Oxidation Characteristics and Electrical Properties of Doped Mn-Co Spinel Reaction Layer for Solid Oxide Fuel Cell Metal Interconnects

To prevent Cr poisoning of the cathode and to retain high conductivity during solid oxide fuel cell (SOFC) operation, Cu or La doped Co-Mn coatings on a metallic interconnect is deposited and followed by oxidation at 750 ◦C. Microstructure and composition of coatings after preparation and oxidation is analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). High energy micro arc alloying process, a low cost technique, is used to prepare Cu or La doped Co-Mn coatings with the metallurgical bond. When coatings oxidized at 750 ◦C in air for 20 h and 100 h, Co3O4 is the main oxide on the surface of Co-38Mn-2La and Co-40Mn coatings, and (Co,Mn)3O4 spinel continues to grow with extended oxidation time. The outmost scales of Co-33Mn-17Cu are mainly composed of cubic MnCo2O4 spinel with Mn2O3 after oxidation for 20 h and 100 h. The average thickness of oxide coatings is about 60–70 μm after oxidation for 100 h, except that Co-40Mn oxide coatings are a little thicker. Area-specific resistance of Cu/La doped Co-Mn coatings are lower than that of Co-40Mn coating. (Mn,Co)3O4/MnCo2O4 spinel layer is efficient at blocking the outward diffusion of chromium and iron.


Introduction
Co-Mn-based spinel coatings and conversion coatings are the potential candidates for metallic interconnect coating materials of solid oxide fuel cell (SOFC) [1][2][3][4][5].Some techniques have been applied to prepare these coatings such as electrophoretic deposition, plasma spraying, dip-coating, sol-gel method, screen printing, and physical vapor deposition.The high energy micro arc alloying process (HEMAA) [6][7][8] is a low cost technique to produce metallurgically bonded coatings.It is economically attractive for coating preparation to significantly reduce the cost of SOFC.In previous study [9], the Cr/La alloying layer is successively obtained on 430 stainless steel (SS) surface by HEMAA using metallic Cr electrode and metallic La electrode, respectively.The result shows that a good protective perovskite coating with an acceptable electrical contact resistance forms on the substrate steel after coating is thermally grown.
Reactive elements have been added to improve the high temperature oxidation characteristics of Co-Mn-based coatings.Mn-Co-Y-O spinel coatings [10] were prepared on AISI 430 and Crofer 22 APU steels by the magnetron sputtering.A combined Ce/Co alloy coatings [11] were deposited on metallic interconnects by physical vapor deposition process, and Co/CeO 2 composite coatings [12] were electroplated on the ferritic stainless steels Crofer 22 APU and Crofer 22 H. Lanthanum (La) is usually synthesized as perovskite cathode and interconnect material for intermediate temperature solid oxide fuel cell [13][14][15][16].High temperature oxidation characteristics of active element Lanthanum have been investigated in alloys.The result shows that the formation of La oxides remarkably improves oxidation resistance of alloys [17,18].
The dopant of Cu and/or Ni into Mn-Co spinel generally affects the high temperature properties of spinel oxide because of crystal structure and phase of spinel oxide after sintering changed [19,20].Cu doped Mn-Co oxide was sintered into pellets after the High Energy Ball Milling process [21].The results show that cubic Mn-Co spinel stability, spinel sintering behavior and electrical conductivity are improved by Cu addition.Cu-Mn-Co spinel coatings prepared via the slurry dip-coating [20,22,23] and the screen-printing technique [24] exhibit high electrochemical performance and oxidation characteristics.However, the adherence to substrate for spinel coatings is still a problem.HEMAA process [25] produces the metallurgically bonded alloy coatings and forms spinel coatings via in situ oxidation.
In this test, lanthanum/copper added into Co-Mn spinel coating for metallic interconnect is studied in detail.This work aims to investigate high-temperature oxidation characteristics and electrical properties of Co-33Mn-17Cu and Co-38Mn-2La coatings prepared by HEMAA for intermediate temperature SOFC interconnects application.
Oxidation measurement for deposited coatings is carried out at 750 • C in air for 20 h and 100 h.Area specific resistance (ASR) measurement of Co-33Mn-17Cu, Co-38Mn-2La and Co-40Mn coatings after oxidation is performed by the four-probe direct current (DC) method [3,8].The applied voltage is 5 V and contact area of Pt electrode is about 10 mm 2 bonding with Pt paste.The measured sample is placed in the constant temperature zone of horizontal tube furnace at room temperature, and then is heated up to setting temperature (500-800 • C).The value of ASR is taken when every setting temperature keeps steady.
X-ray diffractometer (XRD, D/MAX-3B, Rigaku Co., Tokyo, Japan) is conducted for phase analyses, and surface roughness analyses are performed by 3D microscope (VHX-900, KEYENCE, Tokyo, Japan) on the top surfaces.Specimens are mounted in a cold-setting epoxy resin to avoid cracking and give support for polishing.Cross-section surfaces and quantitative elemental analyses are examined by scanning electron microscopy (SEM, JSM-5800, JEOL, Tokyo, Japan) with energy dispersive X-ray spectroscopy (EDS).

