# 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 about 7 h at an RH of 63%, the stagnation of this curve can be observed, whereas the curve of the cumulative video event count started to stagnate later, after about 9.8 h at an RH of 51%, which was around the third exchange of the saturated salt solutions. Although there was a lack of recorded videos, as explained above, the described stagnation of the cumulative video event count was considered valid, as the curve flattened significantly and remained constant at a very low level between hour 9.8 and hour 10.6. The corresponding video of the onset of the stagnation of the cumulative AE hit energy showed that the water of the droplet had already evaporated. Moreover, small pits had formed on the corresponding surface of the specimen. However, although the surface of the specimen seemed to be dry, small bubbling was still observed, which was also detected by the video processing algorithm. This bubbling took place in and in the near vicinity of already existing and emerging pits. This observation demonstrates that a seemingly dried-out surface does not mean that corrosion is no longer taking place. The electrolyte can remain in the pits and allow corrosion to progress further. This was expected for two reasons: (i) the drying of the pits simply takes longer, e.g., corrosion products around the pits build a barrier to the environment, or (ii), due to the deposits and surface conditions of the specimen, the equilibrium RH is reduced inside the pits [5; 6]. At hour 23.5 when the RH reached its preliminary minimum of 33% and started to force the RH to rise again, the corresponding video showed a dry specimen and no more bubbles could be seen in the pits (cf. Figure 5b). About 20 min later at an RH of about 44%, again, small bubbling at the pits was observed, which led to a slightly progressive increase in the cumulative video event count until hour 51, when the RH increased to about 74%. At this point, small droplets could be seen on the surface of the specimen. In addition, the large NaCl crystals began to deliquesce due to the high RH that was very close to its equilibrium or deliquesce relative humidity (DRH) (cf. Table 1). From the point where the NaCl crystals started to deliquesce and thus become a liquid solution again (see Figure 5c), the curve of the cumulative video event count again increased significantly, as the liquid electrolyte promoted corrosion. Thus, more and probably larger bubbles formed that were detected by the video processing algorithm. The progress of corrosion was further observed by the increasing size of several pits (cf. Figure 5b-d). The curve of the cumulative AE hit energy started to increase again at hour 55 at an RH of 79%. The final and maximum RH was about 85%, which was reached at hour 76.7. Thereafter, the RH was rapidly decreased to a low level using silica gel. After a further 40 min, the cumulative AE hit energy clearly stopped increasing. The same held for the cumulative video event count. Again, about 40 min later, the RH reduction stagnation of the cumulative video event count was observed. Thus, due to the rapid reduction in the RH, the AE and video activity stopped simultaneously, i.e., the electrolyte in the pits also evaporated rapidly. These findings clearly confirm that corrosion took place during the period of significant AE and bubble activity and vice versa. The curve of the cumulative AE hit energy showed its steepest increase at the beginning and clearly flattened after 3.7 h. At hour 55, the curve started to increase with a slope similar to that observed between hour 4 and hour 7, i.e., during the evaporation of the droplet. These similar slopes seem plausible since similar conditions prevailed at these times (similar amount of highly concentrated electrolyte at similar locations). Thereafter, the curve gradually increased again and between hour 67 and hour 69, a slope that was almost as steep as at the beginning was observed. From hour 70 onward, the curve began to flatten.