- Email: [email protected]
Manufacture of Co-containing coating on AISI430 stainless steel via pack cementation approach for SOFC interconnect
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 3 2 8 e3 0 3 3 8
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Manufacture of Co-containing coating on AISI430 stainless steel via pack cementation approach for SOFC interconnect Kun Liu a,1, Shuai Xu a,1, Fei Teng a, Jie Yang a, Jianzhong Man a, Jinlong Cui b,**, Juncai Sun a,* a
Key Lab of Ship-Machinery Maintenance & Manufacture, Dalian Maritime University, Dalian 116026, China School of Chemistry and Chemical Engineering, Inner Mongolia University of Science & Technology, Baotou 014010, China
b
highlights Co-contained coating is made on surface of 430 SS interconnect by pack cementation. CoFe2O4 spinel coating limits inward diffusion of O2 and outward diffusion of Cr2þ. Coated samples with 2 h and 2% have better oxidation resistance. Coated sample with 2 h has a weight change of 0.231 mg/cm2 after 650 h of oxidation. The area specific resistance of coated sample with 2% is 90.21 mU cm2 after 450 h.
article info
abstract
Article history:
Stainless steel can be applied as interconnect materials in solid oxide fuel cells (SOFCs) at
Received 31 July 2019
operating temperatures 600e800 C. Chromium (Cr)-forming stainless steel as an inter-
Received in revised form
connect plate possesses a low oxidation resistance at high temperature and electrical
14 September 2019
conductivity, and volatility of Cr oxide scale can poison the cathode material. One effective
Accepted 18 September 2019
strategy is to use a surface coating to improve interconnect performance. This work is to
Available online 18 October 2019
form cobalt (Co)-containing coatings on the surface of AISI 430 ferritic stainless steel interconnect via pack cementation approach. The resultant coating is extremely effective
Keywords:
at heightening the oxidation resistance and electrical conductivity of AISI 430 ferritic
Solid oxide fuel cells
stainless steel. The area specific resistance of samples was measured as a function of time.
Interconnect
The area specific resistance of coated sample with 2% of activator content and holding time
Cobalt-containing coatings
of 2 h is 90.21 and 108.32 mU cm2 after 450 h of oxidation in air, respectively. Additionally,
AISI 430 ferritic stainless steel
the coated sample with 2% of activator content and holding time of 2 h has a weight change
Pack cementation approach
of merely 0.299 and 0.231 mg/cm2 after 650 h of isothermal oxidation at 800 C, separately.
CoFe2O4 spinel coating
The results displayed that the formation of CoFe2O4 spinel coating enhanced oxidation resistance by inhibiting the outward diffusion of Cr cations and the inward diffusion of oxygen anions. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. ** Corresponding author. Linghai Road 1#, Gaoxin District, Dalian maritime University, Dalian 116034, China. E-mail addresses: [email protected] (J. Cui), [email protected] (J. Sun). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.ijhydene.2019.09.144 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 3 2 8 e3 0 3 3 8
30329
Introduction Solid oxide fuel cell (SOFC) is identified as an appealing candidate material for energy conversion owing to its higher electrical efficiency and decreased emissions from the traditional energy-conversion devices [1e3]. Minimizing the operating temperature of SOFC to 600e800 C results in the possibility of applying cost-effective stainless steel as replacements rather than conventional lanthanum chromate ceramics [4,5]. At such a low temperature, more environmentally viable and mechanically sturdy materials can be applied [6]. Ferritic stainless steels are deemed as be among the most prospect candidate alloys thanks to their excellent mechanical behavior, higher electrical conductivity and low cost [7e9]. Nevertheless, inevitable high temperature oxidation for the cathode operating environment and the incessant growth of Cr-rich oxide scales at the metallic interconnects surface can bring not solely about an increase in the electrical resistance but also lead to the possibility of potential spallation [10,11]. In addition, Cr2O3 can react with water molecules by shaping volatile compounds. These Cr-containing species can poison the cathode material and result in cathode degradation [12e14], which seriously restricts practical application of the SOFC. In order to deal with the above impediments, pack cementation technique is the most efficacious method for heightening interconnect behaviors, which can be employed a surface coating to enhance conductivity, decreased scale growth and the formation of volatile Cr. Recently, more researchers have absorbed in the application of conductive oxide and protective coatings. For instance, Saeidpour et al. have reported an electroplating technique for the preparation of Co and CoeZrO2 coatings [15]. Zhao et al. used FeCoNi coating on SUS 430 stainless steel via magnetron sputtering, followed by high temperature oxidation to transform the FeCoNi coating to (Fe,Co,Ni)3O4 spinel oxides [16]. Waluyo et al. coated Mn2CuO4 on metallic interconnects (Cr ofer 22 APU) by a plasma spray (PS) process [17]. Tsai et al. manufactured La0.6Sr0.4Co0.2Fe0.8O3 protective coatings with screen printing method [18]. Several techniques have been exploited to employ spinel coatings to ferritic stainless steels, including electrodeposition [15,19], magnetron sputtering [16,20], plasma spraying [17,21], screen printing [18,22] and pack cementation [23]. Among them, pack cementation approach has captured tremendous interests due to its inexpensive cost, reliable, simplicity of preparation as well as excellent adherence between the coating and substrate [24]. In this method, Co powder reacts with ammonium chloride (NH4Cl) to form halides at high temperature, halide reacts with the substrate to deposit Co on the substrate and then the Co forms intermetallic compounds with substrate at high temperature. Schematic representation of the pack cementation cobaltizing process as exhibited in Fig. 1. To our best knowledge, there are rare reports about Co-containing coatings used as SOFC interconnects via pack cementation approach to prevent the Crvolatilization, enhance oxidation behavior and electrical conductivity of AISI 430 ferritic stainless steel.
Fig. 1 e Schematic representation of the pack cementation cobaltizing process.
The objective of the current investigation is to contrast the effects of four holding times (1h, 2h, 3h and 5h) and three activator (NH4Cl) contents (1%, 2% and 3%) on Co-containing coatings on the AISI 430 ferritic stainless steels substrate. The Co-containing coatings are manufactured by pack cementation approach. The effects of Co-containing coatings on the isothermal oxidation and cyclic oxidation were detailly studied. Additionally, the electrical property was assessed as a function of time to evaluate the area specific resistance (ASR)
30330
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 3 2 8 e3 0 3 3 8
under different holding times and different NH4Cl contents, separately.
Experimental sections Preparation of samples The commercial AISI 430 stainless steel sheet was cut in 10 mm 10 mm 2 mm as the substrate and the composition was listed in Table 1. Each sample was ground with SiC paper of 320-grit to 2000-grit and polished, then cleaned in an ultrasound acetone bath and dried in room temperature. In the current research, the pack cementation method was used to deposit Co onto the substrate. The powder mixture contained Al2O3 (inert filler), NH4Cl (activator) and Co powder (deposition source). Subsequently, the as-prepared samples were buried in powder mixture in a quartz tube, the quartz tube was plugged with a quartz plug and sealed with clay, then heated at 800 C for different of holding time in a muffle furnace (in air) and cooled to ambient temperature naturally. To optimize the quality of the formed coating, the two valid factors of the pack cementation process were studied by altering the factors, the two factors included holding time (1 h, 2 h, 3 h and 5 h) and activator content (1%, 2% and 3%). Generally speaking, inappropriate amount of activator led to many voids to the coating or deposited difficulties on the surface of stainless steel, which caused the lower oxidation resistance and higher values of ASR. In addition, higher holding time can result in thicker coating or form a brittle layer. Here, Co-containing coating sample with holding time of 2 h and activator content of 2% was named as AS-2h, AS2%, respectively. For comparison, the Co-containing coating samples with holding time of 1 h, 3 h and 5 h, defined as AS1h, AS-3h and AS-5h, separately, and the obtained Cocontaining coating samples were labeled as AS-1% and AS3%, corresponding to the added activator content of 1% and 3%, respectively.
Oxidation testing The isothermal oxidation was conducted at 800 C for 650 h in static air. The bare and coated samples were placed in a 15 ml corundum crucible and this crucible was placed in another bigger corundum crucible with volume of 50 ml. Crucibles were taken out every 25 h, samples were weighted with the smaller corundum crucible in order to avoid the change of mass when sample was moved during weighting process. In addition, the coated with different holding time (1, 2, 3 and 5 h) and uncoated samples were submitted to cyclic oxidation test. 50 cycles were exerted and each cycle consisted of repeating of heating and cooling at room temperature.
Table 1 e Chemical composition of AISI 430 ferritic stainless steel (wt.%). Wt% AISI 430
Cr
Mn
Ni
C
Si
P
Fe
16 .53
0.33
0.12
0.16
0.58
0.03
Bal
Area specific resistance measurements To study the electrical properties of samples, the area specific resistance (ASR) was accomplished by pseudo four-point DC method with a constant current density of 100 mA. Platinum (Pt) wires and meshes were applied to bond the samples and test instrument, and silver paste was painted on the surface of the sample to make it fully contact with the Pt meshes. Samples were measured at 800 C for 450 h in static air and the voltage date was recorded every 25 h. Fig. 2 shows the schematic diagram of the setup applied for ASR measurement.
