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CrNx dual layer coatings for ferritic stainless steel interconnects
Results in Physics 12 (2019) 1598–1605
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Preparation and performances of Ni-Fe/CrNx dual layer coatings for ferritic stainless steel interconnects
T
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P.F. You, X. Zhang , X.G. Yang, H.L. Zhang, L.X. Yang, C.L. Zeng Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Solid oxide fuel cell Interconnect NiFe2O4 CrNx Oxidation resistance Area specific resistance
Dual layer coatings CrNx and Ni-Fe alloy have been developed for metallic interconnects application by a combined approach. The CrNx layer was deposited on ferritic stainless steel using multi-arc ion plating, and the Ni-Fe alloy coating was electroplated on the CrNx coated steel subsequently. After oxidation at 800 °C in air for 5 h, a multi-layer scale composed of NiFe2O4, Cr2O3, Ni-Fe-Cr alloy and CrNx from outside to inside was formed on the CrNx/substrate with a 1.6 μm thick Ni-32 at%Fe alloy coating. Further oxidation led to the decomposition of the CrNx layer, but no pores could be observed near the scale/substrate interface even after an oxidation of 1000 h. The CrNx layer acted as an effective barrier against the mutual diffusion of elements between the coating and the substrate either before or after decomposition. The employment of the Ni-Fe/CrNx dual layer coating decreased the growth rate of the Cr2O3 scale and thus a lower scale area specific resistance (ASR). The oxidation behavior of the Ni-Fe/CrNx coated substrate and the influence mechanism of the CrNx barrier layer were discussed.
Introduction Solid oxide fuel cells (SOFCs) are power generation systems that can convert chemical energy directly into electricity with high efficiency and reduced emissions [1]. Recent progress in SOFC technology has made it possible to reduce the cell operation temperatures down to the range of 600–800 °C [2–4]. This allows the replacement of the traditional LaCrO3-based ceramics by metallic interconnects. Among the high temperature alloys, ferritic stainless steels are the most promising candidates as interconnect materials, due to the advantages of low cost, high electrical and thermal conductivity, matched coefficient of thermal expansion (CTE) with other SOFC components [5]. However, the main obstacles for the application of ferritic stainless steel interconnects are the deterioration of the cathode performance caused by the volatile Cr species (CrO3 or CrO2(OH)2) and a relatively fast increase in the electrical resistance resulted from the formation of oxide scales [6,7]. Applying conductive spinel coatings onto ferritic stainless steels is an effective method to solve these problems. Several fabrication techniques, such as screen printing, slurry spraying, plasma spraying and magnetron sputtering, have been developed for preparing spinel coatings and have been thoroughly reviewed by Shaigan et al. [8]. Among these techniques, electroplating an alloy layer followed by oxidation treatment has attracted much attention due to the advantages such as low cost and suitability for mass production on substrates with ⁎
complicated geometry [9]. Challenge with electrodeposition of alloys is that the deposition potential of each component should be similar. Unfortunately, as one of the most promising candidates for interconnect application, the precursors of (Co,Mn)3O4 coatings, i.e., Co-Mn alloys, possess components with considerably large potential difference (E0(Mn2+/ 2+ Mn) = −1.18 VSHE, E0(Co /Co) = −0.28 VSHE). By comparison, Ni and Fe exhibit quite similar deposition potentials and thus excellent process stability for electroplating Ni-Fe alloys. It is reported that the CTE of NiFe2O4 spinel (10.8 × 10−6 K−1) is extremely close to that of the ferritic stainless steels (11 × 10−6 K−1) [10]. Although compared with (Co,Mn)3O4 (e.g., 60 S cm−1 for MnCo2O4 [10]), NiFe2O4 presents a much lower conductivity of 0.26 S cm−1[10], it is still several orders of magnitude higher than that of Cr2O3 (0.02 S cm−1 [11]) and accordingly the inner Cr2O3 scale should be the determinant for the conductivity of the coated steel [12]. Some researchers have investigated the effect of (Ni,Fe)3O4 coatings on the performance of ferritic stainless steel interconnects and the results showed a decreased area specific resistance (ASR) and a retarded outward migration of Cr for the (Ni,Fe)3O4 coated steels [5,13–15]. Consequently, (Ni,Fe)3O4 should be a promising coating material for the metallic interconnect application. Unfortunately, there is a drawback for the application of (Ni,Fe)3O4 coatings on ferritic stainless steels. During oxidation, many pores would
Corresponding author at: Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China. E-mail address: [email protected] (X. Zhang).
https://doi.org/10.1016/j.rinp.2019.01.061 Received 12 December 2018; Received in revised form 7 January 2019; Accepted 16 January 2019 Available online 30 January 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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appear beneath the oxide scale of the coated steel and their sizes would increase with extending exposure time [5,13–15]. Because the diffusion of Ni from the coating facilitates the formation of an austenite zone beneath the scale, and the diffusion coefficients of Ni, Fe and Cr in austenitic iron matrix are two orders of magnitude smaller than those in ferrite at 800 °C, atoms of Ni and Fe are easier to migrate from austenite to ferrite, leaving point defects and thus pores in austenite [5,16]. The existence of pores beneath the scale plays a negative role in the coating/ substrate adhesion and the conductivity of interconnect. Our previous investigation has successfully reduced the pores beneath the scales by adding Ce element to the Ni-Fe coatings [5]. The reduction in the pores is due to the reactive element effect (REE), which means that the rare earth element can provide vacancy sinks at the CeO2/alloy interface and can change the transport mechanism from a predominantly outward Cr diffusion to a principally inward oxygen transport. In this work, our aim is to inhibit the formation of the pores by suppressing the migration of Ni from the coating and thus a reduced austenite zone beneath the scale. Hovsepian et al. [17] prepared a ceramic based CrN/ NbN coating on P92 steel using high power impulse magnetron sputtering deposition method. The coating exhibited superior oxidation resistance and inhibition effect against the migration of H and O after oxidation at 600 °C in a high pressure (50 bar) 100% steam atmosphere for 1000 h. Panjan et al. [18] obtained a CrN/(Cr,V)N coating by DC unbalanced magnetron sputtering. After oxidation at 600–750 °C for 220 min, the surface was uniformly covered with a thin chromium oxide layer which could hinder the out-diffusion of vanadium. Therefore, it can be concluded that both CrN and its oxidation product, i.e., Cr2O3, can act as an effective barrier against the diffusion of elements. In this study, a CrNx diffusion barrier has been prepared between the ferritic stainless steel and the Ni-Fe alloy layer, and the oxidation behavior and electrical resistance of the Ni-Fe/CrNx coated steels have been investigated, with an attempt to reveal the effect of CrNx on the performances of the Ni-Fe coating.
Table 1 Electroplating parameters for alloy coatings with different Fe contents. Alloy composition Ni-55 at%Fe Ni-32 at%Fe Ni-25 at%Fe
Current density −2
10 mA cm 20 mA cm−2 10 mA cm−2
Temperature 40 °C 60 °C 60 °C
Oxidation experiments The coated steels were pre-oxidized at 800 °C in air in a high temperature muffle furnace to analyze the initial stage of oxidation. Further oxidation measurements of the coated steels were carried out at 800 °C in air for up to 1000 h. The specimens were placed in alumina crucibles. After oxidation for various periods, the alumina crucibles containing samples were taken out from the furnace, cooled to room temperature and weighed. The specimens were then replaced in the furnace for further oxidation. Three parallel samples were used for oxidation kinetics analysis. The phase structures of the oxide scales were characterized by X-ray diffraction (XRD, Phillips, PW-1700) with a Cu Ka radiation source. The microstructures and chemical compositions of the oxide scales were analyzed by scanning electron microscopy (SEM, FEI Inspect FSEM) coupled with an energy dispersive X-ray spectroscopy (EDS). The operating voltage was 25 kV, accelerated by a field emission gun. Quantitative analysis of EDS results was carried out without standard samples. Area specific resistance measurements The area specific resistances (ASR) is commonly used to evaluate the electrical performance of SOFC interconnects, as can be given by:
ASR =
R·S 2
(1)
where R is the electrical resistance [mΩ], S is the contact area [cm2]. Since the electrical resistance of the alloy substrate is negligible compared with that of the oxide scale, the oxide scale plays a main role in the ASR for interconnects. The area specific resistances of the coated samples were measured using a setup reported in the Ref. [19]. Before measurement, platinum paste was brushed onto the two sides of the sample surfaces with a soft brush, followed by heat treatment at 800 °C for 10 min. The platinum foils, each spot-welded with a platinum wire, were used as current collectors and placed on the top of the platinum paste. The resulting resistances of Pt wires and foils were deducted from the original results. The change of the scale ASR with oxidation time was investigated.
