Effect of the Composition of Co-W Oxide Conversion Layer on the Cr-Diffusion Barrier Property for SOFC Interconnect
Authors: Saeki Isao , Gan LU , Murakami Hideyuki 
Published: February 2020

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.

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.

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.