•MnCu coating has been sputtered on bare and on pre-oxidized SUS 430 steel.•(Mn,Cu)3O4 spinel atop Cr-rich oxide has been thermally grown on the coated steels.•Cr2O3 formed on the pre-oxidized steel has suppressed outward diffusion of Fe.•(Mn,Cu)3O4 outer layer has effectively inhibited Cr outward migration.•The double-layer surface scale has exhibited a high electrical conductivity.MnCu (Mn:Cu = 1:1, atomic ratio) metallic coatings have been deposited by magnetron sputtering on bare and on 100 h pre-oxidized SUS 430 steel for planar solid oxide fuel cells interconnects application. After oxidation at 800 °C in air, the MnCu coating directly deposited on the bare steel has been thermally converted to (Mn,Cu)3O4 spinel with Fe, containing discrete CuO on the outer surface. Nevertheless, the converted (Mn,Cu)3O4/CuO layer from the MnCu coating deposited on the pre-oxidized steelis almost free of Fe. A double-layer oxide structure with a main (Mn,Cu)3O4 spinel layer atop a Cr-rich oxide layer has been developed on the bare and pre-oxidized steel samples with MnCu coatings after thermal exposure. The outer layer mainly consisted of (Mn,Cu)3O4 spinel has not only significantly suppressed Cr outward migration to the scale surface, but also effectively reduced the area specific resistance (ASR) of the scale. The sputtered MnCu metallic coating is a very promising candidate for steel interconnect coating material.

•Cu coating was deposited on ferritic stainless steel by means of electroplating.•The surface scale of CuO atop Cr2O3 was thermally developed on the coated steel.•The outer layer of CuO reduced the growth rate of Cr2O3 inner layer.•CuO acted as an effective barrier against the outward diffusion of Cr.•The surface scale on the coated steel exhibited high electrical conductivity.A Cu coating was deposited on SUS 430 ferritic stainless steel by a low-cost method of electroplating which was employed for the protection of solid oxide fuel cell metallic interconnects. After oxidation, a two-layer oxide scale consisting of an external layer of CuO and an internal layer of Cr2O3 was primarily formed on the coated steel, and a spinel layer consisting of Cu, Cr, Fe and Mn was formed at the CuO/Cr2O3 interface. The CuO outer layer not only suppressed the Cr2O3 scale growth but also served as an effective obstacle against Cr outward diffusion. The coated steels had lower scale area-specific resistances (ASRs) compared to the uncoated steel. The oxidation mechanism and surface scale electrical properties of the coated steel were discussed.

A sputtered coating of a low-Cr alloy without Si was deposited on the cast alloy with the same composition. The short term (100 h) oxidation behavior of the sputtered coating and the cast alloy was evaluated in air at 800 °C. The results indicated that the sputtered coating exhibited a higher oxidation resistance than the cast alloy. It was found that the mass gain of the cast alloy increased continuously with oxidation time and was higher than that of the sputtered coating, which demonstrated only a slight increase in mass gain with oxidation time after 5 h thermal exposure. During the initial thermal exposure of 0.5 h, the oxide scale formed on the cast alloy consisted of Fe2O3 and (Fe,Co,Cr)3O4 spinel with a small amount of Cr. However, (Fe,Co,Cr)3O4 spinel and Fe2O3 were thermally grown on the sputtered coating. After oxidation for 100 h, the oxide scale formed on the cast alloy consisted of Co3O4 and (Fe,Co)3O4 with internal oxide of Cr, while a double-layer oxide consisting of an outer (Fe,Co,Cr)3O4 spinel layer and an inner Cr2O3 layer was developed on the sputtered coating.

