polymer electrolyte membrane fuel cell, J. Power Sources 189 (2009) 966–971. [5] M. Stüber, S. Ulrich, H. Leiste, H. Holleck, Magnetron sputtered nanocrystalline metastable (V,Al) (C,N) hard coatings, Surf. Coat. Technol. 206 (2011) 610–616. [6] S.H. Lee, S.P. Woo, N. Kakati, Y.N. Lee, Y.S. Yoon, Corrosion and electrical properties of carbon/ceramic multilayer coated on stainless steel bipolar plates, Surf. Coat. Technol. 303 (2016) 162–169. [7] S.H. Huang, C.Y. Tong, T.E. Hsieh, J.W. Lee, Microstructure and mechanical properties evaluation of cathodic arc deposited CrCN/ZrCN multilayer coatings, J. Alloy. Comp. 803 (2019) 1005–1015. [8] Y. Fu, G.Q. Lin, M. Hou, B. Wu, H.K. Li, L.X. Hao, Z.G. Shao, B.L. Yi, Optimized Crnitride film on 316L stainless steel as proton exchange membrane fuel cell bipolar plate, Intel. J. Hydrogen Energy 34 (2009) 453–458. [9] K. Suzuki, T. Kaneko, H. Yoshida, Y. Obi, H. Fujimori, Synthesis of the compound CrN by DC reactive sputtering, J. Alloy. Comp. 280 (1998) 294–298. [10] D.M. Zhang, L. Guo, L.T. Duan, W.H. Tuan, Preparation of Cr-based multilayer coating on stainless steel as bipolar plate for PEMFCs by magnetron sputtering, Intel. J. Hydrogen Energy 36 (2011) 2184–2189. [11] S.H. Lee, N. Kakati, J. Maiti, S.H. Jee, D.J. Kalita, Y.S. Yoon, Corrosion and electrical properties of CrN- and TiN-coated 316L stainless steel used as bipolar plates for polymer electrolyte membrane fuel cells, Thin Solid Films 529 (2013) 374–379. [12] O.V. Maksakova, S. Simoẽs, A.D. Pogrebnjak, O.V. Bondar, Y.O. Kravchenko, T.N. Koltunowicz, Z.K. Shaimardanov, Multilayered ZrN/CrN coatings with enhanced thermal and mechanical properties, J. Alloy. Comp. 776 (2019) 679–690. [13] P. Mandal, U.K. Chanda, S. Roy, A review of corrosion resistance method on stainless steel bipolar plate, Materials Today: Proceedings 5 (2018) 17852–17856. [14] D.G. Nam, H.C. Lee, Thermal nitridation of chromium electroplated AISI316L stainless steel for polymer electrolyte membrane fuel cell bipolar plate, J. Power Sources 170 (2007) 268–274. [15] J.G. Buijnsters, P. Shankar, J. Sietsma, J.J. ter Meulen, Gas nitriding of chromium in NH3-H2atmosphere, Mater. Sci. Eng. A 341 (2003) 289–295. [16] M.P. Brady, K. Weisbrod, I. Paulauskas, R.A. Buchanan, K.L. More, H. Wang, M. Wilson, F. Garzon, L.R. Walker, Preferential thermal nitridation to form pin-hole free Cr-nitrides to protect proton exchange membrane fuel cell metallic bipolar plates, Scr. Mater. 50 (2004) 1017–1022. [17] M.P. Brady, H. Wang, B. Yang, J.A. Turner, M. Bordignon, R. Molins, M. Abd Elhamid, L. Lipp, L.R. Walker, Growth of Cr-nitrides on commercial Ni-Cr and Fe-Cr base alloys to protect PEMFC bipolar plates, Int. J. Hydrog. Energy 32 (2007) 3778–3788. [18] I.E. Paulausks, M.P. Brady, H.M. Meyer III, R.A. Buchanan, L.R. Walker, Corrosion behavior of CrN, Cr2N and π phase surfaces on nitrided Ni-50Cr for proton exchange membrane fuel cell bipolar plates, Corros. Sci. 48 (2006) 3157–3171. [19] D.H. Han, W.H. Huang, H.S. Choi, J.J. Lee, Inductively coupled plasma nitriding of chromium electroplated AISI 316L stainless steel for PEMFC bipolar plate, Int. J. Hydrog. Energy 34 (2009) 2387–2395.