Experimental Preparation of Ni-Fe/CrNx alloy coatings A type of 430 stainless steel (430SS) with a nominal composition (wt%) of 17.0Cr, 1.0Mn, 1.0Si, 0.12C, 0.04P, 0.03S and balance Fe was used as substrate alloy. The steel plates were cut into coupons with a size of 10 mm × 15 mm × 2 mm. The coupons were mechanically ground and polished to a surface roughness of 1.5 μm, and then rinsed with deionized water, ethanol and acetone in an ultrasonic cleaner, respectively and eventually dried at ambient temperature in a vacuum oven. CrNx films were deposited on 430SS using a multi-arc ion plating system. Protective gas (Ar, 40 sccm) and reactive gas (N2, 1.0 Pa) were continuously introduced around the target to accelerate the reaction of plasma. Prior to deposition, the chamber was pumped down to a base pressure of 8 × 10−3 Pa. To remove the thin oxide layer and contaminants, the substrates were etched by ion bombardments for 3 min with a substrate bias voltage of −900 V. Deposition was conducted at 200 °C with applying a substrate bias voltage of −600 V for 20 min. Chromium target with a current of 70 A was triggered to fabricate CrNx layers in argon and nitrogen plasma. A two-electrode system with pure Fe and pure Ni as the anode was employed for galvanostatic electrodeposition of Ni-Fe alloy layers on the CrNx coated 430SS substrate. An electroplating solution composed of 200 g·L−1 NiSO4·6H2O, 60 g·L−1 FeSO4·6H2O, 40 g·L−1 H3BO3, 30 g·L−1 NaCl, 30 g·L−1 Na3C6H5O7·2H2O, 0.3 g·L−1 C7H5O3NS and 0.3 g·L−1 C12H25NaSO4 were used to deposit Ni-Fe alloy coatings. The pH of the electroplating solution was adjusted to 3.5 with 20 vol% H2SO4. Electroplating parameters for alloy coatings with different Fe contents are shown in Table 1.
Results and discussion Preparation of NiFe2O4 spinel coatings on CrNx/430SS Since CrNx exhibits excellent electrical conductivity [20,21], electrodeposition method can be employed to deposit Ni-Fe alloys on the CrNx coated steels. Fig. 1 shows the cross-sectional morphology (a) and EDS line scan (b) of CrNx/430SS with a Ni-Fe alloy coating electroplated at the current density of 20 mA cm−2 at 60 °C for 3.5 min. The CrNx layer prepared by multi-arc ion plating is adherent and compact, with a thickness of around 1.7 μm. The EDS result for the CrNx layer (point 1) is 54.03Cr-37.33N-6.26Fe-1.14Ni-0.85Al-0.38Si (at%). The detected Fe and Ni signals can be ascribed to the influence of the Ni-Fe alloy layer and the steel substrate. The Al and Si signals are impurities probably introduced during fabrication. The electroplated Ni-Fe alloy coating also exhibits an adherent and homogenous morphology. This indicates the suitability of using electrodeposition method to prepare Ni-Fe alloys on CrNx/430SS. 1599
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Fig. 1. Cross-sectional morphology (a) and EDS line scan (b) of CrNx/430SS with a Ni-Fe alloy coating electroplated at the current density of 20 mA cm−2 at 60 °C for 3.5 min.
to those of NiFe2O4, the existence of Fe2O3 rather than NiFe2O4 was confirmed by EDS analysis. The chromia shown in Fig. 2 for the Ni-55 at %Fe coating is actually a (Cr,Fe)2O3 structure, with characteristic peaks slightly shifting to a lower angle. The Cr element in (Cr,Fe)2O3 comes from the thermal decomposition of CrNx. The formation of the (Cr,Fe)2O3 scale inhibits the further oxidation of the inner Ni-Fe alloy layer, leading to a surplus of Fe content in the outer layer. The thickness of the (Cr,Fe)2O3 layer is about 1.2 μm, which is comparatively large and will impair the conductivity of the steel. Accordingly, the thickness and the Ni content of the Ni-Fe alloy coating need be increased to some extent to further optimize the oxidized Ni-Fe layer. For the 1.6 μm thick Ni-32 at%Fe coated steel (Fig. 3b,e), a multi-layer structure with NiFe2O4, Cr2O3, Ni-Fe-Cr alloy and undecomposed CrNx from outside to inside is formed on the steel substrate after oxidation at 800 °C for 5 h. Slight amounts of Fe2O3 and NiO are existed in the NiFe2O4 layer, with Fe2O3 mainly in the outside and NiO mainly in the inside. The phase structure of the Ni-Fe-Cr alloy layer for the Ni-32 at%Fe coated steel is different from that for the Ni-55 at%Fe coated steel, probably due to the difference in the thickness and composition of the coating. Some pores appear at the Ni-Fe-Cr/CrNx interface, probably caused by the decomposition of CrNx. Additionally, a thin Ni-rich metallic layer can be observed in the CrNx layer, which may be produced by the elemental diffusion from the Ni-Fe alloy layer. Further reducing the Fe content to 25at% facilitates the formation of a Ni-rich (Ni,Fe)3O4 outer layer. It should be pointed out that for all the samples, no Ni element can be detected beneath the CrNx layer, which indicates that CrNx can effectively block the mutual diffusion between the Ni-Fe coating and the 430SS substrate. By comparison, 1.6 μm thick Ni-32 at%Fe alloy should be the optimal alloy layer for preparing NiFe2O4 coated CrNx/430SS and was selected as the research subject in the following study.
Fig. 2. XRD patterns of CrNx/430SS with 0.7 μm thick Ni-55 at%Fe, 1.6 μm thick Ni-32 at%Fe and 1.6 μm thick Ni-25 at%Fe alloy coatings oxidized at 800 °C in air for 44 h, 5 h and 5 h, respectively.
To obtain a NiFe2O4 layer on the CrNx/430SS, Ni-Fe alloy layers with various contents and thicknesses have been investigated. Figs. 2 and 3 presents the XRD patterns and SEM morphologies of CrNx/430SS with 0.7 μm thick Ni-55 at%Fe, 1.6 μm thick Ni-32 at%Fe and 1.6 μm thick Ni-25 at%Fe alloy coatings oxidized at 800 °C in air for 44 h, 5 h and 5 h, respectively. It can be seen that the CrNx barrier layer deposited using multi-arc ion plating consists of CrN and Cr2N. All the samples exhibit good adhesion with the substrates. After oxidation at 800 °C for 44 h, the 0.7 μm thick Ni-55 at%Fe coating is transformed into a four-layer structure consisted of Fe2O3, (Cr,Fe)2O3, Ni-Fe-Cr alloy and undecomposed CrNx (CrN and Cr2N) from outside to inside, as shown in Fig. 3d. The Ni-Fe-Cr alloy probably comes from the unreacted Ni-Fe alloy layer and the diffusion of Cr to the Ni-Fe alloy. Since the crystal structure and the lattice constant of Fe2O3 are quite similar
Oxidation products and kinetics Fig. 4 gives the oxidation kinetics of the Ni-Fe/CrNx coated 430SS oxidized at 800 °C in air for various periods of time. The oxidation of the coated steel follows approximately a parabolic rate law during the experimental duration of 1000 h. The following equation based on Wagner's oxidation theory was used to calculate the corresponding value of the parabolic rate constant (kp):
Δw 2 ⎛ ⎞ = k p· t + C ⎝ S ⎠
(2)
where Δw is the mass change during oxidation, S stands for the surface area of the specimen, t is oxidation time and C is an integration constant 1600
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Fig. 3. Surface and cross-sectional morphologies of CrNx/430SS with 0.7 μm thick Ni-55 at%Fe (a,d), 1.6 μm thick Ni-32 at%Fe (b,e) and 1.6 μm thick Ni-25 at%Fe (c,f) alloy coatings oxidized at 800 °C in air for 44 h, 5 h and 5 h, respectively.
