# 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
Figure 7: High-resolution XPS of Zn2p (**a**) and Sn3d (**b**) spectra of Cu5 Zn5 Al1 Sn alloy surface without and with addition of inhibitors in 3.5 wt.% NaCl solution. The deconvoluted O1s spectra of the Cu5 Zn5 Al1 Sn alloy in the 3.5 wt.% NaCl solution without and with the Cys inhibitor are shown in Figure 8b. There are three distinct peaks for the blank alloy surface in 3.5 wt.% NaCl solution, with the first peak located at 530.74 eV being attributed to O\({}^{2-}\), which could be related to oxygen atoms bound to the constituent metal oxides [48; 59; 60]. Since the major corrosion product on the outermost part of the alloy is derived from copper, the dominant metal oxide relies on copper [8], whereas the contribution of other constituent metals (Zn, Sn, and Al) cannot be ignored. The presence of Zn and Sn oxides are confirmed by their respective XPS spectrum. On the other hand, it is impossible to obtain peak analysis for Al (neither Al2p nor Al2s can be used) because of the peak overlapping with Cu3p and Cu3s [8]. The second peak observed at 531.57 eV may be ascribed to OH\({}^{-}\), which can be associated with the occurrence of hydrous copper, zinc, tin, and aluminum oxides. The third peak at 532.33 eV may be assigned to atmospheric oxygen, which is similar to the carbon contamination [42]. With the addition of 10\({}^{-4}\) M of Cys, the O1s peak is deconvoluted into three peaks located at 530.74, 531.48, and 532.09 eV, which could be assigned to metal oxides, hydrous metal oxides, and C=O or -CON arising from cysteine molecules, respectively. The presence of metal oxide can be ascribed to partial coverage of the metal surface by the inhibitor film [42; 61; 62]. The O1s spectrum of the sample containing 10\({}^{-2}\) M Cys may be deconvoluted into three peaks at 531.12, 531.87, and 532.79 eV, which can be attributed to C-O from carboxyl and C=O or -CON, respectively [39]. In contrast to the uninhibited and the 10\({}^{-4}\) M Cys inhibited alloy surface, the alloy inhibited with the high Cys (10\({}^{-2}\) M) concentration shows no peak associated with the presence of metal oxide. It is  
Figure 8: High-resolution XPS C1s (**a**), O1s (**b**), S2p, and N1s spectra of the Cu5 Zn5 Al1 Sn alloy surface before and after addition of inhibitors in 3.5 wt.% NaCl solution (**c**,**d**). indicated that the alloy surface is fully covered by the inhibitor film after the addition of 10\({}^{-2}\) M cysteine [63]. Figure 8c shows the deconvoluted S2p spectrum of Cu5 Zn5 Al1 Sn alloy in the absence and presence of cysteine. There is no peak found on the S2p spectrum of the blank Cu5 Zn5 Al1 Sn alloy in 3.5 wt.% NaCl attributed to the nonappearance of any sulfur species. In the presence of 10\({}^{-4}\) M Cys, two peaks located at 162.95 and 167.95 eV can be recognized, and for the case of 10\({}^{-2}\) M Cys, the three deconvoluted peaks are located at 162.45, 163.42, and 168.1 eV. The lower binding energies (162.95 and 162.45) may be assigned to the metal-sulfur interaction and the upper ones (167.95 and 168.10) are ascribed to the -SH group of the cysteine molecule [42; 61; 63; 64]. The deconvoluted peaks at 162.45 and 163.42 eV obtained from the 10\({}^{-2}\) M Cys inhibited surface result from band broadening and because of the presence of more sulfur content they are considered to be peaks derived from the same component [43; 65]. The intensity of S2p peaks and percentage ratio of S (Table 3) increases with cysteine concentration, suggesting that the efficiency of cysteine is concentration-dependent and more Cys molecules are adsorbed on the Cu5 Zn5 Al1 Sn alloy surface at the higher inhibitor concentration. The N1s spectrum for the uninhibited/inhibited surface has only one deconvoluted peak, as shown in Figure 8d, for all cases. The peak at 400.02 eV obtained from the blank solution can be assigned to N raised from the atmosphere during sample preparation.