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Characteristics and properties of CreN compound layer produced by plasma nitriding of Cr-electroplated of AISI 304 stainless steel
T
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Hongyu Shen, Liang Wang , Juncai Sun Department of Materials Science and Engineering, Dalian Maritime University, Dalian 116026, China
A R T I C LE I N FO
A B S T R A C T
Keywords: CreN compound Cr coating Corrosion Plasma nitriding Electrical resistance
In this work, plasma nitriding of the Cr-electroplated coating on a 304 stainless steel (304 SS) was carried out at 700 °C in pure ammonia for different times (2 h, 3 h and 4 h). After plasma nitriding, a thin (1–2 μm) and compact chromium nitride layer mainly composed of Cr2N phase was formed on the surface region of Crelectroplated coating. The corrosion resistance of CreN layers was examined in the proton exchange membrane fuel cell working environment to evaluate the possible application of the duplex-treated 304 stainless steel for bipolar plates. Before and after corrosion test the interfacial contact resistance was measured to determine the effect of nitriding on the electrical conductivity of all samples. The results show that the CreN compound layer has a very low passive current density 0.13 μA cm−2 and interfacial contact resistance 4.5 mΩ cm2.
1. Introduction Bipolar plates play crucial roles in determining the performance of proton exchange membrane fuel cell system (PEMFCs). Their cost and weight account a great proportion of the whole PEMFCs [1–2]. Compared with graphite material, metallic bipolar plates (BPs) have many advantages such as low cost and high electrical conductivity. However, the conductivity of metals is always reduced if they are corroded with the corrosion products or a passive film adhering to the surface. Hence, the contact electrical resistance of metallic bipolar plates will be increased as a result of corrosion or passive film formation in PEMFCs working environment. Sometimes, metals may be dissolved into solution as metallic ions to contaminate PEMFCs. These two cases can greatly degrade the performance of PEMFCs. Austenitic stainless steel is considered one of many promising candidate materials for bipolar plates due to its high corrosion resistance, good electrical and thermal conductivity with lower manufacturing cost. But in PEMFC environment, its corrosion resistance is not enough to resist the aggressive attack of corrosive medium [3–4]. However, the corrosion behavior and contact resistance of bipolar plates are closely related to their surface properties. So, the bipolar plates may work well if a coating or layer with both high corrosion resistance and electrical conductivity is generated on their surfaces. Some of transition metal nitrides are highly chemical stability and electrical conductivity because the chemical bonds holding metal and nitrogen atoms together are composed of covalent and metallic bindings [5]. Thus, these compounds are
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commonly used to modify the surface of metallic bipolar plates. Considering that they are easily produced and good performance, chromium nitride coatings formed on the surface of bipolar plates or other alloys have been extensively investigated [6–13]. It is known that there are two different kinds of CreN compounds, Cr2N with hexagonal structure and CrN with face-centered cubic structure. Compared with CrN, the electrical resistance of Cr2N is much lower [14]. Therefore, Cr2N layers have been used to modify the surface characteristics of austenitic stainless steels and other alloys by different ways including physical vapour deposition (PVD), thermal nitridation of NieCr or high Cr-containing alloys and plasma or gas nitridiation of Cr-electroplated coating [15–19]. Recently, the research showed that the corrosion resistance and conductivity of 304 stainless steel was greatly improved by combining physical deposition of a very thin (5–200 nm) Cr coating followed by plasma nitriding at low temperature of 450 °C [20]. Although these compounds are highly chemical stable in nature, their resistance to corrosion mainly depends on the quality of coatings, since various defects often exist in coatings especially deposited by physical vapour deposition. Corrosion problems during application these coatings generally related to pinholes or other defects. Local corrosion may occur in those sites and cause an accelerated degradation of the metallic substrate [21]. Their very good resistance to corrosion is most often obtained by preventing or reducing defects in the coatings. Therefore, a dense CreN layer without or with very few defects on the surface of 304 SS will help it to find application as bipolar plate. In this paper, a thin CreN layer mainly composed of Cr2N was prepared by
Corresponding author. E-mail address: [email protected] (L. Wang).
https://doi.org/10.1016/j.surfcoat.2020.125450 Received 12 November 2019; Received in revised form 4 February 2020; Accepted 6 February 2020 Available online 07 February 2020 0257-8972/ © 2020 Elsevier B.V. All rights reserved.
