1. Introduction.

Solid-oxide fuel cells (SOFC) seems to the most reasonable solution for the next power generation because of high efficiency, low CO₂ emission, and fuel flexibility. In the operation condition of SOFC, however, Cr in the stainless steel will react with O₂ and H₂O in the atmosphere to form CrO₂(OH)₂ or CrO₃ gas which cause detrimental effects on cathode materials. Formation of Cr₂O₃ will also lead to high electrical resistance which decreases the cells efficiency. We found that a CoWO₄ layer formed by the oxidation of Co-2.4 at% W electroplated stainless steel immediately in oxidation at 800 ˚C. This layer blocks the outward diffusion of Cr ion for it does not accommodate Cr³⁺ ion. As a result, no Cr was found at the oxide surface, and the thickness of inner Cr₂O₃ layer kept thin. However, we utilized only Co-2.4 at% W plating in the previous work, and the effect of plating composition was known. In this study, Co-W electrodeposition to obtain a wide range of W content was examined, then the high-temperature oxidation and area specific resistance (ASR) of the oxidized specimen was tested.

2. Experimental.

Type 430 (Fe-17 mass% Cr) and 445 (Fe-23 mass% Cr-1 mass% Mo) stainless steels were used as a substrate. The cathodic current density was 1-5 A dm^-2 and plating time was adjusted to obtain a 10 µm of electroplating. Electroplating of Co-W alloy was carried out with sodium tungstate and citric acid solution at pH range of 2-10. Cathodic polarization and constant potential deposition were carried out with a Cu working electrode (cathode), Pt counter electrode (anode) and an Ag-AgCl reference electrode immersed in a saturated KCl solution. A thin Co layer was electroplated to enhance the adherence before Co-W electroplating at constant current density. After the electroplating at constant current densities, the samples were oxidized at 800 ºC in the air with 3 vol% water vapor with the flow rate of 100 cm³ min-1. X-ray diffraction (XRD) was used to identify crystal phase before and after the oxidation test. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) were used to observe and analyze morphology and distribution of the element to the depth direction of the oxide. ASR was measured at 800 ˚C for 100 h in air at the constant current of 0.5 A dm^-2. Samples were pressurized at 30 kPa during the measurement.

3. Results.

(1) A uniformly distributed Co-W electroplating film with 2.4-30 at% W was deposited on Type 430 and 445 stainless steel. The alloy composition was determined only by the solution pH. Cathodic current density did not affect to the composition.

(2) Co-W alloy plating with less than 5 at% W is enough to obtain Cr barrier layer. For the specimen with W content higher than 5 at% formed an W enriched layer within the substrate layer. Whether the formation of this layer is detrimental to oxidation and mechanical properties is not known, the thickness and the compactness of formed CoWO₄ layer after oxidation are similar for the electroplating containing >5 at% W.

(3) Both the increasing W content in deposits and decreasing Fe fraction in the substrate will lead to the reduction of Fe outward diffusion. Also, the increasing in W content lead to higher ASR.