Background
Stress corrosion cracking (SCC) can occur in materials from the combined influence of tensile stress and a corrosive environment. It has previously been shown in the literature, that copper can be sensitive to SCC in the presence of sulfide containing water. Since both tensile stresses are present as well as a material were SCC can occur, SCC could potentially be a problem for the copper canisters intended to be used for final storage of spent nuclear fuel. Testing materials susceptibility of SCC can be performed by accelerating the tensile stresses, the environment or both. A testing method frequently used within the nuclear industry for screening tests concerningS CC is Slow Strain Rate Testing (SSRT). In that test a material is slowly deformed plastically in an environment where presence of SCC can be spotted by decreased mechanical performance or cracks at the surface of the specimen. Other testing methods are also available for evaluating SCC susceptibility like constant strain or constant load testing. I this study only the first testing method, SSRT, has been employed to study SCC susceptibility of oxygen free copper sulfide containing deoxygenated water.
Objectives
In the present SSRT experiments the objective were to identify if thresholds in regards to tensile stress and sulfide concentration under which SCC does not occur in the designed experiment could be identified. This implicate that SSRT experiments was conducted to a certain level of strain where after presence of cracking on the specimen surface were evaluated.
Results
Based on the results in this study, SCC in oxygen free copper material intended for final storage of spent nuclear fuel have been observed in chloride containing water at a sulfide concentration of ~1·10−3 M and at a true stress threshold value of approximately 160 MPa. At lower sulfide concentration (~1·10−4 M), no SCC was observed on specimens. To some degree these results are consistent with other results found in the literature, even if the found sulfide threshold concentration is lower in this work. The sulfide content which resulted in SCC in the present study can be compared to sulfide contents between 10−8 M to 10−4 M found in groundwater of the Forsmark area. The results in this study indicates consequently that SCC of canister would not occur in the planned repository. However, based on the results in this study SSM finds that considerably more research is needed before SCC of oxygen free copper can be excluded in sulfide containing repository environments.