Chang Soo Kim1, Ki Yong Lee2, Kwang Ho Song2, Young-Gon Yoon3, Gyeong-Taek Gong1, Kye Sang Yoo1, Kwang-Deog Jung1, and Hoggon Kim1. (1) Energy and Environmental Technology Division, Korea Institute of Science and Technology, Hawolgokdong 39-1, Seongbukgu, Seoul, 139-791, South Korea, (2) Department of Chemical and Biological Engineering, Korea University, Anamdong, Seongbukgu, Seoul, 136-701, South Korea, (3) Department of Chemical Engineering, Yonsei University, 134 Shinchon-dong, Seogaemun-gu, Seoul, 120-749, South Korea
The process of Sulfur-Iodine (SI) thermochemical cycle was developed by General Atomics Co. in 1970s in order to produce hydrogen by water splitting at a temperature lower than 1000oC. It has been regared as one of promising processes to massively generate hydrogen from water using high-temperature heat transferred from a VHTR (very high temperature gas-cooled reactor). SI cycle is composed of three steps: Bunsen reaction ( x I2 + SO2 + 2 H2O = 2 HIx + H2SO4 ), HIx decomposition ( 2 HI = H2 + I2 ), and H2SO4 decomposition ( H2SO4 = H2O + SO2 + 1/2 O2 ). Acidic sulfur and iodine compounds involved in these reactions are known highly corrosive. Most of commercial metallic alloys proposed for the unit of H2SO4 vaporization are expected to be corroded by the mixture of sulfuric acid, SO3 and H2O. In addition, the corrosion seems accelerated under the decomposition reaction condition of high temperature over 800oC and high pressure over 20 atm. There have been lots of efforts to find proper and cheap materials for the vaporization and the decomposition of H2SO4. In a quick test of screening metallic materials adoptable to sulfuric acid vaporization or decomposition, certain alloys such as SUS304 and SUS316 showed better corrosion resistance to sulfuric acid than more expensive alloys such as alloy 600. However, long-term tests showed different order of corrosion rates because the corrosion behaviors of metallic alloys are changing along with exposure time. This indicated that the short-term screening test sometimes mislead a conclusion. In this study, corrosion behaviors of most commonly available metallic alloys were investigated in sulfuric acid solutions of 40wt% at 115oC and 95wt% at 325oC in quartz chambers with reflux for a sufficient period, and compared with those of silicon carbides and silicon nitride in the same solutions. Applicability of Hastelloy-X and C276 to the H2SO4 vaporizor was reviewed. Changes of surface microstructure and metallic species produced on the surface were also investigated and compared.