602 イオンプレーティング処理を施した7075合金中の水素の挙動解析(材料と組織)

Abstract

Fuel cell vehicles (FCVs) are ideal in terms of solving the global warming problem. In the FCVs, high-pressure (35MPa) hydrogen has been being used as the fuel, being contained in a tank composed of 6061 aluminum-alloy liner and surrounding carbon-fiber-reinforced-plastics layer. Replacing the 6061 alloy by a high-strength aluminum alloy is demanded to increase the distance per a fuel supply by raising the initial pressure to 70MPa. In some high-strength aluminum alloys, however, hydrogen embrittlement has been reported when the materials are tensile-deformed in a moist air or the materials are cathode-charged with hydrogen. To confirm the safety of the hydrogen tank in the FCVs, investigation on the behavior of hydrogen followed by the study on the hydrogen embrittlement is needed. Hydrogen microprint technique (HMPT) has been known to be effective to investigate the behavior of hydrogen, in which hydrogen atoms can be detected as silver particles through a reaction between hydrogen atom and silver halide. Up to date, hydrogen invasion from the environment to the inside of the aluminum base materials as well as the hydrogen emission from the materials to the materials has been considered to be significantly affected by the surface oxide layer. To confirm this issue, in this study the effect of surface coating, ion-plating and vacuum-vapor deposition, on the behavior of hydrogen has been examined in a typical high-strength aluminum alloy, 7075, by means of HMPT. Sheet specimens ion-plated and vacuum-deposited both with nickel was charged with hydrogen gas of 2MPa with and without tensile stress above the yield stress on the coated-side surface, and HMPT image was observed on the uncoated-side surface. The ion-plated specimen had a larger amount of hydrogen emission than the vacuum-deposited specimen when tensile stress was applied.