decomposed CrNx from outside to inside. Compared with the sample oxidized for 5 h (Fig. 3e), the original Fe2O3 and NiO doped in the NiFe2O4 layer have fully reacted and the Cr2O3 layer has grown to approximately 1.5 μm thick after oxidized for 200 h. Most CrNx has decomposed, leaving a slight amount of undecomposed CrNx (light contrast) discretely distributed beneath the Ni-Fe-Cr alloy layer. No pores at the interface of the Ni-Fe-Cr alloy and the CrNx layer can be observed any more. The EDS result for the decomposed CrNx is 48.3Cr43.2Fe-5.5Ni-2.1Si-0.9Al (at%) for point 3, indicating that the Ni and Fe atoms in the Ni-Fe coating and the Fe element in the substrate have diffused to the original CrNx zone. The disappearance of the pores at the Ni-Fe-Cr/CrNx interface is due to the decomposition of CrNx and subsequent filling up of the pores by the migrating Ni and Fe atoms. The
defining the onset of parabolic kinetics. The parabolic rate constant, i.e., the slope of the curve in Fig. 4b, is 4.7 × 10−14 g2 cm−4 s−1. This is lower than our previous results for the bare and Ni-Fe coated 430SS, whose parabolic rate constants were 7.4 × 10−14 and 7.8 × 10−14 g2 cm−4 s−1, respectively [5]. The decrease in the parabolic rate constant for the Ni-Fe/CrNx coated 430SS demonstrates that the CrNx diffusion barrier can enhance the oxidation resistance of the steel. Fig. 5 gives the surface and cross-sectional morphologies of the NiFe/CrNx coated 430SS after oxidation at 800 °C in air for 200 h. The sample surface presents a dense structure, with grain size of a few hundred nanometers (Fig. 5a). After oxidized for 200 h, the coating remains a multi-layer structure with NiFe2O4, Cr2O3, Ni-Fe-Cr alloy and
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EDS results for the Ni-Fe-Cr alloy layer (point 2) and the substrate beneath the CrNx layer (point 4) are 63.2Fe-22.2Cr-14.6Ni (at%) and 77.2Fe-17.1Cr-5.7Ni (at%), respectively. Slight amounts of Ni can be detected in the substrate beneath the CrNx layer. The Cr element in the Ni-Fe-Cr alloy layer comes from the decomposition of CrNx. The Ni content in the Ni-Fe-Cr layer is higher than that of the substrate, demonstrating that the decomposed CrNx can still hinder the mutual diffusion of elements between the coating and the substrate. Since N elements and a higher Cr content can be detected in the particles (dark contrast) along the grain boundaries of the substrate (Fig. 5(b,d)), these particles may be CrN and/or Cr2N generated from the reaction of Cr with the migrating N from the decomposed CrNx. Fig. 6 gives the surface and cross-sectional morphologies of the NiFe/CrNx coated 430SS after oxidation at 800 °C in air for 500 h. The coating exhibits a similar microstructure to that oxidized for 200 h. The thickness of the Cr2O3 layer remains increasing to up to 2 μm. The original CrNx layer has fully decomposed, with chemical composition of 58.6Cr-35.4Fe-2.7Si-2.3Ni-1Al (at%) for point 6. The Si element in the decomposed CrNx zone may be ascribed to the migrating Si from the substrate and the impurities brought during fabrication. As previous mentioned, since the 430SS used in this work doesn’t contain Al, the Al element in the decomposed CrNx should be the impurities introduced during preparation. The chemical compositions of the Ni-Fe-Cr alloy layer (point 5) and the substrate beneath the decomposed CrNx (point 7) are 70.3Fe-23.4Cr-6.3Ni (at%) and 79.3Fe-15.3Cr-5.4Ni (at%), respectively. By comparison, the concentration of Ni in the Ni-Fe-Cr alloy layer after oxidized for 500 h is lower than that oxidized for 200 h, but the Ni content in the substrate has no significant increase. This illustrates that the decomposed CrNx can still retard the migration of the elements from the Ni-Fe-Cr alloy layer to the substrate and the Ni element beneath the decomposed CrNx layer can rapidly diffuse deep to the internal substrate. Figs. 7 and 8 shows the XRD pattern and SEM morphologies of the Ni-Fe/CrNx coated 430SS after oxidation at 800 °C in air for 1000 h. After an oxidation of 1000 h, the coating still remains a NiFe2O4
Fig. 4. Weight changes of the Ni-Fe/CrNx/430SS oxidized at 800 °C in air and its parabolic plot.
Fig. 5. Surface (a) and cross-sectional (b, c, d) morphologies of Ni-Fe/CrNx/430SS oxidized at 800 °C in air for 200 h. 1602
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Fig. 6. Surface (a) and cross-sectional (b, c, d) morphologies of Ni-Fe/CrNx/430SS oxidized at 800 °C in air for 500 h.
(at%), respectively. The concentration of Ni in the decomposed CrNx zone is lower than those in the nearby Ni-Fe-Cr alloy layer and substrate. This is probably because Ni can react with the latter two phases to form structures with higher thermal stability and/or it can only diffuse along the grain boundaries or defects in the decomposed CrNx zone. ASR of oxide scales The electrical resistance of the metallic substrate is negligible in comparison with that of the oxide scale formed during exposure. Consequently, the oxide scale has a major influence on the ASR for interconnects. Fig. 9 displays the ASR of the Ni-Fe/CrNx coated 430SS oxidized at 800 °C in air as a function of time. The ASR increases over the oxidation time, but its growth rate shows a decreasing tendency. The ASR of the sample increases by nearly 12 mΩ cm2 from an oxidation of 200 h to 1000 h. After oxidation for 1000 h, the ASR for the NiFe/CrNx coated steel reaches to 58.8 mΩ cm2. According to our previous study, the scale ASR for the bare and Ni-Fe coated steels after oxidized for 1000 h are 101 and 72 mΩ cm2, respectively [5]. The decrease in the scale ASR for the Ni-Fe/CrNx coated steel is attributed to the formation of a highly conductive and protective NiFe2O4 top layer and a significantly reduced growth of a poorly conductive Cr2O3 inner layer. The elimination of the pores near the coating/substrate interface may also plays a role in reducing the ASR of the Ni-Fe/CrNx coated steel. However, the coating still exhibits higher ASR values than the required electrical resistances for SOFC interconnect after long-term operation. Assuming that the growth of the Cr2O3 inner layer is diffusion-controlled and obeys parabolic rate kinetics ASR2 = a·t + b during the operation period of 40,000 h (where t is the operation time, a and b are constants), the ASR for the Ni-Fe/CrNx coated steel should be 257 mΩ cm2 after 40,000 h operation, which is higher than the generally accepted upper limit of 100 mΩ cm2 for SOFC interconnect. Accordingly, the properties of the Ni-Fe/CrNx coating, especially the compactness of the CrNx layer, should be further improved.
Fig. 7. XRD pattern of Ni-Fe/CrNx/430SS oxidized at 800 °C in air for 1000 h.
structure, indicating the thermal stability of this spinel phase. Compared with those oxidized for 200 h and 500 h, the grain size on the sample surface increases to about 1 μm after oxidation for 1000 h (Fig. 8a). The coating is generally compact, uniform and well adherent to the substrate. The thickness of the Cr2O3 sublayer grows with oxidation time, and reaches to about 2.6 μm after oxidized for 1000 h. It should be noted that the thicknesses of the Cr2O3 scale for the Ni-Fe coated 430SS is approximately 4 μm after oxidized 800 °C in air for 1000 h, as reported in our previous work [5]. This indicates that the exploitation of the CrNx layer can reduce the growth rate of the Cr2O3 scale. Additionally, The chemical compositions for the Ni-Fe-Cr alloy layer (point 8), the decomposed CrNx zone (point 9) and the base alloy near the coating/substrate interface (point 10) is 73.5Fe-21.0Cr-5.5Ni (at%), 60.0Cr-36.1Fe-1.6Al-1.6Si-0.7Ni (at%) and 82.0Fe-13.5Cr-4.5Ni 1603
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Fig. 8. Surface (a) and cross-sectional (b, c) morphologies of Ni-Fe/CrNx/430SS oxidized at 800 °C in air for 1000 h.
can be NiFe2O4, Fe2O3 and NiO, depending on the atomic ratio of Ni/ Fe) at the upmost surface, but the CrNx inner layer remains intact. Possible reactions of CrNx in air at 800 °C are as follows: 2CrN = 2Cr + N2 (g)
(3)
2CrN + 1.5O2 (g) = Cr2O3 + N2 (g)
(4)
2Cr2N = 4Cr + N2 (g)
(5)
2Cr2N + 3O2 (g) = 2Cr2O3 + N2 (g)
(6)
Based on the thermodynamic data in the HSC6.0 software, the Gibbs free energies for Reactions (3), (4), (5) and (6) are 16.27, −187.20, 21.53 and −386.21 kJ mol−1, respectively, among which only Reactions (4) and (6) can occur spontaneously. Before the sufficient oxidation of the Ni-Fe alloy layer, the Cr atoms in the steel substrate migrate to the unreacted Ni-Fe alloy through the grain boundaries of CrNx, forming a Ni-Fe-Cr alloy layer beneath the Ni-Fe oxide scale. The oxygen pressure at the Ni-Fe oxide scale/alloy interface is sufficiently high for the external oxidation of Cr to Cr2O3. With extended oxidation, Cr diffuses outward from Ni-Fe-Cr alloy to contribute to the formation of a continuous Cr2O3 scale beneath the Ni-Fe oxides. After an oxidation of 5 h at 800 °C, the coating of the Ni-Fe/CrNx coated steel presents a multi-layer structure with NiFe2O4, Cr2O3, Ni-FeCr alloy and undecomposed CrNx from outside to inside, as shown in Fig. 3e. The formation of the continuous Cr2O3 scale blocks the inward migration of O and the outward migration of Ni and Fe, and the CrNx inner layer hinders the mutual diffusion between the Ni-Fe-Cr alloy and the substrate, thus leaving the Ni-Fe-Cr alloy sublayer remain unoxidized. Actually, the Gibbs free energies for the Reactions (3)–(6) are calculated under standard state, and would vary with the change of the state. The doping of the alloy elements, e.g., Fe and/or Mn, to CrNx may boost the decomposition of CrNx to Cr and N2. The diffusion of N2 to deep inside the alloy may also promote Reactions (3) and (5) to proceed in a forward direction. Therefore, the decomposition of CrNx to Cr and N2 may probably serve as another Cr source for the growth of the Cr2O3
Fig. 9. The ASR of the Ni-Fe/CrNx/430SS oxidized at 800 °C in air for various time.
