Copper is the intended canister material for the disposal of spent nuclear fuel in Sweden. At repository depth the groundwater may contain dissolved sulfide
Copper rods were tested for stress corrosion cracking in chloride solutions with 1.0 mM sulphide. The pH of the test solution was kept near neutral by addition of 10 mM phosphate or borate buffers of pH 7.2. Slow strain rate tests were performed at 60°C and 90°C. The extension rates were 5x10-7 s-1 or 1x10-7 s-1, and in some tests interrupted before rupture. Cross sections of the specimens were investigated with SEM.
Stress-strain curves do not reveal any signs of stress corrosion cracking. Intergranular corrosion in the shape of cracks developed early during the test. Tests interrupted after a few days of strain revealed a number of cracks preferentially located towards the ends of the gauge length of the test rod. At final rupture of the test rods, after about 14 days of continuous strain at 5x10-7 s-1, the cracks were more evenly distributed along the test rod. Necking and final rupture occurred close to the middle of the gauge length and not at the location where the first cracks appeared.
A mechanistic description is formulated, and the hypotheses about the processes are tested. The main features of the mechanism consist of:
- Intergranular corrosion with a possible spatial separation between anodic and cathodic sites.
- One role of the strain is suggested to be cracking of a layer of corrosion products that otherwise would be protecting. Another role of stress and strain would be to aggravate intergranular corrosion at sites where gaps in the protective layer coincide with a susceptible grain boundary.
- Intergranular corrosion seems to produce dissolved copper sulfide species as primary corrosion products. The more stable Cu2S(s) forms outside the cavities, initially. The rate of propagation is then limited by the rate of diffusion of soluble corrosion products from the front of the cavity to the mouth of the cavity. Eventually, solid Cu2S forms also inside the cavity. The Cu2S is predicted to form in increasing proportions, inside the cavity, as more surface become available for crystal growth. Because of the volume increase when copper metal is corroded to Cu2S, the cavity will tend to fill up with Cu2S. Finally, the cavity will be filled with solid corrosion products and unless continued strain creates new aqueous volume in the cavity, propagation will stop.
Copper corrosion sulfide testing ssrt
