Surface-softened clad steel （SCS） was developed to achieve both high strength and high resistance to liquid ammonia stress corrosion cracking （SCC）, and SCS plates with tensile strength （TS） of 600 MPa were applied to two ammonia tanks more than 30 years ago. In order to grasp the long-term SCC resistance, the open inspection results were analyzed. During 30 years of service, one of the SCS plate tanks had no crack, and only six crack incidents were detected cumulatively in the other tank. Compared with the conventional TS600 MPa steel plate tank, SCC of the SCS plate tanks was remarkably suppressed, suggesting that the SCS plate tank has excellent long-term SCC resistance. The cracks in the SCS plates tanks were only in the welded parts, as in the same case of TS440 MPa SLA325A commonly applied to liquid ammonia tanks. It can be said that the TS600 MPa SCS plate tank has high resistance to liquid ammonia SCC equivalent to the TS440 MPa SLA325A plate tank.

This study aims to elucidate the hydrogen entry behavior into partially Ni-coated steel sheet in a 0.1 M NaCl aqueous solution. Electrochemical hydrogen permeation tests and hydrogen visualization tests using a polyaniline-based hydrogenochromic sensor were conducted to investigate corrosion-induced hydrogen entry. While the hydrogen permeation current was not detected during the electrochemical hydrogen permeation test, the hydrogenochromic sensor successfully visualized the hydrogen entry in situ. It was revealed that hydrogen entry is accelerated in the pits formed in the area where the steel substrate is exposed. Hydrogen entry was not confirmed in the Ni-coated area. The visualization tests also demonstrated time-lag between the corrosion initiation and the detection of hydrogen entry. The highly sensitive hydrogenochromic sensor provides insight into the spatial distribution of hydrogen entry sites, which is not readily detectable by conventional electrochemical hydrogen permeation methods. The findings clarified that the critical role of solution chemistry on the corroded area in hydrogen entry into Ni-coated steel.