Krystaufeux Williams
Published: 2014
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The work discussed in this dissertation is an experimental validation of a body of research that was created to model stress corrosion cracking phenomenon for 304 stainless steels in boiling water reactors. This coupled environment fracture model (CEFM) incorporates the natural laws of the conservation of charge and the differential aeration hypothesis to predict the amount of stress corrosion crack growth as a function of many external environmental variables, including potential, stress intensity, solution conductivity, oxidizer concentrations, and various other environmental parameters. Out of this approach came the concept of the coupling current; a local corrosion current that flows from within cracks, crevices, pits, etc... of a metal or alloy to the external surface. Because of the deterministic approach taken in the mentioned research, the coupling current analysis and CEFM model can be applied to the specific problem of SCC in aluminum alloy 5083 (the alloy of interest for this dissertation that is highly sought after today because of its corrosion resistance and high strength to weight ratio). This dissertation research is specifically devoted to the experimental verification of the coupling current, which results from a coupling between the crack's internal and external environments, by spatially resolving them using the scanning vibrating probe (SVP) as a tool. Hence, through the use of a unique fracture mechanics setup, simultaneous mechanical and local electrochemical data may be obtained, in situ.The SVP is an alternating current device designed to obtain highly localized potential gradients (with a best resolution of microns) in a solution conductivity of 100s of [mu]S/cm. In order to enhance resolution of the SVP maps as much as possible, without being too far away from the desired test conditions of 0.6M saltwater (utilized in the lab as a substitute for seawater), dilution of the saltwater by an order of magnitude (0.06 M) was used throughout all experiments unless otherwise noted. Initial experiments of localized corrosion events from 10s of micron to mm-sized galvanic couples were first mapped in order to obtain confidence in the ability to map the current flowing through the solution above a stress corrosion crack. Furthermore, because of these feasibility studies, the current density that flows between an alloys matrix to or from an intermetallic compound can be spatially mapped as well.Standard fracture mechanics of AA5083-H116 bend bars were performed in diluted saltwater (0.06M) to obtain the critical fracture toughness for cracking in air and in saltwater. During loading in a bending test setup, standard load and crack mouth opening signals were obtained until the sample broke, or until the crack arrested. It was discovered that when the data was analyzed by plotting the load as a function of crack mouth opening displacement, the critical stress intensity (air), or threshold stress intensity (electrolyte) could be determined by identifying the decreasing of unload/reload slopes from a constant value. Transitions to and from different characteristic zones of cracking (different characteristics for different environments) are observed in the provided light and scanning electron microscopy images.The posited existence of the coupling current provides a different and more convenient marker for analyzing stress corrosion cracking, corrosion fatigue, and other forms of localized corrosion attack involving applied loads. For the first time, through the research in this dissertation, the positive current flowing from a crack were mapped, in situ, providing the changes in the anodic current density through the solution as a function of position. A collage of SVP maps were constructed on a grand scale according to their observed location with the light microscope by using features (such as the notch, or the growing crack) from the larger SVP maps taken at lower fracture toughness values with less resolution. These larger maps were then lined up with the extremes of the same features in smaller, SVP maps taken at a later time with a new location and higher resolution to keep up with the advancing crack tip. From that collage, a crack length vs. time graph was plotted to examine the characteristics of the crack growth. Finally, a crack growth rate vs. fracture toughness trend was constructed to compare with the CEFM and other studies on stress corrosion cracking of AA5083.In order to accommodate the expansion of the CEFM to predict crack growth in heavily sensitized AA5083 specimens, more electrochemical data (i.e., polarization scans taken from recent literature and compiled in this dissertation) was needed. The experimental findings from this dissertation also contributed to the model's expansion with the spatial analysis of the crack internal and external environments. Future work will incorporate testing of the more sensitive specimen orientations, acoustic emission analysis for probing micro-fracture processes, and coating effectiveness for SCC mitigation.