Discussion As previous mentioned, a drawback for the application of (Ni,Fe)3O4 coatings on ferritic stainless steels is the appearance of a large number of pores beneath the oxide scales after oxidation. This is because the inward diffusion of Ni from the coating facilitates the formation of an austenite zone near the coating/substrate interface, and the diffusion coefficients of Ni, Fe and Cr in austenite are significantly smaller than those in ferrite at high temperature. The outward migration of elements from the substrate also boosts the generation of the pores. By comparison, for the Ni-Fe/CrNx coated steel, no pores can be observed near the interface of the CrNx layer and the substrate throughout the experiment. At the initial oxidation stage, the Ni-Fe alloy outer layer reacts with oxygen, forming a continuous oxide layer (possible products 1604
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still serve as an effective barrier against the mutual diffusion of elements between the coating and the substrate. During the experimental duration of 1000 h, no pores could be observed near the scale/substrate interface. The application of the CrNx diffusion barrier beneath the NiFe alloy coating could reduce its oxidation rate to 4.7 × 10−14 g2 cm−4 s−1. The Ni-Fe/CrNx dual layer coating significantly retarded the growth of the less conductive Cr2O3 scale and effectively eliminated the pores beneath the scale, leading to a decreased ASR of the steel.
scale. After an oxidation of 200 h, most CrNx has decomposed. CrNx at the Ni-Fe alloy/CrNx interface interact with O2, forming Cr2O3 particles in the CrNx layer. Since the oxygen pressures for the Cr/Cr2O3 equilibrium is extremely low, the Cr2O3 product can exist stably instead of decomposing into Cr and O2. A slight amount of Ni can be detected in the substrate beneath the scale, with chemical composition of 77.2Fe17.1Cr-5.7Ni (at%) for point 4 (Fig. 5c). According to the phase diagram of the Fe-Cr-Ni ternary alloy at 1000 K (from FactSage database by thermodynamic calculation), alloy with composition of 77.2Fe17.1Cr-5.7Ni (at%) falls into the area quite near the boundary line between the α + γ phase region and the γ phase region. The phase transformation line of the phase diagram is calculated under the condition that the Gibbs free energies of the parent phase (Gα) and the new phase (Gγ) are identical. Actually, the thermodynamic condition for phase transformation is the free energy difference (△Gα→γ) of the two phases. A larger △Gα→γ contributes to a higher phase transformation driving force. Another requirement for the transition from α to γ is to overcome the phase transformation barrier caused by overcoming the attractive forces among atoms during lattice reorganization. These obstacles would make the γ single phase region in the actual Fe-Cr-Ni phase diagram smaller than the thermodynamic calculated one. Therefore, unlike the Ni-Fe coated steel [5], the phase structure of the substrate right beneath the scale would contain large amounts of ferrite for the Ni-Fe/CrNx coated alloy. Further oxidation to 500 h makes the CrNx fully decomposed, but the Ni content in the substrate near the decomposed CrNx remains almost unchanged. Even after an oxidation of 1000 h, the Ni content in the substrate is still very low, with a value of only 4.5 at% for point 10 (Fig. 8c). It can be concluded that either before or after decomposition, the CrNx layer plays an important role in reducing the diffusion rate of Ni to the substrate and eliminating its enrichment beneath the CrNx layer. Since the Ni content of the substrate beneath the scale remains a low value, which is not enough to maintain a large amount of austenite, the point defects introduced by the enormous difference in the diffusion coefficients of austenite and ferrite can be drastically decreased. The employment of CrNx diffusion barrier can not only reduce the interface defects, but also decrease the growth rate of Cr2O3. As previously mentioned, the rate constant for the Ni-Fe/CrNx coated steel during the experimental duration of 1000 h is 4.7 × 10−14 g2 cm−4 s−1, nearly half of that for the Ni-Fe coated steel of 7.8 × 10−14 g2 cm−4 s−1. For the oxidized Ni-Fe/CrNx/430SS, the Cr content in the Ni-Fe-Cr alloy layer beneath the Cr2O3 scale is higher than that in the 430SS substrate, so the low growth rate of Cr2O3 in this circumstance is not caused by a decreased Cr content in the nearby region, but probably resulted from the interference of N. The N atoms derived from the decomposed CrNx can migrate inward to the substrate, depositing along the grain boundaries by forming CrNx and/or Cr2N particles with the Cr atoms. It can be accordingly inferred that the N atoms generated from the decomposed CrNx migrate outward to the NiFe-Cr alloy layer, and inhibit the diffusion of Cr to the Cr2O3 scale during oxidation.
Acknowledgement This work was supported by the National Natural Science Foundation of China under the Grant Nos. 51601202 and 51471179. References [1] Tariq S, Marium A, Raza R, Ahmad MA, Khan MA, Abbas G, et al. Comparative study of Ce0.80Sm0.20Ba0.80Y0.20O3-δ (YB-SDC) electrolyte by various chemical synthesis routes. Results Phys 2018;8:780–4. [2] Szymczewska D, Chrzan A, Karczewski J, Molin S, Jasinski P. Spray pyrolysis of doped-ceria barrier layers for solid oxide fuel cells. Surf Coat Technol 2017;313:168–76. [3] Kumar SS, Nalluri A, Anandan C, Prakash BS, Aruna ST. Deposition and evaluation of Mn-Co oxide protective sputtered coating on SOFC interconnects and current collectors. J Electrochem Soc 2016;163:F905–12. [4] Montero X, Tietz F, Sebold D, Buchkremer HP, Ringuede A, Cassir M. MnCo1.9Fe0.1O4 spinel protection layer on commercial ferritic steels for interconnect applications in solid oxide fuel cells. J Power Sources 2008;184:172–9. [5] You PF, Zhang X, Zhang HL, Liu HJ, Zeng CL. Effect of CeO2 on oxidation and electrical behaviors of ferritic stainless steel interconnects with Ni-Fe coatings. Int J Hydrogen Energy 2018;43:7492–500. [6] Bateni MR, Wei P, Deng XH, Petric A. Spinel coatings for UNS 430 stainless steel interconnects. Surf Coat Technol 2007;201:4677–84. [7] Zhang X, You PF, Zhang HL, Yang XG, Luo MQ, Zeng CL. Preparation and performances of Cu-Co spinel coating on ferritic stainless steel for solid oxide fuel cell interconnect. Int J Hydrogen Energy 2018;43:3273–9. [8] Shaigan N, Qua W, Iveyb DG, Chen WX. A review of recent progress in coatings, surface modifications and alloy developments for solid oxide fuel cell ferritic stainless steel interconnects. J Power Sources 2010;195:1529–42. [9] Molin S. Evaluation of electrodeposited Mn-Co protective coatings on Crofer 22 APU steel. Int J Appl Ceram Technol 2017;15:349–60. [10] Petric A, Ling H. Electrical conductivity and thermal expansion of spinels at elevated temperatures. J Am Ceram Soc 2007;90:1515–20. [11] Hiroshi N, Ken O. Effect of TiO2 on the sintering and the electrical conductivity of Cr2O3. J Am Ceram Soc 1989;72:400–3. [12] Goebel C, Fefekos A, Svensson J, Froitzheim J. Does the conductivity of interconnect coatings matter for solid oxide fuel cell applications. J Power Sources 2018;383:110–4. [13] Liu Y, Chen DY. Protective coatings for Cr2O3-forming interconnects of solid oxide fuel cells. Int J Hydrogen Energy 2009;34:9220–6. [14] Geng SJ, Qi SJ, Zhao QC, Zhu SL, Wang FH. Electroplated Ni-Fe2O3 composite coating for solid oxide fuel cell interconnect application. Int J Hydrogen Energy 2012;37:10850–6. [15] You PF, Zhang X, Zhang HL, Liu HJ, Zeng CL. Oxidation behavior of NiFe2O4 spinel coated interconnects in wet air. Oxid Met 2018;90:499–513. [16] LeClaire AD, Neumann G. Diffusion in solid metals and alloys. Germany: Springer; 1990. p. 7. [17] Hovsepian PE, Ehiasarian AP, Purandare YP, Mayr P, Abstoss KG, Feijoo M, et al. Novel HIPIMS deposited nanostructured CrN/NbN coatings for environmental protection of steam turbine component. J Alloys Compd 2018;746:583–93. [18] Panjan P, Drnovšek A, Kovač J, Gselman P, Bončina T, Paskvale S, et al. Oxidation resistance of CrN/(Cr, V)N hard coatings deposited by DC magnetron sputtering. Thin Solid Films 2015;591:323–9. [19] Feng ZJ, Zeng CL. Oxidation behavior and electrical property of ferritic stainless steel interconnects with a Cr-La alloying layer by high-energy micro-arc alloying process. J Power Sources 2010;195:7370–4. [20] Lee SH, Pukha VE, Vinogradov VE, Kakati N, Jee SH, et al. Nanocomposite-carbon coated at low-temperature: a new coating material for metallic bipolar plates of polymer electrolyte membrane fuel cells. Int J Hydrogen Energy 2013;38:14284–94. [21] Bi FF, Yi YY, Zhou T, Peng LF, Lai XM. Effects of Al incorporation on the interfacial conductivity and corrosion resistance of CrN film on SS316L as bipolar plates for proton exchange membrane fuel cells. Int J Hydrogen Energy 2015;40:9790–802.
