Characterization of a Cobalt-Tungsten Interconnect

A ferritic steel interconnect for a solid oxide fuel cell must be coated in order to prevent chromium evaporation from the steel substrate. The Technical University of Denmark and Topsoe Fuel Cell have developed an interconnect coating based on a cobalt-tungsten alloy. The purpose of the coating is to act both as a diffusion barrier for chromium and provide better protection against high temperature oxidation than a pure cobalt coating.

This work presents a characterization of a cobalt-tungsten alloy coating electrodeposited on the ferritic steel Crofer 22 H which subsequently was oxidized in air for 300 h at 800 °C. The coating was characterized with Glow Discharge Optical Spectroscopy (GDOES), Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD).

The oxidation properties were evaluated by measuring weight change of coated samples of Crofer 22 H and Crofer 22 APU as a function of oxidation time. The coating had completely oxidized during the 300 h oxidation time. GDOES measurements showed that the tungsten was located in an inner zone in the coating/substrate interface.

The outer layer of the coating did not contain any tungsten after oxidation but consisted mainly of cobalt and oxygen with smaller amounts of iron and manganese. The iron and manganese had diffused from the steel into the coating during oxidation. XRD measurements showed that tungsten reacts with cobalt and oxygen to form CoWO4.

Cobalt oxide in the outer layer was a spinel of either Co3O4 or Co3-y(Mn,Fe)yO4. Chromium in the steel had oxidized to form a thin layer of almost pure chromium oxide underneath the coating. The coating appears to be an effective diffusion barrier for chromium as a very small amount of chromium was measured in the coating after oxidation.

The cobalt-tungsten coated samples oxidized slightly slower than the cobalt coated samples. An interconnect used in a fuel cell stack was also investigated with SEM/EDS. The interconnect from the fuel cell stack was different from the samples oxidized in the furnace with respect to the location of the tungsten.

The tungsten in the interconnect coating was present in the chromium oxide layer instead of as CoWo4 on top of it.