# Monitoring of Atmospheric Corrosion of Aircraft Alloy AA2024 by Acoustic Emission Measurements
## 3 Results and Discussion
### Frequency Analysis of Atmospheric Corrosion AE Data  
The frequency analysis of the AE data of the atmospheric corrosion experiment revealed further AE activity that was not detected by the simple amplitude threshold approach. Therefore, a short-time Fourier transform (STFT) of the complete AE data, which also included the time periods where only noise without hits was measured, was conducted. Due to the very long duration of the entire AE measurement signal (approximately 4 days) compared to the duration of the individual AE hits (approximately 5 ms), a visual representation in the form of a spectrogram is not meaningful, and the dominant frequencies were instead plotted. The results of the STFT \(Z(t,f)\) were divided into \(n\) time segments, each of a \(\Delta t=0.1\) s duration. For each segment, the frequency \(\hat{f}_{i}\) of the maximum of the absolute value of \(Z(t,f)\), i.e.,  
\[\hat{f}_{i}=\underset{f}{\arg\max}|Z(t,f)|\qquad t\in[i\Delta t,(i+1)\Delta t] \tag{7}\]  
with \(i=0\ldots n-1\), was extracted. The STFT was calculated using the command signal.stft in the Python package scipy. The Hann window with a window length of \(w=2500\) samples and window overlap of \(w/2\) was used, leading to a time resolution of 62.5 us and a frequency resolution of 8 kHz. Figure 9 shows the results of this frequency analysis, together with the curve of the RH. Figure 9: RH and results of the STFT-based frequency analysis of the entire AE data of the atmospheric corrosion experiment. The colored vertical dashed lines represent the times at which the cumulative AE hit energy and cumulative video event count indicate the stopping and starting of the corrosion activity. The areas marked in blue represent the night times from 8 pm to 7 am. The detailed view highlights the different frequencies at the day and night times, as well as the transitions between frequencies indicating the corrosion activity and constant frequencies due to background noise. Clear differences in the calculated frequencies between the wet and dry phases, i.e., the phases of high and low RH, can be seen. In particular, at the beginning, when the initially deposited droplet was present on the specimen, frequencies up to 1.5 MHz were yielded. The reduction in the RH, and thus the electrolyte on the specimen's surface, also resulted in a reduction in the frequency. Between hour 12 and hour 48, the same frequencies were consistently observed. In this time span, a low RH prevailed and the NaCl crystals had not yet started to deliquesce. A detailed analysis of these frequencies (using the experimental measurement of the electromechanical impedance spectrum of the specimen with the applied PWAS; see [42] for this measurement method) revealed that some of them (e.g., 24 kHz, 40 kHz, 64 kHz, 88 kHz) fit to the natural frequencies of the specimen (e.g., 25.8 kHz, 42.3 kHz, 63.7 kHz, 85.9 kHz). Moreover, it was observed that the day (7 am to 8 pm) and night times (8 pm to 7 am) clearly influenced these frequencies (cf. detail view in Figure 9). It was assumed that this was related to the building technology of the laboratory (heating control, electrical infrastructure, lighting, etc.). However, the specific reason could not be identified. This influence had to be considered when attempting to determine the transition from frequencies indicative of corrosion activity to frequencies due to background noise. The transition indicating the stopping of corrosion due to the lowering of the RH was suggested to have occurred at approximately hour 7.5, which was between the stopping points determined by the cumulative AE hit energy and cumulative video event count. The onset of corrosion activity due to the increase in the RH was determined to have occurred at approximately hour 51.2, which matched the point of the cumulative video event count. The frequency behavior after hour 51.2 was again clearly different from the previous phase with a low RH.