# Monitoring of Atmospheric Corrosion of Aircraft Alloy AA2024 by Acoustic Emission Measurements
Aluminum reacts under hydrogen (H\({}_{2}\)) formation when it comes into contact with aqueous solutions [6; 29; 30]; thus, in the immersion tests, which were further accelerated using polarization techniques, H\({}_{2}\) bubbles were observed. All the research works mentioned above concluded that the sources with the most emissions were related to H\({}_{2}\) formation and the evolution of its bubbles. Consequently, it is expected that AE can detect corrosion as soon as the first H\({}_{2}\) bubbles appear, and thus has the potential to detect corrosion earlier than other acoustic methods. In summary, it has already been shown that under certain corrosion-promoting conditions, the corrosion of aluminum can be detected using AE. However, in atmospheric corrosion without any corrosion-promoting polarization and due to the small amount of electrolyte involved, the formation of H\({}_{2}\) bubbles may not exist or may be very small. Therefore, AE might not be applicable to atmospheric corrosion at all or might have limited applicability with current measurement equipment. Although the authors conducted an extensive literature review, no research was found that demonstrated this applicability. This shortcoming is addressed in the present contribution by investigating whether atmospheric corrosion, typical for the operating conditions of aluminum aircraft structures,can be detected using the AE method. Therefore, a small droplet of electrolyte is used to trigger corrosion on a specimen and the RH is controlled to simulate periodic drying and wetting due to condensation. Aluminum alloy AA2024-T351, which is commonly used in the aircraft industry [1; 6; 29; 30], is used for the investigation. In aircraft applications, alloys with protective aluminum cladding are usually used. However, to focus on the corrosion of the actual structural material, an uncladded AA2024-T351 sheet material is analyzed in the present investigation. Alloys of the 2 XXX series are sensitive to localized corrosion in the form of pitting corrosion, intergranular corrosion, exfoliation corrosion, and stress-corrosion cracking [6]. In atmospheric corrosion environments and with the absence of mechanical loading, the most common form of corrosion is pitting corrosion, which is further enhanced when chlorides (Cl\({}^{-}\)) are present in the corrosive atmosphere [6]. Figure 1 illustrates the mechanism of pitting corrosion under a droplet of an aqueous solution and H\({}_{2}\) bubble activity. The bubble activity can be divided into the following phases: 1 formation of the H\({}_{2}\) bubble, 2 detachment from the metal surface and movement of the bubble in the droplet, 3 arrival of the bubble at the droplet surface and bubble growth, and 4 bursting of the bubble. The corrosion of aluminum is eventually the sum of two electrochemical reactions occurring simultaneously but at different sites, namely anodic oxidation (release of electrons) and cathodic reduction (consumption of electrons). This leads to the overall corrosion reaction of aluminum in aqueous media:  
\[\text{Al}+3\,\text{H}_{2}\text{O}\longrightarrow\text{Al}(\text{OH})_{3}+ \frac{3}{2}\,\text{H}_{2}\,, \tag{1}\]  
where insoluble aluminum hydroxide Al(OH)\({}_{3}\) and H\({}_{2}\) are formed as the final products [6]. However, as the H\({}_{2}\) formation described by Equation (1) is reported to be an important basis for the AE-based corrosion detection of aluminum [26; 27; 28] its derivation in the case of pitting corrosion is presented below [6]. Pitting corrosion is initiated by the adsorption of the aggressive chloride ions (Cl\({}^{-}\)) into the naturally existing oxide film of aluminum. This generally takes place at the weak points of the oxide film, particularly at the microcracks in the neighborhood of the intermetallic particles (cathodic areas). The result is a breakdown of the oxide film at the weak points, and thus the exposure of the bare aluminum (anodic areas) to the solution.