# The Protection Role of Cysteine for Cu-5 Zn-5 Al-1 Sn Alloy Corrosion in 3.5 wt.% NaCl Solution
## 3 Results
### XPS and Auger Results
However, the peak observed on the surface inhibited with 10\({}^{-2}\) M Cys is almost negligible due to low Zn\({}^{2+}\) species on the alloy surface. This phenomenon indicates that the alloy surface is not fully protected by inhibitor molecules in the presence of lower cysteine concentration. Obviously, the dezincification rate of the Cu5 Zn5 Sn1 Al alloy is strongly bounded by the surface film/inhibitor film of the alloy surface. A more dense/integrated surface film leads to a lower dezincification rate. It can also be deduced that the formed inhibited film may rise from other alloying metals of the Cu5 Zn5 Sn1 Al alloy, which will probably be a copper-cysteine film. The Sn3d peak may be deconvoluted into two different peaks: Sn5/2 and Sn3/2. The peaks centered at 486.65 eV, 486.77 eV, and 486.58 eV correspond to the Sn5/2 peaks, which were obtained in blank solution, with 10\({}^{-4}\) M Cys and with 10\({}^{-2}\) M Cys, respectively (Figure 7b). In all cases, the peak values correspond to ionic tin (Sn\({}^{++}\)) [39; 52; 53; 54; 55; 56], which may indicate the formation of the compound of Sn on the alloy surface, meaning, therefore, the contribution of Sn\({}^{2+}\) to the surface film should not be ignored. In addition to the main Sn3d peaks, satellite peaks at higher binding energies of 499.05 and 499.08 eV appear for the blank and the surface-inhibited surface with lower Cys concentration (10\({}^{-4}\) M), respectively, which may be attributed to the presence of more tin ion contents on the alloy surface [57]. Figure 6: High-resolution XPS Cu2p 3/2 spectra (**a**) and Auger spectra (**b**) of the Cu5 Zn5 Al1 Sn alloy surface without and with addition of inhibitors in 3.5 wt.% NaCl. Legend: BE, binding energy. In the meantime, the atomic ratio shown in Table 3 reveals that the inhibitor free alloy surface contains a greater amount of tin. On the other hand, the alloy surface inhibited with 10\({}^{-2}\) M Cys contains an almost insignificant amount of SnO\({}_{2}\), which may result in the development of the inhibitor film and prevent the formation of the corresponding metal oxide. The deconvoluted C1s spectrum illustrated in Figure 8a for the golden alloy in the absence of cysteine may be attributed to three peaks which indicate three chemical forms of the C element present on the alloy surface. The three peaks described above are located at 284.75, 285.73, and 288.59 eV. The component at 284.6 eV is assigned to the non-oxidized carbon containing (C-C) composition. Moreover, the features at BE values of 285.73 eV and 288.59 eV can be assigned to the groups containing a carbon-oxygen bond, i.e., a C-O group like ether or hydroxyl, and adventitious carbon which is usually found on the metal surfaces and results from adsorbed oxidized carbonaceous species from the atmosphere, respectively [44; 45; 58]. For the case involving the presence of 10\({}^{-4}\) M cysteine, the C1s spectrum (Figure 7a) also shows three peaks at 288.54, 286.38, and 284.88 eV, and the three deconvoluted peaks for the case of 10\({}^{-2}\) M Cys are located at 288.68, 286.21, and 284.66 eV, which match with the COO\(-\), C\(-\)NH\({}_{2}\), and C\(-\) SH groups, respectively [39; 40]. The increment of the atomic ratio of carbon (Table 3) of the 10\({}^{-2}\) M Cys inhibited surface can be ascribed to the existence of the high amount of cysteine on the alloy surface. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Element**} & \multicolumn{3}{c}{**Atomic \%**} \\ \cline{2-3}  & **Blank** & **Blank + 10\({}^{-4}\) M Cys** & **Blank + 10\({}^{-2}\) M Cys** \\ \hline C1s & 37.9 & 33.54 & 49.4 \\ N1s & 2.77 & 4.52 & 7.78 \\ O1s & 36.99 & 37.07 & 29.38 \\ S2p & 0 & 2.1 & 3.29 \\ Cu2p & 13.22 & 14.42 & 9.36 \\ Zn2p & 7.98 & 7.26 & 0.63 \\ Sn3d & 1.14 & 1.08 & 0.16 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Composition (atomic %) of the surface of Cu5 Zn5 Al1 Sn alloy samples before and after the addition of inhibitors in a 3.5 wt.% NaCl solution.