Conclusion A compact CrNx layer was deposited on 430SS using multi-arc ion plating, followed by electrodeposition of a Ni-Fe alloy coating on CrNx/ 430SS. An adhesive NiFe2O4 top layer could be obtained after oxidizing the 1.6 μm thick Ni-32 at%Fe coated sample. After oxidized at 800 °C in air for 5 h, the Ni-Fe/CrNx coated steel presented a multi-layer scale with NiFe2O4, Cr2O3, Ni-Fe-Cr alloy and CrNx from outside to inside. The CrNx layer gradually decomposed after further oxidation, but could
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Preparation and performances of Ni-Fe/CrNx dual layer coatings for ferritic stainless steel interconnects
T
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P.F. You, X. Zhang , X.G. Yang, H.L. Zhang, L.X. Yang, C.L. Zeng Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Solid oxide fuel cell Interconnect NiFe2O4 CrNx Oxidation resistance Area specific resistance
Dual layer coatings CrNx and Ni-Fe alloy have been developed for metallic interconnects application by a combined approach. The CrNx layer was deposited on ferritic stainless steel using multi-arc ion plating, and the Ni-Fe alloy coating was electroplated on the CrNx coated steel subsequently. After oxidation at 800 °C in air for 5 h, a multi-layer scale composed of NiFe2O4, Cr2O3, Ni-Fe-Cr alloy and CrNx from outside to inside was formed on the CrNx/substrate with a 1.6 μm thick Ni-32 at%Fe alloy coating. Further oxidation led to the decomposition of the CrNx layer, but no pores could be observed near the scale/substrate interface even after an oxidation of 1000 h. The CrNx layer acted as an effective barrier against the mutual diffusion of elements between the coating and the substrate either before or after decomposition. The employment of the Ni-Fe/CrNx dual layer coating decreased the growth rate of the Cr2O3 scale and thus a lower scale area specific resistance (ASR). The oxidation behavior of the Ni-Fe/CrNx coated substrate and the influence mechanism of the CrNx barrier layer were discussed.
Introduction Solid oxide fuel cells (SOFCs) are power generation systems that can convert chemical energy directly into electricity with high efficiency and reduced emissions [1]. Recent progress in SOFC technology has made it possible to reduce the cell operation temperatures down to the range of 600–800 °C [2–4]. This allows the replacement of the traditional LaCrO3-based ceramics by metallic interconnects. Among the high temperature alloys, ferritic stainless steels are the most promising candidates as interconnect materials, due to the advantages of low cost, high electrical and thermal conductivity, matched coefficient of thermal expansion (CTE) with other SOFC components [5]. However, the main obstacles for the application of ferritic stainless steel interconnects are the deterioration of the cathode performance caused by the volatile Cr species (CrO3 or CrO2(OH)2) and a relatively fast increase in the electrical resistance resulted from the formation of oxide scales [6,7]. Applying conductive spinel coatings onto ferritic stainless steels is an effective method to solve these problems. Several fabrication techniques, such as screen printing, slurry spraying, plasma spraying and magnetron sputtering, have been developed for preparing spinel coatings and have been thoroughly reviewed by Shaigan et al. [8]. Among these techniques, electroplating an alloy layer followed by oxidation treatment has attracted much attention due to the advantages such as low cost and suitability for mass production on substrates with ⁎
complicated geometry [9]. Challenge with electrodeposition of alloys is that the deposition potential of each component should be similar. Unfortunately, as one of the most promising candidates for interconnect application, the precursors of (Co,Mn)3O4 coatings, i.e., Co-Mn alloys, possess components with considerably large potential difference (E0(Mn2+/ 2+ Mn) = −1.18 VSHE, E0(Co /Co) = −0.28 VSHE). By comparison, Ni and Fe exhibit quite similar deposition potentials and thus excellent process stability for electroplating Ni-Fe alloys. It is reported that the CTE of NiFe2O4 spinel (10.8 × 10−6 K−1) is extremely close to that of the ferritic stainless steels (11 × 10−6 K−1) [10]. Although compared with (Co,Mn)3O4 (e.g., 60 S cm−1 for MnCo2O4 [10]), NiFe2O4 presents a much lower conductivity of 0.26 S cm−1[10], it is still several orders of magnitude higher than that of Cr2O3 (0.02 S cm−1 [11]) and accordingly the inner Cr2O3 scale should be the determinant for the conductivity of the coated steel [12]. Some researchers have investigated the effect of (Ni,Fe)3O4 coatings on the performance of ferritic stainless steel interconnects and the results showed a decreased area specific resistance (ASR) and a retarded outward migration of Cr for the (Ni,Fe)3O4 coated steels [5,13–15]. Consequently, (Ni,Fe)3O4 should be a promising coating material for the metallic interconnect application. Unfortunately, there is a drawback for the application of (Ni,Fe)3O4 coatings on ferritic stainless steels. During oxidation, many pores would
Corresponding author at: Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China. E-mail address: [email protected] (X. Zhang).
https://doi.org/10.1016/j.rinp.2019.01.061 Received 12 December 2018; Received in revised form 7 January 2019; Accepted 16 January 2019 Available online 30 January 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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appear beneath the oxide scale of the coated steel and their sizes would increase with extending exposure time [5,13–15]. Because the diffusion of Ni from the coating facilitates the formation of an austenite zone beneath the scale, and the diffusion coefficients of Ni, Fe and Cr in austenitic iron matrix are two orders of magnitude smaller than those in ferrite at 800 °C, atoms of Ni and Fe are easier to migrate from austenite to ferrite, leaving point defects and thus pores in austenite [5,16]. The existence of pores beneath the scale plays a negative role in the coating/ substrate adhesion and the conductivity of interconnect. Our previous investigation has successfully reduced the pores beneath the scales by adding Ce element to the Ni-Fe coatings [5]. The reduction in the pores is due to the reactive element effect (REE), which means that the rare earth element can provide vacancy sinks at the CeO2/alloy interface and can change the transport mechanism from a predominantly outward Cr diffusion to a principally inward oxygen transport. In this work, our aim is to inhibit the formation of the pores by suppressing the migration of Ni from the coating and thus a reduced austenite zone beneath the scale. Hovsepian et al. [17] prepared a ceramic based CrN/ NbN coating on P92 steel using high power impulse magnetron sputtering deposition method. The coating exhibited superior oxidation resistance and inhibition effect against the migration of H and O after oxidation at 600 °C in a high pressure (50 bar) 100% steam atmosphere for 1000 h. Panjan et al. [18] obtained a CrN/(Cr,V)N coating by DC unbalanced magnetron sputtering. After oxidation at 600–750 °C for 220 min, the surface was uniformly covered with a thin chromium oxide layer which could hinder the out-diffusion of vanadium. Therefore, it can be concluded that both CrN and its oxidation product, i.e., Cr2O3, can act as an effective barrier against the diffusion of elements. In this study, a CrNx diffusion barrier has been prepared between the ferritic stainless steel and the Ni-Fe alloy layer, and the oxidation behavior and electrical resistance of the Ni-Fe/CrNx coated steels have been investigated, with an attempt to reveal the effect of CrNx on the performances of the Ni-Fe coating.
Table 1 Electroplating parameters for alloy coatings with different Fe contents. Alloy composition Ni-55 at%Fe Ni-32 at%Fe Ni-25 at%Fe
Current density −2
10 mA cm 20 mA cm−2 10 mA cm−2
Temperature 40 °C 60 °C 60 °C
Oxidation experiments The coated steels were pre-oxidized at 800 °C in air in a high temperature muffle furnace to analyze the initial stage of oxidation. Further oxidation measurements of the coated steels were carried out at 800 °C in air for up to 1000 h. The specimens were placed in alumina crucibles. After oxidation for various periods, the alumina crucibles containing samples were taken out from the furnace, cooled to room temperature and weighed. The specimens were then replaced in the furnace for further oxidation. Three parallel samples were used for oxidation kinetics analysis. The phase structures of the oxide scales were characterized by X-ray diffraction (XRD, Phillips, PW-1700) with a Cu Ka radiation source. The microstructures and chemical compositions of the oxide scales were analyzed by scanning electron microscopy (SEM, FEI Inspect FSEM) coupled with an energy dispersive X-ray spectroscopy (EDS). The operating voltage was 25 kV, accelerated by a field emission gun. Quantitative analysis of EDS results was carried out without standard samples. Area specific resistance measurements The area specific resistances (ASR) is commonly used to evaluate the electrical performance of SOFC interconnects, as can be given by:
ASR =
R·S 2
(1)
where R is the electrical resistance [mΩ], S is the contact area [cm2]. Since the electrical resistance of the alloy substrate is negligible compared with that of the oxide scale, the oxide scale plays a main role in the ASR for interconnects. The area specific resistances of the coated samples were measured using a setup reported in the Ref. [19]. Before measurement, platinum paste was brushed onto the two sides of the sample surfaces with a soft brush, followed by heat treatment at 800 °C for 10 min. The platinum foils, each spot-welded with a platinum wire, were used as current collectors and placed on the top of the platinum paste. The resulting resistances of Pt wires and foils were deducted from the original results. The change of the scale ASR with oxidation time was investigated.
