New corrosion test equipment for high temperature gas of decomposed sulfuric acid was manufactured in order to ascertain flow rate of sulfuric acid in the piping, occurrence of sulfuric acid decomposition reaction in the equipment, and temperature distribution inside the furnace tube. The flow rate of the sulfuric acid solution was constantly measured using an ultrasonic flowmeter. The SO₃ concentration at the inlet of the test equipment was almost the same as that at the inlet of the sulfuric acid decomposer in the hydrogen production plant assuming a high-temperature gas cooled reactor hydrogen-power cogeneration system（GTHTR300C）. On the other hand, during a test, leakage of sulfuric acid occurred from the fitting part at the outlet side. Hence the temperature distribution of the fitting part at the outlet side was investigated using fluid analysis. As a result, it was found that the temperature at the fitting was low enough to use fluorine joint grease when the distance was 0.05 m or more away from the outlet side pipe. An improved furnace tube was manufactured and the temperature was measured again at fitting part of the outlet side. The temperature was lower that the temperature limit of the joint grease, and almost the same as the temperature distribution in the fluid analysis. Leakage of sulfuric acid has not occurred to date.

In expectation of nano-scale nitride layer formation, type 445J1 stainless steels with different grain sizes were cathodically charged in nitric acid-nitrate solution （ECN treatment） and evaluated in terms of interfacial contact resistance against carbon paper （ICR）, anodic polarization behavior, chemical state of the surface and PEFC performance in use as bipolar plates. The ECN treatment reduced ICR with more pronounced effect for the stainless steel with finer grains. Polarization test revealed that corrosion resistance was not degraded by the ECN treatment. XPS analysis proved formation of nitride layer on the surface of the stainless steel. The nitride layer was thicker and more stable against anodic polarization for the stainless steel with finer grains. The ECN treatment was applied to the fine-grained stainless steel for use as bipolar plates. The cell performance was comparable to that with graphite bipolar plates, proving that the ECN treated type 445J1 stainless steel with fine grains is a promising candidate material for PEFC bipolar plates.

Using a three-electrode system, effect of pulse period of controlled potential on the electropolishing behavior of Nb in 30 mass％ H₂SO₄ was examined. The potential pattern consisted of cathodic pulse （－3 V vs. Ag/AgCl for t_c）, anodic pulse （＋1 V vs. Ag/AgCl for t_a） and rest time （0 V vs. Ag/AgCl for 1 ms）. The results of various combinations of t_c and t_a showed that both t_c and t_a were needed to be longer than 0.5 ms for successful polishing. Good performance was obtained when each pulse was balanced and the etching rate became maximum for t_c＝t_a＝ 2 ms. The surface observation indicated that small pits were generated in the early stage of polishing and extended to form dimpled faces. However shorter t_a than t_c resulted in rugged surface, suggesting mechanical damage of Nb matrix by hydrogen. Thus, the combination of t_c and t_a in good balance is important in this polishing.

In order to improve the self-healing property of coating, we try to produce spheroidal capsules by using precursor solutions, whose viscosity was increased by heat treatment. The spheroidal capsules can be obtained under agitation with low speed（300 rpm）. The flattening coefficient of spheroidal capsule synthesized is about 0.7-0.9.