# The Protection Role of Cysteine for Cu-5 Zn-5 Al-1 Sn Alloy Corrosion in 3.5 wt.% NaCl Solution
## 3 Results
### Electrochemical Impedance Spectroscopy  
Nyquist and Bode diagrams of the Cu5 Zn5 Al1 Sn alloy in 3.5 wt.% NaCl solution with and without different amounts of cysteine are illustrated in Figure 2a,b, correspondingly. From Figure 2a, the Nyquist diagram for Cu5 Zn5 Al1 Sn alloy in 3.5 wt.% NaCl solution contains a capacitive loop at intermediate frequency and a straight line in the low frequency region. A straight line at low frequency area may be attributed to the presence of the Walberg constant as a result of the movement of soluble metal species from the alloy surface to the bulk solution [23]. The diameter of the capacitive loop rises in the presence of Cys and the straight line in the low frequency region is removed at higher Cys concentrations, which may be attributed to the inhibitive effect of Cys on the corrosion process [31]. The results of Bode plots (Figure 2b) show that the frequency range with maximum phase becomes larger with increasing Cys concentration. There is a shift in the phase maximum to a lower frequency region and the phase angle is raised to about 80\({}^{\circ}\), in the inhibitor concentration of 10\(-\)2 M. These results suggest that the protective ability of the Cu5 Zn5 Al1 Sn alloy is increased with inhibitor concentration, showing that inhibitor particles effectively adsorb on the alloy surface [32; 33]. The results of the EIS experiment were fitted with a simulated circuit (Figure 3) and the value of electrical elements that make up the equivalent circuit are presented in Table 2. The equivalent circuit parameters are solution resistance (Rs), film resistance (Rf), film capacitance (Qf), charge transfer resistance (Rct), double layer capacitance (QdI), and Warburg resistance (W), which result from ionic diffusion of the corrosion product to the electrode surface. The EIS fitting result of the Cu5 Zn5 Al1 Sn alloy in blank 3.5%wt NaCl solution shows a Warburg resistance and a lower value of Rct, as shown in Figure 2a and Table 2, respectively. The Warburg impedance is included to account for the diffusion of corrosion products to the bulk solution and/or dissolved oxygen to the electrode surface. With a lower \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Cysteine** & **E\({}_{\text{corr}}\) (V versus** & \(\beta\)a & \(\beta\)c & **I\({}_{\text{corr}}\)** & **u\%** \\
**Concentration** & **Ag/AgCl** & **(mV/Decade)** & **(mV/Decade)** & **(μAcm\({}^{-2}\))** & **u\%** \\ \hline Blank (0) & \(-\)0.280 & 69.18 & 102.3 & 3.44 & - \\ \(10^{-5}\) M & \(-\)0.226 & 77.77 & 127.2 & 1.84 & 46.5 \\ \(10^{-4}\) M & \(-\)0.247 & 48.5 & 63.86 & 1.78 & 48.2 \\ \(10^{-3}\) M & \(-\)0.292 & 61.23 & 56.96 & 0.483 & 85.9 \\ \(10^{-2}\) M & \(-\)0.326 & 23.71 & 19.49 & 0.094 & 97.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Polarization parameters for the Cu5 Zn5 Al1 Sn alloy measured in 3.5 wt.% NaCl solution in the absence and presence of different concentrations of cysteine. Cys concentration, the Warburg impedance remains unchanged, and Rf and Qf are observed to address the film formed between the Cys molecules and alloying elements. The presence of W is attributed to the low inhibition efficiency of the lower concentration of Cys, which may be related to the thinner and unstable film formed which can be easily dissolved to the bulk solution. When more than \(10^{-4}\) M Cys is added to the solution, the Warburg impedances vanish and the Rct values become larger. This shows the presence of high resistance to transfer charge from bulk solution to specimen surface and/or and specimen surface to bulk solution; hence, the diffusion process is controlled. This may be attributed to the development of a thick and protective Cys/metal film on the alloy surface.