In-situ sensors, based on electrochemical impedance spectroscopy (EIS), have been developed to detect corrosion and the absorption of moisture in polymeric systems. In the case of a coated metal, moisture intrusion into the coating foretells corrosion of the substrate so that the sensor is sensitive to the very early stages of corrosion. In an adhesive bond, the sensor monitors ingress of moisture into the bondline that causes plasticization of the adhesive and hydration of an aluminum adherend.
Two versions of the sensor have been developed: a painted electrode that is permanently attached to the structure of interest and a hand-held probe that is pressed against the surface only while measurements are being acquired. Both versions give identical results to each other and to conventional EIS measurements during comparison immersion studies. Consequently, the procedures and analyses developed for conventional EIS can be applied directly.
The choice of sensor is dependent on the specific application. The permanent sensor is well suited for monitoring inaccessible areas of a structure and evaluating test panels in environmental chambers, such as a salt fog chamber. The hand-held sensor is best suited for spot checks of specimens without the permanent sensor or in areas where permanent sensors are not desired for reasons of aerodynamics or visual appearance. It is also well suited for inspection of composites. Both versions are suitable for monitoring adhesive bonds.
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Field Inspection: The usefulness of the sensors has been demonstrated to evaluate coatings and structures in the field as shown in three examples. |
| Graphite-Epoxy Composites: Same-side and opposite-side measurements gave similar results as did measurements with the electrodes oriented along the outer fiber direction and at 45° to this direction. The overall impedance values for these composites are generally lower then the impedance values for the glass/polyimide composites. Also, the change in impedance as a function of moisture content is less pronounced in the graphite-epoxy composites as compared with the glass-polyimide specimens. |
Equivalent circuit analysis focused on identifying circuits that were physically significant with the intention of identifying circuit elements particularly sensitive to changes in the quantity of moisture absorbed. The method was based on tracking the changing values of several moisture sensitive circuit elements as a function of moisture content. Identifying and monitoring the trends of more than one circuit element increases confidence in this method of moisture content prediction. The model selected for the monolithic samples is shown here.
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Graphite-Epoxy Honeycomb Composites: For aluminum core honeycomb composite specimens, equivalent-circuit analysis was needed. The model of shown here consists of two loops in series and a third imbedded loop. |
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