Increasing the strength of steel is one of the effective means of reducing the environmental load, as it leads to the reduction of CO₂ emissions by reducing the weight of transportation equipment such as automobiles and the reduction of materials used in buildings. On the other hand, since increasing the strength generally increases the susceptibility to hydrogen embrittlement, suppression of hydrogen embrittlement is important for practical use. Because hydrogen diffuses easily even at room temperature and the hydrogen concentration in steel changes in a complex manner over time, it has traditionally been difficult to understand hydrogen embrittlement behavior in atmospheric corrosive environments. It is difficult to understand the actual situation with the conventional method of measuring the hydrogen concentration by periodically collecting test pieces, hence in-situ measurement is necessary. Furthermore, since localized hydrogen can cause embrittlement, it is important to understand the distribution of hydrogen in the steel.

In the present research, in order to clarify the mechanism of hydrogen embrittlement in atmospheric corrosion environments, the hydrogen entry and hydrogen embrittlement behaviors of high strength steels in various parts of Japan were investigated using in-situ measurement techniques. By in-situ measurement of the hydrogen entry using the hydrogen permeation technique, it was revealed that the amount of hydrogen increased at the beginning of corrosion and when the corrosion reaction was accelerated, such as when a typhoon approached, and that the hydrogen embrittlement occurred when the surface hydrogen concentration exceeds the critical hydrogen concentration. By in-situ measurement of hydrogen embrittlement behavior using the strain gauge method, it is directly demonstrated that hydrogen embrittlement crack initiation, arrest, and propagation are repeated accompanied with hydrogen entry and release due to environmental changes, leading to final fracture. Furthermore, fracture surface observation and hydrogen visualization by secondary ion mass spectrometry (SIMS) suggested that the local hydrogen content was high at locations slightly inside the surface of the U-bend where the local stress was high, resulting in intergranular fracture, while quasi-cleavage fracture occurred near the surface of the U-bend where the strain was high.