ICONE19-43563 Corrosion test of metallic materials in high temperature acidic environments of IS process

Abstract

Japan Atomic Energy Agency (JAEA) has been conducting a research and development on hydrogen production using High Temperature Gas-Cooled Reactor. As a part of this effort, a thermochemical water-splitting cycle featuring iodine- and sulfur-compounds (IS process) is under development considering its potential for large scale CO_2 free hydrogen production. The IS process constitutes very severe environments to the structural materials because of the corrosive nature of process chemicals, especially of the high temperature acidic solutions of sulfuric acid and hydriodic acid dissolving iodine. Therefore, selection of the corrosion-resistant materials and development of the components have been studied as crucial issues of the process development. In terms of workability and corrosion resistance, this paper examined the corrosion resistance of novel metallic materials in high temperature hydriodic acid and sulfuric acid. As for the sulfuric acid environment, corrosion resistances of MoN, Mo-Ta-N, and Mo-Cr-N alloys were examined by constant temperature immersion tests. The experiments were performed for the duration of 5 hour in 90 wt% sulfuric acid at 300 ℃. Although no material showed satisfactory corrosion resistance (ca. 1.5 mm/y in the case of MoN), the addition of Ta or Cr was found to be effective to improve the corrosion resistance. As for the hydriodic acid environment, corrosion resistance of Nb-W alloy was examined by immersing the test pieces as long as 100 hours in the test solutions of 200 ℃, whose compositions were HI/I_2/H_2O (in molar ratio) = 1/2/5.5 and 1/0/5.5, using an autoclave designed for the immersion test. It was observed that the corrosion rates lowered with the progress of immersion time and reached to a stable value of lower than 0.1 mm/year.