# Monitoring of Atmospheric Corrosion of Aircraft Alloy AA2024 by Acoustic Emission Measurements
## 3 Results and Discussion
### Comparison of AE, Bubble Activity, and Atmospheric Corrosion
After the last forced increase in the RH, a small intermediate increase in the AE hit energy was observed at hour 75. The forced drying at hour 76.6 led to the final stagnation of the curve. This trend clearly shows that the most energy was generated during the phases of high RH, i.e., at the beginning when the droplet was still present and during the phase of sufficient deliquescence of the NaCl crystals, thus suggesting the correlation of the AE activity with the amount of electrolyte present on the specimen. This is also illustrated in Figure 8b by the scatter plot of the peak amplitude versus the AE hit energy. The four colored clusters represent the AE signals of the two phases when a large amount of electrolyte was present on the specimen (p1 and p3) and the two phases when no or a small amount of electrolyte was present on the specimen (p2 and p4) during the experiment (cf. annotations in Figure 8a). During phases p1 and p3, signals with high energies and peak amplitudes were detected, leading to the above-described steep increases in the cumulative AE hit energy. However, during phases p2 and p4, AE signals were also detected. These signals were less frequent and had significantly lower energies and peak amplitudes, which describes the stagnation of the cumulative AE hit energy during these two phases, as seen in Figure 8a. The slopes of the cumulative video event counts in the time spans at the beginning (hour 0 to hour 10) and at the start of the deliquescence of the NaCl crystals (hour 51 to hour 59) were similar. In the first time span, the focal plane of the camera was set near the droplet surface, where bubbles in the droplet are clearly visible. In the second time span, the focal plane was set at the specimen's surface, where small bubbles in and near the pits were observed. From hour 59 onward, more and more electrolyte in the form of single droplets were present on the specimen due to the ongoing deliquescence of the NaCl crystals. However, due to the focal plane settings of the camera, bubbles appeared to be blurred in the droplets of the electrolyte and were less well detected by the algorithm. This is considered the reason for the flattening of the cumulative video event count curve between hour 59 and hour 72.5. From hour 72.5 to hour 76, the setting for the focal plane was changed (see the gray marked area in Figure 8a). The focal plane was moved slightly above the specimen's surface. This modification made the bubbles partly visible. However, blurred bubbles were still present on the edges of the droplets, which were not detected by the algorithm. Thus, if there were several small droplets on the specimen, only a portion of the bubbles within these droplets was detected, i.e., changing the focal plane did not significantly change the trend in the cumulative video event count (cf. Figure 8a). From a global perspective, both cumulative curves showed similar trends that were both related to the measured RH. A stopping of the corrosion when the RH reached low values was observed in the videos (no pit growth or formation of new pits) and reflected in the cumulative video event count (no bubble activity). In addition, a re-onset of corrosion when the RH was increased again was observed. It was shown that the stopping and restarting of atmospheric corrosion could also be detected using the AE method, however, with some time delays with respect to the RH and bubble activity. It was observed that in the phases where just a small amount of solution was present on the specimen or only in the pits, fewer and smaller bubbles formed. It was believed that these small bubbles were too weak to generate AE events that exceeded the noise level and thus were not detected by the simple amplitude threshold approach used. The visual observation by the videos strongly suggests that bubbles were the main source of the measured AE signals, as very few events that were not bubbles were observed or detected.