# Monitoring of Atmospheric Corrosion of Aircraft Alloy AA2024 by Acoustic Emission Measurements
## 2 Experimental Investigation
### AE Source Identification Experiment  
_Setup and Procedure_  
The purpose of this experiment is to investigate whether the measured AE signals from the atmospheric corrosion experiment and the resulting findings are truly related to corrosion or whether other effects produce AE signals. Therefore, the difference between the AE caused by a droplet of pure water and the AE caused by a droplet of the corrosive NaCl solution on two equal specimens within the same environment is analyzed.  
The experimental setup was similar to the one explained in Section 2.3 and is shown in Figure 4. On the first specimen, a droplet of Milli-Q water (water of the highest purity) was deposited and on the second specimen, a droplet of the 50 g/L NaCl solution was deposited. Each droplet initially wetted a surface of 9 mm in diameter and had a volume of approximately 0.1 mL. The two specimens, which were each placed on foam material, were put into the tight glass chamber, where both specimens were exposed to the same environmental conditions. The high RH was controlled by two small Petri dishes of saturated potassium chloride (KCl) solutions, with a theoretical equilibrium RH of 85% at room temperature (see Table 1) to prevent the evaporation of the droplets. After about 8.7 h, enough data were believed to have been acquired to draw reliable conclusions (based on previous tests), and the RH was rapidly decreased by exchanging the saturated KCl solution with a silica gel. The experiment was stopped after the complete evaporation of the droplets was visually observed so that dry specimens could be obtained. The sensor cables of both PWASs were led out through a tiny hole at the bottom of the rubber mat and connected via preamplifiers (type Vallen AEP5) with an amplification of 40 dB to the DAQ system (type Vallen AMSY-6). The same measurement equipment as that used for the atmospheric corrosion experiment was used. In addition, the same DAQ settings as those used for the atmospheric corrosion experiment were used, except for the sampling rate, which was 10 MHz for this experiment (the atmospheric corrosion experiment suggested this sample rate to be sufficiently high). The DAQ was controlled by an external notebook. The temperature and RH inside the glass chamber were measured using a data logger (type RS PRO RS-172 TK). In contrast to the atmospheric corrosion experiment, no visual observations by video recording were conducted, as the aim of the experiment was a direct comparison of the AE activity.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Step**} & **Solution Petri** & **Solution Petri** & **Expected RH** & \multirow{2}{*}{**Remark**} \\  & **Dish 1** & **Dish 2** & **(\%)** & \\ \hline
0 & - & - & - & ambient RH \\
1 & Na2 SO4 \(\cdot 10\,\mathrm{H_{2}O}\) & Na2 SO4 \(\cdot 10\,\mathrm{H_{2}O}\) & 93.0 & initial high RH \\
2 & NaCl & NaCl & 76.0 & start of drying \\
3 & NaCl & Mg(NO3)2 \(\cdot 6\,\mathrm{H_{2}O}\) & 65.5 & - \\
4 & Mg(NO3)2 \(\cdot 6\,\mathrm{H_{2}O}\) & Mg(NO3)2 \(\cdot 6\,\mathrm{H_{2}O}\) & 55.0 & - \\
5 & MgCl2 \(\cdot 6\,\mathrm{H_{2}O}\) & MgCl2 \(\cdot 6\,\mathrm{H_{2}O}\) & 33.0 & - \\
6 & Mg(NO3)2 \(\cdot 6\,\mathrm{H_{2}O}\) & Mg(NO3)2 \(\cdot 6\,\mathrm{H_{2}O}\) & 55.0 & start of rewetting \\
7 & NaCl & NaCl & 76.0 & - \\
8 & KCl & - & 85.0 & - \\
9 & Na2 SO4 \(\cdot 10\,\mathrm{H_{2}O}\) & - & 93.0 & - \\
10 & H2O & H2O & \(>\)93.0 & deionized water \\
11 & silica gel & silica gel & \(<\)33.0 & final drying \\ \hline \hline \end{tabular}
\end{table}
Table 2: Sequence of saturated solutions used to control the RH in the glass chamber.