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Effect of Zro2 particles on oxidation and electrical behavior of Co coatings electroplated on ferritic stainless steel interconnect
Corrosion Science 153 (2019) 200–212
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Effect of Zro2 particles on oxidation and electrical behavior of Co coatings electroplated on ferritic stainless steel interconnect
T
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Fatemeh Saeidpoura,b, Morteza Zandrahimia, , Hadi Ebrahimifarc a
Department of Metallurgy and Materials Science, Faculty of Engineering, Shahid Bahonar University of Kerman, Jomhoori Eslami Blvd., Kerman, Iran Department of Materials Engineering, Esfarayen University of Technology, Esfarayen, North Khorasan Province, Iran c Department of Materials Engineering, Faculty of Mechanical and Materials Engineering, Graduate University of Advanced Technology, Kerman, Iran b
A R T I C LE I N FO
A B S T R A C T
Keywords: Crofer 22 APU Co ZrO2 Oxidation Electrical behavior
The application of protective coatings has been proposed to overcome the difficulties in utilizing ferritic stainless steels as interconnects. Co and Co/ZrO2 coatings are electroplated onto Crofer22APU, and their oxidation and electrical behavior are evaluated in this paper. Cyclic oxidation is done in laboratory air at 800 °C for 500 h, oxidation rates and oxide layer microstructures are examined. SEM–EDS and XRD investigations exhibit the thicknesses of Cr2O3 scale, and oxidation rates are decreased for Co/ZrO2 coated steels relative to Co-coated and uncoated steels. The ASR value of the Co/ZrO2 coated steel is 14.5 mΩ cm2 after cyclic oxidation at 800 °C.
1. Introduction Many attempts have been dedicated to the progression of intermediate-temperature solid oxide fuel cells (SOFCs) in order to enhance the lengthy cycle stability, decrease the expenses and supply further cell material selections [1]. Recent developments have reduced the operation temperature of SOFCs to about 600–800 °C, which makes metallic alloys attainable for interconnect, for example, Fe-, Ni-, Cr- and Cobased alloys [2]. Among these alloys, ferritic stainless steels (FSSs) are promising candidates for utilization as SOFC interconnect because of their fairly high-temperature oxidation resistance, good electronic and heat conductivities, acceptable mechanical behavior, relatively low expense and compatible thermal expansion with their neighbor cell components [3–5]. Thus far, a variety of FFS grades have been considered for SOFC interconnect utilization, but Crofer 22 APU is currently one of the most popular FSSs, which was previously studied in [6]. The reason is that FFSs are mainly evaluated to have sufficiently good oxidation resistance at high temperatures and they also contain Cr which reacts with oxygen anions to create a compact and uninterrupted surface oxide layer to prevent the corrosion of the substrate alloy [7]. In spite of these benefits, the development of surface oxide layers over time increases the electrical resistance of the FSS interconnect [8–10]. Furthermore, the evaporation of Cr in the form of CrO2(OH)2 from the interconnect steel into the electrolyte-cathode gas interface, which can deposit on the SOFC cathode and electrolyte, leads to the performance degradation in SOFC stacks [7].
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Some alternative methods consisting of applying coatings and modifying alloy composition have been employed to resolve the difficulties. The application of a conductive coating is relatively easy and less expensive than alloy design [11]. Cubic spinels are a kind of the main favorable coating materials, the conventional formula of which is AB2O4. Depending on the selection of A and B ions and their proportions, spinels can have high electrical conductivity and show compatible thermal expansion with the FSS substrate and other cell components. Moreover, spinel coatings have a good ability for preventing the evaporation of Cr species [12]. For example, MnCo2O4-based coatings are able to obstruct important development of Cr2O3 layers and also Cr migration [13]. However, applying a more efficient coating by a more effective coating method is necessary to increase the high-temperature oxidation resistance compared to cobalt-based spinels, which could improve the lifetime of SOFCs. Reactive elements oxides have been proven to considerably improve layer adhesion and increase oxidation resistance. In spite of increasing electrical conductivity, they are not appropriate as obstacles for Cr vaporization because they are usually thin and porous, and cannot be efficient in suppressing Cr migration toward the surface of interconnect and inhibiting Cr poisoning [14]. Reactive elements are effective since they can be deposited as surface oxide layers [15,16], added as alloying elements and/or spread as oxides [17,18]. An effective coating method is the incorporation of Co-based spinels into the reactive element oxide inside single coating for decreasing Cr migration toward the surface of the metallic interconnect and increasing the oxidation resistance.
Corresponding author. E-mail addresses: [email protected] (F. Saeidpour), [email protected] (M. Zandrahimi), [email protected] (H. Ebrahimifar).
https://doi.org/10.1016/j.corsci.2019.03.034 Received 22 August 2018; Received in revised form 10 March 2019; Accepted 13 March 2019 Available online 26 March 2019 0010-938X/ © 2019 Elsevier Ltd. All rights reserved.
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Table 1 Chemical composition of Crofer 22APU stainless steel. Element
C
Cr
Mn
Si
Ni
Al
Ti
La
Fe
Concentration (wt %)
0.01
22.7
0.38
0.02
0.02
0.02
0.07
0.06
Bal
Previous studies have confirmed the effectiveness of this method [19,20]. In the present work, Co and Co-ZrO2 coatings are fabricated on Crofer 22 APU by using electroplating technique. The oxidation resistance and area specific resistance (ASR) of the Co and Co-ZrO2 coated steels were likewise evaluated by an effort to exhibit the effect of ZrO2 on the properties of the coating. 2. Materials and experimental methods
Fig. 1. Thermal cycle diagram with data on thermal transients for oxidation test for three cycles.
Crofer 22APU stainless steel sheet, manufactured by ThyssenKrupp VDM GmbH (the chemical compositions are listed in Table 1), was used as a metallic substrate for electrodeposition. Crofer 22 APU sheet was wire-cut into 10 × 10 mm2 samples and total thickness was 2 mm. The surfaces of all the samples were ground with 240–2500 grit SiC paper, washed in distilled water and then degreased with acetone in an ultrasonic cleaner for 20 min. Before the electroplating process, the specimens were weighed and, then, pickled in a 5% HNO3 and 25% HCl solution for 2 min to activate the surface. Afterward, they were electroplated in a plating bath with 90 g/l of CoCl2.6H2O and 90 ml/l of 37% HCl to form a very thin strike layer of Co before the coating step. The strike detaches the natural oxide scale from the surface of the sample and, then, creates a Co layer with a very thin thickness and improves the adhering of the following coating layers [19]. The samples were coated by using electroplating technique with pure Co and Co/ZrO2 coatings at 45 °C for 15 min. Electroplating was performed in a modified Watts bath with composition and operating conditions shown in Table 2. For electroplating processes, Crofer 22APU (cathode) was located uprightly inside the bath, parallel to unadulterated Co (anode). The ZrO2 particles were scattered in the bath with a magnetic stirrer during electroplating. All the tests were performed in 100 ml of electrolytes made ready by deionized water. The particle size of ZrO2 in deposition bath was measured by a light phase scatter analyzer (Zetasizer -Vasco3). To modify the electrolyte pH, NH4OH or 20 vol% H2SO4 was utilized. The space between Crofer 22APU and unadulterated Co was 3 cm and the anode to cathode surface proportion was 2, so that the polarization effect was decreased [21]. After electrodeposition, the cyclic oxidation tests of uncoated, Cocoated and Co/ZrO2-coated steels were conducted in a box furnace with a dry static laboratory air atmosphere. Cyclic thermal oxidation tests were also performed for up to 20 cycles. Every cycle included a 25 h duration at 800˚C followed by 20 min of cooling at 25 °C. The heating rate was about 5 °C/min and cooling rate was almost 25 °C/min for oxidation tests. Fig. 1. Shows thermal cycle diagram with data on thermal transients for oxidation test for three cycle. The weight of the
samples was measured before and after the high temperature exposure and the weight gains were calculated as well. Three samples of each steels were oxidized in an electrical furnace for 500 h in laboratory air at 800 °C. The average mass gains were calculated for 3 samples. Individual mass gains were reproducible within ± 5%. Surface chemistry and morphology were analyzed by utilizing the scanning electron microscopy (SEM), (CamscanMV2300) by energy dispersive X-ray (EDX) analysis. The phase composition of the oxidation products created on the uncoated and coated samples was characterized by the means of X-ray diffraction (XRD) with a Philips X’Pert High Score diffractometer by using Cu Ka (k = 1.5405 A°). The cross-sections of the steels were analyzed by utilizing SEM with EDX elements line scan analysis to observe the distribution profile of the elements on the crosssections. The temperature-dependence of the ASR of the Co-coated, Co/ZrO2coated, and uncoated oxidized samples were evaluated within 650–800 °C after 500 h of cyclic oxidation in laboratory air at 800 °C. ASR evaluation tests were conducted by using the setup shown in Fig. 2. Before testing, Ag paste was applied to the sample surface; then, an Ag mesh was placed on it and Ag wires as the electrical conductor were mounted to the samples. During the test, by applying a fixed current with the density of 500 mA cm−2 to the samples, the voltage was inscribed by using an Autolab Pgstat 302. The ASR was obtained by multiplying R in the area of the Ag layer (cm2). 3. Results and discussion 3.1. Microstructures and compositions of as-electroplated coating Fig. 3a and b show the surface morphology of the steel with
Table 2 Composition and operating conditions for electroplating of Co and Co/ZrO2 on Crofer 22APU. Cobalt sulphate heptahydrate, CoSO4.7H2O Cobalt chloride hexahydrate, CoCl2.6H2O Boric acid, H3BO3 Zirconium oxide, ZrO2 pH Temperature Agitation Anode Operating time Current density
100 g/L 10 g/L 20 g/L 0–20 g/L 3 45 °C Magnetic bar from the bottom Pure cobalt 15 min 25 mA/cm2
Fig. 2. Schematic figure of the experimental setup utilized for measuring the ASR. 201
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Fig. 3. Secondary electron image (a,b) and EDX analysis (c) of the surface of the as-deposited Co/ZrO2 coatings on Crofer 22 APU.
