# Monitoring of Atmospheric Corrosion of Aircraft Alloy AA2024 by Acoustic Emission Measurements
## 3 Results and Discussion
### Comparison of AE, Bubble Activity, and Atmospheric Corrosion
The upper part of the images shows the originally captured frame and the lower part shows the corresponding processed frame. The original frame in Figure 7a shows several bubbles in the droplet, and few changes from the previous frame were detected, resulting in an almost black processed frame. Figure 7b shows the subsequent frame. The bubble on the upper-left edge of the droplet has burst, which was detected by the algorithm and is highlighted with a green rectangle. Additionally, two smaller events were detected in the lower-right corner of the frame. The authors decided that a threshold size of 60 pixels was a proper value to avoid the invalid detection of noise or other artificial objects caused by small vibrations or shocks to the setup. However, larger shocks to the setup caused several detections at the same time, which resulted in a heavily disturbed cumulative count (cf. Figure 7c). Such disturbances were removed from the cumulative count in a final processing step. The manual inspection of some processed videos revealed that there was predominantly H\({}_{2}\) bubble activity in the videos and that this activity was well captured by the video processing algorithm. Furthermore, the manual inspection revealed that in most cases, a maximum of three to four valid objects were detected between two consecutive frames. Thus, if five or more objects were detected simultaneously, the corresponding counts were removed. In another cleaning step, a check was performed to see if an event had been detected in the same frame or a nearby position in the previous frame. This avoided the multiple counting of single events of longer durations, e.g., if bubbles slowly moved or rose. The normalized cumulative AE hit energy and normalized cumulative video event count are shown together with the curve of the RH in Figure 8a. The normalization of the two cumulative quantities was performed using their maximum values, which were \(1.99\times 10^{6}\) eu and 82,555 video events, respectively. As mentioned above, there were some periods where no videos were recorded, especially in the time span from hour 10.6 to hour 23.5, which is illustrated by the solid line without markers on the curve of the cumulative video event count in Figure 8a. Figure 8: Results of the atmospheric corrosion experiment. (**a**) Measured relative humidity (RH) and calculated normalized cumulative AE hit energy and normalized cumulative video event count. The vertical dashed lines represent the times when the cumulative quantities indicated the stopping and starting of corrosion activity. The gray area marks a time span in which the focus of the camera was changed. (**b**) Peak amplitude versus AE hit energy of the four phases p1–p4 of the experiment. Figure 7: Example of results of the video processing algorithm. The upper part of the images represents the originally captured frames and the lower part, the processed frames. (**a**) Bubbles in the droplet can be seen in the original frame but the algorithm detected no activity, (**b**) shows the subsequent frame. The bubble on the upper-left edge of the droplet has burst, which was detected by the algorithm and is highlighted with a green rectangle. (**c**) Small vibrations in the test setup caused 37 detections at once; such disturbances were not considered in the subsequent analysis. Figure 8a clearly shows that the cumulative AE hit energy and cumulative video event count were related to the RH, or more precisely, to the resulting presence of electrolyte on the specimen's surface. In the initial phase, when the RH rose sharply to high values so that the droplet of the NaCl solution remained on the specimen and did not evaporate, both cumulative quantities showed a significant increase. After about 3.7 h and an RH drop to 66%, which was after the second exchange of the saturated salt solutions, the curve of the cumulative AE hit energy significantly flattened. The corresponding videos showed that some water of the droplet had already evaporated; thus, less solution was present on the specimen.