chugoku1996

HYDROGEN ENVIRONMENT EMBRITTLEMENT OF MATERIALS FOR HYDROGEN ENERGY SERVICE

K. Yokogawa¹, S. Fukuyama¹, G. Han² and J.He³
1: Chugoku National Industrial Research Institute (CNIRI), AIST, MITI,
2: Postdoctoral Researcher by JRDC in CNIRI,
3: Industrial Technology Researcher by NEDO in CNIRI ,
2-2-2 Hiro-suehiro, Kure, Hiroshima, 737-01, JAPAN

ABSTRACT

The new material test equipment for hydrogen environment embrittlement (HEE) in high pressure hydrogen up to 10 MPa at cryogenic temperatures down for 20 K, and that at elevated temperatures up to 1500 K were developed. HEE of the steels to be used for hydrogen energy service were examined using the equipments in high-pressure hydrogen at low, room and elevated temperatures. It was found that hydrogen degraded the mechanical properties of the materials severely at the temperatures. Although it has been believed that hydrogen embrittlement of steels is not found above around 400 K, it was found that HEE of steels still occurred at 800 K in hydrogen atmosphere. HEE of the materials depended on the metallic phase. Effect of gaseous inhibitors added to hydrogen atmosphere on the crack growth of the materials was also investigated at room temperature to prevent HEE. It was found that the addition of oxygen and carbon mono-oxide was effective to prevent the crack growth but that of hydrogen sulfide accelerated the crack growth.

INTRODUCTION

Hydrogen has been expected as a new clean secondary energy to improve the environment on the Earth and as a energy carrier for long distance transportation in the world. New energy projects, i.e. hydrogen energy, solar energy, coal conversion, geothermal energy, wind energy and etc., have been conducted [1] since the oil shock attacked in the world.
Hydrogen has many advantages in the physical and chemical properties. However, it is well known that the structural materials to sustain hydrogen energy service show hydrogen environment embrittlement (HEE) which degrades the mechanical properties of the materials. It has become important to investigate the evaluation and the prevention of HEE of the materials for the establishment of the safety in hydrogen energy service. Hydrogen has been also used for the fuel for rocket engine, so many systematic studies of HEE were conducted in the space programs for the development of the engines [2,3,4].
In this paper, the new materials test equipments for HEE in high-pressure hydrogen at low, room and elevated temperatures were developed and the evaluation of HEE of steels to be used for hydrogen energy service, i.e. the liquid hydrogen storage container, the hydrogen gas pipe, the hydride reactor, and the hydrogen combustion turbine etc., in high-pressure hydrogen at the temperatures were investigated. The preventive technique for HEE of the steels was also discussed.

2. HEE at low temperatures

2.1 Development of the new materials test equipment in high-pressure hydrogen at cryogenic temperatures [5]

A new materials test equipment was designed in our laboratory to evaluate HEE of the specimen in high-pressure hydrogen up to 10 MPa at the temperature range from room to cryogenic temperatures down for the temperature of liquid hydrogen, 20 K. The materials test equipments in high-pressure hydrogen at cryogenic temperatures down to 77 K [2] and to 111 K [6] using the external cryogenic jacket cooled by liquid nitrogen were developed already in the space programs. However, these equipments could not cover the experiment at the temperatures below 77 K down to the temperature of liquid hydrogen. In our equipment, the specimen was set up in high-pressure hydrogen or helium in the pressure vessel which was cooled down to the 77 K using the thermal siphon connected with the shroud-like vessel filled with liquid nitrogen. The temperature was measured using the thermocouple attached with the specimen directly and was controlled by the heater which surrounded the specimen inside the pressure vessel and by the nitrogen pressure in the thermal siphon. After cooling at 77 K, the nitrogen in the thermal siphon was evacuated and the pressure vessel was cooled down for 20 K by a cryogenic refrigerator using helium. The temperature was controlled by the heater described above. These cooling system was installed in the high-vacuum chamber. The equipment is shown in Fig.1.

2.2 Effect of temperature on HEE of steels at low temperatures [5]

The tensile tests of the austenitic stainless steels, the ferritic stainless steels and the high-strength steels were conducted to select the candidate materials for hydrogen service at cryogenic temperatures using the equipment in 1 MPa hydrogen and helium gas at the temperature from room temperature to 78 K. HEE of Inconel 718 at 77 K [2] and type 347 stainless steel at 111 K [6] were investigated only previously, but HEE of the other materials are still unknown. HEE of the austenitic stainless steels are shown in Fig.2. HEE is shown as the relative reduction of area (F_Hydrogen/ F_Helium), because HEE affects reduction of area of the tensile properties of the steels. Relative reduction of area of 1.0 shows no hydrogen effect on the material and that of 0.0 shows extreme hydrogen effect. HEE of type 304L stainless steel increased with decreasing the temperature from room temperature to 220 K, reached the maximum at around 220 K and decreased with decreasing the temperature. HEE depended on the materials, i.e. HEE of type 316L stainless steel was smaller than that of type 304L stainless steel. HEE of type 304L is as similar as hydrogen embrittlement (HE) of the material charged by hot hydrogen gas [7,8].

