It has been clarified that some of impurities contained in hydrogen gas affect the material strength in the hydrogen environment. However, the studies on the effect of the impurities can only be found in basic fatigue crack growth test or fracture toughness test. In addition, only partial characterizations of the effect of impurities and elucidation of its mechanism have been carried out.
Therefore, for deeper understanding and evaluation of the effect of the impurities on material strength properties of the pressurized hydrogen containment systems, further investigations and discussions should be done.
In this context, the effects of impurities on fretting fatigue strength and fracture toughness in hydrogen environment were studied in this study. In the fretting fatigue, the effects of ppm-levels of oxygen and water vapor to hydrogen environment on fretting fatigue strength were characterized, and its mechanism was elucidated. In the fracture toughness, the influence factors on the effect of impurities, such as amounts and species of impurity, loading rate and material were characterized.
In chapter 2, to realize an accurate measurement of the relative slip range during fretting fatigue in hydrogen, a new measurement method applying MEMS optical encoder was developed.
The results are as follows:
3. The system and procedure of the measurement of the relative slip range during the fretting fatigue test in hydrogen were established.
4. The relative slip range in the hydrogen is significantly lower than that in air. It was presumed that the reduction was caused by the local adhesion between the contacting surface produced during the fretting fatigue in hydrogen.
In chapter 3, to characterize the effect of addition of ppm-levels of oxygen or water vapor on the fretting fatigue properties in hydrogen, the fretting fatigue tests in variety of environments were carried out. A new gas system was developed, and a special procedure for the test environment preparation procedure were established. The results are as follows;
7. The fretting fatigue strength in hydrogen was significantly reduced by the addition of ppm-levels of oxygen or water vapor.
8. Conversely, the crack initiation limit under fretting condition was increased by the addition of oxygen.
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9. The fretting wear was not significant in the high-purity hydrogen. On the other hand, the fretting wear was significant at the contact edge in the oxygen-hydrogen mixture and humidified hydrogen.
10. In the high-purity hydrogen, the original surface oxide layer was removed by the fretting, and no oxides were produced during the fretting fatigue. The no oxide production resulted in the mitigation of the fretting wear. In addition, the no oxide production also caused the pure-metal surface contact between the contacting surfaces and strong adhesion between the contacting surfaces. In the oxygen-hydrogen mixtures and humidified hydrogen, a thin oxide layer was produced by overcoming the removal action of the oxide layer by the fretting. The oxide production enhanced the fretting wear. In addition, the oxide production also prevented the pure-metal surface contact and mitigated the adhesion between the contacting surfaces.
11. The relative slip range in the high-purity hydrogen was significantly lower than that in air. This is because a strong adhesion occurred between the contacting surfaces in the high-purity hydrogen. The relative slip range during fretting fatigue test was increased by the addition of 5 vol. ppm oxygen causing the mitigation of the adhesion between the contacting surfaces. The increase in the relative slip range is one of the reasons for the enhancement of the fretting wear.
12. In the non-oxidative environment, the mechanical condition on the contacting surfaces was kept throughout the fretting fatigue test similar to the initial condition due to the low fretting wear.
Therefore, a high compressive stress field in the specimen was kept, and it suppressed the fatigue crack propagation. On the other hand, in the oxidative environment, the compressive stress field was relieved due to the change in the geometry of the contact by the fretting wear.
This relief of the compressive stress field was the cause of the reduced fretting fatigue strength in the oxygen-hydrogen mixture and humidified hydrogen.
In chapter 4, to evaluate the effect of slight increase in the oxygen content in hydrogen environment during fracture test in hydrogen environment, fracture toughness tests were carried out in hydrogen used a closed gas system and open gas system. To verify the test results, additional fracture toughness tests with varying the oxygen content and loading rate were also carried out. The material was ASTM A333 grade 6 pipe steel. The results are follows:
6. In the closed gas system, oxygen content in the hydrogen gas increased with time. The oxygen content reached about 1 vol. ppm in 30 hours in this test, which corresponds to the testing time
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of the fracture toughness test at a crosshead speed of 2.0 × 10-5 mm/s. On the other hand, in the open gas system, the purity of the hydrogen gas was not changed.
7. Severe hydrogen-assisted degradation of the fracture toughness was observed in the experiment with the open gas system. The fracture toughness in the closed gas system was less affected by hydrogen, because oxygen level increased to 1 vol. ppm during the test.
