Chromium poisoning from metallic interconnect plates degrades solid oxide cell electrodes by vaporizing volatile chromium hexavalent oxide-hydroxide species from high-temperature ferritic stainless steel interconnects, which diffuse through gas channels and electrochemically reduce into solid, insulating chromium oxide deposits that permanently block catalytic reaction sites.
To scale solid oxide technology from individual single cells into commercial multi-kilowatt stacks, multiple planar cells must be connected in series. The electrical and physical separator plates between adjacent cells are known as interconnects. Interconnects must be electrically conductive, mechanically robust, and cost-effective. Consequently, stack manufacturers utilize specialized high-chromium ferritic stainless steels (such as Crofer 22 APU or Sandvik Sanergy HT), which contain 20 to 24 percent chromium. At operating temperatures between 700 and 850 degrees Celsius, chromium naturally diffuses to the alloy surface and oxidizes to form a protective chromium oxide (Cr2O3) scale that prevents catastrophic thermal oxidation while maintaining electrical conductivity.
However, in the presence of oxygen and high-temperature moisture, this chromium oxide scale undergoes reactive vaporization. Hexavalent chromium reacts with oxygen and water vapor to form volatile chromium species—predominantly chromium oxyhydroxide (CrO2(OH)2) and chromium trioxide (CrO3): Cr2O3 (s) + 2 H2O (g) + 1.5 O2 (g) -> 2 CrO2(OH)2 (g). These gaseous chromium molecules travel through the air gas channels into the porous oxygen electrode. Once inside the active electrode layer, where oxygen partial pressure shifts and catalytic reduction occurs, the volatile chromium undergoes electrochemical and chemical reduction: 2 CrO2(OH)2 (g) + 6e- -> Cr2O3 (s) + 2 H2O + 1.5 O2 + 6 O^2-. Solid, non-conductive chromium oxide (Cr2O3) and chromium-manganese spinels precipitate directly onto catalytic reaction sites and triple phase boundaries. Within hundreds of operating hours, these insulating deposits poison oxygen evolution kinetics, quadruple polarization resistance, and permanently choke off cell power output.
To halt chromium evaporation, modern interconnect plates are coated with protective conductive ceramic coatings—specifically manganese-cobalt spinel (Mn,Co)3O4 (MCO) applied via atmospheric plasma spraying or electrophoretic deposition. Dense MCO layers act as an impenetrable physical barrier against chromium outward diffusion while retaining high electrical conductivity across thousands of operating hours.
Reactive element additions such as lanthanum or yttrium in the steel substrate further peg the chromia scale, suppressing scale growth rates and eliminating spallation during cyclic cooling phases.
Materials degradation laboratories, high-temperature oxidation research groups, and electron microscopy facilities accredited by the National Universities Commission conduct transpiration vaporization experiments, characterize spinel coating microstructure via cross-sectional TEM, and model chromium vapor transport in porous electrode media.
Metallurgical manufacturing standards, alloy coating quality specifications, and electrochemical component testing guidelines governed by the Standards Organisation of Nigeria and the Nigerian Electricity Regulatory Commission establish chromium vapor release limits, protective coating adhesion tests, and certified stack lifetime endurance criteria.
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