Substitution of High-Pressure Charge by Electrolysis Charge and Hydrogen Environment Embrittlement Susceptibilities for Inconel 625 and SUS 316L

Abstract

Hydrogen-fuel-cell vehicles have been developed and the gaseous pressure in the current major storage tanks of the vehicles varies from 35 to 70 MPa because of the demand for the increase in running distance. Hydrogen refueling stations are required to be resistant to 100 MPa hydrogen gas and the alloys used for such stations are required to have an excellent resistance to hydrogen environment embrittlement (HEE). The purposes of the present study are to substitute the high-pressure gaseous charge of hydrogen by electrolysis charge and to evaluate hydrogen degradation susceptibilities for Inconel 625 and SUS 316L in the environments substituted by electrolysis charge. Electrolysis hydrogen was charged to Inconel 625 and SUS 316L at various electrolysis fugacities and gaseous hydrogen was charged from 0.3 to 45 MPa hydrogen gas at 90°C. Hydrogen states and contents were compared using thermal desorption analysis (TDA). Hydrogen degradation susceptibilities were evaluated using the slow strain rate technique (SSRT) at a constant extension rate of 8.6×10−6 /s at room temperature. The fundamental properties of thermal hydrogen desorption for Inconel 625 and SUS 316L were first analyzed to compare the hydrogen states after hydrogen charge by electrolysis and high pressure. The peak temperatures and profiles of hydrogen desorption do not change with charging temperature. When hydrogen is charged by electrolysis and high pressure until hydrogen saturation at 90°C, the peak temperatures and profiles are the same in both environments. This means that hydrogen diffusion during and hydrogen states after hydrogen absorption are independent of charging method in spite of the differences in adsorption and dissociation reaction on the specimen surfaces. Using Sieverts law, the fugacity of electrolysis can transform into gaseous pressure. This indicates that high-pressure hydrogen environments in pipes or other components at hydrogen refueling stations can be substituted by electrolysis charge. Fracture strain in Inconel 625 decreases as hydrogen content charged by electrolysis increases, whereas that in SUS 316L does not change regardless of the hydrogen content of 161.5 mass ppm. Grain boundary fracture is observed on the surface of Inconel 625 absorbing a hydrogen content of 27.5 mass ppm, which corresponds to 59.2 MPa hydrogen gas at R.T using Sieverts law. In contrast, the fracture surfaces of SUS 316L hydrogen-charged at extremely high fugacities remain ductile dimples. Thus, hydrogen degradation susceptibility is much lower for SUS 316L than for Inconel 625.