Experimental Preparation of Ni-Fe/CrNx alloy coatings A type of 430 stainless steel (430SS) with a nominal composition (wt%) of 17.0Cr, 1.0Mn, 1.0Si, 0.12C, 0.04P, 0.03S and balance Fe was used as substrate alloy. The steel plates were cut into coupons with a size of 10 mm × 15 mm × 2 mm. The coupons were mechanically ground and polished to a surface roughness of 1.5 μm, and then rinsed with deionized water, ethanol and acetone in an ultrasonic cleaner, respectively and eventually dried at ambient temperature in a vacuum oven. CrNx films were deposited on 430SS using a multi-arc ion plating system. Protective gas (Ar, 40 sccm) and reactive gas (N2, 1.0 Pa) were continuously introduced around the target to accelerate the reaction of plasma. Prior to deposition, the chamber was pumped down to a base pressure of 8 × 10−3 Pa. To remove the thin oxide layer and contaminants, the substrates were etched by ion bombardments for 3 min with a substrate bias voltage of −900 V. Deposition was conducted at 200 °C with applying a substrate bias voltage of −600 V for 20 min. Chromium target with a current of 70 A was triggered to fabricate CrNx layers in argon and nitrogen plasma. A two-electrode system with pure Fe and pure Ni as the anode was employed for galvanostatic electrodeposition of Ni-Fe alloy layers on the CrNx coated 430SS substrate. An electroplating solution composed of 200 g·L−1 NiSO4·6H2O, 60 g·L−1 FeSO4·6H2O, 40 g·L−1 H3BO3, 30 g·L−1 NaCl, 30 g·L−1 Na3C6H5O7·2H2O, 0.3 g·L−1 C7H5O3NS and 0.3 g·L−1 C12H25NaSO4 were used to deposit Ni-Fe alloy coatings. The pH of the electroplating solution was adjusted to 3.5 with 20 vol% H2SO4. Electroplating parameters for alloy coatings with different Fe contents are shown in Table 1.
Results and discussion Preparation of NiFe2O4 spinel coatings on CrNx/430SS Since CrNx exhibits excellent electrical conductivity [20,21], electrodeposition method can be employed to deposit Ni-Fe alloys on the CrNx coated steels. Fig. 1 shows the cross-sectional morphology (a) and EDS line scan (b) of CrNx/430SS with a Ni-Fe alloy coating electroplated at the current density of 20 mA cm−2 at 60 °C for 3.5 min. The CrNx layer prepared by multi-arc ion plating is adherent and compact, with a thickness of around 1.7 μm. The EDS result for the CrNx layer (point 1) is 54.03Cr-37.33N-6.26Fe-1.14Ni-0.85Al-0.38Si (at%). The detected Fe and Ni signals can be ascribed to the influence of the Ni-Fe alloy layer and the steel substrate. The Al and Si signals are impurities probably introduced during fabrication. The electroplated Ni-Fe alloy coating also exhibits an adherent and homogenous morphology. This indicates the suitability of using electrodeposition method to prepare Ni-Fe alloys on CrNx/430SS. 1599
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Fig. 1. Cross-sectional morphology (a) and EDS line scan (b) of CrNx/430SS with a Ni-Fe alloy coating electroplated at the current density of 20 mA cm−2 at 60 °C for 3.5 min.
to those of NiFe2O4, the existence of Fe2O3 rather than NiFe2O4 was confirmed by EDS analysis. The chromia shown in Fig. 2 for the Ni-55 at %Fe coating is actually a (Cr,Fe)2O3 structure, with characteristic peaks slightly shifting to a lower angle. The Cr element in (Cr,Fe)2O3 comes from the thermal decomposition of CrNx. The formation of the (Cr,Fe)2O3 scale inhibits the further oxidation of the inner Ni-Fe alloy layer, leading to a surplus of Fe content in the outer layer. The thickness of the (Cr,Fe)2O3 layer is about 1.2 μm, which is comparatively large and will impair the conductivity of the steel. Accordingly, the thickness and the Ni content of the Ni-Fe alloy coating need be increased to some extent to further optimize the oxidized Ni-Fe layer. For the 1.6 μm thick Ni-32 at%Fe coated steel (Fig. 3b,e), a multi-layer structure with NiFe2O4, Cr2O3, Ni-Fe-Cr alloy and undecomposed CrNx from outside to inside is formed on the steel substrate after oxidation at 800 °C for 5 h. Slight amounts of Fe2O3 and NiO are existed in the NiFe2O4 layer, with Fe2O3 mainly in the outside and NiO mainly in the inside. The phase structure of the Ni-Fe-Cr alloy layer for the Ni-32 at%Fe coated steel is different from that for the Ni-55 at%Fe coated steel, probably due to the difference in the thickness and composition of the coating. Some pores appear at the Ni-Fe-Cr/CrNx interface, probably caused by the decomposition of CrNx. Additionally, a thin Ni-rich metallic layer can be observed in the CrNx layer, which may be produced by the elemental diffusion from the Ni-Fe alloy layer. Further reducing the Fe content to 25at% facilitates the formation of a Ni-rich (Ni,Fe)3O4 outer layer. It should be pointed out that for all the samples, no Ni element can be detected beneath the CrNx layer, which indicates that CrNx can effectively block the mutual diffusion between the Ni-Fe coating and the 430SS substrate. By comparison, 1.6 μm thick Ni-32 at%Fe alloy should be the optimal alloy layer for preparing NiFe2O4 coated CrNx/430SS and was selected as the research subject in the following study.
Fig. 2. XRD patterns of CrNx/430SS with 0.7 μm thick Ni-55 at%Fe, 1.6 μm thick Ni-32 at%Fe and 1.6 μm thick Ni-25 at%Fe alloy coatings oxidized at 800 °C in air for 44 h, 5 h and 5 h, respectively.
To obtain a NiFe2O4 layer on the CrNx/430SS, Ni-Fe alloy layers with various contents and thicknesses have been investigated. Figs. 2 and 3 presents the XRD patterns and SEM morphologies of CrNx/430SS with 0.7 μm thick Ni-55 at%Fe, 1.6 μm thick Ni-32 at%Fe and 1.6 μm thick Ni-25 at%Fe alloy coatings oxidized at 800 °C in air for 44 h, 5 h and 5 h, respectively. It can be seen that the CrNx barrier layer deposited using multi-arc ion plating consists of CrN and Cr2N. All the samples exhibit good adhesion with the substrates. After oxidation at 800 °C for 44 h, the 0.7 μm thick Ni-55 at%Fe coating is transformed into a four-layer structure consisted of Fe2O3, (Cr,Fe)2O3, Ni-Fe-Cr alloy and undecomposed CrNx (CrN and Cr2N) from outside to inside, as shown in Fig. 3d. The Ni-Fe-Cr alloy probably comes from the unreacted Ni-Fe alloy layer and the diffusion of Cr to the Ni-Fe alloy. Since the crystal structure and the lattice constant of Fe2O3 are quite similar
Oxidation products and kinetics Fig. 4 gives the oxidation kinetics of the Ni-Fe/CrNx coated 430SS oxidized at 800 °C in air for various periods of time. The oxidation of the coated steel follows approximately a parabolic rate law during the experimental duration of 1000 h. The following equation based on Wagner's oxidation theory was used to calculate the corresponding value of the parabolic rate constant (kp):
Δw 2 ⎛ ⎞ = k p· t + C ⎝ S ⎠
(2)
where Δw is the mass change during oxidation, S stands for the surface area of the specimen, t is oxidation time and C is an integration constant 1600
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Fig. 3. Surface and cross-sectional morphologies of CrNx/430SS with 0.7 μm thick Ni-55 at%Fe (a,d), 1.6 μm thick Ni-32 at%Fe (b,e) and 1.6 μm thick Ni-25 at%Fe (c,f) alloy coatings oxidized at 800 °C in air for 44 h, 5 h and 5 h, respectively.
