Aim of this study is to make clear both the relaxation of compressive residual stresses and the property change of shot-peened Type-316 stainless steel by thermal aging at 190℃ in air. Here, the Type-316 stainless steel is planned to be the material of dry storage vessel （canister） for the used nuclear fuel. We found that the stress relaxation ratio: m （defined as the ratio of stresses after aging against the stresses before aging or at being peened） was kept to be higher than 0.8, even after the aging over 35,000 hours. We also found that change of material properties such as surface hardening by the thermal aging was weak. We did not observe any intergranular （IG） cracks over 35,000 hour, while severe IG-cracks did occur by aging at 340℃ in air, as reported in the previous paper.

Corrosion of steel materials due to seawater and sea salt particles is a very important problem in steel structures in marine environments, and it is no exaggeration to say that the life of the structure itself is determined by this corrosion. Therefore, by coating the steel material of the main body with titanium, which has excellent corrosion resistance to seawater, a marine structure is constructed in which the durability of the entire structure is greatly improved. On the other hand, although the corrosion resistance of titanium to seawater has been confirmed at the laboratory level, there have been almost no reports of confirmation tests by long-term exposure in actual sea areas. Therefore, the （National Research Institute） Public Works Research Institute started in 1984, “Marine exposure confirmation test of high corrosion resistance metal coating anticorrosion method, etc.” A body was manufactured and a durability confirmation test was conducted.

As a result of this 30-year exposure test, the soundness of not only titanium itself but also special parts such as crevices and welds was confirmed.