# Monitoring of Atmospheric Corrosion of Aircraft Alloy AA2024 by Acoustic Emission Measurements  
[PERSON]  
1 Institute of Structural Lightweight Design, Johannes Kepler University Linz, Altenberger Str. 69, 4040 Linz, Austria 1  
[PERSON]  
1 Institute of Structural Lightweight Design, Johannes Kepler University Linz, Altenberger Str. 69, 4040 Linz, Austria 1  
[PERSON]  
1 Institute of Structural Lightweight Design, Johannes Kepler University Linz, Altenberger Str. 69, 4040 Linz, Austria 1  
Footnote 23: https://www. leakages and spillages of larger amounts of electrolytes, the above-mentioned examples are typical sources of so-called atmospheric corrosion, which is the corrosion of a metal where the electrolyte is present in the form of a thin film, small droplets, or both [1; 5; 6]. Typically, the corrosion of the structural components of aircraft is kept under control through scheduled inspections and clear protocols on how to determine corrosion [3; 7], whereas the aim of the present research is to develop a structural health monitoring (SHM) application for the future detection and monitoring of aircraft corrosion damage. Several research works can be found where corrosion issues of different industries are addressed using the non-destructive testing (NDT) and SHM methods. For example, electrochemical techniques, such as the electrochemical noise technique, show good potential for monitoring corrosion [8; 9]. There are also methods based on electrochemical impedance spectroscopy that were tested in field applications several years ago [10]. Another group of monitoring methods is optical fiber-based techniques [11]. Several studies can be found on the monitoring of the corrosion of reinforced concrete in civil engineering [12], but fiber-optic techniques have also been tested for aircraft [13; 14]. Besides the already mentioned methods, acoustic methods are also of great importance to the monitoring of corrosion. The most popular technique in this context is perhaps ultrasonic testing (UT) [15]. UT is well developed for ground-based NDT applications during the scheduled maintenance of aircraft components. Other acoustic methods that also seem promising for the automated monitoring of corrosion are guided wave (GW) and acoustic emission (AE) [16; 17; 18; 19]. In particular, the AE technique has attracted attention in recent years for the monitoring of the corrosion of metals. It is a passive method that detects the rapid release of energy in the form of transient elastic waves, which are directly caused by the formation or presence of damage. Typical applications of AE can be found in the detection and monitoring of structural damage such as cracks, impact events, and delamination in different kinds of mechanical structures [20]. However, there are also numerous research works and industrial applications for the detection and monitoring of corrosion using AE. For example, AE is used in the petroleum and natural gas industry for the detection of leakages and corrosion of large tanks and pipelines made of different steels [19; 21; 22; 23]. Another sector where AE is used is civil engineering for the monitoring of the corrosion of steel-reinforced concrete [19; 24; 25]. In contrast to steel, to the best of the authors' knowledge, there are no industrial applications and only a small number of research works can be found dealing with the use of AE to monitor the corrosion of aluminum. In [26], for example, the pitting corrosion of aluminum alloy AA2024 was analyzed with accelerated corrosion tests using potentiostatic polarization. The work in [27] investigated the exfoliation corrosion of AA2024 and AA7449 aluminum alloys and [28] examined the pitting corrosion of an AA1050 aluminum alloy using potentiodynamic polarization. The research works found used immersion-like setups for their investigations, i.e., a large volume of electrolyte was in contact with the specimen.