Fig. 4. Particles size dispersion of ZrO2 reinforcement powder by intensity at pH of 3.
coating was 1–5 μm, showing that some of ZrO2 particles were agglomerated during the electroplating process. Mean particle size of ZrO2 powders in bath deposition was measured about 1.5 μm (Fig. 4) whereas the size of ZrO2 particles in the coating was 1–5 μm (Fig. 3a), indicating that ZrO2 particles have agglomerated during electrodeposition. These particles had a spherical shape and seemed poriferous. EDX
composite coatings obtained by means of electroplating technique. Fig. 3a shows that ZrO2 particles were successfully co-deposited into the Co matrix and had a uniform distribution. Surface morphology of coating was continuous, uniform and very dense with none large-scale pores or cracks. According to Fig. 3b, the Co matrix with needle-like grains can be found on the substrates, and ZrO2 particles were clearly observed in the Co matrix coating. The size of the ZrO2 particles in the 202
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Fig. 5. SEM cross-section image and EDS line scan of the: (a, b) Co-coated and (c, d) Co/ZrO2-coated samples.
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Fig. 6. Oxidation kinetics of the steels in laboratory air for 500 h at 800 °C expressed as: (a) weight gain for uncoated, (b) squared weight gain for uncoated, (c) weight gain for Co-coated, (d) squared weight gain for Co-coated, (e) weight gain for Co/ZrO2 -coated and (f) squared weight gain for Co/ZrO2 coated samples as a function of time in cyclic oxidation (time period in which the parabolic rate law is followed, is shown inside the parentheses).
analysis (Fig. 3c) that was carried out on the overall surface of the coating (Fig. 3a) exhibited that Co, Zr and O amounts (weight percent) in the coating were 50. 5%, 33.5%, and 16. 0% respectively, verifying the electroplated coating included Co, Zr and O. Fig. 5 shows SEM cross-section images with EDS line scan of the Cocoated (Fig. 5a, b) and Co/ZrO2-coated (Fig. 5c, d) samples. These coatings were electroplated with the unvarying thicknesses of approximately 5 μm and adhered to the Crofer 22APU substrate well with no cracks or separation in the direction of the interface, although a few small cracks can be seen in the interface of Co-coating and substrate. The ZrO2 particles in the cross-section of the Co/ZrO2 coating are also visible in Fig. 5c. They can be perceived by EDS analysis as well (Fig. 5d).
temperatures due to the volatility of chromium oxides. Therefore, these results are usually utilized for comparing the oxidation resistance of the samples. Fig. 6 shows the kinetics of the oxidation of the uncoated, Co coated and Co/ZrO2 coated steels, indicated weight gains (Fig. 6a,c,e) and squared weight gains (Fig. 6b,d,f) as a function of time oxidation in air at 800 °C. In Fig. 6b, d and f, the square area-specific mass change versus the oxidation time is plotted. Based on Wagner's oxidation theory [22], if the curve of the square area-specific mass change versus the oxidation time is a straight line, the growth is diffusion-controlled and follows parabolic rate kinetics. The oxidation kinetics of the examined specimens almost followed the parabolic kinetics law. The Pilling–Bedworth Eq. (1) was utilized to evaluate the parabolic rate constant (kp) [23].
3.2. Oxidation kinetics
⎛ δm ⎞ = kp × t + C ⎝ A ⎠
2
For evaluating the oxide layer growth rates of the samples, the weight change per unit area was measured as a function of oxidation time. However, the use of weight change for determining the oxidation resistance of chromium-containing alloys is not correct at high
(1)
where δw is the weight change, A is the steel surface area, kp is the parabolic rate constant, t is the oxidation time and C is an integration constant, explaining the start of parabolic kinetics. This behavior of the 204
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Fig. 7. Secondary electron images of: (a,b) uncoated Crofer 22APU, (c,d) Co-coated and (e,f) Co/ZrO2-coated samples after 500 h of cyclic oxidation at 800 °C.
uncoated and coated steels is caused by the fact that Cr2O3 layer growth follows the parabolic rate law [24]. kp values of uncoated steel from 0 to 500 h was 5.3 × 10−13 mg2 cm−4s−1 due to the growth of Cr2O3 scale which slowly increase by increasing the oxidation time (Fig. 6a, b). The Co-coated and Co/ZrO2-coated steels had a first quick mass gain (kp = 3.3 × 10-12 for Co coated (Fig. 6d) and kp = 2.0 × 10−12 for Co/ ZrO2 coated (Fig. 6f) steels) and, afterwards, the mass gain raised very slightly (kp = 3.4 × 10−13 for Co coated (Fig. 6d) and
Table 3 EDS analysis data for the spots shown in Fig. 7b (compositions in weight. %). Spot
1 2
Element Cr
O
Mn
Fe
37.61 39.46
40.4 34.37
21.99 18.98
0.00 7.19
205
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and stopping Cr poisoning [14]. In conformity with the information on the oxidation kinetics of steels acquired after cyclic oxidation experiments, weight gain rates of the uncoated and Co coated steels was higher than the oxidation rate of the coated Crofer 22 APU with Co/ ZrO2 composite coating after 500 h of oxidation time in air at 800 °C (Fig. 6) and the useful effect of ZrO2 particles in Co coating as reinforcement for the thermal protecting of Crofer 22APU was confirmed. 3.3. Microstructure and chemical composition of oxidized samples Failure of the oxide layer is generally increased during cyclic oxidation tests relative to isothermal oxidation tests. Thus, the oxidation behavior in the process of thermal cycling further indicates of what is occurring to the materials at the time of being exposed to operating conditions. Entirely the ability of coatings to remain well adhered during thermal cycling is an important property for SOFC applications. Hence, the cyclic oxidation test was selected for evaluating the effect of oxidation on the microstructure and chemical composition of the samples [22,25–27]. After 500 h of cyclic oxidation, an oxide layer was observed on the uncoated sample surface, as shown in Fig. 7a. The oxide layer was principally made up of a black oxide layer and over this continuous outer layer, some loose pyramidal grains or obvious small islands could be observed in the direction of the grain boundaries of the steel (Fig. 7a, b). It shows more oxidation above the grain boundaries of the substrate. As seen, the scale is covered by the octahedral crystals particles (Fig. 7b). The arrangement of these faceted particles (Fig. 7a, b) proposes strongly that they have formed preferentially at locations where the alloy grain boundaries intersect with the surface. It should be considered that the grain boundary diffusion of transition metals through Cr2O3 is 3–5 orders of magnitude faster than the bulk diffusion [12]. The diameter of these well-developed grains is from around 10 μm to about 50 μm. Both EDS data obtained from surface and cross section of oxidizes sample imply that these 0.5–2.5 μm octahedrons are the MnCr2O4 and FeCr2O4 spinle phases (see Table 3b and Fig. 7b). As these particles consist mainly of Mn, Cr and O together with some Fe, it may inferred that these are cubic chromium manganese spinel with a small amount of FeCr2O4 spinel phase. The formation of MnCr2O4 spinel is caused by fast diffusion of manganese along the ferrite and Cr2O3 grain boundaries; on the surface, Mn reacts with the initially formed Cr2O3 and with the inwardly diffused O anions to form the spinel phase [12]. XRD analysis (Fig. 8a) also confirmed these results. Fig. 7c and d demonstrate the surface morphology of Co-coated steel after 500 h of cyclic oxidation in laboratory air at 800 °C. Oxide surface includes pyramidal particles and the XRD analysis verifies (Fig. 8b) that they are Co3O4 and MnCo2O4 spinels with a little amount of MnCr2O4 and FeCr2O4, which were approximately dense after 500 h of oxidation. The FeCr2O4, Co3O4 and MnCo2O4 phases have the same crystal structure (cubic) and similar lattice parameters: a = b = c = 8.3640 A for FeCr2O4, a = b = c = 8.0850 A for Co3O4, and a = b = c = 8.2000 A for MnCo2O4. Therefore, FeCr2O4, Co3O4 and MnCo2O4 phases can overlap with each other. The oxide layer composed on the surface of the Co/ZrO2-coated sample was denser and had smaller crystal size than the one composed on the Co-coated sample surface (Fig. 7e, f). Furthermore, no important marks of spallation or crack were seen on the surface of the Co/ZrO2coated samples after cyclic oxidation. XRD analysis (Fig. 8c) verified that the compositions of the surface oxide layers created on the Cocoated and Co/ZrO2-coated samples were similar. Some of the ZrO2 particles, visible on the surface as-deposited Co/ZrO2 steel (Fig. 3a, b), were inserted in the Co oxide (Fig. 7e, f), which grew due to the diffusion of metal ions [19], but some of them were still visible on the surface of steel after oxidation tests. Fig. 9a and b show the polished cross-sectional images and EDX elements line scan of the uncoated Crofer 22APU after 500 h cyclic oxidation in laboratory air at 800 °C. As can be seen, the surface scale
Fig. 8. XRD pattern of: (a) uncoated sample, (b) Co-coated sample and (c) Co/ ZrO2-coated sample after 500 h of cyclic oxidation in air at 800 °C.
kp = 1.1 × 10−13 for Co/ZrO2 coated (Fig. 6f) steels) after 100 h of oxidation. The reason could be that the first oxidation of Co was anticipated to occur quickly and was completely oxidized after 100 h in air at 800 °C [19]. This showed that the oxidation production of Co coating layer reduced the oxidation rate that was an effective factor for overlong working cycle because it could result in the development of a Cr2O3 scale with high ASR. Weight gains of the Co/ZrO2-coated sample had a very lower rate than the uncoated and Co-coated steels after 100 h. Cr evaporation was not examined here; however, it would just enhance the distinction between uncoated and coated steels due to the point that the Cr migration from the uncoated steel was seemingly higher. Although the reactive elements oxides remarkably improve coating adhesion and decrease oxidation rate and in addition, increase the electrical conductivity of the surface layer, they are not useful as the limits of Cr evaporation because they have naturally low thickness, are porous and cannot be efficient in preventing Cr migration to the surface 206
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Fig. 9. SEM cross-section image and EDS line scan of: (a, b) uncoated, (c, d) Co-coated and (e, f) Co/ZrO2-coated samples after 500 h of cyclic oxidation at 800 °C.