Characterization of coating properties The scanning electron microscopy (SEM, SUPRA 55 SAPPHIRE, Germany)showed the cross section and surface morphology of coated samples, and the chemical composition and cross sectional composition profile were analyzed by energy dispersive spectroscopy (EDS, X-Max/OXFORD, UK). The phases of the coated samples were analyzed by X-ray diffraction (XRD, Rigaku-D/MAX-3A) using a Co-Ka radiation source (l ¼ 0.1789 nm).
Results and discussion Effect of holding time on coating Phases and morphologies of coated samples Fig. 3 is the XRD patterns of the coated samples prepared at different holding time. It can be found from Fig. 3 that AS-1h, AS-2h, AS-3h and AS-5h samples have similar diffraction peaks. In addition, there are four phases Co, Cr2O3, CoFe2O4 and Co3Fe7 except the substrate phase, which may correspond to the main components of the coating. The existence of Co3Fe7 phase is caused by the chemical reaction of Co and Fe at high temperature. However, the appearance of O element in CoFe2O4 and Cr2O3 phases is due to the fact that a part of the oxygen in air participates in the reaction when samples are heated in a muffle furnace. Fig. 4 presents SEM images and corresponding composition profile along the cross section of coated sample at different holding time (1 h, 2 h, 3 h and 5 h) after chemical cobaltizing treatment. Fig. 4a shows that the coating of AS-1h sample is uniformly distributed and possesses about 10 mm thickness. Additionally, it can be seen from Fig. 4a that Cr2O3 oxide layer has not been formed completely. According to the corresponding cross sectional composition profile, the content of Co element gradually decreases from the surface to the surface at a distance of 6 mm. However, the Cr element has been gathered at coating/substrate interface, and the O element possesses a high content in the coating but does not enter the substrate. When the holding time is increased to 2 h, the coating of AS-2h sample is uniform and dense (Fig. 4b). The total thickness of the coating is added to 20 mm, including a 0e6 mm CoFe2O4 spinel outlayer and a 6e20 mm Cr2O3 oxide inner layer. It can be seen from cross sectional composition profile that the content of Cr element near the coating is significantly lower than the average content of Cr element in the substrate, indicating that the Cr element migrates
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 3 2 8 e3 0 3 3 8
30331
Fig. 2 e Schematic diagram of samples for ASR measurement.
Fig. 3 e The XRD patterns of AS-1h, AS-2h, AS-3h and AS5h samples.
outward to form a Cr2O3 oxide layer. When the holding time is prolonged to 3 h, the AS-3h sample thickens obviously (approximately 30e40 mm), and it is divided into CoFe2O4 spinel outlayer and Cr2O3 oxide innerlayer (Fig. 4c). The thickness of the outer Co-containing coating is about 16e30 mm, and the main components are Co and CoFe2O4, which is well consistent with the result of XRD analysis and the corresponding cross sectional composition profile. Whereas, the thickness of the inner Cr2O3 oxide layer increases significantly (about 8e10 mm) compared with Fig. 4b. In general, the thickness of the coating is not uniform, and there are some voids at coating/substrate interface. By contrast with other samples, the thickness of the coating of AS-5h sample is further increased to approximately 60 mm (Fig. 4d). From the combined results of XRD (Fig. 3) and corresponded to cross sectional composition profile, at a distance of 0e40 mm from the surface, the main phases are Co and CoFe2O4, and a large number of Cr and O elements can be observed at the distance of 50e60 mm from the surface, combined with the XRD pattern, which is the Cr2O3 oxide layer.
However, a brittle layer with obvious cracks can be observed at Cr2O3 oxide layer/substrate interface, according to the corresponding cross sectional composition profile, it is found that a large amount of Si elements are accumulated at coating/substrate interface, which makes that cracks were easily formed. From the results above SEM cross sectional images, we can observe that there are some tiny voids at coating/substrate interface in four samples, and the longer the holding time was, the more obvious the void was, which may be ascribed to the outward diffusion of Cr cations in the substrate during the growth of the oxide film, leaving a large number of vacancies. Meanwhile, as the holding time increased, the thickness of the layer will gradually increased, but if the holding time was too long, the excessive thickness of the coating can cause the poor conductivity of the sample. Moreover, too thick coating was prone to cracking and protrusion during the high-temperature heating process, which may eventually result in the exfoliation of the coating. When the holding time was too long (5 h), the Si element will accumulate at coating/substrate interface to form a brittle layer, which was because the oxide brittleness of Si was extremely enormous and seriously destroyed the adherence between the coating and the substrate. Fig. 5 exhibits the surface morphology of the coating for 1 h, 2h and 5 h. After 1 h of treatment (Fig. 5a), the surface of the AS-1h sample is uniform and flat, without obvious cracks or spallation. It can be observed that the surface is a fine particle phase with a particle size of less than 1 mm. In addition, the coating of the AS-1h sample is not completely formed and is less than 10 mm in SEM cross sectional image (Fig. 4a), and the depth of the Co element diffusing into the substrate is relatively low. When the holding time is prolonged to 2 h (Fig. 5b), the surface of the AS-2h coating is flat and particles grows up to about 1 mm compared with Fig. 5a. There are no defects such as cracks and spallation. Additionally, it can be seen from the SEM cross section image (Fig. 4b) that the voids of the surface do also not appear in the deeper areas of the coating. Compared with AS-2h sample (Fig. 5b), the particles on the surface of AS-3h and AS-5h samples continue to grow, but the morphology does not change significantly. The surface
30332
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 3 2 8 e3 0 3 3 8
Fig. 4 e SEM cross sectional images of (a) AS-1h, (b) AS-2h, (c) AS-3h and (d) AS-5h samples, and corresponding to cross sectional composition profile.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 3 2 8 e3 0 3 3 8
30333
Fig. 5 e SEM micrographs of coating for (a) 1 h, (b) 2 h, (c) 3 h and (d) 5 h, (e) EDS point analysis of AS-2h sample.
particle size of sample with holding time of 3 h is about 2 mm (Fig. 5c), while that of sample with holding time of 5 h is approximately 2.5e3 mm. There are no cracks and spallation on the surfaces of AS-3h and AS-5h samples. However, as the holding time increases, the surface appears looser and the voids become more obvious, which is in well accordance with SEM cross section images of coating (Fig. 4). The EDS point analysis image of AS-2h sample displays the presence of Co, Fe, Cr and O elements (Fig. 5e), which is the same result as that of the XRD displayed in Fig. 3. It can be seen that a small amount of Cr element appears on the surface, which is attributed to the diffusion of Cr element in the substrate to the surface. The presence of the O element is due to the fact that the entire experiment is carried out in a box-type resistance furnace and there is no protective gas. The quartz tube containing the sample is basically in static air. Although the canal orifice is sealed, it cannot completely prevent the entry of air.
Oxidation behavior An isothermal oxidation experiment was carried out in static air at 800 C for 650 h. Fig. 6a portrays the specific weight gain as a function of time for uncoated, AS-1h, AS-2h, AS-3h and AS-5h samples. The initial oxidation rate is strikingly high for the uncoated sample. This is because of the bare substrate in uncoated sample, which oxidizes freely. In all samples, weight gain increases parabolically with the isothermal oxidation time. By contrast with uncoated sample, AS-1h, AS-2h, AS-3h and AS-5h samples display smaller mass gain at all times, which suggests that the Co-containing coating hinders the diffusion of oxygen and limits the growth of the Cr2O3 oxide layer, and enhances the oxidation resistance of the samples at high temperature. The uncoated sample has a weight change
of 1.613 mg/cm2 after 650 h of isothermal oxidation, while the AS-1h, AS-2h, AS-3h and AS-5h samples has a weight change of 0.542 0.299, 1.102 and 1.353 mg/cm2, respectively. For AS-3h and AS-5h samples, the weight gain is still fast after 150 h of isothermal oxidation, this is due to the cracks and even spallation of the coating in the long-term high temperature oxidation, so that the coating loses the role of protecting the substrate. Whereas the weight gains of AS-1h sample is higher than AS-2h sample. Therefore, by above investigation, AS-2h sample has the highest oxidation resistance in five samples. To further determine the oxidation kinetics is parabolic, the oxidation kinetics curves of the all samples are shown in Fig. 6b. It can be calculated from Fig. 6b that the parabolic rate constants of uncoated, AS-1h, AS-2h, AS-3h and AS-5h samples are 7.35 104, 2.10 105, 6.38 105, 3.03 104 and 5.67 104 mg2 cm4 h1, respectively, which fulfills Eq. (1): Dm2 ¼ Kp ,t A
(1)
Where in Eq. (1): Dm is mass gain, A is the sample surface area, Kp is the parabolic rate constant and t is the oxidation time [25]. The uncoated, AS-1h, AS-2h, AS-3h and AS-5h samples were subjected to cyclic oxidation in static air at 800 C. 50 cycles were exerted and each cycle composed of repeating of heating and cooling at room temperature. Fig. 7 delivers the weight gain of samples as a function of cycle number. In all of the cycles, compared with the uncoated sample, the coated samples have less weight gain except AS-5h sample in 25th cycle. This is due to protective CoFe2O4 coating that restricts the oxidation reactions. In addition, due to the excessive thickness of the AS-5h sample, after 15 cycles, the coating
30334
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 3 2 8 e3 0 3 3 8
Fig. 6 e (a)The weight gain and (b) oxidation kinetics for uncoated, AS-1h, AS-2h, AS-3h and AS-5h samples.
