# 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  
In order to further understand the surface composition of the uninhibited and inhibited copper alloy, high-resolution XPS Cu2p and Auger CuLM2 spectra of the Cu5 Zn5 Al1 Sn alloy surface without and with the addition of various concentrations of cysteine were recorded and are shown in Figure 6. The Cu 2p profiles of Cu(0) and Cu(I) are similar, and as a result it is difficult to use the XPS Cu 2p spectrum alone to differentiate them, whereas the binding energy of the Auger peak of Cu(I) is about 2 eV higher than the Auger peak of Cu(0), meaning the CuLM2 spectrum is necessary to further investigate the valence state of copper. In all cases, there are two deconvoluted peaks of Cu2p 3/2 at different binding energies (Figure 6a). The interpretation of the peaks is different as the reaction taking place on the alloy surface is not same in the blank and Cys-inhibited solutions. In the blank solution, the corrosion products may exist as the form of oxides by reactions with the dissolved oxygen. While in the presence of inhibitors, the inhibitor particles have the opportunity to participate in surface reaction. In blank 3.5 wt.% NaCl solution, the deconvoluted Cu2p peaks at 932.99 and 934.83 eV (Figure 6a) may be assigned to Cu/Cu\({}^{+}\) and Cu\({}^{2+}\), respectively. In addition to the XPS result, the Auger spectrum obtained in the blank solution shows an Auger peak at 570.11 eV, which is a typical characteristic of cuprous ion (Cu\({}^{+}\)), with a band broadening to the lower binding energy. The band broadening is an indication of the presence of metallic copper, which may be attributed to the uncovered alloy surface. Generally, the XPS and Auger results of the blank solution show the presence of Cu, Cu\({}^{+}\), and Cu\({}^{2+}\) species on the alloy/solution interface, which may be attributed to the uncovered alloy surface. Figure 5: SEM images of Cu-5 Zn-5 Al-1 Sn alloy before (**a**) and after immersion in 3.5 wt.% NaCl, without (**b**) and with \(10^{-4}\) M of cysteine (**c**) and \(10^{-2}\) M cysteine (**d**), respectively. With regard to the Cu2p 3/2 obtained after immersion in solution with 10\({}^{-4}\) M Cys, the deconvoluted peaks are located at 932.53 and 934.41 eV, respectively. Similarly, when the surface is inhibited with 10\({}^{-2}\) M Cys, the Cu2p 3/2 peak is deconvoluted into peaks located at 932.62 and 934.10 eV. The peaks may be assigned to Cu/Cu\({}^{+}\) and Cu\({}^{2+}\) ions, respectively, in both cases [39; 40; 41; 42]. In the solution containing different concentrations of cysteine (10\({}^{-4}\)and 10\({}^{-2}\) M), the Auger peaks obtained at 571.05 and 571.02 eV may both be assigned to Cu\({}^{+}\). The binding energy of the Cu\({}^{+}\) peak on the Auger spectra increases in the presence of Cys, which may be associated with the formation of a film containing organic inhibitor molecules typically centered at higher binding energies than the binding energy for the oxide species [43; 44; 45; 46; 47]. Both in the absence and presence of Cys, the Cu2p XPS spectra demonstrate the Cu (II) satellite peaks attributed to the presence of copper (II). Furthermore, there is no Auger peak related to Cu\({}^{2+}\) other than a band broadening in the blank solution, which may be ascribed to the lower content of the Cu\({}^{2+}\) species. The uninhibited alloy surface and that of the inhibited one with lower Cys concentration (10\({}^{-4}\) M) have a higher peak intensity, which could be attributed to the poor coverage area of corrosion products and the metal/inhibitor film, respectively [48]. High resolution XPS Zn2p spectra (Figure 7a) show two peaks for the alloy surface, both without and with different concentrations of Cys, which may be assigned to Zn2p 3/2 and Zn2p 1/2. The Zn2p 3/2 peak at 1021.75 and 1021.76 eV in the blank and 10\({}^{-4}\) Cys inhibited surfaces may be assigned to Zn\({}^{2+}\), which is associated with the presence of corrosion product [49; 50; 51].