# Research on Corrosion Damage Evolution of Aluminum Alloy for Aviation
## 2 Materials and Methods
### Methods  
#### 2.2.1 Determination of Corrosion Solution  
The annual average value of atmospheric environmental data obtained from the Hainan, Wanning Natural Environment Test Station through the \"China Corrosion and Protection Network\" is shown in Table 2.  
The formation of water droplets or water films on the surface of aluminum alloys is a necessary condition for atmospheric corrosion [23]. According to the environmental chemistry of water, the following chemical reactions occur when SO\({}_{2}\) and NO\({}_{2}\) are dissolved in water: \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Si** & **Fe** & **Cu** & **Mn** & **Mg** & **Cr** & **Zn** & **Ti** & **Al** \\ \hline
0.40 & 0.50 & 1.2\(\sim\)2 & 0.30 & 2.1\(\sim\)2.9 & 0.18\(\sim\)0.28 & 5.1\(\sim\)6.1 & 0.20 & allowance \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Chemical composition of 7075 aluminum alloy (mass fraction/%). Figure 1: Geometrical dimensions of the test piece. \[\text{SO}_{2}+\text{H}_{2}\text{O}\xrightleftharpoons{k_{1}}\text{H}^{+}+\text{ HSO}_{3}^{-} \tag{1}\]  
\[2\text{NO}_{2}(\text{aq})+\text{H}_{2}\text{O}\xrightleftharpoons{k_{2}}2\text{H} ^{+}+\text{NO}_{2}^{-}+\text{NO}_{3}^{-} \tag{2}\]  
\(\text{SO}_{2}\) and \(\text{NO}_{2}\) have their own dissolution equilibrium and obey [PERSON]'s law, that is, under isothermal and isobaric conditions, the solubility of a certain gas in the solution is proportional to the equilibrium pressure of the gas on the liquid surface. The solubility in water can be expressed by Formula (3):  
\[\text{X}(\text{aq})=K_{H}\text{-}p_{G} \tag{3}\]  
In Formulas (1)-(3) \(k_{1}\) is the ionization equilibrium constant, \(k_{2}\) is the chemical equilibrium constant, \(K_{H}\) is the Henry constant of different gases, and \(p_{G}\) is the partial pressure of different gases. Calculate the solubility of each gas according to Formula (3) and calculate \(\text{SO}_{2}\) and \(\text{NO}_{2}\) according to ionization balance and chemical balance. Under dissolution equilibrium, equivalent calculations of \(\text{H}_{2}\text{SO}_{4}\) and \(\text{HNO}_{3}\) with the same \(\text{H}^{+}\) concentration can obtain their respective molar concentrations. The \(\text{Cl}^{-}\) deposition is shown in Table 2. The formation and rupture of sea water mist and sea bubbles will produce tiny droplets that undergo a series of evolutions such as cracking and evaporation in the air to form atmospheric salt cores and then continue to deposit on land. Therefore, the influence of the corrosion medium \(\text{Cl}^{-}\) must be considered. In this paper, it was simplified, and the approximate mass concentration of NaCl in seawater was 3.5% as the concentration of \(\text{Cl}^{-}\) in the accelerated corrosion solution. According to the abovementioned equal weighting method for the concentration of the corrosion medium, the concentration of each corrosion medium in the accelerated corrosion solution was finally determine. Considering that \(\text{Cl}^{-}\) is treated according to the mass concentration of seawater, it was already serious. So, only the equivalent corrosion media \(\text{H}_{2}\text{SO}_{4}\) and \(\text{HNO}_{3}\) of \(\text{SO}_{2}\) and \(\text{NO}_{2}\) were weighted in equal proportions. The pH of the accelerated corrosion solution was set to 1 and the corrosion was finally obtained by equivalent weight. The concentration of the medium is shown in Table 3.  
According to general research, it is found that the three ions, \(\text{SO}_{4}\text{{}^{2}}\), \(\text{NO}_{3}\text{{}^{-}}\), and \(\text{Cl}^{-}\), have the greatest influence on the atmospheric corrosion of aluminum alloys [24; 25]. So, it was reasonable to introduce these three ions in the accelerated corrosion solution of this experiment. The \(\text{Cl}^{-}\) / \(\text{NO}_{3}\text{{}^{-}}\) was 6.7465, very close to 6.667 in the standard EXCO corrosion solution (EXCO corrosion solution is the standard solution for accelerated corrosion test of 7 xxx series high strength aluminum alloy).