Fig. 7 e Specific weight gain as a function of cycle number during cyclic oxidation.
Fig. 8 e ASR parameter evolution (for 450 h)of uncoated,AS1h, AS-2h, AS-3h and AS-5h samples.
breaks down, which results in direct contact between substrate and oxygen in air, the oxide layer increases sharply, and the weight gain of the sample rapidly increases to a higher level. Compared with the weight gain value of uncoated (1.88 mg/cm2), AS-1h (1.01 mg/cm2), AS-3h (1.26 mg/cm2) and AS-5h samples (1.71 mg/cm2), AS-2h sample has a low weight gain of only 0.65 mg/cm2 after 50 cycles. The result further verifies that AS-2h sample has a high oxidation resistance.
because the presence of the coating decreases the diffusion rate of the Cr and O element. In the late stage of oxidation (150e450 h), the ASR values of all samples increase slowly, especially for AS-2h sample, the ASR value maintains stable with only a mild increase. According to the above results, the ASR values of the samples with coating are lower that of the uncoated sample during the whole oxidation stage, which may be attributed to the fact that CoFe2O4spinel layer can reduce ASR of AISI 430 ferritic stainless steel [29]. Based on the above analysis, it can be verified that AS-2h sample displays much better conductivity compared with other samples.
ASR measurements The ASR for uncoated, AS-1h, AS-2h, AS-3h and AS-5h samples, as a function of time, are presented in Fig. 8. After 450 h of oxidation, ASR parameters for uncoated, AS-1h, AS-2h, AS3h and AS-5h samples are 1786.31, 482.36, 108.32, 230.62 and 384.34 mU cm2, respectively. It is universally known that a small ASR parameter reveals a low resistance and high electrical conductivity [7,26,27]. In the initial stage of oxidation (0e150 h), the growth of ASR parameters is obvious. That is because the oxide scale shaped on the stainless steel surface peeled off partially [28]. The ASR values growth rate of four samples are much lower than that of uncoated sample, this is
Effect of activator (NH4Cl) content on coating Phases and morphologies of coated samples Fig. 9 shows the XRD patterns of the coating with different NH4Cl contents in the pack mixture. When the content of NH4Cl is 1%, XRD patterns of the coatings compose of peaks resulting from the Co3Fe7 and CoCr2O4 phases. No Co peaks can be observed, which may be ascribed to the amount of HCl decompose from the NH4Cl is extremely small when the
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 3 2 8 e3 0 3 3 8
Fig. 9 e XRD patterns of samples with different NH4Cl contents in the pack mixture.
content of NH4Cl is low, and it is rapidly consumed during the pack cementation process. Hence, no sufficient Co elements deposite on the surface of AISI 430 substrate. At the same time, oxygen is inevitably involved in the reaction because the pack cementation process is carried out in air. Finally, a part of Co element on the surface of the sample and Cr element diffuses from the substrate as well as O element in air form CoCr2O4 phase, whereas another part of Co element and Fe element in the substrate form CoeFe intermetallic compound (Co3Fe7), and eventually Co element is consumed completely so that there is no pure Co phase in the coating. The XRD peaks of the coating formed after the addition of 2% NH4Cl are identified as the Co, CoFe2O4 and Co3Fe7 phases, indicating the content of NH4Cl meets the requirement of cobaltizing. This result indicates that the content of NH4Cl (2%) is appropriate so that sufficient Co element can be deposited on the surface of the AISI430 substrate. At the same time, CoFe2O4 and Co3Fe7 are shaped by the reaction of Co in the coating and Fe in the substrate. When the content of NH4Cl is increased to 3%, the XRD peaks of the coating are consisted of Co and Co3Fe7 phases, which may be attributed to the fact that when the content of NH4Cl in the pack mixture continues to increase to 3%, more NH3 and HCl gases are decomposed. These gases are filled with quartz tube, resulting in the outflow of residual air in the pack mixture. Therefore, there is no oxygen element in the coating. SEM images of coating cross-section and corresponding to element depth profiles tested by EDS of samples with different NH4Cl contents in the pack mixture are shown in Fig. 10. SEM image of AS-1% sample (Fig. 10a) displays uniform coating with an average coating thickness of 2e3 mm. No distinct cracks or spallation were found. According to the EDS point analysis, it can be observed that the content of Co element in the coating is lower than the other samples, but the content of Cr element is higher. Additionally, the content of Cr element decreases gradually from inside to outside and exists in the whole coating, this is because the coating is too thin, the outward diffusion of Cr element is relatively easy, which can not restrict the volatilization of Cr element. When the content
30335
of NH4Cl is increased to 2% (Fig. 10b), the coating thickness of AS-2% sample is uniform, about 15 mm. It can be clearly seen that the Cr2O3 oxide layer appears at coating/substrate interface, and the main elements are Co, Fe and O in the coating, which is in consistent with the XRD patterns (Fig. 9). There is only a small amount of Cr element in the interior of the coating, indicating that as the content of NH4Cl increases, the amount of Co element deposited on the surface of the substrate increases, and the outward diffusion of Cr element is limited at coating/substrate interface. It can be found from Fig. 10c that there is still Cr2O3 oxide layer at coating/substrate interface, but it is extremely uneven and has more voids. The Cr element is enriched around the Cr2O3 oxide layer, and there is not a large amount of Cr elements passing through the coating to reach the surface. Additionally, the thickness of coating and the content of Co, Fe and O elements in the coating are similar to those in Fig. 10b. As stated above, when the content of NH4Cl increases from 2% to 3% in the pack mixture, the coating does not continue to thicken, but the interface becomes uneven and the coating has more voids, which may result in uneven stress at coating/substrate interface. And it is prone to cause stress concentration, resulting in cracks inside the coating, thus reduces the adherence at coating/substrate interface. In addition, a small amount of NH4Cl (1%) makes it difficult for Co to deposit in stainless steel.
Oxidation behavior Fig. 11 shows the weight gain curve of AS-1%, AS-2% and AS3% samples during isothermal oxidation. For AS-2% sample, the increasing rate of weight gain is lower than those of AS-1% and AS-3% samples during the whole oxidation time. AS-1% sample exhibits a weight gain of 0.916 mg/cm2 after 650 h of isothermal oxidation, while theAS-2% and AS-3% samples display a weight gain of 0.231 and 0.351 mg/cm2, separately. Due to the thin coating of the AS-1% sample (Fig. 10a), the rate of oxygen passing through the coating is faster, resulting in more severe oxidation of the substrate. Additionally, the weight gain of AS-2% sample is significantly lower than that of AS-3% sample, which indicates that the AS-2% sample possesses better oxidation resistance [30].
ASR measurements In order to investigate the effect of the content of NH4Cl on ASR, the ASR was tested for AS-1%, AS-2% and AS-3% samples. Fig. 12 illustrates the ASR as a function of time. ASR values for AS-1%, AS-2% and AS-3% samples are equal to 221.54, 90.21, 321.35 mU cm2, separately. From the above results, it can be seen that the AS-2% sample exhibits much better electrical conductivity than other samples. From these results above weight gain and ASR, we can see that the AS-2h and AS-2% sample have the best high temperature oxidation resistance and electrical conductivity. The ASR and weight gain results of samples are shown in Table 2. Additionally, the ASR value of the coated sample is much lower than that of the uncoated sample. A possible interpretation is that the CoFe2O4 spinel coating forms on the bare substrate and can effectively reduce the ASR of the AISI 430 ferretic stainless steel. The small ASR of the coated sample is assigned to the formation of a greatly conductive and
30336
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 3 2 8 e3 0 3 3 8
Fig. 10 e SEM micrographs of coating cross section and corresponding to element depth profiles tested by EDS for (a) AS-1%, (b) AS-2% and (c) AS-3% samples.
protective CoFe2O4 spinel coating [10]. The main component of the Co-containing coating is CoFe2O4 spinel. The electrical conductivity of the spinel emerged by hopping of charge between octahedral sites, which has been described in Ref. [31].
Therefore, the main conductive mechanism of CoFe2O4 spinel coating can be interpreted in Eq. (2). Co2þ¼Co3þþe
(2)
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 3 2 8 e3 0 3 3 8
30337
Conclusion
Fig. 11 e Specific weight gain for AS-1%, AS-2% and AS-3% samples as a function of time during isothermal oxidation.