Cr2O3 grain boundaries. Mn reacted with Cr2O3 that was at first created on the surface and the O anions diffused towards the inside in order to create the spinel phase [22]. Fig. 9c and d show the polished cross-sectional images and elements line scan of the Co-coated Crofer 22APU after 500 h cyclic oxidation in laboratory air at 800 °C. Oxide layer composed on the Co-coated Crofer 22APU included the internal layer of Cr-rich oxide with Co oxide and Co-Mn spinle layer on the external surface, corresponding to other studies [22,29]. Fig. 9c shows that the external Co spinel layer was stuck to the internal Cr2O3 layer. But, the entire surface oxide layer adhering to the substrate appeared to be weak. In Co/ZrO2-coated sample, as shown in Fig. 7e and f, the surface oxide layer consisted of a twofold oxide layer with the composition similar to the oxide scale created on the surface of Co-coated sample. XRD analysis (Fig. 8b, c)
composed on the uncoated Crofer 22APU consisted of the internal layer of Cr-rich oxide and Mn-Cr-rich oxide on the external surface, corresponding to other studies [23,24,28]; the thickness of this oxide scale is about 2 μm, as exhibited in Fig. 9a. XRD analysis (Fig. 8a) confirmed that thin inner oxide layer consisted of Cr2O3 and outer oxide layer was composed of MnCr2O4 and FeCr2O4. It could be concluded that surface of the uncoated one was covered with Cr2O3 and, then over this oxide layer, was slowly encased by MnCr2O4 spinels and a little amount of FeCr2O4 spinles as oxidation time increased. Even though Crofer 22 APU steel included lower than 1%Mn, the existence of Mn in the external layer was ascribed to the rapid diffusion of Mn cations compared to other cations (such as Fe and Cr cations) reaching from the Crofer 22APU to the Cr2O3 scale; therefore, the creation of Cr-Mn spinel was caused by the rapid diffusion of Mn in the direction of the steel and 207
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Fig. 10. SEM cross-section image of: (a) Co-coated and (b) Co/ZrO2-coated samples after 100 h of cyclic oxidation in laboratory air at 800 °C.
is anticipated to be proportionate with electrical conductivity, supposing a consistent conductivity of the oxide; it has been shown that the ASR of FSSs electroplated with Co is an approximately linear function of the Cr2O3 layer thickness [32]. A thin Cr2O3 layer is also advantageous since the danger of spallation increases by increasing oxide layer thickness. The spallation of the protective Cr2O3 layer may result in both increased Cr evaporation and creation of non-protective iron oxides. A higher boundary for suitable Cr2O3 layer thickness is about 10 μm in many cases because the danger of oxide spallation and the electrical resistance becomes large above that thickness in a manner that is not acceptable [33]. EDX maps analysis was also performed on the surface of Co-coated (Fig. 11) and Co/ZrO2-coated (Fig. 12) samples after 500 h of cyclic oxidation in laboratory air at 800 °C. These figures undoubtedly are useful for better describing areas of enrichment or depletion in certain elements. The elemental mapping images in Figs. 11 and 12 indicate that there is a small portion of inter-diffusion of ionic species, e.g. Mn, Cr, O, Co and Fe across the interface of coatings and substrate. It is apparently indicated that a distinct layer with significant Co and to the less extent O, Mn, Cr, and Fe was formed on the Crofer 22APU surface (Figs. 11,12). In the Co/ZrO2-coated sample, the presence of ZrO2 particles can be seen in this layer too (Fig. 12b). Formation of a thin layer of Cr and O in the coatings/substrate interface is also in good agreement with the EDS maps (Figs. 11b, c, and 12 c, d). The effect of ZrO2 particles on the development rate of Cr2O3 layer is explained by the reactive element effect, representing several orders of decrease in the oxidation rate of similar kinds of FFS when used as the interconnect [18–20,34]. It is clear that the addition of ZrO2 to the Co coating could inhibit effectively the growth of Cr2O3 scale. Near the ZrO2 particles, Cr2O3 tends to grow inward along the particles of ZrO2 (Fig. 9e). It should be pointed out that only a small number of pores are observed beneath the oxide scales for the Co/ZrO2-coated steels, much fewer than those for the Co alloy coated samples. In research conducted to clarify the effect of the reactive element on the growth of the voids at the oxide scale/alloy interface, adding minor Hf significantly improved the cyclic oxidation resistance of the NiAl alloy by suppressing the growth of voids at the oxide scale/alloy interface in both dry and humid atmospheres [27]. Among the different mechanisms suggested to illustrate the effect of reactive elements (improvement of oxide nucleation [35,36], “sulfur” theory [37,38], “dynamic segregation theory” [39]), the conversion of the oxide scale development mechanism is accepted to present the most advantageous effects. Experiments by utilizing two-stage oxidation tests under 16O2–18O2 [40,41] clearly illustrated that outward diffusion of Cr ions is the prevalent mechanism for Cr2O3 development on Cr2O3-forming alloys. The addition of reactive elements in the Cr2O3 scale modifies the
also verified that the compositions of the surface oxide scales created on the Co-coated and Co/ZrO2-coated samples were similar. Fig. 9e shows that the external Co spinel layer was excellently stuck to the internal Cr2O3 layer. Furthermore, the entire surface oxide layer adhering to the substrate appeared to be intense. The Cr2O3 layer with approximately 2.0 μm thickness was created on the uncoated sample after 500 h oxidation (Fig. 9a), but the internal Cr2O3 layer developed on the Co-coated sample (Fig. 9c) was only around 1.5 μm, exhibiting that the creation of the external Co3O4 and MnCo2O4 spinel layer decreased the development rate of the internal Cr2O3 scale. It is apparently indicated that the external spinel layer has not only limited the outward diffusion of Cr but also repressed the inward diffusion of O, finally resulting in the lower development rate of the internal Cr2O3 scale on Co-coated Crofer 22APU [22,30]. The Cr2O3 layer on the Co/ZrO2-coated sample (Fig. 9e) had thickness of about 800 nm. This was remarkably thinner than the one on the uncoated and Co-coated steels, showing that the incorporation of ZrO2 particles into Co coating decreased the development rate of a Cr2O3 layer on Crofer 22APU, whereas it was placed in SOFC cathode-related situations. For the coated samples, two consecutive oxidation regimes appear (Fig. 6a, b): a rapid oxidation on the first 100 h attributed to the oxidation of Co, and a much slower kinetic regime associated with the oxidation of the substrate. This prove by the cross-section analysis on oxidized samples for about 100 h. As shown in Fig. 10a, the Cr2O3 layer with 1.1 μm average thickness was created on the Co-coated sample after 100 h oxidation, but the internal Cr2O3 layer developed on the Co/ ZrO2-coated sample (Fig. 10b) was only around 0.5 μm. The average thickness of the external Co3O4 and MnCo2O4 spinel layers that were created on the surfaces was about 7 μm after 100 h cyclic oxidation time. The thickness of external layers developed about 2 μm after 100 h in air at 800 °C. Compared to oxidized coated sample for 500 h at 800 °C, the internal Cr2O3 layer thickness was thinner but the external spinel layer have the same thickness. These results show that after 100 h oxidation, the Co coating is completely oxidized and converted into a Co spinel oxide coating, and then, after 500 h of oxidation, the thickness of this outer layer of spinel oxide on both coated samples does not change. But the internal layer of Cr2O3 continues to grow for up to 500 h, then after 100 h, its growth rate, especially for Co/ZrO2-coated sample, reduces significantly. A slow growth rate of the Cr2O3 layer is beneficial due to the fact that Cr2O3 has a low electrical conductivity and causes electrical missing of the SOFC systems. The electric conductivity of Cr2O3 is 0.022 S cm−1 at 800 °C [16], which is considerably lesser than the one of Co oxides (6.7 6.7 and 60 S cm−1) [29]. Therefore, the thickness of the Cr2O3 layer is anticipated to be the most important ingredient for the electrical resistance of the sample [31]. The thickness of the Cr2O3 layer 208
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Fig. 11. EDX maps analysis on the cross section surface of Co-coated sample: (a) Co, (b) Cr, (c) O, (d) Mn, and (e) Fe after 500 h of cyclic oxidation in laboratory air at 800 °C.
interconnect utilization is the ASR of the surface oxide layers. High and steady electrical conductivity is fundamental for metallic interconnects under the work conditions of the SOFC. ASR is utilized to estimate the electrical conductivity of interconnects [42]. ASR evaluations were done for the uncoated, Co-coated and Co/ZrO2-coated samples after cyclic oxidation experiment in laboratory air at 800 °C for 500 h as a function of temperature. The ASR value of the studied samples measured within 650–800 °C increased with temperature (Fig. 13a). Correlation between log (ASR/T) and 1/T was linear (Fig. 13b), which was consistent with the semiconductor behavior, depicted with Arrhenius equation [43]:
ionic species diffusion, and inward O ion diffusion becomes the prevalent transport mechanism. Such conversion in the transport mechanism is attributed to the segregation of reactive element ions or second phase particles at the Cr2O3 grain boundaries. It results in obstructing the outward cation diffusion, diminishing the oxidation rate and improving the oxide scale adherence. Such a change in the Cr2O3 scale growth mechanism results in inhibiting the formation and coalescence of cation vacancies at the metal/oxide interface. Void formation at the metal/oxide interface is a result of the outward transport of metallic ions through vacancies which allow their coalescence at the interface to weaken the adhesion to the substrate [18].
