# Monitoring of Atmospheric Corrosion of Aircraft Alloy AA2024 by Acoustic Emission Measurements
## 3 Results and Discussion
### Evaluation of AE Source Identification Experiment  
The AE data of the particular AE source identification experiment were evaluated by the cumulative AE hit energy of the two AE datasets gained from the specimen subjected to the Milli-Q water droplet and the specimen subjected to the NaCl solution droplet (cf. Figure 4). A threshold value of 25 dB was used for the hit extraction. The threshold was chosen to be 3 dB above the noise level (proven to suppress small noise outliers), which was approximately constant at 22 dB for both AE data streams. Furthermore, for all the detected hits, a 5 ms time signal was used, i.e., a predefined hit duration was applied. The 5 ms duration was enough to fully record most AE events that triggered a hit. Figure 10 shows the cumulative AE hit energy due to the droplet of the Milli-Q water and the droplet of the NaCl solution, as well as the curve of the measured RH in the glass chamber. The starting value of the RH was 38%, which was the ambient RH before the setup was closed. Due to the saturated KCl solution, a steep increase in the RH followed, which was intended to avoid the evaporation of the droplets. After 2.1 h, the equilibrium RH of the NaCl (75.5%) was reached and a final RH of about 80% was generated after 8.7 h. After this period of high RH, the evaporation of the droplets was forced with the use of silica gel and the experiment ended after 10.8 h with a steep decrease in the RH to 28%. The temperature measured by the data logger was nearly constant and varied between 23.6 \({}^{\circ}\)C and 25.1 \({}^{\circ}\)C with a mean value of 24.1 \({}^{\circ}\)C.  
The droplet of the NaCl solution led to significant AE activity of about \(1.187\times 10^{6}\) hits over the 10.8 h of the experiment. Interestingly, the droplet of the Milli-Q water also generated a non-negligible number of about \(1.767\times 10^{5}\) hits, i.e., a difference of a factor of 6.7. However, the cumulative AE hit energy (cf. Figure 10a) showed a clear difference between the Milli-Q water and the NaCl solution. The difference in the cumulative AE hit energy was a factor of \(3.785\times 10^{8}\) eu\(/(8.896\times 10^{6}\) eu\()=42.5\) at the end of the experiment. This significant difference can be easily explained by considering the scatter plots of the peak amplitude versus the AE hit energy presented in Figure 10b, where the hit energy values and peak amplitudes of the signals due to the Milli-Q water can be seen to be much lower. For further examination, the areas of the specimens that were initially wetted with the droplets were analyzed using optical microscopy (type Olympus SZX10 with DP26-CU camera). Figure 11a shows the corresponding surface caused by the NaCl solution droplet. The NaCl crystals that remained after the evaporation of the water of the solution can be seen. Moreover, some pits, indicating that corrosion happened, can be identified on the microscopic image, e.g., the detailed view shows a pit of an approximate size of \(115\,\upmu\mathrm{m}\times 135\,\upmu\mathrm{m}\). Figure 11b shows the corresponding surface caused by the droplet of the Milli-Q water. The microscopic image indicates that due to the Milli-Q water, corrosion took place. This was concluded due to (i) the seemingly small pits that were observed (see the detailed view in Figure 11b with a pit of an approximate size of \(50\,\upmu\mathrm{m}\times 60\,\upmu\mathrm{m}\)), (ii) some distinct discolorations that were observed over a wide area, possibly due to a uniform type of corrosion, and (iii) the white depositions that were observed on the specimen, which was most likely aluminum hydroxide Al(OH)\({}_{3}\) that formed during corrosion (cf. Figure 1 and Equation (1)). Metallographic or scanning electron microscopy (SEM) images could provide further insights into the corrosion (type and extent) that occurred. However, as this was not the primary aim of this study, no such analyses were conducted.