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The sensors have also been used to evaluate coatings and structures in the field as illustrated by the following three examples. In the first, spot inspections were taken of primed areas inside a C-135 that was undergoing maintenance at Tinker AFB. Three areas were chosen: 1) good, original primer; 2) good primer that had been repaired at some time in the past; and 3) deteriorated primer that needed to be repaired. The sensor readily distinguished between the good and bad primers as illustrated in Figure 7. The demonstration showed that there was no problem making a ground connection to the aluminum structure and that the deteriorated primer exhibited low-frequency impedance two-to-three orders of magnitude below that of the good primers. Each measurement took only a few minutes.
The second example involves three land vehicles. One vehicle had been stored inside for several years and was in very good condition. The other two vehicles had been extensively used in a corrosive environment. Sensor measurements were taken in several areas on each vehicle. Although the later two vehicles had considerable rust, care was taken to acquire impedance spectra away from the rust in areas where the coating system looked good. The data of Figure 8 clearly show a difference between the coatings on the new and used vehicles with the impedance de-creasing by two-three orders of magnitude although there were no visual differences in the areas inspected. The measured low-frequency impedance of the two used vehicles seemed to correlate with the general visual appearance - the one with the lowest impedance had the greatest rust over its body.
The final example concerns a series of coated panels that are being exposed on a ship in Hawaii. Periodically, the panels are returned to the laboratory for inspection with the handheld corrosion sensor. Although there is little visual difference in the panels, some showed a significant decrease in the low-frequency impedance after only five months of shipboard exposure. Figure 9 shows the worst case. Here the coating's low-frequency impedance decreased by approximately three orders of magnitude in the first 5 months. At that point, the coating stabilized and little change occurred over the next seven months.
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.