Quest for a Material for Sulfuric Acid Superheater/Decomposer for Sulfur-Iodine Thermochemical Cycle for Hydrogen Production

The sulfur-iodine and hybrid-sulfur thermochemical cycles that can utilize high-temperature heat from advanced nuclear reactors have shown promise economically for large-scale production of hydrogen from water. Both of these cycles employ a step to decompose sulfuric acid to sulfur trioxide by heating it above 723 K followed by the catalytic decomposition to sulfur dioxide at a temperature >1073 K depending on the catalyst used. Successful commercial implementation of these technologies is dependent on the development of suitable materials for use in these highly corrosive environments. In this study, a laboratory-scale superheater/decomposer was constructed and used to study the corrosion resistance of natural diamond, synthetic diamond films treated with boron and titanium, silicon carbide, quartz, aluminum nitride, INCONEL, and platinum to sulfuric acid and SO₃. However, it appeared that some of these materials catalyzed SO₃ to SO₂ and O radicals, which also attacked these materials, increasing their corrosion rates.

Natural diamonds, synthetic diamond films (treated with boron and titanium), aluminum nitride, and INCONEL have unacceptable corrosion rates above 873 K. Both the boron- and titanium-treated diamond samples completely disintegrated at temperatures >973 K. The high corrosion rates may have resulted from carbons in diamond having a higher preference for oxygen free radicals that were formed during the decomposition process. Oxygen free radical concentrations increased as a function of the increasing temperature.

The present study showed that silicon carbide had the best corrosion resistance over the range of conditions at which the superheater would operate. Quartz was also corrosion resistant but became brittle after 30 h of exposure to this harsh environment. Platinum, used as a catalyst to reduce the decomposition temperatures, exhibited almost no corrosion when exposed to decomposition products. However, platinum did corrode when exposed to liquid sulfuric acid at high temperatures.