# Monitoring of Atmospheric Corrosion of Aircraft Alloy AA2024 by Acoustic Emission Measurements
## 3 Results and Discussion  
### Comparison of AE, Bubble Activity, and Atmospheric Corrosion  
Figure 5 shows the curve of the measured temperature \(T\) and RH during the atmospheric corrosion experiment, which lasted about 96 h. The temperature varied periodically with the day-night cycle; the minimum value during the night was 22 \({}^{\circ}\)C and the maximum value during the day was 25 \({}^{\circ}\)C. The initial RH was about 35%. The controlled variation started with a rapid increase in the RH, which approached about 77% RH at hour 2.8 (i.e., at the end of step 1). In steps 2 to 5, a controlled decrease to a low RH of about 33% at hour 23.5 was performed. In steps 6 to 10, the RH slowly increased again to a maximum RH of 85% at hour 76.7. Step 11 started with a rapid drop in the RH and then continued with a slower decrease to a final RH of 7% at hour 95.8. The times that the saturated salt solutions were exchanged, i.e., the changes between the steps, are indicated by vertical dashed lines and can also be seen in the abrupt drops or rises in the RH curve. Although the expected RH values, as given in Table 2, were not achieved for the saturated salt solutions used, the RH variations were suitable for simulating the dry and wet phases of the monitored specimen, as expected in the operation of aircraft components. Moreover, Figure 5 shows several images of the specimen's surface with the deposited droplet (extracted from the recorded videos) and the corresponding time points. Figure 5a shows the droplet at 2.8 h after the start of the experiment, where a high RH of 77% was present. Bubbles of different sizes are clearly visible in the droplet. Due to the absence of other bubble-forming processes and the corrosion taking place (as can be seen later), this was determined to be H\({}_{2}\) produced by corrosion processes. Figure 5b shows the surface of the specimen at hour 23.5, which was the end of the defined decrease of the RH to 33%. The steady decrease in the RH caused the water to slowly evaporate from the specimen's surface, whereas the NaCl crystals of the droplet solution remained on the surface. Furthermore, the formation of pits can be clearly seen in the figure. Figure 5c shows the surface at hour 54.9 at an RH of 79%. Due to deliquescence, the moisture in the humid environment was absorbed by the NaCl crystals, which resulted in numerous smaller droplets of another corrosive NaCl solution. Figure 5d depicts the situation at hour 74.6 with an RH of about 83%, which was only briefly before the maximum RH of 85% was reached. Due to the increasing duration and RH, the droplets increased further in size and partly merged. Moreover, by comparing Figure 5b-d, the growth of the already existing pits and the formation of new pits can be clearly observed. Figure 5e shows a completely dried-out but corroded surface after 95.8 h at an RH of 7%. Significant pits and NaCl crystals can be identified. The minor pit growth observed between Figure 5d,e was also expected, but due to the short time (about 2 h) in which the electrolyte was present on the specimen in this period, this was not visually apparent. After the rapid decrease in the RH at hour 76.7, the pit growth stopped. Figure 4: Experimental setup of the AE source identification experiment to compare the AE caused by the droplets of Milli-Q water and NaCl solution, respectively. Figure 6 illustrates that corrosion did not progress during the dry phases at a low RH, which can be seen by comparing the exemplary images of the corrosion site (extracted from the recorded videos) in these phases. Due to some issues with the video control, some periods exist where no videos were recorded, e.g., in the dry phase from hour 9.8 to hour 23.5. Therefore, Figure 6a shows the specimen's surface only at hour 23.5, i.e., at the end of the defined dry phase, and Figure 6b shows the surface at hour 27.5 at an RH of 47%, where the specimen's surface was also still dry. During this period, neither the pit growth, formation of new pits, nor changes in the deposits were identified.