Major models so far proposed on the mechanism of hydrogen embrittlement are critically reviewed. The theories are classified into two streams, brittle or ductile fracture in nature. The theories for brittle fracture are triggering by internal pressure, surface energy reduction and lattice decohesion. The ductile fracture models presented here are hydrogen-assisted cracking, hydrogen-enhanced localized plasticity, adsorption-induced dislocation emission and autocatalytic effect on plastic instability. Interconnections of hydrogen embrittlement and stress corrosion cracking are also presented. Particular attention is paid on experimental conditions and assumptions on which the theories are based.
The case cited here occurred in a waste heat boiler plant in 1993, which has many features in common with the case of Mihama nuclear power plant in 2004 in terms of piping materials, environmental conditions as well as in the location where the damage took place: very close to an upstream flow-meter. On this case, the cause was analyzed based on the best available knowledge and seemingly the best countermeasures for preventing a recurrence were taken. Seven years after the accident, in 2000, the effectiveness of these countermeasures was examined. The results of the examination and the newly obtained knowledge from the Mihama's case together confirmed the rationality of the opinion that the unusual thinning of pipe wall can be attributed not to the turbulence in fluid flow, but to a macro-cell corrosion.
The corrosion resistance of noble metals and Ti/IrO2-Ta2O5 in hydrochloric acid was evaluated to develop a material for a supercritical water oxidation reactor. The results of the corrosion tests in 20 wt% hydrochloric acid with the autoclaves showed Ir had good corrosion resistance between 300ºC and 600ºC. Although Ti-Pd showed active dissolution at 300ºC, the corrosion rate of Ir was less than 1.8 mm/y. This value was superior to that of Rh (26 mm/y) and Ru (14mm/y). The mass change of Ti/IrO2-Ta2O5 was little. Ti/IrO2-Ta2O5 has superior corrosion resistance to Ti-Pd. Using the continuous flow reactor for 893 h, the detachment of Ti/IrO2-Ta2O5 coatings was observed at 550-580ºC. The detachment of coatings was not observed at 400-450ºC. At 550-580ºC, the thickness of oxide layer on Ti grew greater than 100 μm and the Vickers hardness of it was 660-772. High temperature and long time would cause the growth and brittle of oxide layer on Ti substrate. The study on the growth and the brittle of the oxide layer on Ti substrate is expected to use Ti/IrO2-Ta2O5 at greater than 550ºC for a supercritical water oxidation reactor.