Acoustic Emission Monitoring of Atmospheric Corrosion on Aluminum Aircraft Structures at Varying Relative Humidity

Abstract

Atmospheric corrosion of aluminum aircraft structures occurs due to numerous reasons. A typical phenomenon leading to corrosion is the deliquescence of contaminants such as salts due to changes in relative humidity (RH) caused by aircraft operation at different altitudes and climate zones. Currently, corrosion of aircrafts is controlled by scheduled inspections. In contrast, the present contribution aims for a continuous monitoring approach by using the acoustic emission (AE) method to detect and further evaluate atmospheric corrosion. The AE method is frequently used for corrosion detection at typically immersion-like conditions or for corrosion types where stress-induced cracking is involved. However, it has not yet been demonstrated for atmospheric corrosion at unloaded aluminum structures. To address this question, the present investigation uses small droplets of a corrosive sodium chloride (NaCl) solution to induce atmospheric corrosion on aluminum alloy AA2024-T351. Operating conditions of an aircraft are simulated by a controlled variation in RH. In addition, videos of the corrosion site are recorded to visually observe the corrosion process. Pitting corrosion is generated and clearly measurable AE signals are detected. An automatic video processing algorithm looking for sudden changes on the corrosion site mainly detects hydrogen bubbles formed when aluminum reacts with aqueous solutions. A clear correlation between the observed pitting corrosion, the AE and the hydrogen bubble activity and the RH, i.e., the electrolyte present at the aluminum surface, is found. Thus, the findings demonstrate the applicability of the AE method for monitoring atmospheric corrosion of aluminum aircraft structures by today’s measurement equipment. Numerous potential effects that can cause measurable AE signals are investigated and discussed. Among these, bubble activity is clearly considered to be the most emissive one.

Acoustic emission-based structural health monitoring concept for corrosion of aluminum aircraft structures

“…The sensitive element of a commercially available AE sensor is typically a piezoelectric element, however, such sensors are expensive and bulky, thus, not well suited for SHM. Instead, piezoelectric wafer active sensors (PWAS) are cheap, lightweight, unobtrusive and their applicability for AE monitoring is already shown [13,15]. Corrosion of aluminum due to aqueous electrolytes always reacts under hydrogen (H2) formation.…”

“…i.e., corrosion product aluminum hydroxide (Al(OH) 3 ) and H2 are formed [1]. It is commonly concluded that bubble activity in immersion tests is the main source of the measured AE signals [12,13] and recent results show that this is also the most likely AE source for atmospheric corrosion [13], cf. Fig.…”

“…The proposed SHM concept for corrosion damage identification by AE is based on the assumption that atmospheric corrosion can be detected and a considered structure of interest is equipped with a sufficient number of PWAS to allow adequate localization of the detected hits. The first, detection, was experimentally demonstrated in [13] and results are shortly presented in Fig. 2.…”

“…However, the authors found no research work that addresses AE monitoring of atmospheric corrosion of aluminum. Recent experimental results of the authors' research group demonstrate that AE is also capable to detect atmospheric corrosion of thin-walled aluminum aircraft structures [13]. Aluminum alloy AA2024-T351 was exposed to small droplets of a 50 g/l sodium chloride (NaCl) solution, that caused clear pitting corrosion.…”