Chromium volatility of coated and uncoated steel interconnects for SOFCs

The requirements of low cost and high-temperature corrosion resistance for bipolar interconnect plates in solid oxide fuel cell stacks has directed attention to the use of steel plates with more oxidation resistant compositions. However, volatile Cr species from the Cr2O3-based oxide scales on these steels find their way to the triple-phase boundary, leading to rapid degradation of fuel cell performance. Coatings can serve not only to slow oxidation rates, but also as diffusion barriers for the Cr-derived species from the steel, slowing the degradation process. We have developed a relatively quick, quantitative procedure using Rutherford Backscattering Spectroscopy (RBS) to obtain measurements of the Cr vaporization rates. Chromium-containing vapors from the steel coupons in a tube furnace at 800 °C were transported with various flow rates of humid air to a Si wafer at ∼ 110 °C near the end of a quartz tube in a tube furnace, where the vapors adsorbed on the Si surface. The wafers were subsequently analyzed for Cr surface concentrations using Rutherford backscattering. Results are presented for 430 SS and for CroferAPU22, the latter with and without a Co–Mn–O spinel coating deposited using hybrid filtered-arc deposition assisted electron beam physical vapor deposition. Separate experiments with Cr2O3 powder as the vapor source established the quantitative reliability of this approach.