# Monitoring of Atmospheric Corrosion of Aircraft Alloy AA2024 by Acoustic Emission Measurements
At these anodic sites, aluminum oxidation (dissolution of aluminum)  
\[\text{Al}\longrightarrow\text{Al}^{3+}+3\,\text{e}^{-} \tag{2}\]  
Figure 1: Mechanism of pitting corrosion of aluminum under a droplet of an aqueous solution [6]. H\({}_{2}\) bubble activity is distinguished into the following phases: 1 formation, 2 detachment from metal surface and movement, 3 arrival at droplet surface and growth, and 4 bursting of H\({}_{2}\) bubbles. takes place, leaving its valence electrons in the aluminum matrix. The dissolution stops when equilibrium is reached. However, when the electrons in the matrix are consumed by a cathodic reaction, e.g., by the reduction of oxygen (O\({}_{2}\))  
\[\frac{3}{2}\,\mathrm{O}_{2}+3\,\mathrm{H}_{2}\mathrm{O}+6\,\mathrm{e}^{-} \longrightarrow 6\,\mathrm{OH}^{-} \tag{3}\]  
the dissolution continues and even becomes accelerated. Nevertheless, the O\({}_{2}\) present repassivates the aluminum surface and stops the dissolution at most sites. Only at locations of high Cl\({}^{-}\) concentrations do layers of soluble chloride and hydrochloride complexes form fast enough, e.g., according to  
\[\mathrm{Al}+4\,\mathrm{Cl}^{-}\longrightarrow\mathrm{AlCl}_{4}{}^{-}+3\, \mathrm{e}^{-}, \tag{4}\]  
to cover the bottom of the microcracks (micro pits) and prevent their repassivation. The Al\({}^{3+}\) cations formed continuously by the anodic oxidation at the base of the pits diffuse to the outside of the pits where Al(OH)\({}_{3}\) forms  
\[\mathrm{Al}^{3+}+3\,\mathrm{H}_{2}\mathrm{O}\longrightarrow\mathrm{Al(OH)}_{3 }+3\,\mathrm{H}^{+} \tag{5}\]  
and precipitate at the edges of the pits (cf. single exemplary pit in Figure 1). However, for the reaction in Equation (5), the water (H\({}_{2}\)O) needs to form hydroxide ions (OH\({}^{-}\)). These are formed by oxygen reduction (cf. Equation (3)) and by the dissociation of water. Besides the oxygen reduction (cf. Equation (3)), the reduction of H\({}^{+}\) protons  
\[3\,\mathrm{H}^{+}+3\,\mathrm{e}^{-}\longrightarrow\frac{3}{2}\,\mathrm{H}_{2} \tag{6}\]  
occurs, leading to the formation of H\({}_{2}\). The combination of Equations (5) and (6) results in the overall corrosion reaction given by Equation (1). Besides the pure detectability of atmospheric corrosion using current AE measurement equipment, the present investigation also discusses the sources that generate AE signals. Based on the literature [26; 27; 28] and the presented corrosion process, it is assumed that H\({}_{2}\) bubble activity is an AE source. Thus, the question is addressed of whether H\({}_{2}\) bubble activity is the only source or whether there are other effects that lead to AE signals. Therefore, two experiments on aluminum alloy AA2024-T351 specimens are conducted. The first experiment is designed to imitate atmospheric corrosion by local initiation followed by dry and wet phases, as this could be also expected during the operation of an aircraft. The second experiment is designed to confirm corrosion as the dominant source of AE signals by comparing the AE measurements of a specimen exposed to a corrosive solution and those of another specimen exposed to pure water. For both experiments, the AE signals are permanently acquired and the corrosion is induced by small droplets of a sodium chloride (NaCl) solution (NaCl dissolved in water with Na\({}^{+}\) and Cl\({}^{-}\) ions). Furthermore, the region of the deposited droplet is visually observed by video recording during all the important phases of the atmospheric corrosion experiment. The correlation between the H\({}_{2}\) bubble formation, AE hit energy, RH, and corrosion progress is analyzed.