Microstructural Studies of Coated Samples
Figure 1a shows that Co-38Mn-2La alloy (electrode) is a two-phase structure composed of a Co-based solid solution and an La-rich phase (light phase), which is most localized at grain boundaries.EDS analysis on Co-38Mn-2La alloy reveals that the concentrations of Co and Mn in solid solution are 63.96 and 36.04 in at.%. Figure 1b shows that a continuous Co-38Mn-2La coating has been produced on the substrate surface with a metallurgical bonding and the average thickness of coating is about 30 µm. Figure 1c shows that Co-33Mn-17Cu alloy is composed of a Co-rich phase and a Cu-rich phase (light phase).EDS reveals that Cu concentration is about 40.4 at.% in the Cu-rich phase.Figure 1d shows the continuous Co-33Mn-17Cu coating is deposited and the interface keeps good adherence to the substrate.

Microstructures of Oxide Coatings on the Metallic Interconnects
Figure 2 shows X-ray diffraction and surface morphology of coatings oxidized at 750 °C for 20 h and 100 h.Before oxidation, Co-38Mn-2La alloy coating in Figure 2a is mainly a cubic Co structure and peaks shift toward low angles by referring to pure Co When coating oxidized at 750 °C in air for 20 h, the outmost scales in Figure 2b combined with EDS analysis consist of Co3O4 and small amount of (Co,Mn)3O4, which is an intermediate tetragonal spinel structure between MnCo2O4 (cubic) and Mn3O4 (tetragonal).The previous study [25] discloses that Co3O4 prefer to form under high oxygen partial pressure and only a thin layer is found on the top oxide layer.When Co-38Mn-2La coating oxidized for 100 h, diffraction peak of tetragonal (Co,Mn)3O4 spinel is intensified in Figure 2c, due to Mn outer-diffusion with the extended time.
Before oxidation, Co-33Mn-17Cu alloy coating in Figure 2d is mainly Cu characteristic peak.After oxidation for 20 h in Figure 2e, the outmost scales combined with EDS analysis are mainly composed of cubic MnCo2O4 spinel with Mn2O3 oxide.The same MnCo2O4 and Mn2O3 diffraction peaks are also detected after Co-33Mn-17Cu coating oxidized for 100 h in Figure 2f.Oxidation products of Co-40Mn coating after 20 h and 100 h in Figure 2g,h are the same as Co-38Mn-2La coating.
Figure 2f1 shows surface morphology of Co-33Mn-17Cu coating oxidized for 100 h, which is a typical spinel structure.Element distributions in Table 1 indicate the trace amount of Fe and Cr detected on the oxide surface.Figure 2b1,b2 show surface morphology of Co-38Mn-2La coating oxidized for 20 h.Co3O4 is the main oxides with some fine pores.The size of pores is correlation with Mn content of Co3O4 oxides and will shrink with Mn content increasing [8].Co3O4 scale formed on the surface is a cubic spinel structure with Mn content 5.6 at.% from XRD and EDS analysis in Table 1, which means the scale is a Co-rich Co-Mn spinel phase.Some granular appearance in Figure 2b2 is also seen on the surface and EDS in Table 1 shows that element compositions are the same as Figure 2b1.It is hypothesized that the granular Co3O4 found in local area is due to active element Lanthanum enrichment, which is found in Co-38Mn-2Dy coating but not for Co-40Mn coating [25].When coating oxidized for 100 h, fine pores and needle-like morphology in Figure 2c1 are visible and uniformly distributed in Co3O4 scales.(Co,Mn)3O4 scales in Figure 2c2 are the typical spinel structure and Mn content in (Co,Mn)3O4 is higher than in MnCo2O4 spinel in Table 1, which is in accordance with XRD analysis.The granular appearance is not found after Co-38Mn-2La coating oxidized for 100 h.Surface morphology of Co-40Mn coating oxidized for 20 h and 100 h is showed in Figure 2g1,h1.It is the same morphology as Co-38Mn-2La coating except for the granular appearance.Figure 2f 1 shows surface morphology of Co-33Mn-17Cu coating oxidized for 100 h, which is a typical spinel structure.Element distributions in Table 1 indicate the trace amount of Fe and Cr detected on the oxide surface.Figure 2b 1 ,b 2 show surface morphology of Co-38Mn-2La coating oxidized for 20 h.Co 3 O 4 is the main oxides with some fine pores.The size of pores is correlation with Mn content of Co 3 O 4 oxides and will shrink with Mn content increasing [8].Co 3 O 4 scale formed on the surface is a cubic spinel structure with Mn content 5.6 at.% from XRD and EDS analysis in Table 1, which means the scale is a Co-rich Co-Mn spinel phase.Some granular appearance in Figure 2b 2 is also seen on the surface and EDS in Table 1 shows that element compositions are the same as Figure 2b 1 .It is hypothesized that the granular Co 3 O 4 found in local area is due to active element Lanthanum enrichment, which is found in Co-38Mn-2Dy coating but not for Co-40Mn coating [25].When coating oxidized for 100 h, fine pores and needle-like morphology in Figure 2c 1 are visible and uniformly distributed in Co 3 O 4 scales.(Co,Mn) 3 O 4 scales in Figure 2c 2 are the typical spinel structure and Mn content in (Co,Mn) 3 O 4 is higher than in MnCo 2 O 4 spinel in Table 1, which is in accordance with XRD analysis.The granular appearance is not found after Co-38Mn-2La coating oxidized for 100 h.Surface morphology of Co-40Mn coating oxidized for 20 h and 100 h is showed in Figure 2g 1 ,h 1 .It is the same morphology as Co-38Mn-2La coating except for the granular appearance.Figure 3 shows that mass gain of Co-33Mn-17Cu coating is smallest when coatings oxidized for 20 h and followed by Co-38Mn-2La coating.However, after oxidation for 100 h mass gain of three coatings is almost overlapped.

