# Monitoring of Atmospheric Corrosion of Aircraft Alloy AA2024 by Acoustic Emission Measurements
## 3 Results and Discussion
### Discussion of the AE Signal Source
A possible AE source could be the initiation or growth of microcracks (starting from pits) in the aluminum matrix caused by the formation of H\({}_{2}\) bubbles from individual H\({}^{+}\) ions or residual stresses in the matrix. However, microcracks can be excluded since they would also trigger measurable AE activity, and small H\({}_{2}\) bubbles were observed (i.e., ongoing corrosion) only in some pits (cf. Figure 8 hour 7 to hour 10, and hour 51 to hour 55). Moreover, no significant residual stresses were expected in the simple, small, and thin specimens. A further effect triggering AE activity may be the dissolution into Al\({}_{3}\)\({}^{+}\) ions in the pits (cf. Figure 1). However, this is a continuous process on an atomic level, i.e., low energy for AE, and thus it is also very unlikely to be a potential AE source. To fully identify the occurring corrosion mechanisms, further AE signal analysis by more advanced signal features [19; 23] or post-mortem examination of the corrosion site by, e.g., SEM, is required. However, this is a matter for future research. (iii) AE sources specifically related to bubble activity. As presented in Figure 1, the main phases of a bubble are its formation, detachment from the metal surface, movement, arrival at the droplet surface, growth, and bursting. The sudden formation of gaseous H\({}_{2}\) from single solid-state H\({}^{+}\) ions could potentially generate a pressure wave leading to AE. This can be excluded since no AE hits were detected during the low RH phase when no droplets were present but small bubbles (indicating ongoing corrosion) were observed in some pits (cf. Figure 8 hour 7 to hour 10, and hour 51 to hour 55). Other potential sources are the detachment of bubbles from the specimen and subsequent bubble movement, arrival of bubbles at the electrolyte surface, and bubble growth, e.g., by the merging of the bubbles and the eventual bursting of bubbles (cf. Figure 1). All these effects must also occur when only a small amount of electrolyte is present in the pits; however, no AE hits were detected in this case (cf. Figure 8 hour 7 to hour 10, and hour 51 to hour 55). Therefore, it is assumed that the amount of electrolyte at the corrosion site influences whether or not AE hits can be detected. From the recorded videos, it was observed that a larger amount of electrolyte, i.e., larger droplets, leads to larger bubbles, which are expected to generate AE hits with larger amplitudes and energies. Moreover, a larger amount of electrolyte might create a  
Figure 12: Classes of effects triggering AE activity on AA2024-T351: (**a**) corrosion and AE activity due to the NaCl solution and unknown contributions of different effects, (**b**) reduced corrosion and AE activity due to Milli-Q water, and consequently, reduced contributions of corrosion-related effects. higher counter pressure against the bubbles, resulting in bubble activity with higher energy, i.e., AE hits with higher amplitudes. Thus, the results strongly suggest that bubble activity is the main source of the measured AE signals. Bubble bursting is assumed to be the most emissive source, but also the merging of the bubbles is expected to produce measurable AE signals. Consequently, the present monitoring approach may potentially be applied to other metals where gas bubbles form due to (atmospheric) corrosion, provided a sufficient amount of electrolyte is present.