In this study, AISI 430 ferretic stainless steel with Cocontaining coating was fabricated by a pack cementation method in static air for 800 C. When the holding time and activator content were 2 h and 2%, respectively, AS-2h and AS2% sample had the better high temperature oxidation resistance and electrical conductivity than those of coated and uncoated samples. The oxidation performance, EDS as well as SEM and ASR results offered ensured proof that the oxidation resistance and electrical conductivity. Firstly, the coating of AS-2h and AS-2% sample were extremely dense and uniform as well as smooth compared with other coated samples. Secondly, the value of 90.21 mU cm2 was obtained for AS-2% sample in static air, whereas the ASR value of 108.32 mU cm2 was obtained for AS-2h sample after oxidized for 450 h. In addition, the weight gain of AS-2% sample was 0.299 mg/cm2, the AS-2h sample had a weight change of 0.231 mg/cm2 after 650 h of isothermal oxidation for 800 C in static air. Finally, the CoFe2O4 coating could validly hinder the Cr-diffusion to shape a Cr-rich layer existing between coating and substrate, thus averted the volatilization of Cr and decreased the degradation of SOFC performance. Through the above results, the Co-containing coating prepared in static air exhibits great potential as a SOFC interconnect in the future.
Acknowledgements
Fig. 12 e ASR values as a function of time for AS-1%, AS-2% and AS-3% samples.
It is usually considered that the CoFe2O4 spinel coating can conduct electricity through transition of charge between Co2þ and Co3þ ions on octahedral sites. This suggests that a relative higher Co content is benefit for the improvement of electrical conductivity. Those results demonstrate that the CoFe2O4 spinel coating distinctly improves the electrical conductivity of stainless steel and is more suitable as a SOFC interconnect coating.
Table 2 e The ASR and weight gain results of samples. Sample description
Values Weight gain (mg/cm2) ASR (mU cm2)
uncoated AS-1h AS-2h AS-3h AS-5h AS-1% AS-2% AS-3%
1.613 0.542 0.299 1.102 1.353 0.916 0.231 0.351
1786.31 482.36 108.32 230.62 384.34 221.54 90.21 321.35
This work is financially supported by the National Foundation of Natural Science of China (No. 51479019, 51962027 and 21476035), the National Key Research and Development Program of China (No. 2016YFB0101206), the Fundamental Research Funds for the Central Universities (No. 3132019328), and the Project of Science Foundation of the Educational Department of Inner Mongolia (NJZY19135).
references
[1] Talica B, Molinb S, Wiika K, Hendriksenb PV, Leina HL. Comparison of iron and copper doped manganese cobalt spinel oxides as protective coatings for solid oxide fuel cell interconnects. J Power Sources 2017;372:145e56. [2] Bateni MR, Wei P, Deng XH, Petric A. Spinel coatings for UNS 430 stainless steel interconnects. Surf Coat Technol 2007;201:4677e84. [3] Mah JCW, Muchtar A, Somalu MR, Ghazali MJ. Metallic interconnects for solid oxide fuel cell: a review on protective coating and deposition techniques. Int J Hydrogen Energy 2017;42:9219e29. [4] Choi JP, Weil KS, Chou YM, Stevenson JW, Yang ZG. Development of MnCoO coating with new aluminizing process for planar SOFC stacks. Int J Hydrogen Energy 2011;36:4549e56. [5] Yang Z, Xia GG, Li XH, Stevenson JW. (Mn, Co)3O4 spinel coatings on ferritic stainless steels for SOFC interconnect applications. Int J Hydrogen Energy 2007;32:3648e54. [6] Sreedhar I, Agarwal B, Goyal P, Singh SA. Recent advances in material and performance aspects of solid oxide fuel cells. J Electroanal Chem 2019;848:113315e50.
30338
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 3 2 8 e3 0 3 3 8
[7] Lenka RK, Patro PK, Sharma J, Mahata T, Sinha PK. Evaluation of La0.75Sr0.25Cr0.5Mn0.5O3 protective coating on ferritic stainless steel interconnect for SOFC application. Int J Hydrogen Energy 2016;41:20365e72. [8] Shaigan N, Qu W, Ivey 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;195:1529e42. [9] Wu J, Liu X. Recent development of SOFC metallic interconnect. J Mater Sci Technol 2010;26:293e305. [10] Zhang X, Zhang HL, Yang XG, Zeng CL. Evaluation of electroplated Co-Cu metallic coatings for intermediatetemperature solid oxide fuel cells. Int J Hydrogen Energy 2018;43:22458e66. [11] Swaminathan S, Ko YS, Lee YS, Kim DI. Oxidation behavior and area specific resistance of La, Cu and B alloyed Fe-22Cr ferritic steels for solid oxide fuel cell interconnects. J Power Sources 2017;369:13e26. [12] Molin S, Sabato AG, Bindi M, Leone P, Cempura G, Salvo M, Cabanas PS, Boccaccini AR, Smeacetto F. Microstructural and electrical characterization of Mn-Co spinel protective coatings for solid oxide cell interconnects. J Eur Ceram Soc 2017;37:4781e91. [13] Dheeradhada VS, Cao HB, Alinger MJ. Oxidation of ferritic stainless steel interconnects: thermodynamic and kinetic assessment. J Power Sources 2011;196:1975e82. [14] Yang Z, Guo M, Wang N, Ma C, Wang J, Han M. A short review of cathode poisoning and corrosion in solid oxide fuel cell. Int J Hydrogen Energy 2017;42:24948e59. [15] Saeidpour F, Zandrahimi M, Ebrahimifar H. Effect of ZrO2 particles on oxidation and electrical behavior of Co coatings electroplated on ferritic stainless steel interconnect. Corros Sci 2019;153:200e12. [16] Zhao M, Geng S, Chen G, Wang F. FeCoNi converting coating for solid oxide fuel cell steel interconnect application. J Power Sources 2019;414:530e9. [17] Waluyo NS, Park SS, Song RH, Lee SB, Lim TH, Hong JE, Ryu KH, Im WB, Lee JW. Protective coating based on manganese-copper oxide for solid oxide fuel cell interconnects: plasma spray coating and performance evaluation. Ceram Int 2018;44:11576e81. [18] Tsai MJ, Chu CL, Lee S. La0.6Sr0.4Co0.2Fe0.8O3 protective coatings for solid oxide fuel cell interconnect deposited by screen printing. J Alloy Comp 2010;489:576e81. [19] Harthøj A, Holtb T, Møller P. Oxidation behaviour and electrical properties of cobalt/cerium oxide composite coatings for solid oxide fuel cell interconnects. J Power Sources 2015;281:227e37.
[20] Fu Q, Tietz F, Sebold D, Wessel E, Buchkremer HP. Magnetron-sputtered cobalt-based protective coatings on ferritic steels for solid oxide fuel cell interconnect applications. Corros Sci 2012;54:68e76. [21] Han SJ, Pala Z, Sampath S. Plasma sprayed manganesecobalt spinel coatings: process sensitivity on phase, electrical and protective performance. J Power Sources 2016;304:234e43. [22] Lee S, Chu CL, Tsai MJ, Lee J. High temperature oxidation behavior of interconnect coated with LSCF and LSM for solid oxide fuel cell by screen printing. Appl Surf Sci 2010;256:1817e24. [23] Ebrahimifar H, Zandrahimi M. Mn coating on AISI 430 ferritic stainless steel by pack cementation method for SOFC interconnect applications. Solid State Ion 2011;183:71e9. [24] Chakraborty SP, Banerjee S, Singh K, Sharma IG, Grover AK, Suri AK. Studies on the development of protective coating on TZM alloy and its subsequent characterization. J Mater Process Technol 2008;207:240e7. [25] Safikhani A, Esmailian M, Salmani MR, Aminfard M. Effect of Ni-Mo addition on cyclic and isothermal oxidation resistance and electrical behavior of ferritic stainless steel for SOFCs interconnect. Int J Hydrogen Energy 2014;39:11210e23. [26] Yang Z, Xia G, Simner SP, Stevenson JW. Thermal growth and performance of manganese cobaltite spinel protection layers on ferritic stainless steel SOFC interconnects. J Electrochem Soc 2005;152:A1896e901. [27] Thaheem I, Joh DW, Noh T, Lee KT. Highly conductive and stable Mn1.35Co1.35Cu0.2Y0.1O4 spinel protective coating on commercial ferritic stainless steels for intermediatetemperature solid oxide fuel cell interconnect applications. Int J Hydrogen Energy 2019;44:4293e303. [28] Ou DR, Cheng M, Wang XL. Development of low-temperature sintered Mn-Co spinel coatings on Fe-Cr ferritic alloys for solid oxide fuel cell interconnect applications. J Power Sources 2013;236:200e6. [29] Zhang HH, Zeng CL. Preparation and performances of Co-Mn spinel coating on a ferritic stainless steel interconnect material for solid oxide fuel cell application. J Power Sources 2014;252:122e9. [30] Saeidpour F, Zandrahimi M, Ebrahimifar H. Evaluation of pulse electroplated cobalt/yttrium oxide composite coating on the Crofer 22 APU stainless steel interconnect. Int J Hydrogen Energy 2019;44:3157e69. [31] Cheng F, Sun J. Fabrication of a double-layered Co-Mn-O spinel coating on stainless steel via the double glow plasma alloying process and preoxidation treatment as SOFC interconnect. Int J Hydrogen Energy 2019;44:18415e24.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Manufacture of Co-containing coating on AISI430 stainless steel via pack cementation approach for SOFC interconnect Kun Liu a,1, Shuai Xu a,1, Fei Teng a, Jie Yang a, Jianzhong Man a, Jinlong Cui b,**, Juncai Sun a,* a
Key Lab of Ship-Machinery Maintenance & Manufacture, Dalian Maritime University, Dalian 116026, China School of Chemistry and Chemical Engineering, Inner Mongolia University of Science & Technology, Baotou 014010, China
b
highlights Co-contained coating is made on surface of 430 SS interconnect by pack cementation. CoFe2O4 spinel coating limits inward diffusion of O2 and outward diffusion of Cr2þ. Coated samples with 2 h and 2% have better oxidation resistance. Coated sample with 2 h has a weight change of 0.231 mg/cm2 after 650 h of oxidation. The area specific resistance of coated sample with 2% is 90.21 mU cm2 after 450 h.
