The Stress Corrosion Cracking of Copper:Nuclear Waste Containers
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extent of SCC based on the time dependence of those environmental and material properties known to control the rate of cracking. We are helped in this task by the availability of kinetic expressions for the various SCC mechanisms proposed in the literature [1]. Based on these mechanistic studies and on a knowledge of the evolution of conditions within the disposal vault, we have identified the following parameters as being important in controlling the rate of SCC for Cu containers: the presence of an SCC agent, the supply of oxidant, the crack-tip strain rate and stress intensity factor (K1), temperature, the presence of Cl- and the grade and properties of the material. In this first part of the work, we report on the effects of just one of these parameters, the supply of oxidant, on the SCC susceptibility of two grades of Cu in the as-received condition. Experiments were performed in NO 2 -containing solutions, an environment known to cause the SCC of Cu [1], using two loading techniques. Nitrite could be formed in a disposal vault by
microbial activity or the radiolysis of moist air or be present as a by-product of blasting operations [1].
887 Mat. Res. Soc. Symp. Proc. Vol. 556 ©1999 Materials Research Society
EXPERIMENTAL Compact-tension (CT) specimens were prepared from oxygen-free electronic (OFE., UNS C 10100) and oxygen-free with phosphorus (OFP, Outokumpu Poricopper Oy, Finland) copper plate. Table I gives various properties of the two materials. All specimens were prepared with the same dimensions (1.25W = 19 mm, a/W = 0.5+0.05, B = W/2 [2]) to permit intercomparison of the results but, because of the ductility of the material, the specimens are not sufficiently large to guarantee plane strain conditions. A fatigue pre-crack was introduced into each sample and was sharpened by gradual reduction of the load. For all of the OFE and some of the OFP samples the load was decreased manually. For the OFP samples it was also found possible to allow the load to decrease automatically, i.e., the decrease in load was self-regulated, possibly as a result of strain hardening in the crack-tip region. The specimens were prepared in the T-S orientation, which is equivalent to a circumferential crack propagating through the wall thickness of a container fabricated from rolled plate, with the plate-rolling direction perpendicular to the major axis of the container.
The CT specimens were loaded using two methods. Constant extension rate tests (CERT) were performed at a cross-head speed of 8.46 x 10.6 mm-s' in a custom-designed, relatively hard rig [3]. During the CERT tests, in which K, generally increases as the crack lengthens, the load was continuously monitored. Constant displacement (CD) tests were performed on OFE samples by inserting a hardened ceramic spacer into the mouth of the CT specimen to give an initial K, of 22 MPa.m' . This value of K1 was chosen as the maximum stress intensity achievable without plastically deforming the specimen. The SCC environment was 0.1 mol.dm- NaNO 2 (nominal pH 9.0). CERT tests
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