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Electrochemical impedance spectroscopy (EIS) was used to monitor changes in the integrity of a bismalimide (BMI)/graphite fiber composite subjected to electrochemical and chemical stresses. Experiments included cathodic and anodic potentiostatic polarizations, exposure to caustic solutions, and galvanic coupling with various metals: aluminum, steel, copper, and titanium. EIS was found to provide a rapid and sensitive assessment of changes in the composite. EIS spectra of composite samples cathodically polarized greater than 200 mV from the composite open circuit (OC) indicated a transition from planar to porous electrode behavior. This response was attributed to the breakdown of the fiber/matrix interface possibly caused by hydroxyl ions produced at the fiber surfaces. Cathodic polarization below 200 mV versus OC yielded EIS data which suggested a decrease in the area of exposed fibers. This phenomenon is not well understood. Anodically polarized samples maintained a planar electrode response indicating uniform ablation which may have been caused by fiber and matrix oxidation. Samples exposed to caustic solutions also exhibited planar behavior attributed to general degradation of the BMI matrix. Galvanic couples with active metals yielded data with attributes of porous electrode behavior as well as characteristics associated with the anodic response. Couples with titanium caused changes in the response similar to those for low cathodic overpotentials. It is not known if these changes are harmful or benign.
electrochemical impedance spectroscopy, bismaleimide (BMI), graphite fibers, electrochemical degradation, polarization, composite, galvanic couples, caustic exposure
Research assistant, Center for Electrochemical Science and Engineering, University of Virginia, Charlottesville, VA
Assistant professor, Materials Science and Engineering, Center for Electrochemical Science and Engineering, University of Virginia, Charlottesville, VA
Professor, Materials Center for Electrochemical Science and Engineering, University of Virginia, Charlottesville, VA