article info
abstract
Article history:
Stainless steel can be applied as interconnect materials in solid oxide fuel cells (SOFCs) at
Received 31 July 2019
operating temperatures 600e800 C. Chromium (Cr)-forming stainless steel as an inter-
Received in revised form
connect plate possesses a low oxidation resistance at high temperature and electrical
14 September 2019
conductivity, and volatility of Cr oxide scale can poison the cathode material. One effective
Accepted 18 September 2019
strategy is to use a surface coating to improve interconnect performance. This work is to
Available online 18 October 2019
form cobalt (Co)-containing coatings on the surface of AISI 430 ferritic stainless steel interconnect via pack cementation approach. The resultant coating is extremely effective
Keywords:
at heightening the oxidation resistance and electrical conductivity of AISI 430 ferritic
Solid oxide fuel cells
stainless steel. The area specific resistance of samples was measured as a function of time.
Interconnect
The area specific resistance of coated sample with 2% of activator content and holding time
Cobalt-containing coatings
of 2 h is 90.21 and 108.32 mU cm2 after 450 h of oxidation in air, respectively. Additionally,
AISI 430 ferritic stainless steel
the coated sample with 2% of activator content and holding time of 2 h has a weight change
Pack cementation approach
of merely 0.299 and 0.231 mg/cm2 after 650 h of isothermal oxidation at 800 C, separately.
CoFe2O4 spinel coating
The results displayed that the formation of CoFe2O4 spinel coating enhanced oxidation resistance by inhibiting the outward diffusion of Cr cations and the inward diffusion of oxygen anions. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. ** Corresponding author. Linghai Road 1#, Gaoxin District, Dalian maritime University, Dalian 116034, China. E-mail addresses: [email protected] (J. Cui), [email protected] (J. Sun). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.ijhydene.2019.09.144 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 3 2 8 e3 0 3 3 8
30329
Introduction Solid oxide fuel cell (SOFC) is identified as an appealing candidate material for energy conversion owing to its higher electrical efficiency and decreased emissions from the traditional energy-conversion devices [1e3]. Minimizing the operating temperature of SOFC to 600e800 C results in the possibility of applying cost-effective stainless steel as replacements rather than conventional lanthanum chromate ceramics [4,5]. At such a low temperature, more environmentally viable and mechanically sturdy materials can be applied [6]. Ferritic stainless steels are deemed as be among the most prospect candidate alloys thanks to their excellent mechanical behavior, higher electrical conductivity and low cost [7e9]. Nevertheless, inevitable high temperature oxidation for the cathode operating environment and the incessant growth of Cr-rich oxide scales at the metallic interconnects surface can bring not solely about an increase in the electrical resistance but also lead to the possibility of potential spallation [10,11]. In addition, Cr2O3 can react with water molecules by shaping volatile compounds. These Cr-containing species can poison the cathode material and result in cathode degradation [12e14], which seriously restricts practical application of the SOFC. In order to deal with the above impediments, pack cementation technique is the most efficacious method for heightening interconnect behaviors, which can be employed a surface coating to enhance conductivity, decreased scale growth and the formation of volatile Cr. Recently, more researchers have absorbed in the application of conductive oxide and protective coatings. For instance, Saeidpour et al. have reported an electroplating technique for the preparation of Co and CoeZrO2 coatings [15]. Zhao et al. used FeCoNi coating on SUS 430 stainless steel via magnetron sputtering, followed by high temperature oxidation to transform the FeCoNi coating to (Fe,Co,Ni)3O4 spinel oxides [16]. Waluyo et al. coated Mn2CuO4 on metallic interconnects (Cr ofer 22 APU) by a plasma spray (PS) process [17]. Tsai et al. manufactured La0.6Sr0.4Co0.2Fe0.8O3 protective coatings with screen printing method [18]. Several techniques have been exploited to employ spinel coatings to ferritic stainless steels, including electrodeposition [15,19], magnetron sputtering [16,20], plasma spraying [17,21], screen printing [18,22] and pack cementation [23]. Among them, pack cementation approach has captured tremendous interests due to its inexpensive cost, reliable, simplicity of preparation as well as excellent adherence between the coating and substrate [24]. In this method, Co powder reacts with ammonium chloride (NH4Cl) to form halides at high temperature, halide reacts with the substrate to deposit Co on the substrate and then the Co forms intermetallic compounds with substrate at high temperature. Schematic representation of the pack cementation cobaltizing process as exhibited in Fig. 1. To our best knowledge, there are rare reports about Co-containing coatings used as SOFC interconnects via pack cementation approach to prevent the Crvolatilization, enhance oxidation behavior and electrical conductivity of AISI 430 ferritic stainless steel.
Fig. 1 e Schematic representation of the pack cementation cobaltizing process.
The objective of the current investigation is to contrast the effects of four holding times (1h, 2h, 3h and 5h) and three activator (NH4Cl) contents (1%, 2% and 3%) on Co-containing coatings on the AISI 430 ferritic stainless steels substrate. The Co-containing coatings are manufactured by pack cementation approach. The effects of Co-containing coatings on the isothermal oxidation and cyclic oxidation were detailly studied. Additionally, the electrical property was assessed as a function of time to evaluate the area specific resistance (ASR)
30330
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 3 2 8 e3 0 3 3 8
under different holding times and different NH4Cl contents, separately.
Experimental sections Preparation of samples The commercial AISI 430 stainless steel sheet was cut in 10 mm 10 mm 2 mm as the substrate and the composition was listed in Table 1. Each sample was ground with SiC paper of 320-grit to 2000-grit and polished, then cleaned in an ultrasound acetone bath and dried in room temperature. In the current research, the pack cementation method was used to deposit Co onto the substrate. The powder mixture contained Al2O3 (inert filler), NH4Cl (activator) and Co powder (deposition source). Subsequently, the as-prepared samples were buried in powder mixture in a quartz tube, the quartz tube was plugged with a quartz plug and sealed with clay, then heated at 800 C for different of holding time in a muffle furnace (in air) and cooled to ambient temperature naturally. To optimize the quality of the formed coating, the two valid factors of the pack cementation process were studied by altering the factors, the two factors included holding time (1 h, 2 h, 3 h and 5 h) and activator content (1%, 2% and 3%). Generally speaking, inappropriate amount of activator led to many voids to the coating or deposited difficulties on the surface of stainless steel, which caused the lower oxidation resistance and higher values of ASR. In addition, higher holding time can result in thicker coating or form a brittle layer. Here, Co-containing coating sample with holding time of 2 h and activator content of 2% was named as AS-2h, AS2%, respectively. For comparison, the Co-containing coating samples with holding time of 1 h, 3 h and 5 h, defined as AS1h, AS-3h and AS-5h, separately, and the obtained Cocontaining coating samples were labeled as AS-1% and AS3%, corresponding to the added activator content of 1% and 3%, respectively.
Oxidation testing The isothermal oxidation was conducted at 800 C for 650 h in static air. The bare and coated samples were placed in a 15 ml corundum crucible and this crucible was placed in another bigger corundum crucible with volume of 50 ml. Crucibles were taken out every 25 h, samples were weighted with the smaller corundum crucible in order to avoid the change of mass when sample was moved during weighting process. In addition, the coated with different holding time (1, 2, 3 and 5 h) and uncoated samples were submitted to cyclic oxidation test. 50 cycles were exerted and each cycle consisted of repeating of heating and cooling at room temperature.
Table 1 e Chemical composition of AISI 430 ferritic stainless steel (wt.%). Wt% AISI 430
Cr
Mn
Ni
C
Si
P
Fe
16 .53
0.33
0.12
0.16
0.58
0.03
Bal
Area specific resistance measurements To study the electrical properties of samples, the area specific resistance (ASR) was accomplished by pseudo four-point DC method with a constant current density of 100 mA. Platinum (Pt) wires and meshes were applied to bond the samples and test instrument, and silver paste was painted on the surface of the sample to make it fully contact with the Pt meshes. Samples were measured at 800 C for 450 h in static air and the voltage date was recorded every 25 h. Fig. 2 shows the schematic diagram of the setup applied for ASR measurement.
