Corrosion Performance of Ceramaic Materials in High Temperature Sulfuric Acid Environments

The Sulfur-Iodide (SI) process has been investigated extensively as an alternate process to generate hydrogen through the thermo-chemical decomposition of water. The commercial viability of this process hinges on the durability and efficiency of heat exchangers/decomposers that operate at high temperatures under corrosive environments. In cooperation with the DOE and the University of Nevada, Las Vegas (UNLV), ceramic based micro-channel decomposer concepts are being developed and tested. The performance benefits of a high temperature, micro-channel heat exchanger are realized from the thermal efficiency due to improved effectiveness of micro-channel heat and mass transfer and the corrosion resistance of the ceramic materials. The success of these high temperature processes is dependent on the corrosion properties for the materials of construction. Super-alloys are often considered because of their ability to be manufactured into heat exchangers and reactors by traditional fabrication methods. The creep and oxidation properties of these metals remain problematic due to these extreme temperatures (900C) and corrosive environments. However, ceramic materials have been noted for their excellent corrosion resistance. In order to assess the viability of ceramic materials, extended high temperature exposure tests have been made to characterize the degradation of the mechanical strength and estimate the recession rates due to corrosion. These results indicate that the strength and recession rates for these ceramic materials were excellent, enabling the development and demonstration of the SI process for hydrogen generation. The results of these corrosion studies will be presented with additional analysis including surface and depth profiling was done using high resolution electron microscopy. These discussions will also compare the expected life and possible failure mechanisms of the candidate materials. INTRODUCTION One potential problem with the utilization of the sulfur-iodine thermochemical cycle to produce hydrogen is that the final reaction in the cycle involves the decomposition of sulfuric acid at elevated temperatures. This final step is a potential obstacle, because it creates an environment that varies fairly significantly from most corrosive environments that have previously been used to test the corrosion resistance of materials. Figure 1. Schematic of the sulfur-iodine thermochemical cycle. Previous corrosion resistance studies have been conducted in numerous environments including: combustion, gaseous N2-H2-CO, coal slag, air, dry and wet oxygen and even O2-H2O-CO2 gaseous environments. Despite the fact that several corrosion studies have been conducted, few studies involved exposure to environments of high temperature sulfuric acid decomposition. As a result, little is known about what materials would withstand such a harsh environment. This lack of knowledge regarding materials compatibility with decomposing sulfuric acid is an obstacle since this type of an environment must be endured and, more importantly, contained during a portion of the hydrogen production process. Those corrosion studies that did involve environments similar to the final step of the sulfur-iodine thermochemical cycle are briefly