Microstructures of Oxide Coatings on the Metallic Interconnects
Figure 4 shows cross section (back-scattered electron image) and EDS line scanning of Co-38Mn-2La coating oxidized at 750 °C in air for 100 h.Oxide coating is compact and good adherence to 430 SS substrate.The average thickness of coating is about 64 μm. Figure 4a1 shows cross-section morphology of Co3O4 on the top oxide layer and an Mn-rich oxide layer under the Co3O4 layer.Figure 3 shows that mass gain of Co-33Mn-17Cu coating is smallest when coatings oxidized for 20 h and followed by Co-38Mn-2La coating.However, after oxidation for 100 h mass gain of three coatings is almost overlapped.Figure 4a2 shows cross-section morphology of (Co,Mn)3O4 scales on the top oxide layer.EDS indicates that light phases precipitating in the oxide coating in Figure 4a2 are La-rich oxides.According to line scanning analysis, Mn is enriched in the vicinity of the top oxide layer and Co content is homogeneous in the oxide coating.Enrichment of Cr is detected in the oxide/substrate interface, and Cr content is very low in the coating.Fe outward diffusion is found in the oxide coating in Table 2.  Cross-section morphology of Co-33Mn-17Cu coating oxidized for 100 h is showed in Figure 5.Some micro-pores are found in scales due to the formation of abundant spinel oxide and diffusion of elements.Similarly, coating keeps good adherence to the substrate and its average thickness is   Figure 4a2 shows cross-section morphology of (Co,Mn)3O4 scales on the top oxide layer.EDS indicates that light phases precipitating in the oxide coating in Figure 4a2 are La-rich oxides.According to line scanning analysis, Mn is enriched in the vicinity of the top oxide layer and Co content is homogeneous in the oxide coating.Enrichment of Cr is detected in the oxide/substrate interface, and Cr content is very low in the coating.Fe outward diffusion is found in the oxide coating in Table 2.    Cross-section morphology of Co-33Mn-17Cu coating oxidized for 100 h is showed in Figure 5.Some micro-pores are found in scales due to the formation of abundant spinel oxide and diffusion of   Cross-section morphology of Co-33Mn-17Cu coating oxidized for 100 h is showed in Figure 5.Some micro-pores are found in scales due to the formation of abundant spinel oxide and diffusion of elements.Similarly, coating keeps good adherence to the substrate and its average thickness is 60-70 µm.The line scan discloses the outer part of oxide layer rich in Mn and Co concentrate increasing slightly in the inner oxide layer.The aggregation of element Cu near the surface is not obviously detected.Element Fe/Cr outward diffusion behavior (Table 3) is analogous to the Co-38Mn-2La oxide layer.3) is analogous to the Co-38Mn-2La oxide layer.Table 3. Composition analysis by EDS of the local region in Figure 5. Cross-section morphology of Co-40Mn coating oxidized for 100 h in Figure 6 shows Co3O4-rich layer on the outmost oxides, and Mn-rich oxide layer is still next to the Co3O4 layer, which is the