# Monitoring of Atmospheric Corrosion of Aircraft Alloy AA2024 by Acoustic Emission Measurements
## 2 Experimental Investigation
### Atmospheric Corrosion Experiment--Setup and Procedure  
This experiment represents a comprehensive study of the atmospheric corrosion of an AA2024-T351 alloy plate. It is conceptualized to simulate the operating conditions of an aircraft that promote corrosion and demonstrates the applicability of AE to the monitoring of the atmospheric corrosion of aluminum aircraft components using current measurement equipment. Furthermore, it is intended to investigate the dependency of corrosion on the ambient RH. A visual inspection of the corroding surface is performed to correlate the ambient RH and AE activity with the propagation of the corrosion. Atmospheric corrosion was induced by a small droplet of a corrosive 50 g/L NaCl solution of pH 3 (glacial acetic acid added), which is slightly more aggressive than seawater and commonly used for the corrosion testing of aluminum (cf. DIN EN ISO 9227). The highest sensitivity of the sensor is given when the corrosion site is as close as possible to the PWAS. Therefore, the droplet was placed in the center of the top side of the specimen, i.e., opposite the PWAS (cf. Figure 2). The experimental setup is presented in Figure 3. It consisted of a closed glass chamber with ambient air in which the RH was controlled and logged along with the temperature. The chamber was built using a big Petri dish with a dimension of \(\diameter 200\,\mathrm{mm}\)\(\times\)\(35\,\mathrm{mm}\), which was placed upside down on a rubber mat and had a rubber sealing around the edge of the dish to obtain a tight chamber (cf. Figure 3). Additionally, two clamps were used to press the dish onto the rubber mat to further increase the tightness (not shown in Figure 3 for visibility reasons). Figure 3: Experimental setup of the atmospheric corrosion experiment. AE monitoring of atmospheric corrosion during a controlled variation of the RH and additional visual observation of the corrosion by an industrial camera. The specimen was placed on foam material inside the glass chamber and was cleaned with acetone before the droplet was deposited. The droplet was deposited manually by a small syringe. Initially, a surface of 5 mm in diameter was wetted by the droplet with an approximate volume of 0.03 mL. The sensor cable was led out through a tiny opening at the bottom of the rubber mat and connected via a preamplifier (type Vallen AEP5) with 40 dB amplification to the data acquisition (DAQ) system (type Vallen AMSY-6). The sampling rate was set to 20 MHz and the whole DAQ was controlled by an external notebook. The AE DAQ was performed in continuous mode, i.e., a gapless AE data stream was recorded, except for some time points described later. Consequently, the extraction of the AE hits was conducted during post-processing by a defined amplitude threshold. Furthermore, no digital frequency filtering was applied during the DAQ. The droplet on the specimen was visually observed using an industrial camera (type IDS UI-3370 CP Rev. 2), which was positioned vertically above and outside the Petri dish on a tripod. Extension tubes were mounted between the camera and the lens for the magnification of the droplet (cf. Figure 3). The resolution of the video recording was 2048 px \(\times\) 2048 px. The recording was done in video sequences of 20 min duration and a framerate of 35 fps. The videos were recorded at selected times rather than continuously. The digital output from the camera was used to connect the internal flash signal, triggered at frame capture, to the AE DAQ system to enable synchronization between the AE and video data in post-processing. The camera was connected to a second external PC that controlled the video recording. The RH under the Petri dish was controlled with various saturated salt solutions, a method that is often used in laboratory experiments [31; 40].