ASR = A.T. exp (−Ea/KT) 3.4. Electrical behavior Another
essential
characteristic
for
estimating
the
(2)
where A is the proportional factor, T is the absolute temperature, k is the Boltzmann constant and Ea indicates the activation energy for
SOFC 209
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Fig. 12. EDX maps analysis on the cross section surface of Co/ZrO2-coated sample: (a) Co, (b) Zr, (c) Cr (d) O, (e) Mn and (f) Fe after 500 h of oxidation in laboratory air at 800 °C.
an extra heat treatment to make the coating dense [46,47]. In those studies μm thick Cr2O3 layers were formed due to the heat treatment necessary to increase the density of the powder. As the ASR measurements in Fig. 13 show, this would have a tremendous effect on electrical resistance at 800 °C. Therefore, metallic conversion coatings, as well as coatings deposited with other techniques that do not require a high temperature heat treatment, seem to be the most suitable coating techniques for SOFC. The activation energy for conductivity (Ea) was estimated from the gradient of the log (ASR/T) versus 1/T graph (Fig. 13b) for all the three studied steels. The Ea values were calculated to be 25.14 kJ mol−1 for the uncoated sample, 14.51 kJ mol−1 for Co and 8.571 kJ mol−1 for the Co/ZrO2 coated steel. Co is a p-type dopant that has an important effect on increasing the conductivity of spinel compounds such as MnCo2O4 (60 scm−1), CoCr2O4 (7.4 scm−1) and CoFe2O4 (0.93 scm−1) [48] compared to Cr2O3 (0.02 S cm−1) and (Mn,Cr)3O4 (0.5 S cm−1 for MnCr2O4 and 0.02 S cm−1 for Mn1.2Cr1.8O4) [49]. The ASR values of
conduction. Fig. 13 shows that the ASR measured for all the samples is generally less than the maximum allowed level for use in SOFC interconnects, i.e. 100 mΩ cm2 [44]. The ASR values obtained for Co-coated Co/ZrO2coated steels was far lower than the uncoated one. As an example, the ASR at 800 °C for Co-coated and Co/ZrO2-coated steels was 18.1 and 14.5 mΩ cm2, respectively, while for the uncoated one, it was 35.6 mΩ cm2. This obviously exhibited the desired effect of deposited coating on obstructing the creation and development of high electrical resistance oxides, and creation of Co and Co-Mn spinels, which had less electrical resistance and compatible thermal expansion with Crofer 22APU [19,24]. When ASR values were measured at 650 °C (Fig. 13a), an important increase in ASR could be observed because of the far thicker Cr2O3 layer on the steels [45]. In this paper coated samples exposed to 800 °C showed very low ASR values. The reason for this is the very thin Cr-rich oxide layer. In several studies coatings have been applied as powder, with the need for 210
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• The potential of Co and Co/ZrO • • • •
2 to function as protective coatings on Crofer 22APU stainless steel was evaluated via cyclic oxidation tests. The Co/ ZrO2 coating decreased the Cr2O3 layers' thickness from about 2.0 μm on uncoated steel and around 1.5 μm on Co-coated sample to ˜800 nm after 500 h oxidation in laboratory air at 800 °C. The Co/ZrO2 coatings could decrease the oxidation rates of the steel during exposure to laboratory air at 800˚C after 100–500 h of oxidation, as compared to the uncoated and Co-coated samples. ASR of the uncoated and coated Crofer 22APU steels was measured in the temperature range of 650–800 °C, which revealed a semiconductor behavior. ASR evaluations at 800 °C in laboratory air exhibited that the lowest ASR values were acquired for the Co/ZrO2-coated steels and was equal to 14.5 mΩ cm2 at 800 °C after 500 h cyclic oxidation in laboratory air.
References [1] X. Huang, et al., Electrochemical evaluation of double perovskite PrBaCo2xMnxO5+δ (x = 0, 0.5, 1) as promising cathodes for IT-SOFCs, Int. J. Hydrogen Energy 43 (2018) 8962–8971. [2] Q. Qi, et al., One new route to optimize the oxidation resistance of TiC/hastelloy (Ni-based alloy) composites applied for intermediate temperature solid oxide fuel cell interconnect by increasing graphite particle size, J. Power Sources 362 (2017) 57–63. [3] Z. Xu, W. Xu, E. Stephens, B. Koeppel, Mechanical reliability and life prediction of coated metallic interconnects within solid oxide fuel cells, Renew. Energy 113 (2017) 1472–1479. [4] S. Geng, S. Qi, D. Xiang, S. Zhu, F. Wang, Oxidation and electrical behavior of ferritic stainless steel interconnect with Fe–Co–Ni coating by electroplating, J. Power Sources 215 (2012) 274–278. [5] R. Wang, Z. Sun, U.B. Pal, S. Gopalan, S.N. Basu, Mitigation of chromium poisoning of cathodes in solid oxide fuel cells employing CuMn1.8O4 spinel coating on metallic interconnect, J. Power Sources 376 (2018) 100–110. [6] J.G. Grolig, J. Froitzheim, J.-E. Svensson, Effect of cerium on the electrical properties of a cobalt conversion coating for solid oxide fuel cell interconnects—a study using impedance spectroscopy, Electrochim. Acta 184 (2015) 301–307. [7] S.T. Hashemi, A.M. Dayaghi, M. Askari, P.E. Gannon, Sol-gel synthesis of Mn1.5Co1.5O4 spinel nano powders for coating applications, Mater. Res. Bull. 102 (2018) 180–185. [8] Y. Fang, C. Wu, X. Duan, S. Wang, Y. Chen, High-temperature oxidation process analysis of MnCo2O4 coating on Fe–21Cr alloy, Int. J. Hydrogen Energy 36 (2011) 5611–5616. [9] W.Z. Zhu, S.C. Deevi, Opportunity of metallic interconnects for solid oxide fuel cells: a status on contact resistance, Mater. Res. Bull. 38 (2003) 957–972. [10] S.C. Paulson, V.I. Birss, Chromium poisoning of LSM-YSZ SOFC cathodes, J. Electrochem. Soc. 151 (2004) A1961–A1968. [11] M. Stanislowski, E. Wessel, K. Hilpert, T. Markus, L. Singheiser, Chromium vaporization from high-temperature alloys, J. Power Sources 154 (2007) 578–589. [12] S.N. Hosseini, F. Karimzadeh, M.H. Enayati, N.M. Sammes, Oxidation and electrical behavior of CuFe2O4 spinel coated Crofer 22 APU stainless steel for SOFC interconnect application, Solid State Ion. 289 (2016) 95–105. [13] N.S. Waluyo, et al., Protective coating based on manganese–copper oxide for solid oxide fuel cell interconnects: plasma spray coating and performance evaluation, Ceram. Int. 44 (2018) 11576–11581. [14] S. Geng, et al., Sputtered MnCu metallic coating on ferritic stainless steel for solid oxide fuel cell interconnects application, Int. J. Hydrogen Energy 42 (2017) 10298–10307. [15] P.F. You, X. Zhang, H.L. Zhang, H.J. Liu, C.L. Zeng, Effect of CeO2 on oxidation and electrical behaviors of ferritic stainless steel interconnects with NiFe coatings, Int. J. Hydrogen Energy 43 (2018) 7492–7500. [16] Y. Li, S. Geng, G. Chen, Electrodeposited Ni/CeO2 mulriple coating on SUS 430 steel interconnect, Int. J. Hydrogen Energy 43 (2018) 12811–12816. [17] D. Szymczewska, S. Molin, M. Chen, P.V. Hendriksen, P. Jasinski, Ceria based protective coatings for steel interconnects prepared by spray pyrolysis, Procedia Eng. 98 (2014) 93–100. [18] S. Chevalier, J.P. Larpin, Influence of reactive element oxide coatings on the high temperature cyclic oxidation of chromia-forming steels, Mater. Sci. Eng. A 363 (2003) 116–125. [19] A. Harthøj, T. Holt, P. Møller, Oxidation behaviour and electrical properties of cobalt/cerium oxide composite coatings for solid oxide fuel cell interconnects, J. Power Sources 281 (2015) 227–237. [20] S. Canovic, et al., Oxidation of Co- and Ce-nanocoated FeCr steels: a microstructural investigation, Surf. Coat. Technol. 215 (2013) 62–74. [21] Z. Yang, G.-G. Xia, G.D. Maupin, J.W. Stevenson, Conductive protection layers on oxidation resistant alloys for SOFC interconnect applications, Surf. Coat. Technol. 201 (2006) 4476–4483. [22] A.M. Dayaghi, M. Askari, P. Gannon, Pre-treatment and oxidation behavior of sol–gel Co coating on 430 steel in 750 °C air with thermal cycling, Surf. Coat.