typical morphology for Co-38Mn-2La and Co-40Mn coatings.Fe content is also high in local area (point 4, Table 4).Additionally, the average thickness of Co-40Mn oxide coating with a Co3O4-rich layer is larger than that of Co-38Mn-2La and Co-33Mn-17Cu oxide coating.Cross-section morphology of Co-40Mn coating oxidized for 100 h in Figure 6 shows Co 3 O 4 -rich layer on the outmost oxides, and Mn-rich oxide layer is still next to the Co 3 O 4 layer, which is the typical morphology for Co-38Mn-2La and Co-40Mn coatings.Fe content is also high in local area (point 4, Table 4).Additionally, the average thickness of Co-40Mn oxide coating with a Co 3 O 4 -rich layer is larger than that of Co-38Mn-2La and Co-33Mn-17Cu oxide coating.

ASR Measurements
Co-38Mn-2La coating after oxidation for 100 h is showed in Figure 7.The surface of oxide coatings is generally highly irregular due to the HEMAA process, and surface microtopography and roughness of oxides for three coatings are close.The values of surface roughness disclose that surface become relative smooth with extended oxidation time.Arithmetical average roughness height (Sa) of Co-38Mn-2La oxide coating is about 7.21 μm for 0 h, 6.34 μm after oxidation for 20 h and 5.53 μm for 100 h.Measurement of area specific resistance is bonded with Pt paste, and the value is little affected by surface roughness.
ASR of three coatings after oxidation at 750 °C for 20 h and 100 h is showed in Figure 8.All ASR values decrease gradually with increasing temperature from 500 to 800 °C.It is typical for electric conductivity of a semiconductor oxide as a function of temperature [24,27], in good agreement with the results of Mn-Co-based spinel in literature.ASR can make a corresponding response to the electrical conductivity and thicknesses of thermally grown oxide layers.Thus, ASR of three coatings after oxidation for 100 h is usually a little higher than that for 20 h, especially at low temperature, such as 500 °C and 600 °C.ASR of Co-40Mn oxide coating is the largest among three oxide coatings for both 100 h and 20 h (Figure 8, line 1 and line 2), ASR of Co-38Mn-2La oxide coating (line 3 and line 4) is smaller than that of Co-40Mn oxide coating, and ASR of Co-33Mn-17Cu oxide coating stays the smallest.Especially when Co-33Mn-17Cu oxide coating oxidized for 20 h, ASR values at low temperature keep in a very small range (line 6).ASR at 800 °C of Co-40Mn oxide coating is 79.2 mΩ cm 2 for 20 h and 43.2 mΩ cm 2 for 100 h, the value obviously decreasing with oxidation time.The reason may be that the structure and composition of oxide products change along with oxidation time, such as some (Co,Mn)3O4 spinel forming.ASR at 800 °C for Co-38Mn-2La oxide coating is about 19.6 mΩ cm 2 for 20 h and 24 mΩ cm 2 for 100 h, the value slowly increasing with oxidation time.ASR at 800 °C for Co-33Mn-17Cu oxide coating is about 7.5 mΩ cm 2 for 20 h and 41 mΩ cm 2 for 100 h, the value obviously increasing with oxidation time.The reason has not been discussed in detail, but,