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(EDS). The phase composition of nitride layer was revealed by X-ray diffraction (XRD) on a Rigaku D/Max-Ultima diffractometer in θ-2θ Bragg–Brentano geometry with Co Kα radiation (λ = 1.79 nm). The corrosion resistance of samples was evaluated by potentiodynamic and potentiostatic polarization curves measured on a CHI660C electrochemical workstation in PEMFCs working conditions (70 °C 0.05 M H2SO4 + 2 ppm F−) at anode (bubbled hydrogen) and cathode (bubbled air). The potentiostatic test was carried out at anode potential of −0.1 V (versus saturated calomel electrode (SCE)) and cathode potential of 0.6 V for 4 h. One 4 h nitrided and an untreated 304 stainless steel samples were tested at 0.6 V for 100 h. Interfacial contact resistance (ICR) of all samples before and after corrosion tests was measured using the method proposed by Wang et al. [22]. In this method, the sample was sandwiched by two pieces of conductive carbon papers (Toray, Inc.) and two copper plates. By recording the voltage, the electrical resistance of the sample can be calculated.
Table 1 Chemical composition of 304 stainless steel (wt%). C
Cr
Ni
S
Si
Mn
P
Fe
0.038
18.16
8.40
0.001
0.356
1.37
0.022
Balance
plasma nitriding of Cr-electroplated 304 stainless steel plate. The properties of corrosion and electrical conductivity were measured in PEMFCs environment.
2. Experimental The Cr coating with the thickness of 25 μm was deposited on a 5 mm thick AISI304 stainless steel plate in an electrolyte solution (chromic acid 250 g/L, sulfuric acid 2.5 g/L, 40 °C, 20 A/dm2) for 2 h and then cut into the samples with a size of 10 mm × 10 mm. Chemical composition (wt%) of 304 stainless steel substrate measured by a QSN750 (OBLF) optical emission spectroscopy is given in Table 1. Plasma nitriding was carried out in an ion nitriding furnace in an ammonia (NH3) atmosphere with a pressure of 300–400 Pa. A 600 V voltage and 5.5 A current were applied to maintain the temperature at 700 °C for nitriding 2–4 h. The surface morphology and cross-sections of the chromium nitride layer were analyed using a SUPRA 55 SAPPHIRE (ZEISS) scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry
3. Results and discussion Fig. 1 shows the surface morphologies of the Cr-electroplated 304 SS samples before and after plasma nitriding. It is clearly seen that some of cracks distributed on the surface of original Cr-electroplated coating (Fig. 1(a)). The reason for appearing these cracks was the high tension stress developed in the coating deposition on the substrate. These cracks may provide easy ways for aggressive ions in the solution to
Fig. 1. SEM surface morphology of samples (a) Cr-electroplated coating on 304 stainless steel, (b) nitrided for 2 h, (c) nitrided for 3 h and (d) nitrided for 4 h. 2
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Fig. 2. Cross-sectional morphology of samples (a) Cr-electroplated coating on 304 stainless steel, (b) nitrided for 2 h, (c) nitrided for 3 h and (d) nitrided for 4 h.
