# 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
Inhibition efficiency was obtained by using Equation (2) [34; 35; 36], the values of which are listed in Table 2. The inhibition efficiency of Cys for Cu5 Zn5 Al1 Sn alloy corrosion in 3.5 wt.% solution is seen to increases with its concentration, and a maximum inhibition efficiency value of 96.44% is recorded for the case of the Cys concentration being \(10^{-2}\) M. The inhibition efficiencies obtained from polarization \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Cys Conc. (M)** & **Rs (\(\Omega\))** & **Qf** & **Rf (\(\Omega\))** & **Qct** & **Rct (K\(\Omega\))** & **W** & **Rp (K\(\Omega\))** & **\#\%** \\ \hline Blank & 2.668 & & \(1.55\times 10^{-4}\) & 692.7 & 0.0032 & 0.695 & - \\ \(10^{-5}\) & 3.224 & \(8.09\times 10^{-5}\) & 588.1 & \(1.64\times 10^{-4}\) & 1169 & 0.0053 & 1.197 & 40.76 \\ \(10^{-4}\) & 2.894 & \(1.05\times 10^{-4}\) & 212.7 & \(1.61\times 10^{-4}\) & 1.986 & & 2.201 & 67.78 \\ \(10^{-3}\) & 3.345 & \(8.24\times 10^{-5}\) & 2382 & \(1.10\times 10^{-4}\) & 6.399 & & 8.784 & 91.92 \\ \(10^{-2}\) & 3.067 & \(5.28\times 10^{-5}\) & 12.02 & \(4.41\times 10^{-6}\) & 19.940 & & 19.955 & 96.44 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Impedance parameters for the Cu5 Zn5 Al1 Sn alloy in 3.5 wt.% NaCl with and without different concentrations of cysteine obtained by using ZSimpWin 3.50. Figure 3: Electrical equivalence circuit for Cu5 Zn5 Al1 Sn alloy in 3.5 wt.% NaCl without R(Q(RW)) (**a**), with \(10^{-5}\) M Cys, R(Q(R(Q(RW)))) (**b**) and \(10^{-4}\)–\(10^{-2}\) M Cys, R(Q(R(QR))) (**c**). Legend: solution resistance (Rs), film resistance (Rf), film capacitance (Qf), charge transfer resistance (Rct), double layer capacitance (Qdl), and Warburg resistance (W). Figure 2: [PERSON] (**a**) and [PERSON] (**b**) plots for the Cu5 Zn5 Al1 Sn alloy in 3.5 wt.% NaCl with and without different concentrations of cysteine (\(10^{-5}\) to \(10^{-2}\)) (scattered: experimental data; lines: fitting data). and EIS investigations are similar and follow the same trend, which implies that the explanation for Cu5 Zn5 Al1 Sn alloy corrosion characteristics on the basis of Icorr and Rp could be well-thought-out as valid and reliable. \[\upmu\%=\frac{\text{Rp(inh)}-\text{Rp}}{\text{Rp(inh)}}\times 100\% \tag{2}\]  
where \(\upmu\), Rp, and Rp (inh) are inhibition efficiency, and polarization resistance for uninhibited and inhibited surfaces, respectively. The inhibition efficiency results obtained from potentiodynamic (PD) and EIS experiments were used to observe the adsorption behavior of cysteine on the Cu5Z5 Al1 Sn alloy surface. Therefore, different adsorption isotherms were tested to determine the best suitable model to explain surface phenomena [23; 37; 38]. \[\frac{\text{C}}{\uptheta}=\frac{1}{\text{Kads}}+\text{C} \tag{3}\]  
\[\uptheta=\frac{\text{RT}}{\text{b}}\text{lnKads}+\frac{\text{RT}}{\text{b}} \text{lnC} \tag{4}\]  
In this equation, C, \(\uptheta\), Kads, and b are inhibitor concentration, the inhibitor's surface coverage, the equilibrium constant related to the interfacial molecular adsorption-desorption phenomena, and a constant relevant to the properties of both adsorbent and adsorbate in the Temkin model, respectively. Figure 4 shows plots of Langmuir and Temkin adsorption isotherms determined by the respective equations listed in Equations (3) and (4), respectively. As observed in Figure 4a, a slope of 0.99 with R\({}^{2}\) = 0.99 derived from EIS and PD results which is close to unity can be observed from a C/\(\uptheta\) versus C linear plot for the case of the Langmuir adsorption isotherm. On the other hand, a regression of 0.89 and 0.83 acquired from EIS and PDP, respectively, was obtained from the plot of \(\uptheta\) versus In C based on the Temkin adsorption isotherm (Figure 4b). By comparing the two isotherms, the Langmuir adsorption isotherm, which was seen to have less fitting error, was chosen as the more suitable model to describe the surface inhibition mechanism of cysteine for Cu5Z5 Al1 Sn alloy corrosion.