ASR Measurements
Co-38Mn-2La coating after oxidation for 100 h is showed in Figure 7.The surface of oxide coatings is generally highly irregular due to the HEMAA process, and surface microtopography and roughness of oxides for three coatings are close.The values of surface roughness disclose that surface become relative smooth with extended oxidation time.Arithmetical average roughness height (S a ) of Co-38Mn-2La oxide coating is about 7.21 µm for 0 h, 6.34 µm after oxidation for 20 h and 5.53 µm for 100 h.Measurement of area specific resistance is bonded with Pt paste, and the value is little affected by surface roughness.
ASR of three coatings after oxidation at 750 • C for 20 h and 100 h is showed in Figure 8.All ASR values decrease gradually with increasing temperature from 500 to 800 • C. It is typical for electric conductivity of a semiconductor oxide as a function of temperature [24,27], in good agreement with the results of Mn-Co-based spinel in literature.ASR can make a corresponding response to the electrical conductivity and thicknesses of thermally grown oxide layers.Thus, ASR of three coatings after oxidation for 100 h is usually a little higher than that for 20 h, especially at low temperature, such as 500 • C and 600 • C. ASR of Co-40Mn oxide coating is the largest among three oxide coatings for both Coatings 2018, 8, 42 8 of 11 100 h and 20 h (Figure 8, line 1 and line 2), ASR of Co-38Mn-2La oxide coating (line 3 and line 4) is smaller than that of Co-40Mn oxide coating, and ASR of Co-33Mn-17Cu oxide coating stays the smallest.Especially when Co-33Mn-17Cu oxide coating oxidized for 20 h, ASR values at low temperature keep in a very small range (line 6).ASR at 800 • C of Co-40Mn oxide coating is 79.2 mΩ cm 2 for 20 h and 43.2 mΩ cm 2 for 100 h, the value obviously decreasing with oxidation time.The reason may be that the structure and composition of oxide products change along with oxidation time, such as some (Co,Mn) 3 O 4 spinel forming.ASR at 800 • C for Co-38Mn-2La oxide coating is about 19.6 mΩ cm 2 for 20 h and 24 mΩ cm 2 for 100 h, the value slowly increasing with oxidation time.ASR at 800 • C for Co-33Mn-17Cu oxide coating is about 7.5 mΩ cm 2 for 20 h and 41 mΩ cm 2 for 100 h, the value obviously increasing with oxidation time.The reason has not been discussed in detail, but, according to existing results, it may be the formation of many isolated for 100 h in the oxide layer, besides the thickness increasing.according to existing results, it may be the formation of many isolated micro-pores for 100 h in the oxide layer, besides the thickness increasing.

Discussion
In previous study [3,8,25], the results disclose that ambient dissociation pressure of oxygen at high temperature will favor the formation of some specific oxides.When coating oxidized in air, partial pressure of oxygen gradually decreases from gas/scale interface to oxide/alloy interface; therefore, a thin Co3O4 layer forms at high oxygen partial pressure and is usually found on the top oxide layer (Figures 4a1 and 6).Rapid cation diffusion can be caused in the mono-oxide such as CoO and MnO, but the continuous (Co,Mn)3O4 spinel oxide layer causes a significant reduction in the according to existing results, it may be the formation of many isolated micro-pores for 100 h in the oxide layer, besides the thickness increasing.