increasing to 4 h, all the micro-cracks were fully closed. Many reports in the previous publications had shown that plasma nitriding could cause the cracks in the Cr coating disappear or close and ascribed this result to the volume expansion accompanied by transforming Cr-coating into Cr2N or CrN layer during nitriding [27,28]. In fact, according to lattice parameters of Cr (bcc, a = 0.2895 nm) and Cr2N (hcp, a = 0.4805 nm and c = 0.4479 nm), the same amount of Cr is transformed into Cr2N, the relative volume expansion will be 250% [15]. The cross-sections of Cr-electroplated and nitrided samples are shown in Fig. 2. The thickness of Cr-electroplated coating was 25 μm (Fig. 2(a)). Several microcracks almost perpendicular to the surface of substrate are observed in the coating. After plasma nitriding for 2 h, 3 h and 4 h, a thin and dense CreN layer was formed on the surface region of Cr-electroplated coatings, which is 1.2 μm, 1.6 μm and 2.3 μm thick respectively (Fig. 2(b), (c) and (d)). The thickness of nitride layer was raised slightly with the nitriding time. There were no microcracks or other else evident defects in the chromium nitride layer. Such chromium nitride layers are thick enough From above results, plasma nitriding of Cr-electroplated coatings at 700 °C for 4 h could produce a chromium nitride layer which was thick enough for improving the corrosion resistance of substrate. The previous research showed that a 1.8 μm thick CreN layer was formed after gas nitriding of 40–60 μm Crelectroplated coating in a gas mixture of NH3 and Ar at 750 °C for 2 h [29]. Another study reported that a CreN layer about 8.5 μm was formed on the Cr-electroplated coating by plasma nitriding at 720 °C for 20 h in a gas with ratio of 5 CH4 + 95 NH3 [30].
Fig. 3. XRD patterns of samples (a) Cr-electroplated coating on 304 stainless steel, (b) nitrided for 2 h, (c) nitrided for 3 h and (d) nitrided for 4 h.
reach the substrate and cause the Cr-coating lose its protective function [23–26]. In contrast, a large amount of micro-particles appeared on the nitrided sample surface, and some cracks was fully closed (Fig. 1(b)) after plasma nitriding at 700 °C for 2 h. With the nitriding times 3
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Fig. 4. EDS spectra and chemical composition of sample nitrided for 4 h measured on (a) the surface and (b) the cross section of CreN layer.
conditions used in this work. The chemical composition of CreN layer measured by EDS on the surface and cross-section of CreN layer was shown in Fig. 4. The ratio of chromium to nitrogen on the cross-section is near to that of Cr2N compound. While, the ratio obtained on the surface is near to that of CrN. This difference indicates that a very thin CrN layer was formed on the outer surface and the CreN layer is composed of a mixture of Cr2N and CrN in accordance with the results of XRD patterns. It is free energy that determines whether or not one phase is stable relative to another phase. The free energy of formation is almost the same for CrN and Cr2N phase at the nitriding temperature used in this study [35]. In addition to, the structure of phases changed from Cr to Cr2N and CrN with increasing nitrogen content in a working atmosphere for the deposition of CreN coating by PVD [36–39]. The contact angle of deionized water on the surfaces of 304 SS and CreN layer is shown in Fig. 5. The contact angle increased from 59° on 304 substrate to 102° on the sample nitrided for 2 h. This value was further increased to 109° as nitriding time increased to 4 h. In general, the material is hydrophobic if the contact angle with water is larger than 90°, otherwise it is hydrophilic. In the PEMFCs working environment, bipolar plates with hydrophobic surface not only favor the water removed quickly, but also enhance the reaction of electrode [40]. The corrosion rate of materials is proportional to the corrosion current density recorded by polarization test [41–43]. The potentiodynamic polarization curves of the bare 304 SS substrate and nitrided layer in cathode side (air purge) and anode side (hydrogen purge) of PEMFCs environment are shown in Fig. 6. The data of the corrosion
Fig. 3 shows the XRD patterns of the Cr-electroplated 304 SS sample and nitrided for 2 h, 3 h and 4 h, respectively. Among these diffraction peaks from nitrided samples, except a very weak peak at 2θ = 77.5° from Cr coating, another three weak peaks at angles of 44.1°, 51.1° and 74.2° may be ascribed to CrN phase. All other peaks correspond with different planes of Cr2N phase indicating that the CreN layer was mainly composed of Cr2N phase. A very strong preferred orientation of (300) plane of Cr2N appeared at 80.6°. With the nitriding time increase, the intensity of (300) diffraction peak of Cr2N phase obviously became stronger. The preferred orientation of crystal plane of coating/layer is affected by many factors. Several researchers had reported the preferred orientation of (300) plane of Cr2N phase in both CreN coatings deposited by physical vapour deposition and layers produced by nitriding of Cr-electroplated coatings. A CreN coating with strongly (300) preferred orientation of Cr2N phase was deposited by vacuum ARC evaporation in a nitrogen partial pressure of 0.5 Pa [31]. An extraordinarily strong (300) texture of Cr2N phase was appeared when the arc-deposited CreN coating containing 34 at.% nitrogen [32]. Similar preferred orientation of CreN layer was reported after plasma nitriding of electroplated chromium at 700 °C for 1 h [33]. After gas nitridation of electroplated chromium on AISI 316 stainless steel at temperatures of 700–900 °C for 10 h, a strong (300) preferred orientation of Cr2N phase in CreN layer was observed [34]. The reasons for this (300) preferred orientation of Cr2N are not completely clear. It can be concluded from these results that Cr2N phase accounts the principal part of CreN layer formed by plasma nitriding under 4
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Fig. 5. Contact angle of the samples with deionized water for (a) untreated 304 stainless steel, (b) nitrided for 2 h, (c) nitrided for 3 h and (d) nitrided for 4 h.
