Degradation of materials under conditions of the sulfur-iodine thermochemical cycle

The sulfur-iodine (SI) thermochemical cycle is one of the main candidate methods to produce hydrogen from non-fossil sources like nuclear (GEN IV reactor) or solar thermal power. A major issue for the successful implementation of the SI cycle is the selection of technically viable and economic materials for construction of process components, particularly heat exchangers. Challenging conditions are encountered in each of the three sections of the SI cycle due to the corrosive chemicals present, including sulfuric acid, iodine, hydroiodic acid, sulfur dioxide, hydrogen, oxygen, and others. In Section I, aqueous, non-aqueous (liquid iodine), and gaseous multi-component phases are present at about 120 oC and 0.7 MPa. Section II involves gaseous and aqueous phases of sulfuric acid, sulfur dioxide, and oxygen at up to about 830 oC and 3.6 MPa. In Section III , aqueous, non-aqueous, and gaseous phases, at up to 310 oC and 2.2 MPa, containing iodine, hydroiodic acid, hydrogen, and others are present. Numerous materials have been suggested for use in structural applications for the SI cycle. They can be divided into the following generic categories: refractory metals, reactive metals, superalloys, glassy metals, ceramics, cermets, polymers, composites, and coatings. A capsule method has been developed to rapidly quantify the decomposition rate of the candidate materials under the target conditions of temperature, pressure, and fluid composition of Section I and Section III. This paper describes experimental results obtained to date. Recommendations and rationale for the material selections for the measurements are given (also applicable to copper-chlorine cycle). (author)