Discussion
In previous study [3,8,25], the results disclose that ambient dissociation pressure of oxygen at high temperature will favor the formation of some specific oxides.When coating oxidized in air, partial pressure of oxygen gradually decreases from gas/scale interface to oxide/alloy interface; therefore, a thin Co3O4 layer forms at high oxygen partial pressure and is usually found on the top oxide layer (Figures 4a1 and 6).Rapid cation diffusion can be caused in the mono-oxide such as CoO

Discussion
In previous study [3,8,25], the results disclose that ambient dissociation pressure of oxygen at high temperature will favor the formation of some specific oxides.When coating oxidized in air, partial pressure of oxygen gradually decreases from gas/scale interface to oxide/alloy interface; therefore, a thin Co 3 O 4 layer forms at high oxygen partial pressure and is usually found on the top oxide layer (Figures 4a 1 and 6).Rapid cation diffusion can be caused in the mono-oxide such as CoO and MnO, but the continuous (Co,Mn) 3 O 4 spinel oxide layer causes a significant reduction in the oxide rate and cation diffusion.Co-Mn spinel oxides first dispersedly precipitate along grain boundaries of oxide matrix and then evolve into a continuous spinel oxide layer when manganese contents are increasing.Therefore, a continuous (Co,Mn) 3 O 4 spinel layer may be formed in the outer part of the scale because of the relatively high oxygen activity and the extended oxidation time.This result is found in both Co-38Mn-2La and Co-40Mn coatings in Figure 2. When coating oxidized for 100 h, (Co,Mn) 3 O 4 spinel continues to grow.
Partial Mn-Co-O phase diagram in air relative to different Co concentration and temperature is discussed in reference [28], and it shows that MnCo 2 O 4 spinel is the stable phase in the operation temperature of SOFC.In this study, the main spinel phase (Mn,Co) 3 O 4 after Co-38Mn-2La coating and Co-40 Mn coating oxidized at 750 • C for 100 h is different of Mn-Co ratio than 1:2, and Mn content is a little higher than that in MnCo 2 O 4 spinel.This becomes complicated because the original concentration ratio of Co and Mn, the thickness of alloy coating, oxidation temperature and element diffusion in oxides all play an important role in the formation of the (Co,Mn) 3 O 4 spinel oxide.When Cu was added into Co-Mn coating, MnCo 2 O 4 spinel successfully forms on the surface of oxides for both 20 h and 100 h.In general, MnCo 2 O 4 spinel has the higher electrical conductivity than (Co,Mn) 3 O 4 spinel, so Cu dopant is helpful for improving high temperature electrical behavior of Co-Mn coating, especially for 500-700 • C.However, the formation of micro-pores in oxides will also affect high temperature behavior of coating.
According to EDS in Tables 2-4 outward diffusion of Cr is suppressed by composited oxide coatings.Fe diffuses into oxide coating but is suppressed by (Mn,Co) 3 O 4 /MnCo 2 O 4 spinel and then aggregates under the (Mn,Co) 3 O 4 /MnCo 2 O 4 layer (points 1 and 2 in Tables 2 and 3, points 3 and 4 in Table 4).It is apparent that the presence of the (Mn,Co) 3 O 4 /MnCo 2 O 4 spinel layer blocks outward diffusion of Fe.When chromium from substrate forms a continuous oxide at the interface, Fe outward diffuses will be also suppressed (points 3 in Tables 2 and 3).Metallic Co diffuses both outward into the oxide layer and inward into 430 SS substrate during oxidation (point 4 in Table 2).

Conclusions
The metallurgical bond Co-33Mn-17Cu, Co-38Mn-2La and Co-40Mn coatings are successfully deposited on the surface of 430 SS using high-energy micro-arc alloying process.MnCo 2 O 4 and Mn 2 O 3 are mainly oxide products when Co-33Mn-17Cu coating oxidized at 750 • C for 20 h and 100 h.ASR of Co-33Mn-17Cu oxide coating is the smallest among the three coatings when oxidized for 20 h.La-rich oxides are locally precipitated in the oxide layer after Co-38Mn-2La coating oxidized for 100 h, and some granular appearance of Co 3 O 4 oxide on the top surface are found for 20 h, which is not found in Co-33Mn-17Cu and Co-40Mn coatings.A thin Co 3 O 4 spinel layer with a small amount of Mn oxides favors forming on the top oxide layer, and the (Mn,Co) 3 O 4 spinel between MnCo 2 O 4 (cubic) and Mn 3 O 4 (tetragonal) gradually grows on the surface of oxide layer with the extended oxidation time for Co-38Mn-2La and Co-40Mn coatings.The value of area-specific resistance at 800 • C for Co-38Mn-2La coating oxidized for 100 h is about 24 mΩ cm 2 , which is the smallest among three coatings.Outward diffusion of Fe and Cr from substrate is remarkably suppressed by (Mn,Co) 3 O 4 /MnCo 2 O 4 spinel oxide layers.The results demonstrate that Cu/La dopant leads to different structures and properties of Mn-Co spinel coating, which is possibly used as coating material for the intermediate temperature SOFC.