Fig. 6. Potentiodynamic polarization curves of samples measured in 0.05 M H2SO4 + 2ppmF+ solution at 70 °C with (a) a cathode condition and (b) an anode condition.
continuing to shift towards the more positive direction to 186 mV with a further decrease in corrosion current density for 3 h nitrided sample. In anodic environment the polarization curves of all samples were similar to those in the cathodic environment confirming that the CreN layer was highly corrosion resistant in both cathodic and anodic conditions. On the other hand, a small active peak appeared at a potential just above corrosion potential and an evident pitting potential about 800 mV was observed from the curve of bare 304 SS substrate. For nitrided samples, there were no active-passive transition regions. They entered the passive region after the potential was immediately above their corrosion potentials and remained passivation in whole measuring scope up to 1600 mV. These results may be attributed to the CreN layer more stable and without defects. Furthermore, the corrosion performance of the layer with thickness of 1.6 and 2.3 μm was slightly better than that of 1.2 μm thick, which is confirmed by having a lower current density in the passive region as well as the higher corrosion potential. In order to meet the requirement of practical application, the metallic bipolar plates must have a stable corrosion resistance for a long term in PEMFCs working condition. Potentiostatic corrosion test can be
Table 2 Data of potentiodynamic and potentiostatic polarization tests in cathodic and anodic environments. Sample
Bare 304 SS 2 h nitrided 3 h nitrided 4 h nitrided
Cathodic environment
Anodic environment
Ecorr (mV)
Icorr (μA/ cm2)
I (0.6v) (μA/ cm2)
Ecorr (mV)
Icorr (μA/ cm2)
I (−0.1v) (μA/cm2)
−335 −69 186 97
43.4 0.15 0.11 0.06
36.7 0.52 0.24 0.13
−242 155 230 155
29.5 0.13 0.09 0.05
9.3 −0.92 −0.89 −1.85
tests is listed in Table 2. In cathodic environment, the corrosion potential (Ecorr) of the 2 h nitrided sample was −69 mV which was more positive than that of untreated 304 SS substrate (−335 mV vs. SCE), and corrosion current density (Icorr) was decreased by almost two orders of magnitude. With the nitriding time increasing, the Ecorr was 5
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H. Shen, et al.
Fig. 7. Potentiostatic polarization curves of samples measured at 70 °C in 0.05 M H2 SO4 + 2ppmF− solution at (a) a cathode condition (0.6 V) for 4 h and (b) an anode condition (−0.1 V) for 4 h. (c) a cathode condition (0.6 V) for 100 h.
Fig. 8. SEM surface micrographs of samples after potentiostatic polarization test at 0.6 V for 100 h (a) untreated 304 SS substrate and (b) nitrided for 4 h.
