Electrochemical Impedance Spectroscopy (EIS) has long been used to evaluate coatings and study corrosion in the laboratory [1-8]. Correlations have been made with EIS parameters, such as low-frequency impedance and breakpoint frequency, and performance under different conditions. Initially, a coating exhibits capacitive behavior with very high impedance at low frequency. As the coating absorbs moisture, the low-frequency impedance decreases and becomes independent of frequency (Figure 1). The low-frequency impedance is a sensitive measure of coating and substrate health; it decreases by several orders of magnitude as moisture is absorbed and substrate corrosion occurs. This change occurs well before any visual indication of deterioration.
Conventional EIS requires immersion of a specimen into an electrolyte and the use of remote counter and reference electrodes. This procedure is suited for the laboratory as long as small specimens and immersion conditions are suitable for the study. Evaluation of larger specimens is possible with the use of flat cells (beakers without bottoms) that can be clamped to a specimen or structure and filled with electrolyte. Counter and reference electrodes are then inserted into the electrolyte and EIS spectra acquired.
This procedure allows measurements to be taken in the field in addition to the laboratory, [9-11] but requires an accessible, flat, smooth, and (preferably) horizontal surface. It provides a local indication of the coating and substrate health; the measurement probes only the area wetted by the electrolyte. The process generally is time-consuming and involves mounting of the cells, handling of the fragile electrodes, allowing the specimen to come into quasi-equilibrium with the electrolyte, acquiring data, removing and storing the cells and electrodes, and rinsing the surface. In some cases, the several-hour exposure to the electrolyte can cause artifactual damage to a coating during subsequent ambient exposure (Figure 2) .
Neither conventional immersion nor flat cell EIS is suitable for measuring ambient moisture up-take in a composite or adhesive bond. The lack of a metallic substrate to act as the working electrode would prevent EIS from being acquired from a composite with the possible exception of graphite composites. In the case of an adhesive bond, lack of electrolyte access to the adhesive would also prevent EIS from probing the bonded area. For both applications, immersing the composite or joint would cause additional moisture absorption and prevent accurate determination of moisture content.
A new, In-Situ corrosion sensor has been developed that addresses these issues and allows EIS measurements to be taken in the field or laboratory under ambient or accelerated testing conditions [12-19]. It is suitable for coated metals, composites, and adhesive bonds. As such, it extends the applicability of EIS to aging structures enabling condition-based maintenance (CBM) and to coating development and screening in exposures ranging from ambient to salt fog (ASTM B117) to cyclic accelerated testing (e.g., Prohesion, Ford APG, GM 9540P, and SAE J2334). The in-situ sensor is compared to conventional EIS in Table 1.
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 (Figure 3). 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 environ-mental 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.
Conventional nondestructive evaluation (NDE) commonly requires extensive corrosion to detect corrosion products or thinning of the structure. NDE of composites and adhesive joints generally detect voids, delaminations, and other gross defects. Detecting moisture in a composite or an adhesive bond before serious damage is very difficult, if not impossible. Sensors, such as the ones described here, serve to provide early warning of the initial stages of damage.
Other types of corrosion sensors have also been developed. Several of these are witness-type sensors that detect corrosion of one or more components of the sensor. One example involves a bimetallic strip. The charge transfer induced by the galvanic attack of the anodic metal by the cathodic metal is monitored . Another incorporates a bimetallic pair of electrodes onto a silicon chip along with the necessary electronics . Others involve metallic coatings on optical fibers and monitor the corrosion of the metallic coating . Although each operates on a different principle, the key commonality that distinguishes them from the technology reported here is that they monitor the corrosion of the sensor itself and not of the structure of interest. Accordingly, the sensors are consumed and, because they typically involve thin films, have a limited lifetime (in the case of a harsh environment, the lifetime can be very short). The information they provide is related to the corrosivity of the environment. They do not provide the information that is desired - the health of the actual coated structure.
Because they determine the corrosivity of the environment, these sensors are best suited for estimating the likelihood of a structure to corrode. Corrosivity measurements must be taken over time, they cannot be used to take a "snapshot" of a structure's health or condition. Nonetheless, such sensors can be used as part of a condition-based maintenance procedure although they give only indirect information. They cannot be used to evaluate the effectiveness of different coatings. The alternative sensors are suitable for adhesive bonding only if they are incorporated into the bond itself during fabrication. This has been demonstrated in the case of coated fiber optics, but the invasive procedure is clearly not possible after the joint is made. None of these sensors is suitable for monitoring moisture in a composite. The only procedure currently used is to monitor the moisture released as the composite is dried in an oven.
In-Situ Sensor to Detect Moisture Intrusion and Degradation of Coatings, Composites, and Adhesive Bonds, G.D. Davis, C.M. Dacres, and L.A. Krebs, DACCO SCI Inc.