decomposed CrNx from outside to inside. Compared with the sample oxidized for 5 h (Fig. 3e), the original Fe2O3 and NiO doped in the NiFe2O4 layer have fully reacted and the Cr2O3 layer has grown to approximately 1.5 μm thick after oxidized for 200 h. Most CrNx has decomposed, leaving a slight amount of undecomposed CrNx (light contrast) discretely distributed beneath the Ni-Fe-Cr alloy layer. No pores at the interface of the Ni-Fe-Cr alloy and the CrNx layer can be observed any more. The EDS result for the decomposed CrNx is 48.3Cr43.2Fe-5.5Ni-2.1Si-0.9Al (at%) for point 3, indicating that the Ni and Fe atoms in the Ni-Fe coating and the Fe element in the substrate have diffused to the original CrNx zone. The disappearance of the pores at the Ni-Fe-Cr/CrNx interface is due to the decomposition of CrNx and subsequent filling up of the pores by the migrating Ni and Fe atoms. The
defining the onset of parabolic kinetics. The parabolic rate constant, i.e., the slope of the curve in Fig. 4b, is 4.7 × 10−14 g2 cm−4 s−1. This is lower than our previous results for the bare and Ni-Fe coated 430SS, whose parabolic rate constants were 7.4 × 10−14 and 7.8 × 10−14 g2 cm−4 s−1, respectively [5]. The decrease in the parabolic rate constant for the Ni-Fe/CrNx coated 430SS demonstrates that the CrNx diffusion barrier can enhance the oxidation resistance of the steel. Fig. 5 gives the surface and cross-sectional morphologies of the NiFe/CrNx coated 430SS after oxidation at 800 °C in air for 200 h. The sample surface presents a dense structure, with grain size of a few hundred nanometers (Fig. 5a). After oxidized for 200 h, the coating remains a multi-layer structure with NiFe2O4, Cr2O3, Ni-Fe-Cr alloy and
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EDS results for the Ni-Fe-Cr alloy layer (point 2) and the substrate beneath the CrNx layer (point 4) are 63.2Fe-22.2Cr-14.6Ni (at%) and 77.2Fe-17.1Cr-5.7Ni (at%), respectively. Slight amounts of Ni can be detected in the substrate beneath the CrNx layer. The Cr element in the Ni-Fe-Cr alloy layer comes from the decomposition of CrNx. The Ni content in the Ni-Fe-Cr layer is higher than that of the substrate, demonstrating that the decomposed CrNx can still hinder the mutual diffusion of elements between the coating and the substrate. Since N elements and a higher Cr content can be detected in the particles (dark contrast) along the grain boundaries of the substrate (Fig. 5(b,d)), these particles may be CrN and/or Cr2N generated from the reaction of Cr with the migrating N from the decomposed CrNx. Fig. 6 gives the surface and cross-sectional morphologies of the NiFe/CrNx coated 430SS after oxidation at 800 °C in air for 500 h. The coating exhibits a similar microstructure to that oxidized for 200 h. The thickness of the Cr2O3 layer remains increasing to up to 2 μm. The original CrNx layer has fully decomposed, with chemical composition of 58.6Cr-35.4Fe-2.7Si-2.3Ni-1Al (at%) for point 6. The Si element in the decomposed CrNx zone may be ascribed to the migrating Si from the substrate and the impurities brought during fabrication. As previous mentioned, since the 430SS used in this work doesn’t contain Al, the Al element in the decomposed CrNx should be the impurities introduced during preparation. The chemical compositions of the Ni-Fe-Cr alloy layer (point 5) and the substrate beneath the decomposed CrNx (point 7) are 70.3Fe-23.4Cr-6.3Ni (at%) and 79.3Fe-15.3Cr-5.4Ni (at%), respectively. By comparison, the concentration of Ni in the Ni-Fe-Cr alloy layer after oxidized for 500 h is lower than that oxidized for 200 h, but the Ni content in the substrate has no significant increase. This illustrates that the decomposed CrNx can still retard the migration of the elements from the Ni-Fe-Cr alloy layer to the substrate and the Ni element beneath the decomposed CrNx layer can rapidly diffuse deep to the internal substrate. Figs. 7 and 8 shows the XRD pattern and SEM morphologies of the Ni-Fe/CrNx coated 430SS after oxidation at 800 °C in air for 1000 h. After an oxidation of 1000 h, the coating still remains a NiFe2O4
Fig. 4. Weight changes of the Ni-Fe/CrNx/430SS oxidized at 800 °C in air and its parabolic plot.
Fig. 5. Surface (a) and cross-sectional (b, c, d) morphologies of Ni-Fe/CrNx/430SS oxidized at 800 °C in air for 200 h. 1602
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Fig. 6. Surface (a) and cross-sectional (b, c, d) morphologies of Ni-Fe/CrNx/430SS oxidized at 800 °C in air for 500 h.
(at%), respectively. The concentration of Ni in the decomposed CrNx zone is lower than those in the nearby Ni-Fe-Cr alloy layer and substrate. This is probably because Ni can react with the latter two phases to form structures with higher thermal stability and/or it can only diffuse along the grain boundaries or defects in the decomposed CrNx zone. ASR of oxide scales The electrical resistance of the metallic substrate is negligible in comparison with that of the oxide scale formed during exposure. Consequently, the oxide scale has a major influence on the ASR for interconnects. Fig. 9 displays the ASR of the Ni-Fe/CrNx coated 430SS oxidized at 800 °C in air as a function of time. The ASR increases over the oxidation time, but its growth rate shows a decreasing tendency. The ASR of the sample increases by nearly 12 mΩ cm2 from an oxidation of 200 h to 1000 h. After oxidation for 1000 h, the ASR for the NiFe/CrNx coated steel reaches to 58.8 mΩ cm2. According to our previous study, the scale ASR for the bare and Ni-Fe coated steels after oxidized for 1000 h are 101 and 72 mΩ cm2, respectively [5]. The decrease in the scale ASR for the Ni-Fe/CrNx coated steel is attributed to the formation of a highly conductive and protective NiFe2O4 top layer and a significantly reduced growth of a poorly conductive Cr2O3 inner layer. The elimination of the pores near the coating/substrate interface may also plays a role in reducing the ASR of the Ni-Fe/CrNx coated steel. However, the coating still exhibits higher ASR values than the required electrical resistances for SOFC interconnect after long-term operation. Assuming that the growth of the Cr2O3 inner layer is diffusion-controlled and obeys parabolic rate kinetics ASR2 = a·t + b during the operation period of 40,000 h (where t is the operation time, a and b are constants), the ASR for the Ni-Fe/CrNx coated steel should be 257 mΩ cm2 after 40,000 h operation, which is higher than the generally accepted upper limit of 100 mΩ cm2 for SOFC interconnect. Accordingly, the properties of the Ni-Fe/CrNx coating, especially the compactness of the CrNx layer, should be further improved.
Fig. 7. XRD pattern of Ni-Fe/CrNx/430SS oxidized at 800 °C in air for 1000 h.
structure, indicating the thermal stability of this spinel phase. Compared with those oxidized for 200 h and 500 h, the grain size on the sample surface increases to about 1 μm after oxidation for 1000 h (Fig. 8a). The coating is generally compact, uniform and well adherent to the substrate. The thickness of the Cr2O3 sublayer grows with oxidation time, and reaches to about 2.6 μm after oxidized for 1000 h. It should be noted that the thicknesses of the Cr2O3 scale for the Ni-Fe coated 430SS is approximately 4 μm after oxidized 800 °C in air for 1000 h, as reported in our previous work [5]. This indicates that the exploitation of the CrNx layer can reduce the growth rate of the Cr2O3 scale. Additionally, The chemical compositions for the Ni-Fe-Cr alloy layer (point 8), the decomposed CrNx zone (point 9) and the base alloy near the coating/substrate interface (point 10) is 73.5Fe-21.0Cr-5.5Ni (at%), 60.0Cr-36.1Fe-1.6Al-1.6Si-0.7Ni (at%) and 82.0Fe-13.5Cr-4.5Ni 1603
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Fig. 8. Surface (a) and cross-sectional (b, c) morphologies of Ni-Fe/CrNx/430SS oxidized at 800 °C in air for 1000 h.
can be NiFe2O4, Fe2O3 and NiO, depending on the atomic ratio of Ni/ Fe) at the upmost surface, but the CrNx inner layer remains intact. Possible reactions of CrNx in air at 800 °C are as follows: 2CrN = 2Cr + N2 (g)
(3)
2CrN + 1.5O2 (g) = Cr2O3 + N2 (g)
(4)
2Cr2N = 4Cr + N2 (g)
(5)
2Cr2N + 3O2 (g) = 2Cr2O3 + N2 (g)
(6)
Based on the thermodynamic data in the HSC6.0 software, the Gibbs free energies for Reactions (3), (4), (5) and (6) are 16.27, −187.20, 21.53 and −386.21 kJ mol−1, respectively, among which only Reactions (4) and (6) can occur spontaneously. Before the sufficient oxidation of the Ni-Fe alloy layer, the Cr atoms in the steel substrate migrate to the unreacted Ni-Fe alloy through the grain boundaries of CrNx, forming a Ni-Fe-Cr alloy layer beneath the Ni-Fe oxide scale. The oxygen pressure at the Ni-Fe oxide scale/alloy interface is sufficiently high for the external oxidation of Cr to Cr2O3. With extended oxidation, Cr diffuses outward from Ni-Fe-Cr alloy to contribute to the formation of a continuous Cr2O3 scale beneath the Ni-Fe oxides. After an oxidation of 5 h at 800 °C, the coating of the Ni-Fe/CrNx coated steel presents a multi-layer structure with NiFe2O4, Cr2O3, Ni-FeCr alloy and undecomposed CrNx from outside to inside, as shown in Fig. 3e. The formation of the continuous Cr2O3 scale blocks the inward migration of O and the outward migration of Ni and Fe, and the CrNx inner layer hinders the mutual diffusion between the Ni-Fe-Cr alloy and the substrate, thus leaving the Ni-Fe-Cr alloy sublayer remain unoxidized. Actually, the Gibbs free energies for the Reactions (3)–(6) are calculated under standard state, and would vary with the change of the state. The doping of the alloy elements, e.g., Fe and/or Mn, to CrNx may boost the decomposition of CrNx to Cr and N2. The diffusion of N2 to deep inside the alloy may also promote Reactions (3) and (5) to proceed in a forward direction. Therefore, the decomposition of CrNx to Cr and N2 may probably serve as another Cr source for the growth of the Cr2O3
Fig. 9. The ASR of the Ni-Fe/CrNx/430SS oxidized at 800 °C in air for various time.