SS substrate rapidly decreased from about 45 μA cm−2 at the initial stage within 20 min showing a change from active to passive state, then gradually reduced to 38 μA cm−2 after 2 h. Beyond this scope, it remained almost constant until the end of test. However, for nitrided samples, the current density was very small and almost unchanged with testing times. With the thickness increase, it decreased from 0.52 μA cm−2 to 0.13 μA cm−2. Such low current density indicated that
utilized to assess this performance. Fig. 7 shows the time-current density curves of potentiostatic corrosion test of samples for 4 h in PEMFCs working environment. The samples of nitrided for 4 h and untreated 304 SS substrate were test at 0.6 V for 100 h. The untreated 304 SS substrate presented a very different corrosion behavior from that of nitrided samples at 0.6 V (in cathodic environment). From Fig. 7(a), it was clearly shown that the corrosion current density of untreated 304 6
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H. Shen, et al.
Fig. 9. ICR values of untreated and nitrided samples of (a) before corrosion test, (b) after corrosion test at a cathode condition for 4 h and (c) after corrosion test at an anode condition for 4 h.
corrosion test, the ICR was 68.5 mΩ cm2 for untreated 304 SS substrate under compaction force 140 N cm−2. While, much smaller values were measured from the nitrided samples, 5.5 mΩ cm2, 4.3 mΩ cm2 and 4.1 mΩ cm2 was respectively corresponding to samples nitrided for 2 h, 3 h and 4 h. After 4 h potentiostatic corrosion test in PEMFCs cathodic and anodic working condition, the ICR of untreated 304 SS substrate was dramatically increased to 84.5 mΩ cm2 (cathode), 80.2 mΩ cm2 (anode), because a passive film or the corrosion product was formed on the surface. In contrast, the ICR of sample nitrided for 4 h was slightly increased to 4.5 mΩ cm2 and 4.3 mΩ cm2 respectively.
the corrosion resistance of 304 stainless steel was markedly improved by CreN layer formed on the Cr-electroplated coating. At −0.1 V (in anodic environment, Fig. 7(b)), the current density of untreated 304 SS substrate was 9.27 μA cm−2 (at cathodic polarization state) while that of nitrided samples remained negative ranging from −0.92 to −1.85 μA cm−2 (at the anode protection state) because −0.1 V was lower than their corrosion potentials. Fig. 7(c) shows the variation of current density tested at 0.6 V for 100 h. The current density of untreated 304 SS substrate was very unstable after testing for 40 h. During the test, a lot of corrosion products were accumulated on the surface of 304 SS and continuously dropped into the solution. The surface morphologies after corrosion test showed that a number of pits with different size and shape were left on the surface of 304 SS substrate indicating that severely corroded process was occurred during testing (Fig. 8(a)). On the contrary, the current density of sample nitrided for 4 h remained at a very value and almost unchanged from the begin to the end of the test. After 100 h corrosion test, no evident corrosion phenomena were observed on the surface sample nitrided for 4 h. The surface morphology remained almost unchanged (Fig. 8(b)). The results of potentiostatic corrosion tests in either cathodic or anodic condition revealed that the CreN layer produced in this study had a high and stable corrosion resistance in PEMFC working environment. The ICR curves of untreated 304 SS substrate and nitrided samples before and after potentiostatic polarization test in cathodic and anodic environments are shown in Fig. 9. The ICR was decreased with the compaction force for all the samples because increasing the compaction force will create a large contact surface area. Before potentiostatic
4. Conclusion Chromium nitride layer with about 2 μm thick was prepared on the surface region of Cr-electroplated coating on 304 stainless steel substrate by plasma nitriding at 700 °C for 4 h. This layer was mainly composed of Cr2N phase. Compared with the bare 304 substrate, the corrosion potential was raised from −335 mV to 97 mV as well as the corrosion current density was reduced from 43.38 to 0.06 μA cm−2. These results confirmed that plasma nitriding of Cr-electroplated coating could greatly improve the corrosion resistance of 304 stainless steel in PEMFC working environment. The ICR measurements showed that the CreN layer had a very low interface contact resistance of 4.0 mΩ cm2 (under 140 N cm−2) much lower than that of 304 stainless steel (68.5 mΩ cm2) and was slightly increased to 4.5 mΩ cm2 after corrosion test for 4 h nitrided sample. Both corrosion resistance and electrical conductivity of CreN layer provide a possibility for application of bipolar plates in PEMFCs. 7
Surface & Coatings Technology 385 (2020) 125450
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CRediT authorship contribution statement
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