Figure 2
Figure2shows X-ray diffraction and surface morphology of coatings oxidized at 750 • C for 20 h and 100 h.Before oxidation, Co-38Mn-2La alloy coating in Figure2ais mainly a cubic Co structure and peaks shift toward low angles by referring to pure Co When coating oxidized at 750 • C in air for 20 h, the outmost scales in Figure2bcombined with EDS analysis consist of Co 3 O 4 and small amount of (Co,Mn) 3 O 4 , which is an intermediate tetragonal spinel structure between MnCo 2 O 4 (cubic) and Mn 3 O 4 (tetragonal).The previous study[25] discloses that Co 3 O 4 prefer to form under high oxygen partial pressure and only a thin layer is found on the top oxide layer.When Co-38Mn-2La coating oxidized for 100 h, diffraction peak of tetragonal (Co,Mn) 3 O 4 spinel is intensified in Figure2c, due to Mn outer-diffusion with the extended time.Before oxidation, Co-33Mn-17Cu alloy coating in Figure2dis mainly Cu characteristic peak.After oxidation for 20 h in Figure2e, the outmost scales combined with EDS analysis are mainly composed of cubic MnCo 2 O 4 spinel with Mn 2 O 3 oxide.The same MnCo 2 O 4 and Mn 2 O 3 diffraction peaks are also detected after Co-33Mn-17Cu coating oxidized for 100 h in Figure2f.Oxidation products of Co-40Mn coating after 20 h and 100 h in Figure2g,h are the same as Co-38Mn-2La coating.Figure2f1 shows surface morphology of Co-33Mn-17Cu coating oxidized for 100 h, which is a typical spinel structure.Element distributions in Table1indicate the trace amount of Fe and Cr detected on the oxide surface.Figure2b1 ,b 2 show surface morphology of Co-38Mn-2La coating oxidized for 20 h.Co 3 O 4 is the main oxides with some fine pores.The size of pores is correlation with Mn content of Co 3 O 4 oxides and will shrink with Mn content increasing[8].Co 3 O 4 scale formed on the surface is a cubic spinel structure with Mn content 5.6 at.% from XRD and EDS analysis in Table1, which means the scale is a Co-rich Co-Mn spinel phase.Some granular appearance in Figure2b2 is also seen on the surface and EDS in Table1shows that element compositions are the same as Figure2b1 .It is hypothesized that the granular Co 3 O 4 found in local area is due to active element Lanthanum enrichment, which is found in Co-38Mn-2Dy coating but not for Co-40Mn coating[25].When coating oxidized for 100 h, fine pores and needle-like morphology in Figure2c1 are visible and uniformly distributed in Co 3 O 4 scales.(Co,Mn) 3 O 4 scales in Figure2c2 are the typical spinel structure and Mn content in (Co,Mn) 3 O 4 is higher than in MnCo 2 O 4 spinel in Table1, which is in accordance with XRD analysis.The granular appearance is not found after Co-38Mn-2La coating oxidized for 100 h.Surface morphology of Co-40Mn coating oxidized for 20 h and 100 h is showed in Figure2g1 ,h 1 .It is the same morphology as Co-38Mn-2La coating except for the granular appearance.

Figure 3 .
Figure 3. Mass gain of oxidized coatings at 750 • C in air.