Characterization of coating properties The scanning electron microscopy (SEM, SUPRA 55 SAPPHIRE, Germany)showed the cross section and surface morphology of coated samples, and the chemical composition and cross sectional composition profile were analyzed by energy dispersive spectroscopy (EDS, X-Max/OXFORD, UK). The phases of the coated samples were analyzed by X-ray diffraction (XRD, Rigaku-D/MAX-3A) using a Co-Ka radiation source (l ¼ 0.1789 nm).
Results and discussion Effect of holding time on coating Phases and morphologies of coated samples Fig. 3 is the XRD patterns of the coated samples prepared at different holding time. It can be found from Fig. 3 that AS-1h, AS-2h, AS-3h and AS-5h samples have similar diffraction peaks. In addition, there are four phases Co, Cr2O3, CoFe2O4 and Co3Fe7 except the substrate phase, which may correspond to the main components of the coating. The existence of Co3Fe7 phase is caused by the chemical reaction of Co and Fe at high temperature. However, the appearance of O element in CoFe2O4 and Cr2O3 phases is due to the fact that a part of the oxygen in air participates in the reaction when samples are heated in a muffle furnace. Fig. 4 presents SEM images and corresponding composition profile along the cross section of coated sample at different holding time (1 h, 2 h, 3 h and 5 h) after chemical cobaltizing treatment. Fig. 4a shows that the coating of AS-1h sample is uniformly distributed and possesses about 10 mm thickness. Additionally, it can be seen from Fig. 4a that Cr2O3 oxide layer has not been formed completely. According to the corresponding cross sectional composition profile, the content of Co element gradually decreases from the surface to the surface at a distance of 6 mm. However, the Cr element has been gathered at coating/substrate interface, and the O element possesses a high content in the coating but does not enter the substrate. When the holding time is increased to 2 h, the coating of AS-2h sample is uniform and dense (Fig. 4b). The total thickness of the coating is added to 20 mm, including a 0e6 mm CoFe2O4 spinel outlayer and a 6e20 mm Cr2O3 oxide inner layer. It can be seen from cross sectional composition profile that the content of Cr element near the coating is significantly lower than the average content of Cr element in the substrate, indicating that the Cr element migrates
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 3 2 8 e3 0 3 3 8
30331
Fig. 2 e Schematic diagram of samples for ASR measurement.
Fig. 3 e The XRD patterns of AS-1h, AS-2h, AS-3h and AS5h samples.
outward to form a Cr2O3 oxide layer. When the holding time is prolonged to 3 h, the AS-3h sample thickens obviously (approximately 30e40 mm), and it is divided into CoFe2O4 spinel outlayer and Cr2O3 oxide innerlayer (Fig. 4c). The thickness of the outer Co-containing coating is about 16e30 mm, and the main components are Co and CoFe2O4, which is well consistent with the result of XRD analysis and the corresponding cross sectional composition profile. Whereas, the thickness of the inner Cr2O3 oxide layer increases significantly (about 8e10 mm) compared with Fig. 4b. In general, the thickness of the coating is not uniform, and there are some voids at coating/substrate interface. By contrast with other samples, the thickness of the coating of AS-5h sample is further increased to approximately 60 mm (Fig. 4d). From the combined results of XRD (Fig. 3) and corresponded to cross sectional composition profile, at a distance of 0e40 mm from the surface, the main phases are Co and CoFe2O4, and a large number of Cr and O elements can be observed at the distance of 50e60 mm from the surface, combined with the XRD pattern, which is the Cr2O3 oxide layer.
However, a brittle layer with obvious cracks can be observed at Cr2O3 oxide layer/substrate interface, according to the corresponding cross sectional composition profile, it is found that a large amount of Si elements are accumulated at coating/substrate interface, which makes that cracks were easily formed. From the results above SEM cross sectional images, we can observe that there are some tiny voids at coating/substrate interface in four samples, and the longer the holding time was, the more obvious the void was, which may be ascribed to the outward diffusion of Cr cations in the substrate during the growth of the oxide film, leaving a large number of vacancies. Meanwhile, as the holding time increased, the thickness of the layer will gradually increased, but if the holding time was too long, the excessive thickness of the coating can cause the poor conductivity of the sample. Moreover, too thick coating was prone to cracking and protrusion during the high-temperature heating process, which may eventually result in the exfoliation of the coating. When the holding time was too long (5 h), the Si element will accumulate at coating/substrate interface to form a brittle layer, which was because the oxide brittleness of Si was extremely enormous and seriously destroyed the adherence between the coating and the substrate. Fig. 5 exhibits the surface morphology of the coating for 1 h, 2h and 5 h. After 1 h of treatment (Fig. 5a), the surface of the AS-1h sample is uniform and flat, without obvious cracks or spallation. It can be observed that the surface is a fine particle phase with a particle size of less than 1 mm. In addition, the coating of the AS-1h sample is not completely formed and is less than 10 mm in SEM cross sectional image (Fig. 4a), and the depth of the Co element diffusing into the substrate is relatively low. When the holding time is prolonged to 2 h (Fig. 5b), the surface of the AS-2h coating is flat and particles grows up to about 1 mm compared with Fig. 5a. There are no defects such as cracks and spallation. Additionally, it can be seen from the SEM cross section image (Fig. 4b) that the voids of the surface do also not appear in the deeper areas of the coating. Compared with AS-2h sample (Fig. 5b), the particles on the surface of AS-3h and AS-5h samples continue to grow, but the morphology does not change significantly. The surface
30332
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 3 2 8 e3 0 3 3 8
Fig. 4 e SEM cross sectional images of (a) AS-1h, (b) AS-2h, (c) AS-3h and (d) AS-5h samples, and corresponding to cross sectional composition profile.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 3 2 8 e3 0 3 3 8
30333
Fig. 5 e SEM micrographs of coating for (a) 1 h, (b) 2 h, (c) 3 h and (d) 5 h, (e) EDS point analysis of AS-2h sample.
particle size of sample with holding time of 3 h is about 2 mm (Fig. 5c), while that of sample with holding time of 5 h is approximately 2.5e3 mm. There are no cracks and spallation on the surfaces of AS-3h and AS-5h samples. However, as the holding time increases, the surface appears looser and the voids become more obvious, which is in well accordance with SEM cross section images of coating (Fig. 4). The EDS point analysis image of AS-2h sample displays the presence of Co, Fe, Cr and O elements (Fig. 5e), which is the same result as that of the XRD displayed in Fig. 3. It can be seen that a small amount of Cr element appears on the surface, which is attributed to the diffusion of Cr element in the substrate to the surface. The presence of the O element is due to the fact that the entire experiment is carried out in a box-type resistance furnace and there is no protective gas. The quartz tube containing the sample is basically in static air. Although the canal orifice is sealed, it cannot completely prevent the entry of air.
Oxidation behavior An isothermal oxidation experiment was carried out in static air at 800 C for 650 h. Fig. 6a portrays the specific weight gain as a function of time for uncoated, AS-1h, AS-2h, AS-3h and AS-5h samples. The initial oxidation rate is strikingly high for the uncoated sample. This is because of the bare substrate in uncoated sample, which oxidizes freely. In all samples, weight gain increases parabolically with the isothermal oxidation time. By contrast with uncoated sample, AS-1h, AS-2h, AS-3h and AS-5h samples display smaller mass gain at all times, which suggests that the Co-containing coating hinders the diffusion of oxygen and limits the growth of the Cr2O3 oxide layer, and enhances the oxidation resistance of the samples at high temperature. The uncoated sample has a weight change
of 1.613 mg/cm2 after 650 h of isothermal oxidation, while the AS-1h, AS-2h, AS-3h and AS-5h samples has a weight change of 0.542 0.299, 1.102 and 1.353 mg/cm2, respectively. For AS-3h and AS-5h samples, the weight gain is still fast after 150 h of isothermal oxidation, this is due to the cracks and even spallation of the coating in the long-term high temperature oxidation, so that the coating loses the role of protecting the substrate. Whereas the weight gains of AS-1h sample is higher than AS-2h sample. Therefore, by above investigation, AS-2h sample has the highest oxidation resistance in five samples. To further determine the oxidation kinetics is parabolic, the oxidation kinetics curves of the all samples are shown in Fig. 6b. It can be calculated from Fig. 6b that the parabolic rate constants of uncoated, AS-1h, AS-2h, AS-3h and AS-5h samples are 7.35 104, 2.10 105, 6.38 105, 3.03 104 and 5.67 104 mg2 cm4 h1, respectively, which fulfills Eq. (1): Dm2 ¼ Kp ,t A
(1)
Where in Eq. (1): Dm is mass gain, A is the sample surface area, Kp is the parabolic rate constant and t is the oxidation time [25]. The uncoated, AS-1h, AS-2h, AS-3h and AS-5h samples were subjected to cyclic oxidation in static air at 800 C. 50 cycles were exerted and each cycle composed of repeating of heating and cooling at room temperature. Fig. 7 delivers the weight gain of samples as a function of cycle number. In all of the cycles, compared with the uncoated sample, the coated samples have less weight gain except AS-5h sample in 25th cycle. This is due to protective CoFe2O4 coating that restricts the oxidation reactions. In addition, due to the excessive thickness of the AS-5h sample, after 15 cycles, the coating
30334
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 3 2 8 e3 0 3 3 8
Fig. 6 e (a)The weight gain and (b) oxidation kinetics for uncoated, AS-1h, AS-2h, AS-3h and AS-5h samples.
