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Tuesday, March 16, 2010: 8:00 AM
214 C (Henry B. Gonzales Convention Center)
Predicting the occurrence and rate of stress corrosion cracking in materials of construction is one of the most critical pathways for assuring the reliability of light water nuclear reactor plants. It is the general intention of operators of nuclear plants that they continue performing satisfactorily for times of 60 to 80 years at least. Such times are beyond existing experience, and there are no bases for choosing credible predictions. Present bases for predicting SCC rely on anecdotal experience for predicting what materials sustain SCC in specified environments and on phenomenological correlations using such parameters as K (stress intensity), 1/T (temperature), Ecorr (corrosion potential), pH, [x]a (concentration), other established quantities, and statistical correlations. While these phenomenological correlations have served the industry well in the past, they have also allowed grievous mistakes. Further, such correlations are flawed in their fundamental credibility.
Predicting SCC in aqueous solutions means to predict its dependence upon the seven primary variables: potential, pH, species, alloy composition, alloy structure, stress and temperature. A serious prediction of SCC upon these seven primary variables can only be achieved by moving to fundamental nano elements. Unfortunately, useful predictability from the nano approach cannot be achieved quickly or easily; thus, it will continue to be necessary to rely on existing phenomenology. However, as the nano approach evolves, it can contribute increasingly to the quantitative capacity of the phenomenological approach.
The nano approach will require quite different talents and thinking than are now applied to the prediction of SCC; while some of the boundary conditions of phenomenology must continue to be applied, elements of the nano approach will include accounting for at least, typically, the following multiple elements as they apply at the sites of initiation and at tips of cracks: discrete molecular environments, hydrogen entry and movement in metals, vacancy formation at dissolving surfaces and from dislocation interactions, effects of hydrogen on dislocation movements, preferential dissolution of alloying elements, very local stress distributions, epitaxial relationship between films and substrates, formation of very thin films, band structure, double layer interactions, reactivity of dislocations, slip interactions with surfaces, atomic bond strength as affected by environmental species, vacancy coalescence, and structure of grain and second phase boundaries as well as interactions with dislocations and solutes. It is unlikely that any one of these elements will supply quantitative predictability. Such predictability can be produced only by the integration of significant nano processes.
Predicting SCC in aqueous solutions means to predict its dependence upon the seven primary variables: potential, pH, species, alloy composition, alloy structure, stress and temperature. A serious prediction of SCC upon these seven primary variables can only be achieved by moving to fundamental nano elements. Unfortunately, useful predictability from the nano approach cannot be achieved quickly or easily; thus, it will continue to be necessary to rely on existing phenomenology. However, as the nano approach evolves, it can contribute increasingly to the quantitative capacity of the phenomenological approach.
The nano approach will require quite different talents and thinking than are now applied to the prediction of SCC; while some of the boundary conditions of phenomenology must continue to be applied, elements of the nano approach will include accounting for at least, typically, the following multiple elements as they apply at the sites of initiation and at tips of cracks: discrete molecular environments, hydrogen entry and movement in metals, vacancy formation at dissolving surfaces and from dislocation interactions, effects of hydrogen on dislocation movements, preferential dissolution of alloying elements, very local stress distributions, epitaxial relationship between films and substrates, formation of very thin films, band structure, double layer interactions, reactivity of dislocations, slip interactions with surfaces, atomic bond strength as affected by environmental species, vacancy coalescence, and structure of grain and second phase boundaries as well as interactions with dislocations and solutes. It is unlikely that any one of these elements will supply quantitative predictability. Such predictability can be produced only by the integration of significant nano processes.