Sputtered coating of a low-Cr Fe–Co–Ni alloy with 0.36wt.% Si has been prepared on the same cast alloy via magnetron sputtering method. The sputtered low-Cr alloy coating with columnar nanocrystalline structure is thermally oxidized in air at 800 °C corresponding to the cathode atmosphere of solid oxide fuel cell (SOFC). It is found that the oxidation resistance of the sputtered low-Cr alloy coating is significantly improved in comparison with the cast low-Cr alloy. A double-layer oxide scale is thermally developed on the sputtered low-Cr alloy coating after oxidation for 12 weeks. The outer layer is (Co,Fe)3O4 spinel containing small amount of Ni and Cr. The inner layer is a continuous Cr2O3 layer, followed by oxides of CrNbO4 and Nb2O5. The surface scale formed on the sputtered coating after 12-week thermal exposure demonstrates an area specific resistance (ASR) of 31.97 mΩ cm2. The sputtered low-Cr alloy nanocrystaline coating exhibits a promising perspective for intermediate temperature SOFC interconnects application.Highlights► Sputtered coating of a low-Cr Fe–Co–Ni alloy with 0.36wt.% Si was prepared. ► Oxidation resistance of the sputtered coating was extremely improved in air at 800 °C. ► The surface scale structure with a low-Cr containing (Fe,Co)3O4 spinel atop Cr2O3 formed on the sputtered coating. ► The spinel layer served as an effective barrier to Cr outward migration. ► The surface scale exhibited a good electrical conductivity.

Fe–Co–Ni coating is deposited on ferritic stainless steel using a cost-effective technique of electroplating for intermediate-temperature solid oxide fuel cell (SOFC) interconnects application. The steel with Fe–Co–Ni coating has been evaluated in air at 800 °C corresponding to the cathode environment of SOFC. The results indicate that the steel with Fe–Co–Ni coating experiences an initially large mass gain, and then the mass gain increases slightly after the first-week rapid oxidation stage. After thermal exposure in air at 800 °C, the Fe–Co–Ni coating has been converted into (Fe,Co,Ni)3O4 spinel layer underneath which a Cr2O3 layer is developed from the steel substrate. The outer layer of (Fe,Co,Ni)3O4 spinel has not only suppressed Cr migration outward but also reduced the growth rate of the inner layer of Cr2O3. The steel with Fe–Co–Ni coating exhibits a stable surface oxide scale area specific resistance (ASR) which is much lower than that of the bare steel. (Fe,Co,Ni)3O4 spinel is a promising protective coating for SOFC steel interconnect.Highlights► Fe–Co–Ni coating has been electroplated on ferritic stainless steel. ► (Fe,Co,Ni)3O4 spinel atop Cr2O3 was thermally grown on the steel with coating. ► The double-layer surface scale exhibited a high electrical conductivity. ► (Fe,Co,Ni)3O4 outer layer acted as an effective barrier to Cr outward migration. ► (Fe,Co,Ni)3O4 outer layer reduced the growth of the Cr2O3 inner layer.

Ni–Fe2O3 composite coating was applied onto ferritic stainless steel using the cost-effective method of electroplating for intermediate temperature solid oxide fuel cell (SOFC) interconnects application. By comparison, the coated and bare steels were evaluated at 800 °C in air corresponding to the cathode environment of SOFC. The oxidation investigations indicated that the oxidation rate of the coated steel was close to that of the bare steel after initially rapid mass gain. The mass gain of the coated steel was higher than that of the bare steel owing to the formation of double-layer oxide structure with an outer layer of (Ni,Fe)3O4/NiO atop an inner layer of Cr2O3. The area specific resistance (ASR) of the double-layer oxide scale was lower than that of the Cr2O3 scale thermally grown on the bare steel.Highlights► Ni–Fe2O3 composite coating has been electroplated on ferrtitic stainless steel. ► Oxides of NiO/(Ni,Fe)3O4 atop Cr2O3 was thermally grown on the coated steel. ► Cr-free outer layer exhibited a high electrical conductivity. ► Cr-free outer layer offered an effective barrier to Cr outward migration.

Fe–Ni alloy is electrodeposited on ferritic stainless steel for intermediate-temperature solid oxide fuel cell (SOFC) interconnects application. The oxidation behavior of Fe–Ni alloy coated steel has been investigated at 800 °C in air corresponding to the cathode environment of SOFC. It is found that the oxidation rate of the Fe–Ni alloy coated steel becomes similar to that of the uncoated steel after the first week thermal exposure, although the mass gain of the coated steel is higher than that of the uncoated steel. Oxide scale formed on the uncoated steel mainly consists of Cr2O3 with (Mn,Cr)3O4 spinel. However, a double-layer oxide structure with a Cr-free outer layer of Fe2O3/NiFe2O4 and an inner layer of Cr2O3 is developed on the Fe–Ni alloy coated steel. The scale area specific resistance (ASR) for the Fe–Ni alloy coated steel is lower than that of the scale for the uncoated steel.