Fig. 7 e Specific weight gain as a function of cycle number during cyclic oxidation.
Fig. 8 e ASR parameter evolution (for 450 h)of uncoated,AS1h, AS-2h, AS-3h and AS-5h samples.
breaks down, which results in direct contact between substrate and oxygen in air, the oxide layer increases sharply, and the weight gain of the sample rapidly increases to a higher level. Compared with the weight gain value of uncoated (1.88 mg/cm2), AS-1h (1.01 mg/cm2), AS-3h (1.26 mg/cm2) and AS-5h samples (1.71 mg/cm2), AS-2h sample has a low weight gain of only 0.65 mg/cm2 after 50 cycles. The result further verifies that AS-2h sample has a high oxidation resistance.
because the presence of the coating decreases the diffusion rate of the Cr and O element. In the late stage of oxidation (150e450 h), the ASR values of all samples increase slowly, especially for AS-2h sample, the ASR value maintains stable with only a mild increase. According to the above results, the ASR values of the samples with coating are lower that of the uncoated sample during the whole oxidation stage, which may be attributed to the fact that CoFe2O4spinel layer can reduce ASR of AISI 430 ferritic stainless steel [29]. Based on the above analysis, it can be verified that AS-2h sample displays much better conductivity compared with other samples.
ASR measurements The ASR for uncoated, AS-1h, AS-2h, AS-3h and AS-5h samples, as a function of time, are presented in Fig. 8. After 450 h of oxidation, ASR parameters for uncoated, AS-1h, AS-2h, AS3h and AS-5h samples are 1786.31, 482.36, 108.32, 230.62 and 384.34 mU cm2, respectively. It is universally known that a small ASR parameter reveals a low resistance and high electrical conductivity [7,26,27]. In the initial stage of oxidation (0e150 h), the growth of ASR parameters is obvious. That is because the oxide scale shaped on the stainless steel surface peeled off partially [28]. The ASR values growth rate of four samples are much lower than that of uncoated sample, this is
Effect of activator (NH4Cl) content on coating Phases and morphologies of coated samples Fig. 9 shows the XRD patterns of the coating with different NH4Cl contents in the pack mixture. When the content of NH4Cl is 1%, XRD patterns of the coatings compose of peaks resulting from the Co3Fe7 and CoCr2O4 phases. No Co peaks can be observed, which may be ascribed to the amount of HCl decompose from the NH4Cl is extremely small when the
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 3 2 8 e3 0 3 3 8
Fig. 9 e XRD patterns of samples with different NH4Cl contents in the pack mixture.
content of NH4Cl is low, and it is rapidly consumed during the pack cementation process. Hence, no sufficient Co elements deposite on the surface of AISI 430 substrate. At the same time, oxygen is inevitably involved in the reaction because the pack cementation process is carried out in air. Finally, a part of Co element on the surface of the sample and Cr element diffuses from the substrate as well as O element in air form CoCr2O4 phase, whereas another part of Co element and Fe element in the substrate form CoeFe intermetallic compound (Co3Fe7), and eventually Co element is consumed completely so that there is no pure Co phase in the coating. The XRD peaks of the coating formed after the addition of 2% NH4Cl are identified as the Co, CoFe2O4 and Co3Fe7 phases, indicating the content of NH4Cl meets the requirement of cobaltizing. This result indicates that the content of NH4Cl (2%) is appropriate so that sufficient Co element can be deposited on the surface of the AISI430 substrate. At the same time, CoFe2O4 and Co3Fe7 are shaped by the reaction of Co in the coating and Fe in the substrate. When the content of NH4Cl is increased to 3%, the XRD peaks of the coating are consisted of Co and Co3Fe7 phases, which may be attributed to the fact that when the content of NH4Cl in the pack mixture continues to increase to 3%, more NH3 and HCl gases are decomposed. These gases are filled with quartz tube, resulting in the outflow of residual air in the pack mixture. Therefore, there is no oxygen element in the coating. SEM images of coating cross-section and corresponding to element depth profiles tested by EDS of samples with different NH4Cl contents in the pack mixture are shown in Fig. 10. SEM image of AS-1% sample (Fig. 10a) displays uniform coating with an average coating thickness of 2e3 mm. No distinct cracks or spallation were found. According to the EDS point analysis, it can be observed that the content of Co element in the coating is lower than the other samples, but the content of Cr element is higher. Additionally, the content of Cr element decreases gradually from inside to outside and exists in the whole coating, this is because the coating is too thin, the outward diffusion of Cr element is relatively easy, which can not restrict the volatilization of Cr element. When the content
30335
of NH4Cl is increased to 2% (Fig. 10b), the coating thickness of AS-2% sample is uniform, about 15 mm. It can be clearly seen that the Cr2O3 oxide layer appears at coating/substrate interface, and the main elements are Co, Fe and O in the coating, which is in consistent with the XRD patterns (Fig. 9). There is only a small amount of Cr element in the interior of the coating, indicating that as the content of NH4Cl increases, the amount of Co element deposited on the surface of the substrate increases, and the outward diffusion of Cr element is limited at coating/substrate interface. It can be found from Fig. 10c that there is still Cr2O3 oxide layer at coating/substrate interface, but it is extremely uneven and has more voids. The Cr element is enriched around the Cr2O3 oxide layer, and there is not a large amount of Cr elements passing through the coating to reach the surface. Additionally, the thickness of coating and the content of Co, Fe and O elements in the coating are similar to those in Fig. 10b. As stated above, when the content of NH4Cl increases from 2% to 3% in the pack mixture, the coating does not continue to thicken, but the interface becomes uneven and the coating has more voids, which may result in uneven stress at coating/substrate interface. And it is prone to cause stress concentration, resulting in cracks inside the coating, thus reduces the adherence at coating/substrate interface. In addition, a small amount of NH4Cl (1%) makes it difficult for Co to deposit in stainless steel.
Oxidation behavior Fig. 11 shows the weight gain curve of AS-1%, AS-2% and AS3% samples during isothermal oxidation. For AS-2% sample, the increasing rate of weight gain is lower than those of AS-1% and AS-3% samples during the whole oxidation time. AS-1% sample exhibits a weight gain of 0.916 mg/cm2 after 650 h of isothermal oxidation, while theAS-2% and AS-3% samples display a weight gain of 0.231 and 0.351 mg/cm2, separately. Due to the thin coating of the AS-1% sample (Fig. 10a), the rate of oxygen passing through the coating is faster, resulting in more severe oxidation of the substrate. Additionally, the weight gain of AS-2% sample is significantly lower than that of AS-3% sample, which indicates that the AS-2% sample possesses better oxidation resistance [30].
ASR measurements In order to investigate the effect of the content of NH4Cl on ASR, the ASR was tested for AS-1%, AS-2% and AS-3% samples. Fig. 12 illustrates the ASR as a function of time. ASR values for AS-1%, AS-2% and AS-3% samples are equal to 221.54, 90.21, 321.35 mU cm2, separately. From the above results, it can be seen that the AS-2% sample exhibits much better electrical conductivity than other samples. From these results above weight gain and ASR, we can see that the AS-2h and AS-2% sample have the best high temperature oxidation resistance and electrical conductivity. The ASR and weight gain results of samples are shown in Table 2. Additionally, the ASR value of the coated sample is much lower than that of the uncoated sample. A possible interpretation is that the CoFe2O4 spinel coating forms on the bare substrate and can effectively reduce the ASR of the AISI 430 ferretic stainless steel. The small ASR of the coated sample is assigned to the formation of a greatly conductive and
30336
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 3 2 8 e3 0 3 3 8
Fig. 10 e SEM micrographs of coating cross section and corresponding to element depth profiles tested by EDS for (a) AS-1%, (b) AS-2% and (c) AS-3% samples.
protective CoFe2O4 spinel coating [10]. The main component of the Co-containing coating is CoFe2O4 spinel. The electrical conductivity of the spinel emerged by hopping of charge between octahedral sites, which has been described in Ref. [31].
Therefore, the main conductive mechanism of CoFe2O4 spinel coating can be interpreted in Eq. (2). Co2þ¼Co3þþe
(2)
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 3 2 8 e3 0 3 3 8
30337
Conclusion
Fig. 11 e Specific weight gain for AS-1%, AS-2% and AS-3% samples as a function of time during isothermal oxidation.
