Stress Corrosion Cracking of Nuclear Pressure Vessel Steels in Pressurized High Temperature Waters

The quantitative evaluation of the subcritical erack growth behavior of nuclear pressure vessels and piping steels in pressurized high temperature water has been receiving extensive attention to ensure the structual integrity of nuclear pressure vessels and piping. Recent research progress in this field suggests the significance of the sulfur content of steels in assessing susceptibility to environmentally assisted cracking of these steels in high temperature water from the view point of solution chemistry at a crack tip, where dissolution of MnS, non-metallic inclusion, results in an enrichment of S anion, such as SO₄^2-. Hence, it is neccesary to simulate the solution chemisty at a crack tip for the slow strain rate test (SSRT) by use of a smooth specimen which has been commonly used as a useful tool to see stress corrosion cracking characteristics in a short time. In this study, SSRT tests are performed in simulated BWR or PWR environments, and specimen electrochemical potentials are controlled chemically or potentiostatically. The cracking potential of this material/environments systems are examined quantitatively as a function of temperature, sulfur content in steels and also SO₄^2- contents in solution, simulating crack tip solution chemistry. Cracking potential showed a lower value of about 250°C, rather than 288°C. Sulfur content of steel and SO₄^2- concentration in solution can drastically influence the cracking potential and some amount of sulfur in steels or SO₄^2- anion at a crack tip make it possible to crack at a BWR or PWR condition.