8. In air, the crack tip shape was fully blunted, and its fracture surface was dimple. Conversely, in open gas system with hydrogen, the crack tip remained sharp after the fracture toughness test, and the fracture surface was quasi-cleavage. On the other hand, when the closed gas system was used, the crack tip shape was not as sharp as in the experiment with the open gas system, nor was it as blunted as in air. The fracture surface exhibited both dimple and quasi-cleavage.
Therefore, change in crack growth mode due to hydrogen was suppressed in the closed gas system.
9. The inhibitory effect of oxygen on hydrogen-assisted degradation of the fracture toughness depended not only on the oxygen content but also on the loading rate. The results obtained in this study correspond well to the trend predicted by the model proposed by Somerday et al.
10. It was verified that 1 vol. ppm oxygen partially inhibits the hydrogen-assisted degradation of fracture toughness.
In chapter 5, to characterize the inhibitory effect of carbon monoxide on hydrogen-assisted degradation of fracture toughness, the fracture toughness tests were carried out. The material used this study was A333 grade 6 and A106 grade B pipe steels. The results are as follows:
5. The addition of carbon monoxide inhibited the hydrogen-assisted degradation of the fracture toughness of the A333 and A106 steels. The inhibitory effect of carbon monoxide increased with an increase in the carbon monoxide content, and more than 1000 vol. ppm carbon monoxide completely inhibited the hydrogen-assisted degradation in both pipe steels with a crosshead speed of 2.0×10-3 mm/s and a gas pressure of 0.6 MPa.
6. Based on the observation of the crack tip shape after the fracture toughness test and fracture surface, it was found that the prevention of change in the crack growth mode by the addition of carbon monoxide is the reason for the mitigation of the hydrogen-assisted degradation. This is the same mechanism as the inhibitory effect of oxygen.
7. The inhibitory effect of carbon monoxide in the A333 was higher than that in the A106. It can
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be considered that since the hydrogen sensitivity of the A106 was higher than that of the A333, the inhibitory effect of carbon monoxide apparently decreased in the A106 compared to the A333.
8. The inhibitory effect of carbon monoxide was weaker than that of oxygen. This result can be theoretically explained by the potential energy behavior of hydrogen, oxygen and carbon monoxide molecules for iron surface.
142 Acknowledgement
This thesis is a summary of the results of the research that I carried out at the doctoral course of the Department of Hydrogen Energy Systems in Graduate School of Engineering of Kyushu University.
I would like to express my special appreciation and thanks to my advisors Professor Hisao Matsunaga, Professor Masanobu Kubota and Professor Kaneaki Tsuzaki. You gave me a lot of enthusiastic and courteous advices in not only my research but also an attitude toward scientist.
Without your great helps, this thesis would not have been accomplished. You have been a tremendous mentor for me. I am convinced that your superb advices and helps will be a great force for my future research carrier.
In Chapter 2, Professor Renshi Sawada in Kyushu University provided me an optical micro-encoder fabricated by MEMS technology developed by him. I would like to express my gratitude to Professor Sawada. In addition, I deeply thank to Mr. Nobutomo Morita and Mr. Fumiya Nakashima, who are member of Professor Sawada’s laboratory. They gave me a precious cooperation to conduct the study. Especially, without the cooperation of Mr. Morita, we could not have executed this study.
In Chapter 3, I gave a lot of priceless advices from Dr. Jader Furtado (Air liquid, Paris-Saclay Research Center). I would like to express my appreciation to Dr. Furtado. Your advices sophisticated this study a lot.
The X-ray photoelectron spectroscopy analysis in Chapter 3 was carried out with great help of Professor Joichi Sugimura and Professor Masayoshi Tanaka. I would express my appreciation.
The studies in Chapters 4 and 5 were carried out with great cooperation of Dr. Francoise Barbier, Dr. Jader Furtado (Air liquid, Paris-Saclay Research Center), Dr. Patrick Ginet, Dr. Takuya Matsumoto, Dr. Jun Sonobe (Air Liquide Laboratories) and Professor Aleksandar Staykov (International Institute for Carbon Neutral Energy Research). I am deeply grateful to all of them.
I appreciate to Mr. Katsuya Shimada of technical staff for machining many specimens and test jigs. I also thank for your courteous guidance how to use machine tools.
My sincere thanks also goes to technical staffs Mr. Takashi Soejima, Mr. Takao Taniguchi, and Mr. Sugao Takatsu. They gave me a various and generous helps.