Fig. 13. ASR of the uncoated, Co coated and Co/ZrO2 coated samples as a function of temperature after cyclic oxidation test in laboratory air at 800 °C for 500 h.
Co/ZrO2-coated Crofer 22APU were lower than ASR values acquired for the Co-coated steels, which showed that the ZrO2 particles did not have any damaging effect on the ASR. The ASR was increased as the highly resistive Cr2O3 scale on the steels raised in thickness [50]. On the other hand, the oxide layer (about 2 μm thick) grown on the uncoated sample was thicker than that (around 1.5 μm thick for Co-coated and 800 nm for Co/ZrO2-coated Crofer 22APU) formed on the steel with the Co and Co/ZrO2 electroplated coatings. As mentioned, adding ZrO2 could improve the interfacial contact by suppressing the formation of voids at the metal/oxide interface (Fig. 9c, e) which might result in lower ASR. The Cr2O3 layer is expected to dominate the ASR of the oxide scale. Thus it is conceived that adding reactive element, which results in a thinner Cr2O3 scale, have a beneficial effect on ASR. Earlier studies of Co- and Ce/Co-coated Sanergy HT at 800 °C have shown that adding Ce lowers the electrical resistance significantly. The Cr2O3 scale thicknesses in this work for the Co and Ce/Co coated material after 500 h at 750 °C was < 1 μm [44]. In a study by Qu et al [51] the ferritic stainless steel 430 SS was dip-coated with Co, Y, and Y/ Co that Y/Co-coatings, which showed an even better oxidation resistance than the Ce/Cocoated material. And also, in contrast to the results presented in this work, the Cr2O3 scale had grown significantly thicker (2.25 mm) after 500 h at 750 °C for the exclusively Co-coated material, compared to the Y/Co-coated material (0.75 μm). It is not trivial to compare ASR values since in most cases the different setups and methods are used. However, what can be concluded from the findings in this work and the above- cited studies is that as long as the Cr2O3 scale is ˜1 μm or thinner, low ASR values can be expected irrespective of the chemical composition, or coating method, of the Co-spinel or MCO-coating [44]. 4. Conclusions
• Well adherent Co/ZrO
2 coatings were electroplated on the ferritic stainless steel Crofer 22APU.
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Effect of Zro2 particles on oxidation and electrical behavior of Co coatings electroplated on ferritic stainless steel interconnect
T
⁎
Fatemeh Saeidpoura,b, Morteza Zandrahimia, , Hadi Ebrahimifarc a
Department of Metallurgy and Materials Science, Faculty of Engineering, Shahid Bahonar University of Kerman, Jomhoori Eslami Blvd., Kerman, Iran Department of Materials Engineering, Esfarayen University of Technology, Esfarayen, North Khorasan Province, Iran c Department of Materials Engineering, Faculty of Mechanical and Materials Engineering, Graduate University of Advanced Technology, Kerman, Iran b
A R T I C LE I N FO
A B S T R A C T
Keywords: Crofer 22 APU Co ZrO2 Oxidation Electrical behavior
The application of protective coatings has been proposed to overcome the difficulties in utilizing ferritic stainless steels as interconnects. Co and Co/ZrO2 coatings are electroplated onto Crofer22APU, and their oxidation and electrical behavior are evaluated in this paper. Cyclic oxidation is done in laboratory air at 800 °C for 500 h, oxidation rates and oxide layer microstructures are examined. SEM–EDS and XRD investigations exhibit the thicknesses of Cr2O3 scale, and oxidation rates are decreased for Co/ZrO2 coated steels relative to Co-coated and uncoated steels. The ASR value of the Co/ZrO2 coated steel is 14.5 mΩ cm2 after cyclic oxidation at 800 °C.
1. Introduction Many attempts have been dedicated to the progression of intermediate-temperature solid oxide fuel cells (SOFCs) in order to enhance the lengthy cycle stability, decrease the expenses and supply further cell material selections [1]. Recent developments have reduced the operation temperature of SOFCs to about 600–800 °C, which makes metallic alloys attainable for interconnect, for example, Fe-, Ni-, Cr- and Cobased alloys [2]. Among these alloys, ferritic stainless steels (FSSs) are promising candidates for utilization as SOFC interconnect because of their fairly high-temperature oxidation resistance, good electronic and heat conductivities, acceptable mechanical behavior, relatively low expense and compatible thermal expansion with their neighbor cell components [3–5]. Thus far, a variety of FFS grades have been considered for SOFC interconnect utilization, but Crofer 22 APU is currently one of the most popular FSSs, which was previously studied in [6]. The reason is that FFSs are mainly evaluated to have sufficiently good oxidation resistance at high temperatures and they also contain Cr which reacts with oxygen anions to create a compact and uninterrupted surface oxide layer to prevent the corrosion of the substrate alloy [7]. In spite of these benefits, the development of surface oxide layers over time increases the electrical resistance of the FSS interconnect [8–10]. Furthermore, the evaporation of Cr in the form of CrO2(OH)2 from the interconnect steel into the electrolyte-cathode gas interface, which can deposit on the SOFC cathode and electrolyte, leads to the performance degradation in SOFC stacks [7].
⁎
Some alternative methods consisting of applying coatings and modifying alloy composition have been employed to resolve the difficulties. The application of a conductive coating is relatively easy and less expensive than alloy design [11]. Cubic spinels are a kind of the main favorable coating materials, the conventional formula of which is AB2O4. Depending on the selection of A and B ions and their proportions, spinels can have high electrical conductivity and show compatible thermal expansion with the FSS substrate and other cell components. Moreover, spinel coatings have a good ability for preventing the evaporation of Cr species [12]. For example, MnCo2O4-based coatings are able to obstruct important development of Cr2O3 layers and also Cr migration [13]. However, applying a more efficient coating by a more effective coating method is necessary to increase the high-temperature oxidation resistance compared to cobalt-based spinels, which could improve the lifetime of SOFCs. Reactive elements oxides have been proven to considerably improve layer adhesion and increase oxidation resistance. In spite of increasing electrical conductivity, they are not appropriate as obstacles for Cr vaporization because they are usually thin and porous, and cannot be efficient in suppressing Cr migration toward the surface of interconnect and inhibiting Cr poisoning [14]. Reactive elements are effective since they can be deposited as surface oxide layers [15,16], added as alloying elements and/or spread as oxides [17,18]. An effective coating method is the incorporation of Co-based spinels into the reactive element oxide inside single coating for decreasing Cr migration toward the surface of the metallic interconnect and increasing the oxidation resistance.
Corresponding author. E-mail addresses: [email protected] (F. Saeidpour), [email protected] (M. Zandrahimi), [email protected] (H. Ebrahimifar).
https://doi.org/10.1016/j.corsci.2019.03.034 Received 22 August 2018; Received in revised form 10 March 2019; Accepted 13 March 2019 Available online 26 March 2019 0010-938X/ © 2019 Elsevier Ltd. All rights reserved.
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Table 1 Chemical composition of Crofer 22APU stainless steel. Element
C
Cr
Mn
Si
Ni
Al
Ti
La
Fe
Concentration (wt %)
0.01
22.7
0.38
0.02
0.02
0.02
0.07
0.06
Bal
Previous studies have confirmed the effectiveness of this method [19,20]. In the present work, Co and Co-ZrO2 coatings are fabricated on Crofer 22 APU by using electroplating technique. The oxidation resistance and area specific resistance (ASR) of the Co and Co-ZrO2 coated steels were likewise evaluated by an effort to exhibit the effect of ZrO2 on the properties of the coating. 2. Materials and experimental methods
Fig. 1. Thermal cycle diagram with data on thermal transients for oxidation test for three cycles.