Figure 4
Figure 4 shows cross section (back-scattered electron image) and EDS line scanning of Co-38Mn-2La coating oxidized at 750 • C in air for 100 h.Oxide coating is compact and good adherence to 430 SS substrate.The average thickness of coating is about 64 µm.Figure 4a 1 shows cross-section morphology of Co 3 O 4 on the top oxide layer and an Mn-rich oxide layer under the Co 3 O 4 layer.Figure4a2 shows cross-section morphology of (Co,Mn) 3 O 4 scales on the top oxide layer.EDS indicates that light phases precipitating in the oxide coating in Figure4a2 are La-rich oxides.According to line scanning analysis, Mn is enriched in the vicinity of the top oxide layer and Co content is homogeneous in the oxide coating.Enrichment of Cr is detected in the oxide/substrate interface, and Cr content is very low in the coating.Fe outward diffusion is found in the oxide coating in Table2.

Figure 3 .
Figure 3. Mass gain of oxidized coatings at 750 °C in air.

Figure 4 .
Figure 4. Cross-section morphology and EDS line scanning of Co-38Mn-2La coating oxidized at 750 °C in air for 100 h.The line scan position is shown in the cross-sectional image.(a) Cross section; (b) EDS line scanning.

Figure 4 .
Figure 4. Cross-section morphology and EDS line scanning of Co-38Mn-2La coating oxidized at 750 • C in air for 100 h.The line scan position is shown in the cross-sectional image.(a) Cross section; (b) EDS line scanning.

Coatings 2018, 8 ,
x FOR PEER REVIEW 6 of 11 60-70 μm.The line scan discloses the outer part of oxide layer rich in Mn and Co concentrate increasing slightly in the inner oxide layer.The aggregation of element Cu near the surface is not obviously detected.Element Fe/Cr outward diffusion behavior (Table

Figure 5 .
Figure 5. Cross-section morphology and EDS line scanning of Co-33Mn-17Cu coating oxidized at 750 °C in air for 100 h.The line scan position is shown in the cross-sectional image.

Figure 5 .
Figure 5. Cross-section morphology and EDS line scanning of Co-33Mn-17Cu coating oxidized at 750 • C in air for 100 h.The line scan position is shown in the cross-sectional image.

Figure 6 .
Figure 6.Cross-section morphology and EDS line scanning of Co-40Mn coating oxidized at 750 °C in air for 100 h.The line scan position is shown in the cross-sectional image.

Figure 6 .
Figure 6.Cross-section morphology and EDS line scanning of Co-40Mn coating oxidized at 750 • C in air for 100 h.The line scan position is shown in the cross-sectional image.

Figure 8 .
Figure 8. ASR of oxide coatings with the temperature range of 500-800 °C. 1 and 2: Co-40Mn coating oxidized at 750 °C in air for 100 h and 20 h; 3 and 4: Co-38Mn-2La coating oxidized at 750 °C in air for 100 h and 20 h; 5 and 6: Co-33Mn-17Cu coating oxidized at 750 °C in air for 100 h and 20 h.

Figure 8 .
Figure 8. ASR of oxide coatings with the temperature range of 500-800 °C. 1 and 2: Co-40Mn coating oxidized at 750 °C in air for 100 h and 20 h; 3 and 4: Co-38Mn-2La coating oxidized at 750 °C in air for 100 h and 20 h; 5 and 6: Co-33Mn-17Cu coating oxidized at 750 °C in air for 100 h and 20 h.

Figure 8 .
Figure 8. ASR of oxide coatings with the temperature range of 500-800 • C. 1 and 2: Co-40Mn coating oxidized at 750 • C in air for 100 h and 20 h; 3 and 4: Co-38Mn-2La coating oxidized at 750 • C in air for 100 h and 20 h; 5 and 6: Co-33Mn-17Cu coating oxidized at 750 • C in air for 100 h and 20 h.

Table 1 .
Composition analysis by energy dispersive X-ray spectroscopy (EDS) of local region in Figure2.

Table 1 .
Composition analysis by energy dispersive X-ray spectroscopy (EDS) of local region in Figure2.

Table 2 .
Composition analysis by EDS of the local region inFigure 4.

Table 2 .
Composition analysis by EDS of the local region in Figure4.

Table 4 .
Composition analysis by EDS of the local region in Figure6.

Table 3 .
Composition analysis by EDS of the local region in Figure5.

Table 4 .
Composition analysis by EDS of the local region in Figure6.