In this study, AISI 430 ferretic stainless steel with Cocontaining coating was fabricated by a pack cementation method in static air for 800 C. When the holding time and activator content were 2 h and 2%, respectively, AS-2h and AS2% sample had the better high temperature oxidation resistance and electrical conductivity than those of coated and uncoated samples. The oxidation performance, EDS as well as SEM and ASR results offered ensured proof that the oxidation resistance and electrical conductivity. Firstly, the coating of AS-2h and AS-2% sample were extremely dense and uniform as well as smooth compared with other coated samples. Secondly, the value of 90.21 mU cm2 was obtained for AS-2% sample in static air, whereas the ASR value of 108.32 mU cm2 was obtained for AS-2h sample after oxidized for 450 h. In addition, the weight gain of AS-2% sample was 0.299 mg/cm2, the AS-2h sample had a weight change of 0.231 mg/cm2 after 650 h of isothermal oxidation for 800 C in static air. Finally, the CoFe2O4 coating could validly hinder the Cr-diffusion to shape a Cr-rich layer existing between coating and substrate, thus averted the volatilization of Cr and decreased the degradation of SOFC performance. Through the above results, the Co-containing coating prepared in static air exhibits great potential as a SOFC interconnect in the future.
Acknowledgements
Fig. 12 e ASR values as a function of time for AS-1%, AS-2% and AS-3% samples.
It is usually considered that the CoFe2O4 spinel coating can conduct electricity through transition of charge between Co2þ and Co3þ ions on octahedral sites. This suggests that a relative higher Co content is benefit for the improvement of electrical conductivity. Those results demonstrate that the CoFe2O4 spinel coating distinctly improves the electrical conductivity of stainless steel and is more suitable as a SOFC interconnect coating.
Table 2 e The ASR and weight gain results of samples. Sample description
Values Weight gain (mg/cm2) ASR (mU cm2)
uncoated AS-1h AS-2h AS-3h AS-5h AS-1% AS-2% AS-3%
1.613 0.542 0.299 1.102 1.353 0.916 0.231 0.351
1786.31 482.36 108.32 230.62 384.34 221.54 90.21 321.35
This work is financially supported by the National Foundation of Natural Science of China (No. 51479019, 51962027 and 21476035), the National Key Research and Development Program of China (No. 2016YFB0101206), the Fundamental Research Funds for the Central Universities (No. 3132019328), and the Project of Science Foundation of the Educational Department of Inner Mongolia (NJZY19135).
references
[1] Talica B, Molinb S, Wiika K, Hendriksenb PV, Leina HL. Comparison of iron and copper doped manganese cobalt spinel oxides as protective coatings for solid oxide fuel cell interconnects. J Power Sources 2017;372:145e56. [2] Bateni MR, Wei P, Deng XH, Petric A. Spinel coatings for UNS 430 stainless steel interconnects. Surf Coat Technol 2007;201:4677e84. [3] Mah JCW, Muchtar A, Somalu MR, Ghazali MJ. Metallic interconnects for solid oxide fuel cell: a review on protective coating and deposition techniques. Int J Hydrogen Energy 2017;42:9219e29. [4] Choi JP, Weil KS, Chou YM, Stevenson JW, Yang ZG. Development of MnCoO coating with new aluminizing process for planar SOFC stacks. Int J Hydrogen Energy 2011;36:4549e56. [5] Yang Z, Xia GG, Li XH, Stevenson JW. (Mn, Co)3O4 spinel coatings on ferritic stainless steels for SOFC interconnect applications. Int J Hydrogen Energy 2007;32:3648e54. [6] Sreedhar I, Agarwal B, Goyal P, Singh SA. Recent advances in material and performance aspects of solid oxide fuel cells. J Electroanal Chem 2019;848:113315e50.
30338
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 3 0 3 2 8 e3 0 3 3 8
[7] Lenka RK, Patro PK, Sharma J, Mahata T, Sinha PK. Evaluation of La0.75Sr0.25Cr0.5Mn0.5O3 protective coating on ferritic stainless steel interconnect for SOFC application. Int J Hydrogen Energy 2016;41:20365e72. [8] Shaigan N, Qu W, Ivey 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;195:1529e42. [9] Wu J, Liu X. Recent development of SOFC metallic interconnect. J Mater Sci Technol 2010;26:293e305. [10] Zhang X, Zhang HL, Yang XG, Zeng CL. Evaluation of electroplated Co-Cu metallic coatings for intermediatetemperature solid oxide fuel cells. Int J Hydrogen Energy 2018;43:22458e66. [11] Swaminathan S, Ko YS, Lee YS, Kim DI. Oxidation behavior and area specific resistance of La, Cu and B alloyed Fe-22Cr ferritic steels for solid oxide fuel cell interconnects. J Power Sources 2017;369:13e26. [12] Molin S, Sabato AG, Bindi M, Leone P, Cempura G, Salvo M, Cabanas PS, Boccaccini AR, Smeacetto F. Microstructural and electrical characterization of Mn-Co spinel protective coatings for solid oxide cell interconnects. J Eur Ceram Soc 2017;37:4781e91. [13] Dheeradhada VS, Cao HB, Alinger MJ. Oxidation of ferritic stainless steel interconnects: thermodynamic and kinetic assessment. J Power Sources 2011;196:1975e82. [14] Yang Z, Guo M, Wang N, Ma C, Wang J, Han M. A short review of cathode poisoning and corrosion in solid oxide fuel cell. Int J Hydrogen Energy 2017;42:24948e59. [15] Saeidpour F, Zandrahimi M, Ebrahimifar H. Effect of ZrO2 particles on oxidation and electrical behavior of Co coatings electroplated on ferritic stainless steel interconnect. Corros Sci 2019;153:200e12. [16] Zhao M, Geng S, Chen G, Wang F. FeCoNi converting coating for solid oxide fuel cell steel interconnect application. J Power Sources 2019;414:530e9. [17] Waluyo NS, Park SS, Song RH, Lee SB, Lim TH, Hong JE, Ryu KH, Im WB, Lee JW. Protective coating based on manganese-copper oxide for solid oxide fuel cell interconnects: plasma spray coating and performance evaluation. Ceram Int 2018;44:11576e81. [18] Tsai MJ, Chu CL, Lee S. La0.6Sr0.4Co0.2Fe0.8O3 protective coatings for solid oxide fuel cell interconnect deposited by screen printing. J Alloy Comp 2010;489:576e81. [19] Harthøj A, Holtb T, Møller P. Oxidation behaviour and electrical properties of cobalt/cerium oxide composite coatings for solid oxide fuel cell interconnects. J Power Sources 2015;281:227e37.
[20] Fu Q, Tietz F, Sebold D, Wessel E, Buchkremer HP. Magnetron-sputtered cobalt-based protective coatings on ferritic steels for solid oxide fuel cell interconnect applications. Corros Sci 2012;54:68e76. [21] Han SJ, Pala Z, Sampath S. Plasma sprayed manganesecobalt spinel coatings: process sensitivity on phase, electrical and protective performance. J Power Sources 2016;304:234e43. [22] Lee S, Chu CL, Tsai MJ, Lee J. High temperature oxidation behavior of interconnect coated with LSCF and LSM for solid oxide fuel cell by screen printing. Appl Surf Sci 2010;256:1817e24. [23] Ebrahimifar H, Zandrahimi M. Mn coating on AISI 430 ferritic stainless steel by pack cementation method for SOFC interconnect applications. Solid State Ion 2011;183:71e9. [24] Chakraborty SP, Banerjee S, Singh K, Sharma IG, Grover AK, Suri AK. Studies on the development of protective coating on TZM alloy and its subsequent characterization. J Mater Process Technol 2008;207:240e7. [25] Safikhani A, Esmailian M, Salmani MR, Aminfard M. Effect of Ni-Mo addition on cyclic and isothermal oxidation resistance and electrical behavior of ferritic stainless steel for SOFCs interconnect. Int J Hydrogen Energy 2014;39:11210e23. [26] Yang Z, Xia G, Simner SP, Stevenson JW. Thermal growth and performance of manganese cobaltite spinel protection layers on ferritic stainless steel SOFC interconnects. J Electrochem Soc 2005;152:A1896e901. [27] Thaheem I, Joh DW, Noh T, Lee KT. Highly conductive and stable Mn1.35Co1.35Cu0.2Y0.1O4 spinel protective coating on commercial ferritic stainless steels for intermediatetemperature solid oxide fuel cell interconnect applications. Int J Hydrogen Energy 2019;44:4293e303. [28] Ou DR, Cheng M, Wang XL. Development of low-temperature sintered Mn-Co spinel coatings on Fe-Cr ferritic alloys for solid oxide fuel cell interconnect applications. J Power Sources 2013;236:200e6. [29] Zhang HH, Zeng CL. Preparation and performances of Co-Mn spinel coating on a ferritic stainless steel interconnect material for solid oxide fuel cell application. J Power Sources 2014;252:122e9. [30] Saeidpour F, Zandrahimi M, Ebrahimifar H. Evaluation of pulse electroplated cobalt/yttrium oxide composite coating on the Crofer 22 APU stainless steel interconnect. Int J Hydrogen Energy 2019;44:3157e69. [31] Cheng F, Sun J. Fabrication of a double-layered Co-Mn-O spinel coating on stainless steel via the double glow plasma alloying process and preoxidation treatment as SOFC interconnect. Int J Hydrogen Energy 2019;44:18415e24.