Discussion As previous mentioned, a drawback for the application of (Ni,Fe)3O4 coatings on ferritic stainless steels is the appearance of a large number of pores beneath the oxide scales after oxidation. This is because the inward diffusion of Ni from the coating facilitates the formation of an austenite zone near the coating/substrate interface, and the diffusion coefficients of Ni, Fe and Cr in austenite are significantly smaller than those in ferrite at high temperature. The outward migration of elements from the substrate also boosts the generation of the pores. By comparison, for the Ni-Fe/CrNx coated steel, no pores can be observed near the interface of the CrNx layer and the substrate throughout the experiment. At the initial oxidation stage, the Ni-Fe alloy outer layer reacts with oxygen, forming a continuous oxide layer (possible products 1604
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still serve as an effective barrier against the mutual diffusion of elements between the coating and the substrate. During the experimental duration of 1000 h, no pores could be observed near the scale/substrate interface. The application of the CrNx diffusion barrier beneath the NiFe alloy coating could reduce its oxidation rate to 4.7 × 10−14 g2 cm−4 s−1. The Ni-Fe/CrNx dual layer coating significantly retarded the growth of the less conductive Cr2O3 scale and effectively eliminated the pores beneath the scale, leading to a decreased ASR of the steel.
scale. After an oxidation of 200 h, most CrNx has decomposed. CrNx at the Ni-Fe alloy/CrNx interface interact with O2, forming Cr2O3 particles in the CrNx layer. Since the oxygen pressures for the Cr/Cr2O3 equilibrium is extremely low, the Cr2O3 product can exist stably instead of decomposing into Cr and O2. A slight amount of Ni can be detected in the substrate beneath the scale, with chemical composition of 77.2Fe17.1Cr-5.7Ni (at%) for point 4 (Fig. 5c). According to the phase diagram of the Fe-Cr-Ni ternary alloy at 1000 K (from FactSage database by thermodynamic calculation), alloy with composition of 77.2Fe17.1Cr-5.7Ni (at%) falls into the area quite near the boundary line between the α + γ phase region and the γ phase region. The phase transformation line of the phase diagram is calculated under the condition that the Gibbs free energies of the parent phase (Gα) and the new phase (Gγ) are identical. Actually, the thermodynamic condition for phase transformation is the free energy difference (△Gα→γ) of the two phases. A larger △Gα→γ contributes to a higher phase transformation driving force. Another requirement for the transition from α to γ is to overcome the phase transformation barrier caused by overcoming the attractive forces among atoms during lattice reorganization. These obstacles would make the γ single phase region in the actual Fe-Cr-Ni phase diagram smaller than the thermodynamic calculated one. Therefore, unlike the Ni-Fe coated steel [5], the phase structure of the substrate right beneath the scale would contain large amounts of ferrite for the Ni-Fe/CrNx coated alloy. Further oxidation to 500 h makes the CrNx fully decomposed, but the Ni content in the substrate near the decomposed CrNx remains almost unchanged. Even after an oxidation of 1000 h, the Ni content in the substrate is still very low, with a value of only 4.5 at% for point 10 (Fig. 8c). It can be concluded that either before or after decomposition, the CrNx layer plays an important role in reducing the diffusion rate of Ni to the substrate and eliminating its enrichment beneath the CrNx layer. Since the Ni content of the substrate beneath the scale remains a low value, which is not enough to maintain a large amount of austenite, the point defects introduced by the enormous difference in the diffusion coefficients of austenite and ferrite can be drastically decreased. The employment of CrNx diffusion barrier can not only reduce the interface defects, but also decrease the growth rate of Cr2O3. As previously mentioned, the rate constant for the Ni-Fe/CrNx coated steel during the experimental duration of 1000 h is 4.7 × 10−14 g2 cm−4 s−1, nearly half of that for the Ni-Fe coated steel of 7.8 × 10−14 g2 cm−4 s−1. For the oxidized Ni-Fe/CrNx/430SS, the Cr content in the Ni-Fe-Cr alloy layer beneath the Cr2O3 scale is higher than that in the 430SS substrate, so the low growth rate of Cr2O3 in this circumstance is not caused by a decreased Cr content in the nearby region, but probably resulted from the interference of N. The N atoms derived from the decomposed CrNx can migrate inward to the substrate, depositing along the grain boundaries by forming CrNx and/or Cr2N particles with the Cr atoms. It can be accordingly inferred that the N atoms generated from the decomposed CrNx migrate outward to the NiFe-Cr alloy layer, and inhibit the diffusion of Cr to the Cr2O3 scale during oxidation.
Acknowledgement This work was supported by the National Natural Science Foundation of China under the Grant Nos. 51601202 and 51471179. References [1] Tariq S, Marium A, Raza R, Ahmad MA, Khan MA, Abbas G, et al. Comparative study of Ce0.80Sm0.20Ba0.80Y0.20O3-δ (YB-SDC) electrolyte by various chemical synthesis routes. Results Phys 2018;8:780–4. [2] Szymczewska D, Chrzan A, Karczewski J, Molin S, Jasinski P. Spray pyrolysis of doped-ceria barrier layers for solid oxide fuel cells. Surf Coat Technol 2017;313:168–76. [3] Kumar SS, Nalluri A, Anandan C, Prakash BS, Aruna ST. Deposition and evaluation of Mn-Co oxide protective sputtered coating on SOFC interconnects and current collectors. J Electrochem Soc 2016;163:F905–12. [4] Montero X, Tietz F, Sebold D, Buchkremer HP, Ringuede A, Cassir M. MnCo1.9Fe0.1O4 spinel protection layer on commercial ferritic steels for interconnect applications in solid oxide fuel cells. J Power Sources 2008;184:172–9. [5] You PF, Zhang X, Zhang HL, Liu HJ, Zeng CL. Effect of CeO2 on oxidation and electrical behaviors of ferritic stainless steel interconnects with Ni-Fe coatings. Int J Hydrogen Energy 2018;43:7492–500. [6] Bateni MR, Wei P, Deng XH, Petric A. Spinel coatings for UNS 430 stainless steel interconnects. Surf Coat Technol 2007;201:4677–84. [7] Zhang X, You PF, Zhang HL, Yang XG, Luo MQ, Zeng CL. Preparation and performances of Cu-Co spinel coating on ferritic stainless steel for solid oxide fuel cell interconnect. Int J Hydrogen Energy 2018;43:3273–9. [8] Shaigan N, Qua W, Iveyb DG, Chen WX. A review of recent progress in coatings, surface modifications and alloy developments for solid oxide fuel cell ferritic stainless steel interconnects. J Power Sources 2010;195:1529–42. [9] Molin S. Evaluation of electrodeposited Mn-Co protective coatings on Crofer 22 APU steel. Int J Appl Ceram Technol 2017;15:349–60. [10] Petric A, Ling H. Electrical conductivity and thermal expansion of spinels at elevated temperatures. J Am Ceram Soc 2007;90:1515–20. [11] Hiroshi N, Ken O. Effect of TiO2 on the sintering and the electrical conductivity of Cr2O3. J Am Ceram Soc 1989;72:400–3. [12] Goebel C, Fefekos A, Svensson J, Froitzheim J. Does the conductivity of interconnect coatings matter for solid oxide fuel cell applications. J Power Sources 2018;383:110–4. [13] Liu Y, Chen DY. Protective coatings for Cr2O3-forming interconnects of solid oxide fuel cells. Int J Hydrogen Energy 2009;34:9220–6. [14] Geng SJ, Qi SJ, Zhao QC, Zhu SL, Wang FH. Electroplated Ni-Fe2O3 composite coating for solid oxide fuel cell interconnect application. Int J Hydrogen Energy 2012;37:10850–6. [15] You PF, Zhang X, Zhang HL, Liu HJ, Zeng CL. Oxidation behavior of NiFe2O4 spinel coated interconnects in wet air. Oxid Met 2018;90:499–513. [16] LeClaire AD, Neumann G. Diffusion in solid metals and alloys. Germany: Springer; 1990. p. 7. [17] Hovsepian PE, Ehiasarian AP, Purandare YP, Mayr P, Abstoss KG, Feijoo M, et al. Novel HIPIMS deposited nanostructured CrN/NbN coatings for environmental protection of steam turbine component. J Alloys Compd 2018;746:583–93. [18] Panjan P, Drnovšek A, Kovač J, Gselman P, Bončina T, Paskvale S, et al. Oxidation resistance of CrN/(Cr, V)N hard coatings deposited by DC magnetron sputtering. Thin Solid Films 2015;591:323–9. [19] Feng ZJ, Zeng CL. Oxidation behavior and electrical property of ferritic stainless steel interconnects with a Cr-La alloying layer by high-energy micro-arc alloying process. J Power Sources 2010;195:7370–4. [20] Lee SH, Pukha VE, Vinogradov VE, Kakati N, Jee SH, et al. Nanocomposite-carbon coated at low-temperature: a new coating material for metallic bipolar plates of polymer electrolyte membrane fuel cells. Int J Hydrogen Energy 2013;38:14284–94. [21] Bi FF, Yi YY, Zhou T, Peng LF, Lai XM. Effects of Al incorporation on the interfacial conductivity and corrosion resistance of CrN film on SS316L as bipolar plates for proton exchange membrane fuel cells. Int J Hydrogen Energy 2015;40:9790–802.
Conclusion A compact CrNx layer was deposited on 430SS using multi-arc ion plating, followed by electrodeposition of a Ni-Fe alloy coating on CrNx/ 430SS. An adhesive NiFe2O4 top layer could be obtained after oxidizing the 1.6 μm thick Ni-32 at%Fe coated sample. After oxidized at 800 °C in air for 5 h, the Ni-Fe/CrNx coated steel presented a multi-layer scale with NiFe2O4, Cr2O3, Ni-Fe-Cr alloy and CrNx from outside to inside. The CrNx layer gradually decomposed after further oxidation, but could
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