Crofer 22APU stainless steel sheet, manufactured by ThyssenKrupp VDM GmbH (the chemical compositions are listed in Table 1), was used as a metallic substrate for electrodeposition. Crofer 22 APU sheet was wire-cut into 10 × 10 mm2 samples and total thickness was 2 mm. The surfaces of all the samples were ground with 240–2500 grit SiC paper, washed in distilled water and then degreased with acetone in an ultrasonic cleaner for 20 min. Before the electroplating process, the specimens were weighed and, then, pickled in a 5% HNO3 and 25% HCl solution for 2 min to activate the surface. Afterward, they were electroplated in a plating bath with 90 g/l of CoCl2.6H2O and 90 ml/l of 37% HCl to form a very thin strike layer of Co before the coating step. The strike detaches the natural oxide scale from the surface of the sample and, then, creates a Co layer with a very thin thickness and improves the adhering of the following coating layers [19]. The samples were coated by using electroplating technique with pure Co and Co/ZrO2 coatings at 45 °C for 15 min. Electroplating was performed in a modified Watts bath with composition and operating conditions shown in Table 2. For electroplating processes, Crofer 22APU (cathode) was located uprightly inside the bath, parallel to unadulterated Co (anode). The ZrO2 particles were scattered in the bath with a magnetic stirrer during electroplating. All the tests were performed in 100 ml of electrolytes made ready by deionized water. The particle size of ZrO2 in deposition bath was measured by a light phase scatter analyzer (Zetasizer -Vasco3). To modify the electrolyte pH, NH4OH or 20 vol% H2SO4 was utilized. The space between Crofer 22APU and unadulterated Co was 3 cm and the anode to cathode surface proportion was 2, so that the polarization effect was decreased [21]. After electrodeposition, the cyclic oxidation tests of uncoated, Cocoated and Co/ZrO2-coated steels were conducted in a box furnace with a dry static laboratory air atmosphere. Cyclic thermal oxidation tests were also performed for up to 20 cycles. Every cycle included a 25 h duration at 800˚C followed by 20 min of cooling at 25 °C. The heating rate was about 5 °C/min and cooling rate was almost 25 °C/min for oxidation tests. Fig. 1. Shows thermal cycle diagram with data on thermal transients for oxidation test for three cycle. The weight of the
samples was measured before and after the high temperature exposure and the weight gains were calculated as well. Three samples of each steels were oxidized in an electrical furnace for 500 h in laboratory air at 800 °C. The average mass gains were calculated for 3 samples. Individual mass gains were reproducible within ± 5%. Surface chemistry and morphology were analyzed by utilizing the scanning electron microscopy (SEM), (CamscanMV2300) by energy dispersive X-ray (EDX) analysis. The phase composition of the oxidation products created on the uncoated and coated samples was characterized by the means of X-ray diffraction (XRD) with a Philips X’Pert High Score diffractometer by using Cu Ka (k = 1.5405 A°). The cross-sections of the steels were analyzed by utilizing SEM with EDX elements line scan analysis to observe the distribution profile of the elements on the crosssections. The temperature-dependence of the ASR of the Co-coated, Co/ZrO2coated, and uncoated oxidized samples were evaluated within 650–800 °C after 500 h of cyclic oxidation in laboratory air at 800 °C. ASR evaluation tests were conducted by using the setup shown in Fig. 2. Before testing, Ag paste was applied to the sample surface; then, an Ag mesh was placed on it and Ag wires as the electrical conductor were mounted to the samples. During the test, by applying a fixed current with the density of 500 mA cm−2 to the samples, the voltage was inscribed by using an Autolab Pgstat 302. The ASR was obtained by multiplying R in the area of the Ag layer (cm2). 3. Results and discussion 3.1. Microstructures and compositions of as-electroplated coating Fig. 3a and b show the surface morphology of the steel with
Table 2 Composition and operating conditions for electroplating of Co and Co/ZrO2 on Crofer 22APU. Cobalt sulphate heptahydrate, CoSO4.7H2O Cobalt chloride hexahydrate, CoCl2.6H2O Boric acid, H3BO3 Zirconium oxide, ZrO2 pH Temperature Agitation Anode Operating time Current density
100 g/L 10 g/L 20 g/L 0–20 g/L 3 45 °C Magnetic bar from the bottom Pure cobalt 15 min 25 mA/cm2
Fig. 2. Schematic figure of the experimental setup utilized for measuring the ASR. 201
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Fig. 3. Secondary electron image (a,b) and EDX analysis (c) of the surface of the as-deposited Co/ZrO2 coatings on Crofer 22 APU.
Fig. 4. Particles size dispersion of ZrO2 reinforcement powder by intensity at pH of 3.
coating was 1–5 μm, showing that some of ZrO2 particles were agglomerated during the electroplating process. Mean particle size of ZrO2 powders in bath deposition was measured about 1.5 μm (Fig. 4) whereas the size of ZrO2 particles in the coating was 1–5 μm (Fig. 3a), indicating that ZrO2 particles have agglomerated during electrodeposition. These particles had a spherical shape and seemed poriferous. EDX
composite coatings obtained by means of electroplating technique. Fig. 3a shows that ZrO2 particles were successfully co-deposited into the Co matrix and had a uniform distribution. Surface morphology of coating was continuous, uniform and very dense with none large-scale pores or cracks. According to Fig. 3b, the Co matrix with needle-like grains can be found on the substrates, and ZrO2 particles were clearly observed in the Co matrix coating. The size of the ZrO2 particles in the 202
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Fig. 5. SEM cross-section image and EDS line scan of the: (a, b) Co-coated and (c, d) Co/ZrO2-coated samples.
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Fig. 6. Oxidation kinetics of the steels in laboratory air for 500 h at 800 °C expressed as: (a) weight gain for uncoated, (b) squared weight gain for uncoated, (c) weight gain for Co-coated, (d) squared weight gain for Co-coated, (e) weight gain for Co/ZrO2 -coated and (f) squared weight gain for Co/ZrO2 coated samples as a function of time in cyclic oxidation (time period in which the parabolic rate law is followed, is shown inside the parentheses).
analysis (Fig. 3c) that was carried out on the overall surface of the coating (Fig. 3a) exhibited that Co, Zr and O amounts (weight percent) in the coating were 50. 5%, 33.5%, and 16. 0% respectively, verifying the electroplated coating included Co, Zr and O. Fig. 5 shows SEM cross-section images with EDS line scan of the Cocoated (Fig. 5a, b) and Co/ZrO2-coated (Fig. 5c, d) samples. These coatings were electroplated with the unvarying thicknesses of approximately 5 μm and adhered to the Crofer 22APU substrate well with no cracks or separation in the direction of the interface, although a few small cracks can be seen in the interface of Co-coating and substrate. The ZrO2 particles in the cross-section of the Co/ZrO2 coating are also visible in Fig. 5c. They can be perceived by EDS analysis as well (Fig. 5d).
temperatures due to the volatility of chromium oxides. Therefore, these results are usually utilized for comparing the oxidation resistance of the samples. Fig. 6 shows the kinetics of the oxidation of the uncoated, Co coated and Co/ZrO2 coated steels, indicated weight gains (Fig. 6a,c,e) and squared weight gains (Fig. 6b,d,f) as a function of time oxidation in air at 800 °C. In Fig. 6b, d and f, the square area-specific mass change versus the oxidation time is plotted. Based on Wagner's oxidation theory [22], if the curve of the square area-specific mass change versus the oxidation time is a straight line, the growth is diffusion-controlled and follows parabolic rate kinetics. The oxidation kinetics of the examined specimens almost followed the parabolic kinetics law. The Pilling–Bedworth Eq. (1) was utilized to evaluate the parabolic rate constant (kp) [23].
3.2. Oxidation kinetics
⎛ δm ⎞ = kp × t + C ⎝ A ⎠
2
For evaluating the oxide layer growth rates of the samples, the weight change per unit area was measured as a function of oxidation time. However, the use of weight change for determining the oxidation resistance of chromium-containing alloys is not correct at high
(1)
where δw is the weight change, A is the steel surface area, kp is the parabolic rate constant, t is the oxidation time and C is an integration constant, explaining the start of parabolic kinetics. This behavior of the 204
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Fig. 7. Secondary electron images of: (a,b) uncoated Crofer 22APU, (c,d) Co-coated and (e,f) Co/ZrO2-coated samples after 500 h of cyclic oxidation at 800 °C.
uncoated and coated steels is caused by the fact that Cr2O3 layer growth follows the parabolic rate law [24]. kp values of uncoated steel from 0 to 500 h was 5.3 × 10−13 mg2 cm−4s−1 due to the growth of Cr2O3 scale which slowly increase by increasing the oxidation time (Fig. 6a, b). The Co-coated and Co/ZrO2-coated steels had a first quick mass gain (kp = 3.3 × 10-12 for Co coated (Fig. 6d) and kp = 2.0 × 10−12 for Co/ ZrO2 coated (Fig. 6f) steels) and, afterwards, the mass gain raised very slightly (kp = 3.4 × 10−13 for Co coated (Fig. 6d) and
Table 3 EDS analysis data for the spots shown in Fig. 7b (compositions in weight. %). Spot
1 2
Element Cr
O
Mn
Fe
37.61 39.46
40.4 34.37
21.99 18.98
0.00 7.19
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and stopping Cr poisoning [14]. In conformity with the information on the oxidation kinetics of steels acquired after cyclic oxidation experiments, weight gain rates of the uncoated and Co coated steels was higher than the oxidation rate of the coated Crofer 22 APU with Co/ ZrO2 composite coating after 500 h of oxidation time in air at 800 °C (Fig. 6) and the useful effect of ZrO2 particles in Co coating as reinforcement for the thermal protecting of Crofer 22APU was confirmed. 3.3. Microstructure and chemical composition of oxidized samples Failure of the oxide layer is generally increased during cyclic oxidation tests relative to isothermal oxidation tests. Thus, the oxidation behavior in the process of thermal cycling further indicates of what is occurring to the materials at the time of being exposed to operating conditions. Entirely the ability of coatings to remain well adhered during thermal cycling is an important property for SOFC applications. Hence, the cyclic oxidation test was selected for evaluating the effect of oxidation on the microstructure and chemical composition of the samples [22,25–27]. After 500 h of cyclic oxidation, an oxide layer was observed on the uncoated sample surface, as shown in Fig. 7a. The oxide layer was principally made up of a black oxide layer and over this continuous outer layer, some loose pyramidal grains or obvious small islands could be observed in the direction of the grain boundaries of the steel (Fig. 7a, b). It shows more oxidation above the grain boundaries of the substrate. As seen, the scale is covered by the octahedral crystals particles (Fig. 7b). The arrangement of these faceted particles (Fig. 7a, b) proposes strongly that they have formed preferentially at locations where the alloy grain boundaries intersect with the surface. It should be considered that the grain boundary diffusion of transition metals through Cr2O3 is 3–5 orders of magnitude faster than the bulk diffusion [12]. The diameter of these well-developed grains is from around 10 μm to about 50 μm. Both EDS data obtained from surface and cross section of oxidizes sample imply that these 0.5–2.5 μm octahedrons are the MnCr2O4 and FeCr2O4 spinle phases (see Table 3b and Fig. 7b). As these particles consist mainly of Mn, Cr and O together with some Fe, it may inferred that these are cubic chromium manganese spinel with a small amount of FeCr2O4 spinel phase. The formation of MnCr2O4 spinel is caused by fast diffusion of manganese along the ferrite and Cr2O3 grain boundaries; on the surface, Mn reacts with the initially formed Cr2O3 and with the inwardly diffused O anions to form the spinel phase [12]. XRD analysis (Fig. 8a) also confirmed these results. Fig. 7c and d demonstrate the surface morphology of Co-coated steel after 500 h of cyclic oxidation in laboratory air at 800 °C. Oxide surface includes pyramidal particles and the XRD analysis verifies (Fig. 8b) that they are Co3O4 and MnCo2O4 spinels with a little amount of MnCr2O4 and FeCr2O4, which were approximately dense after 500 h of oxidation. The FeCr2O4, Co3O4 and MnCo2O4 phases have the same crystal structure (cubic) and similar lattice parameters: a = b = c = 8.3640 A for FeCr2O4, a = b = c = 8.0850 A for Co3O4, and a = b = c = 8.2000 A for MnCo2O4. Therefore, FeCr2O4, Co3O4 and MnCo2O4 phases can overlap with each other. The oxide layer composed on the surface of the Co/ZrO2-coated sample was denser and had smaller crystal size than the one composed on the Co-coated sample surface (Fig. 7e, f). Furthermore, no important marks of spallation or crack were seen on the surface of the Co/ZrO2coated samples after cyclic oxidation. XRD analysis (Fig. 8c) verified that the compositions of the surface oxide layers created on the Cocoated and Co/ZrO2-coated samples were similar. Some of the ZrO2 particles, visible on the surface as-deposited Co/ZrO2 steel (Fig. 3a, b), were inserted in the Co oxide (Fig. 7e, f), which grew due to the diffusion of metal ions [19], but some of them were still visible on the surface of steel after oxidation tests. Fig. 9a and b show the polished cross-sectional images and EDX elements line scan of the uncoated Crofer 22APU after 500 h cyclic oxidation in laboratory air at 800 °C. As can be seen, the surface scale
Fig. 8. XRD pattern of: (a) uncoated sample, (b) Co-coated sample and (c) Co/ ZrO2-coated sample after 500 h of cyclic oxidation in air at 800 °C.
