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In order to establish the safe use of hydrogen energy, it is of urgent necessity to
clarify the effects of hydrogen on mechanical properties of structural materials.
Typical metallic materials for structural use are ferrous-alloys such as chromiummolybdenum
(chromoly) steel. Due to its high strength and durability, chromoly
steel has been used for various structures that are subjected to harsh load
conditions as in pressure vessels. Its application to structural bodies which are
exposed directly to gaseous hydrogen is, on the other hand, yet to be generally
accepted because the effect of gaseous hydrogen on the subcritical-crack growth
property and the microscopic mechanism is not undersood well.