# Monitoring of Atmospheric Corrosion of Aircraft Alloy AA2024 by Acoustic Emission Measurements
## 4 Conclusions  
In the present research, the AE method is investigated for the continuous monitoring of the atmospheric corrosion of AA2024-T351 aluminum alloy, commonly used for aircraft structures. The atmospheric corrosion is induced by small droplets of a corrosive NaCl solution. The operating conditions of an aircraft are simulated by controlled variations in the RH. Video recording of the corrosion site and AE measurements by an adhesively bonded PWAS are used to monitor the corrosion process. The AE data are analyzed by the AE hit energy, peak amplitude, and STFT-based frequency analysis. The recorded videos are processed by an algorithm to automatically detect sudden events occurring between consecutive video frames. The following conclusions can be drawn:  
* The clear formation of pits, the stopping of pit growth at a low RH, and the re-onset of pit growth at an increasing RH are shown. Furthermore, the RH directly correlates with the observed amount of electrolyte present on the specimen. * Similarly, the cumulative AE hit energy stagnates and starts to increase again with a sufficient amount of electrolyte on the surface. In addition, the STFT-based frequency analysis of the AE data indicates the AE signal change in the presence of an electrolyte. * Consequently, the correlation between the AE activity and corrosion progress is given, leading to the conclusion that atmospheric corrosion can be reliably detected using current AE measurement equipment. * The video processing algorithm mostly detects bubbles of H\({}_{2}\) that are formed during corrosion. * The evaluation of the AE data, processed video data, RH, and observed progress of corrosion strongly suggest that H\({}_{2}\) bubble activity is the predominant source for the measured AE hits. * For the considered atmospheric corrosion, the corrosion-related AE signal amplitude and energy increase with the amount of electrolyte on the specimen's surface. * The amount of electrolyte at the corrosion site has to be large enough for the AE measurement system to detect bubbles or corrosion. This is also expected to be the case for the operating conditions of aircraft structures. Thus, the AE method may be applicable for the online and continuous corrosion monitoring of aluminum aircraft components. * It is shown that AE sources unrelated to corrosion cannot be fully excluded but are of minor importance to the observed results. Future research will identify the characteristics of the corrosion-related AE signals (e.g., by multivariate analysis) to enable the identification of corrosion in noisy environments. Moreover, advanced experiments will be designed and further methods applied (e.g., SEM) to identify the true effects that trigger corrosion-related AE activity, thereby also enabling a better understanding of the signal characteristics. Future work will also address the localization of corrosion-triggered AE signals in structural components using more than one PWAS and the evaluation of the corrosion type and extent. **Author Contributions:** Conceptualization, T.E. and C.K. ; methodology, T.E. and C.K. ; software, T.E. ; formal analysis, T.E. ; investigation, T.E. ; resources, T.E. and C.K. ; writing--original draft preparation, T.E. ; writing--review and editing, T.E., C.K., and M.S. ; visualization, T.E. ; project administration, C.K. ; funding acquisition, C.K. and M.S. All authors have read and agreed to the published version of the manuscript. **Funding:** The research leading to these results received funding from the Take Off program (Project no.:881095). Take Off is a research, technology, and innovation funding program of the Ministry of Climate Action, The Republic of Austria. The Austrian Research Promotion Agency (FFG) has been authorized for the program's management. **Institutional Review Board Statement:** Not applicable. **Informed Consent Statement:** Not applicable. **Data Availability Statement:** The data presented in this study are available upon request from the corresponding author.