kp = 1.1 × 10−13 for Co/ZrO2 coated (Fig. 6f) steels) after 100 h of oxidation. The reason could be that the first oxidation of Co was anticipated to occur quickly and was completely oxidized after 100 h in air at 800 °C [19]. This showed that the oxidation production of Co coating layer reduced the oxidation rate that was an effective factor for overlong working cycle because it could result in the development of a Cr2O3 scale with high ASR. Weight gains of the Co/ZrO2-coated sample had a very lower rate than the uncoated and Co-coated steels after 100 h. Cr evaporation was not examined here; however, it would just enhance the distinction between uncoated and coated steels due to the point that the Cr migration from the uncoated steel was seemingly higher. Although the reactive elements oxides remarkably improve coating adhesion and decrease oxidation rate and in addition, increase the electrical conductivity of the surface layer, they are not useful as the limits of Cr evaporation because they have naturally low thickness, are porous and cannot be efficient in preventing Cr migration to the surface 206
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Fig. 9. SEM cross-section image and EDS line scan of: (a, b) uncoated, (c, d) Co-coated and (e, f) Co/ZrO2-coated samples after 500 h of cyclic oxidation at 800 °C.
Cr2O3 grain boundaries. Mn reacted with Cr2O3 that was at first created on the surface and the O anions diffused towards the inside in order to create the spinel phase [22]. Fig. 9c and d show the polished cross-sectional images and elements line scan of the Co-coated Crofer 22APU after 500 h cyclic oxidation in laboratory air at 800 °C. Oxide layer composed on the Co-coated Crofer 22APU included the internal layer of Cr-rich oxide with Co oxide and Co-Mn spinle layer on the external surface, corresponding to other studies [22,29]. Fig. 9c shows that the external Co spinel layer was stuck to the internal Cr2O3 layer. But, the entire surface oxide layer adhering to the substrate appeared to be weak. In Co/ZrO2-coated sample, as shown in Fig. 7e and f, the surface oxide layer consisted of a twofold oxide layer with the composition similar to the oxide scale created on the surface of Co-coated sample. XRD analysis (Fig. 8b, c)
composed on the uncoated Crofer 22APU consisted of the internal layer of Cr-rich oxide and Mn-Cr-rich oxide on the external surface, corresponding to other studies [23,24,28]; the thickness of this oxide scale is about 2 μm, as exhibited in Fig. 9a. XRD analysis (Fig. 8a) confirmed that thin inner oxide layer consisted of Cr2O3 and outer oxide layer was composed of MnCr2O4 and FeCr2O4. It could be concluded that surface of the uncoated one was covered with Cr2O3 and, then over this oxide layer, was slowly encased by MnCr2O4 spinels and a little amount of FeCr2O4 spinles as oxidation time increased. Even though Crofer 22 APU steel included lower than 1%Mn, the existence of Mn in the external layer was ascribed to the rapid diffusion of Mn cations compared to other cations (such as Fe and Cr cations) reaching from the Crofer 22APU to the Cr2O3 scale; therefore, the creation of Cr-Mn spinel was caused by the rapid diffusion of Mn in the direction of the steel and 207
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Fig. 10. SEM cross-section image of: (a) Co-coated and (b) Co/ZrO2-coated samples after 100 h of cyclic oxidation in laboratory air at 800 °C.
is anticipated to be proportionate with electrical conductivity, supposing a consistent conductivity of the oxide; it has been shown that the ASR of FSSs electroplated with Co is an approximately linear function of the Cr2O3 layer thickness [32]. A thin Cr2O3 layer is also advantageous since the danger of spallation increases by increasing oxide layer thickness. The spallation of the protective Cr2O3 layer may result in both increased Cr evaporation and creation of non-protective iron oxides. A higher boundary for suitable Cr2O3 layer thickness is about 10 μm in many cases because the danger of oxide spallation and the electrical resistance becomes large above that thickness in a manner that is not acceptable [33]. EDX maps analysis was also performed on the surface of Co-coated (Fig. 11) and Co/ZrO2-coated (Fig. 12) samples after 500 h of cyclic oxidation in laboratory air at 800 °C. These figures undoubtedly are useful for better describing areas of enrichment or depletion in certain elements. The elemental mapping images in Figs. 11 and 12 indicate that there is a small portion of inter-diffusion of ionic species, e.g. Mn, Cr, O, Co and Fe across the interface of coatings and substrate. It is apparently indicated that a distinct layer with significant Co and to the less extent O, Mn, Cr, and Fe was formed on the Crofer 22APU surface (Figs. 11,12). In the Co/ZrO2-coated sample, the presence of ZrO2 particles can be seen in this layer too (Fig. 12b). Formation of a thin layer of Cr and O in the coatings/substrate interface is also in good agreement with the EDS maps (Figs. 11b, c, and 12 c, d). The effect of ZrO2 particles on the development rate of Cr2O3 layer is explained by the reactive element effect, representing several orders of decrease in the oxidation rate of similar kinds of FFS when used as the interconnect [18–20,34]. It is clear that the addition of ZrO2 to the Co coating could inhibit effectively the growth of Cr2O3 scale. Near the ZrO2 particles, Cr2O3 tends to grow inward along the particles of ZrO2 (Fig. 9e). It should be pointed out that only a small number of pores are observed beneath the oxide scales for the Co/ZrO2-coated steels, much fewer than those for the Co alloy coated samples. In research conducted to clarify the effect of the reactive element on the growth of the voids at the oxide scale/alloy interface, adding minor Hf significantly improved the cyclic oxidation resistance of the NiAl alloy by suppressing the growth of voids at the oxide scale/alloy interface in both dry and humid atmospheres [27]. Among the different mechanisms suggested to illustrate the effect of reactive elements (improvement of oxide nucleation [35,36], “sulfur” theory [37,38], “dynamic segregation theory” [39]), the conversion of the oxide scale development mechanism is accepted to present the most advantageous effects. Experiments by utilizing two-stage oxidation tests under 16O2–18O2 [40,41] clearly illustrated that outward diffusion of Cr ions is the prevalent mechanism for Cr2O3 development on Cr2O3-forming alloys. The addition of reactive elements in the Cr2O3 scale modifies the
also verified that the compositions of the surface oxide scales created on the Co-coated and Co/ZrO2-coated samples were similar. Fig. 9e shows that the external Co spinel layer was excellently stuck to the internal Cr2O3 layer. Furthermore, the entire surface oxide layer adhering to the substrate appeared to be intense. The Cr2O3 layer with approximately 2.0 μm thickness was created on the uncoated sample after 500 h oxidation (Fig. 9a), but the internal Cr2O3 layer developed on the Co-coated sample (Fig. 9c) was only around 1.5 μm, exhibiting that the creation of the external Co3O4 and MnCo2O4 spinel layer decreased the development rate of the internal Cr2O3 scale. It is apparently indicated that the external spinel layer has not only limited the outward diffusion of Cr but also repressed the inward diffusion of O, finally resulting in the lower development rate of the internal Cr2O3 scale on Co-coated Crofer 22APU [22,30]. The Cr2O3 layer on the Co/ZrO2-coated sample (Fig. 9e) had thickness of about 800 nm. This was remarkably thinner than the one on the uncoated and Co-coated steels, showing that the incorporation of ZrO2 particles into Co coating decreased the development rate of a Cr2O3 layer on Crofer 22APU, whereas it was placed in SOFC cathode-related situations. For the coated samples, two consecutive oxidation regimes appear (Fig. 6a, b): a rapid oxidation on the first 100 h attributed to the oxidation of Co, and a much slower kinetic regime associated with the oxidation of the substrate. This prove by the cross-section analysis on oxidized samples for about 100 h. As shown in Fig. 10a, the Cr2O3 layer with 1.1 μm average thickness was created on the Co-coated sample after 100 h oxidation, but the internal Cr2O3 layer developed on the Co/ ZrO2-coated sample (Fig. 10b) was only around 0.5 μm. The average thickness of the external Co3O4 and MnCo2O4 spinel layers that were created on the surfaces was about 7 μm after 100 h cyclic oxidation time. The thickness of external layers developed about 2 μm after 100 h in air at 800 °C. Compared to oxidized coated sample for 500 h at 800 °C, the internal Cr2O3 layer thickness was thinner but the external spinel layer have the same thickness. These results show that after 100 h oxidation, the Co coating is completely oxidized and converted into a Co spinel oxide coating, and then, after 500 h of oxidation, the thickness of this outer layer of spinel oxide on both coated samples does not change. But the internal layer of Cr2O3 continues to grow for up to 500 h, then after 100 h, its growth rate, especially for Co/ZrO2-coated sample, reduces significantly. A slow growth rate of the Cr2O3 layer is beneficial due to the fact that Cr2O3 has a low electrical conductivity and causes electrical missing of the SOFC systems. The electric conductivity of Cr2O3 is 0.022 S cm−1 at 800 °C [16], which is considerably lesser than the one of Co oxides (6.7 6.7 and 60 S cm−1) [29]. Therefore, the thickness of the Cr2O3 layer is anticipated to be the most important ingredient for the electrical resistance of the sample [31]. The thickness of the Cr2O3 layer 208
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Fig. 11. EDX maps analysis on the cross section surface of Co-coated sample: (a) Co, (b) Cr, (c) O, (d) Mn, and (e) Fe after 500 h of cyclic oxidation in laboratory air at 800 °C.
