# Monitoring of Atmospheric Corrosion of Aircraft Alloy AA2024 by Acoustic Emission Measurements
## 3 Results and Discussion
### Discussion of the AE Signal Source  
To systematically determine the source of the measured AE signals, all possible effects that may have provoked AE activity during the experiments are discussed. Figure 12a presents the classification of these effects. It is shown that atmospheric corrosion can be triggered on AA2024-T351 by a small droplet of a corrosive NaCl solution, as the visual observation clearly identified pits after some time. Hydrogen that formed during corrosion led to bubble activity, which was also seen in the videos, and the AE monitoring measured significant increases in the cumulative AE energy. All three observed phenomena, i.e., corrosion, bubbles, and increase in AE energy, were associated with the RH (cf. Figures 5 and 8a), strongly suggesting that bubble activity (origination, detachment, movement, arrival at droplet surface, growth, bursting) was the main source of the measured AE signals, which has been repeatedly claimed in the literature for different kinds of corrosion tests [26, 27, 28]. However, it cannot be excluded that there were other unknown effects that generated measurable AE signals. These other effects may or may not have been related to corrosion (cf. Figure 12). Since we attempted to answer the question of whether atmospheric corrosion can be clearly detected by AE, sources not related to corrosion should ideally be excluded or known, considering that Figures 8 and 10 show that the detected AE signals were dominated by corrosion-related effects. These results show that a sufficient amount of electrolyte (NaCl solution) results in corrosion and a large number of AE signals with comparatively high energies and peak amplitudes. In contrast, the different conditions with just a very small amount of electrolyte or Milli-Q water on the surface resulted in no or very little corrosion (cf. Figures 6 and 11) and at the same time, generated only a small number of AE signals with significantly lower energies and peak amplitudes. However, the dominance of corrosion-related effects in the AE signals can be supported by considering Figure 12b. Figure 12b presents the classification of all possible effects that may have provoked AE activity during the experiments, but reduces the representation of the AE activity and its triggering effects, which are suppressed by the use of Milli-Q water instead of NaCl solution. The Milli-Q water generated corrosion and also measurable AE signals. However, as observed above, the corrosion caused by the Milli-Q water was found to be significantly less, which meant less formation of H\({}_{2}\) (cf. Equation (1)), i.e., less bubble  
Figure 11: Microscopic images of the corroded surface of the specimen in the AE source identification experiment, i.e., after the evaporation of the droplets of **(a)** the NaCl solution and **(b)** the Milli-Q water. The detailed views show the exemplary pits that formed. activity and also fewer other effects that triggered AE activity related to corrosion. Thus, it can be concluded that the measured AE signals were strongly dominated by corrosion-related effects. However, non-corrosion-related effects that contributed to the measured AE activity cannot be fully excluded but were shown to be of minor importance. There are various effects that have the potential to be sources of AE. These effects include (i) AE sources unrelated to corrosion. Environmental noise and interference were uncommon, as the experiments were conducted under laboratory conditions. Nevertheless, electric interference, which typically produces spike-like AE signals, was filtered out by not considering hits with counts less than five. AE sources caused by the effects of the used saturated salt solutions and the silica gel can also be excluded, as the AE data did not show sudden changes due to the exchanging of the salt solutions or the silica gel. (ii) AE sources related to corrosion but not directly related to bubble activity.