title: Study of corrosion of materials in the sulfur-iodine hydrogen production cycle
Creators: name: Ho, Thao Trung
Publisher: name: University of Nevada, Las Vegas
Year: 2009

description: Hydrogen is of great interest since the availability of traditional fossil fuels is in decline. Strictly speaking, hydrogen is not a primary source of energy but is an energy carrier, since energy typically must be used from another source (electricity, natural gas, coal, etc.) to produce it. Of hydrogen production techniques, the Sulfur-Iodine thermochemical water splitting process (S-I cycle), which was proposed by General Atomics (GA), is promising with its simplicity and high efficiency. Most of the chemicals are recycled except water. However, the S-I cycle operates in a harsh, corrosive environment in the presence of a mixture of iodine (I 2 ), hydroiodic acid (HI), and water (called HIx in the literature), phosphoric acid (H 3 PO 4 ) and sulfuric acid (H 2 SO 4 ) at different sections of the cycle. Hence, the corrosion performance of structural materials plays an important role in the success of the process. 

In a search for structural materials for the S-I cycle, a systematic material suitability study was conducted at GA and Ceramatec Inc. As a result of the preliminary study, corrosion-resistant materials such as tantalum alloys, niobium alloys, and ceramics were exposed to corrosive environments for extended periods. These environments were similar to the hydrogen iodide decomposition and sulfuric acid decomposition sections of the Sulfur-Iodine thermochemical hydrogen production cycle. 

Post mortem studies were conducted of parts from the recent Sulfur-Iodine Integrated Laboratory Scale (SI-ILS) test at GA. A HI distillation column boiler made of bulk tantalum alloy tube failed during refurbishment under shutdown conditions, and a few tantalum-coated steel parts (swage fittings) were found to have unexpected corrosion residues. 

This thesis reports the result of the microscopic and spectroscopic study of these exposed materials as well as the results of the post mortem investigations described above. The refractory tantalum and niobium alloys developed their metal oxide layers in the harsh corrosive environment of HI x /H 3 PO 4 at 120 Â°C. The study results indicate that there is a relation between the stability of these oxides and the corrosion of the alloys. The corrosion rate of the tantalum alloy decreases with exposure time due to the formation of a stable protective tantalum pentoxide film. On the other hand, the corrosion of niobium alloys is complicated by the formation of several oxides. 

The tantalum boiler section showed severe material degradation, and the observation of multiple contaminants at the fresh fracture surface leads to the suggestion that cracking of the tantalum led to the degradation. The cracking may be due to hydrogen embrittlement or another process. The curious form of the lateral cracking (delamination) may indicate investigations of the crystal form and fabrication technique of the tubing could be in order. 

In the silicon-based ceramic samples tested, silicon oxides were formed as a protective layer on the samples' surface (except alumina). The type and structure of the silicon oxide depend on the fabrication process of the ceramic. Alumina showed a different corrosion mechanism than the silicon based ceramics. 

The study of corrosion of the refractory alloys in HI x /H 3 PO 4 and of ceramic materials in H 2 SO 4 reiterates the important role of oxidized layers, which formed on the surface of these materials as corrosion products, on their corrosion behavior. The XPS and SEM analytical techniques have proved their strength in the study of corrosion as well as in the failure analysis. The results from this study and from other studies (visual examinations, weight change measurements, and mechanical property tests) contributed to the material selection for the S-I cycle test as well as to the success of the test.