3. HEE at elevated temperatures

3.1 Development of the new materials test equipment in high-pressure hydrogen at elevated temperatures

A new materials test equipment was designed in our laboratory to evaluate HEE of the specimen in high-pressure hydrogen up to 20 MPa at elevated temperatures up to 1500 K. The materials test equipment in high-pressure hydrogen at elevated temperatures up to 950 K using the internal heater were developed already in space program [2,6]. However, the equipment could not cover the experiment at the temperatures above 950 K to around 1800 K of the service temperature of hydrogen combustion turbine. In our equipment, the specimen was set up in high-pressure hydrogen or argon in the pressure vessel and was heated up by the heater which surrounded the specimen and was installed inside the pressure vessel using Mo wire. The temperature was measured using the thermocouple attached with the specimen directly and was controlled by the heater. The equipment is shown in Fig.3.

3.2 Effect of temperature on HEE of steels at elevated temperatures [9,10]

The tensile tests of the austenitic stainless steels, the ferritic stainless steels and the high-strength steels, which show extreme hydrogen susceptibility, were conducted to select the candidate materials for hydrogen service at elevated temperatures in 19.7 MPa hydrogen and argon gas at the temperature from room temperature to 773 K. HEE of Ni-base superalloys [2,3] and type 347 stainless steel [6] at 950 K were investigated only previously, but HEE of many steels are unknown yet. HEE of the steels are shown in Fig.4. HEE is shown as the relative reduction of area ( Hydrogen/ Argon). HEE of the steels increased with increasing the temperature. It has been believed as the general concept of HE that HE mainly occurs in the temperature range between arund 200 and 400 K. HEE of Type 304 followed the general concept of HE, i.e. HEE of the material showed 0.52 and almost 1.0 above 373 K. HEE of the material depended on the formation of the deformation induced martensite phase which normally occurred below around 373 K. HEE of the other steels depended on the metallic phase of the materials. In the case of high-strength steels, HEE of AISI4340 and 18-Ni Maraging steels showed almost 0.0 below 373 K and increased a little even at 673 K. It is remarkable that HEE of Fe-30Cr alloy aged ferritic steel showed 0.54 still at 773 K. It is evident that HEE of steels still occurred above the temperature of 400 K. The maximum temperate of HEE of steels depended on the metallic phase of the materials. The results will be important for the materials evaluation of the materials to be used for the hydrogen combustion turbines for future.

4. Prevention of HEE [11,12]

On the basis above results, it is obvious that HEE of the materials is the serious problem for the structural materials for hydrogen energy service. Hydrogen causes the crack initiation and accelerates the crack growth of the materials. It becomes important that the preventive technique to arrest the crack growth of the materials. The effect of gaseous inhibitors added to 1.1 MPa hydrogen on the fatigue crack growth of high-strength steel of AISI4340 steel [11] and the pressure vessel materials for the chemical reactor at elevated temperatures of 2.25Cr-1Mo steel [12] are shown in Fig.5 and Fig.6, respectively. The inhibitive effect is shown as (da/dN)_Inhibitor/(da/dN)_Hydrogen. It is found that the addition of oxygen, carbon mono-oxide, or sulfur dioxide had much preventive effect on hydrogen assisted fatigue crack growth, however that of water vapor, methane, carbon dioxide or methyl mercaptan had little effect, but, that of hydrogen sulfide had accelerative effect for AISI4340. It is also found that the addition of oxygen or carbon mono-oxide had much preventive effect on the fatigue crack growth, however that of sulfur dioxide, methane or carbon dioxide had little effect, but that of water vapor, methyl mercaptan, or hydrogen sulfide had accelerative effect for 2.25Cr-1Mo steel. Frandsen found the inhibitive effect of oxygen and carbon mono-oxide and no effect of carbon dioxide and methane on the fatigue crack growth of A514B steel [13]. Brazil found the accelerative effect of hydrogen sulfide on fatigue crack growth of 2.25Cr-1Mo steel [14]. It is evident that the addition of oxygen or carbon mono-oxide had preventive effect, but that of hydrogen sulfide had accelerative effect on the fatigue crack growth. The effect of the addition of water vapor or methyl mercaptan on the fatigue crack growth depended on the materials.

5. Conclusion

The new material test equipment for HEE in high pressure hydrogen up to 10 MPa at cryogenic temperatures down for 20 K, and that at elevated temperatures up to 1500 K were developed in our laboratory. HEE of the steels were examined using the equipments in high-pressure hydrogen at low, room and elevated temperatures.
It was found that hydrogen degraded the mechanical properties of the materials severely at low, room and elevated temperatures. Although it has been believed that HE of steels is not found above around 400 K, it was found that HEE of steels still occurred at 800 K in hydrogen atmosphere. HEE of the materials depended on the metallic phase.
Effect of gaseous inhibitors added to hydrogen atmosphere on the crack growth of the materials was also investigated at room temperature to prevent HEE. It was found that the addition of oxygen and carbon mono-oxide was effective to prevent the crack growth but that of hydrogen sulfide accelerated the crack growth.

Acknowledgment

The study has been conducted in Sub-task 6 of WE-NET project and supported financially by The New Energy R/D in AIST, MITI, Japan.

References

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