interconnect utilization is the ASR of the surface oxide layers. High and steady electrical conductivity is fundamental for metallic interconnects under the work conditions of the SOFC. ASR is utilized to estimate the electrical conductivity of interconnects [42]. ASR evaluations were done for the uncoated, Co-coated and Co/ZrO2-coated samples after cyclic oxidation experiment in laboratory air at 800 °C for 500 h as a function of temperature. The ASR value of the studied samples measured within 650–800 °C increased with temperature (Fig. 13a). Correlation between log (ASR/T) and 1/T was linear (Fig. 13b), which was consistent with the semiconductor behavior, depicted with Arrhenius equation [43]:
ionic species diffusion, and inward O ion diffusion becomes the prevalent transport mechanism. Such conversion in the transport mechanism is attributed to the segregation of reactive element ions or second phase particles at the Cr2O3 grain boundaries. It results in obstructing the outward cation diffusion, diminishing the oxidation rate and improving the oxide scale adherence. Such a change in the Cr2O3 scale growth mechanism results in inhibiting the formation and coalescence of cation vacancies at the metal/oxide interface. Void formation at the metal/oxide interface is a result of the outward transport of metallic ions through vacancies which allow their coalescence at the interface to weaken the adhesion to the substrate [18].
ASR = A.T. exp (−Ea/KT) 3.4. Electrical behavior Another
essential
characteristic
for
estimating
the
(2)
where A is the proportional factor, T is the absolute temperature, k is the Boltzmann constant and Ea indicates the activation energy for
SOFC 209
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Fig. 12. EDX maps analysis on the cross section surface of Co/ZrO2-coated sample: (a) Co, (b) Zr, (c) Cr (d) O, (e) Mn and (f) Fe after 500 h of oxidation in laboratory air at 800 °C.
an extra heat treatment to make the coating dense [46,47]. In those studies μm thick Cr2O3 layers were formed due to the heat treatment necessary to increase the density of the powder. As the ASR measurements in Fig. 13 show, this would have a tremendous effect on electrical resistance at 800 °C. Therefore, metallic conversion coatings, as well as coatings deposited with other techniques that do not require a high temperature heat treatment, seem to be the most suitable coating techniques for SOFC. The activation energy for conductivity (Ea) was estimated from the gradient of the log (ASR/T) versus 1/T graph (Fig. 13b) for all the three studied steels. The Ea values were calculated to be 25.14 kJ mol−1 for the uncoated sample, 14.51 kJ mol−1 for Co and 8.571 kJ mol−1 for the Co/ZrO2 coated steel. Co is a p-type dopant that has an important effect on increasing the conductivity of spinel compounds such as MnCo2O4 (60 scm−1), CoCr2O4 (7.4 scm−1) and CoFe2O4 (0.93 scm−1) [48] compared to Cr2O3 (0.02 S cm−1) and (Mn,Cr)3O4 (0.5 S cm−1 for MnCr2O4 and 0.02 S cm−1 for Mn1.2Cr1.8O4) [49]. The ASR values of
conduction. Fig. 13 shows that the ASR measured for all the samples is generally less than the maximum allowed level for use in SOFC interconnects, i.e. 100 mΩ cm2 [44]. The ASR values obtained for Co-coated Co/ZrO2coated steels was far lower than the uncoated one. As an example, the ASR at 800 °C for Co-coated and Co/ZrO2-coated steels was 18.1 and 14.5 mΩ cm2, respectively, while for the uncoated one, it was 35.6 mΩ cm2. This obviously exhibited the desired effect of deposited coating on obstructing the creation and development of high electrical resistance oxides, and creation of Co and Co-Mn spinels, which had less electrical resistance and compatible thermal expansion with Crofer 22APU [19,24]. When ASR values were measured at 650 °C (Fig. 13a), an important increase in ASR could be observed because of the far thicker Cr2O3 layer on the steels [45]. In this paper coated samples exposed to 800 °C showed very low ASR values. The reason for this is the very thin Cr-rich oxide layer. In several studies coatings have been applied as powder, with the need for 210
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• The potential of Co and Co/ZrO • • • •
2 to function as protective coatings on Crofer 22APU stainless steel was evaluated via cyclic oxidation tests. The Co/ ZrO2 coating decreased the Cr2O3 layers' thickness from about 2.0 μm on uncoated steel and around 1.5 μm on Co-coated sample to ˜800 nm after 500 h oxidation in laboratory air at 800 °C. The Co/ZrO2 coatings could decrease the oxidation rates of the steel during exposure to laboratory air at 800˚C after 100–500 h of oxidation, as compared to the uncoated and Co-coated samples. ASR of the uncoated and coated Crofer 22APU steels was measured in the temperature range of 650–800 °C, which revealed a semiconductor behavior. ASR evaluations at 800 °C in laboratory air exhibited that the lowest ASR values were acquired for the Co/ZrO2-coated steels and was equal to 14.5 mΩ cm2 at 800 °C after 500 h cyclic oxidation in laboratory air.
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Fig. 13. ASR of the uncoated, Co coated and Co/ZrO2 coated samples as a function of temperature after cyclic oxidation test in laboratory air at 800 °C for 500 h.
Co/ZrO2-coated Crofer 22APU were lower than ASR values acquired for the Co-coated steels, which showed that the ZrO2 particles did not have any damaging effect on the ASR. The ASR was increased as the highly resistive Cr2O3 scale on the steels raised in thickness [50]. On the other hand, the oxide layer (about 2 μm thick) grown on the uncoated sample was thicker than that (around 1.5 μm thick for Co-coated and 800 nm for Co/ZrO2-coated Crofer 22APU) formed on the steel with the Co and Co/ZrO2 electroplated coatings. As mentioned, adding ZrO2 could improve the interfacial contact by suppressing the formation of voids at the metal/oxide interface (Fig. 9c, e) which might result in lower ASR. The Cr2O3 layer is expected to dominate the ASR of the oxide scale. Thus it is conceived that adding reactive element, which results in a thinner Cr2O3 scale, have a beneficial effect on ASR. Earlier studies of Co- and Ce/Co-coated Sanergy HT at 800 °C have shown that adding Ce lowers the electrical resistance significantly. The Cr2O3 scale thicknesses in this work for the Co and Ce/Co coated material after 500 h at 750 °C was < 1 μm [44]. In a study by Qu et al [51] the ferritic stainless steel 430 SS was dip-coated with Co, Y, and Y/ Co that Y/Co-coatings, which showed an even better oxidation resistance than the Ce/Cocoated material. And also, in contrast to the results presented in this work, the Cr2O3 scale had grown significantly thicker (2.25 mm) after 500 h at 750 °C for the exclusively Co-coated material, compared to the Y/Co-coated material (0.75 μm). It is not trivial to compare ASR values since in most cases the different setups and methods are used. However, what can be concluded from the findings in this work and the above- cited studies is that as long as the Cr2O3 scale is ˜1 μm or thinner, low ASR values can be expected irrespective of the chemical composition, or coating method, of the Co-spinel or MCO-coating [44]. 4. Conclusions
• Well adherent Co/ZrO
2 coatings were electroplated on the ferritic stainless steel Crofer 22APU.
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