**4. Discussion**

The corrosion characteristics of the Cu5Zn5Al1Sn alloy in 3.5 wt.% NaCl solution were examined by different electrochemical and surface analysis techniques. It has been proven from electrochemical tests that the corrosion of the Cu5Zn5Al1Sn alloy in 3.5 wt.% NaCl solution decreases on the addition of the cysteine inhibitor ( Figure 1; Figure 2). The effect of cysteine concentration was also investigated and the highest inhibition efficiencies of 97.2% and 96.44% were obtained at a cysteine concentration of 10−<sup>2</sup> M based on potentiodynamic and EIS investigations, respectively. Morphological analysis based on SEM shows a better protected alloy surface in the presence of the highest concentration (10−<sup>2</sup> M) of cysteine. Furthermore, composition and the inhibition mechanism were further determined by carrying out elemental analysis using XPS techniques.

From potentiodynamic testing it was determined that the current density decreases upon addition of the inhibitor, which can be accredited to the inhibitive effect of cysteine. In addition, the corrosion potential was observed to shift to the negative direction for the inhibited samples, with a potential difference between the blank and inhibited samples of less than 85 mV, which can be ascribed to the mixed inhibitive effect of cysteine for the Cu5Zn5Al1Sn alloy in 3.5 wt.% NaCl solution and which has a greater effect to retard cathodic reaction [33,67]. A diversity of adsorption isotherms like Langmuir, Temkin, Freundlich, and Frumkin are often used to model experimental results to understand surface reactions [23,37]. An adsorption model to describe the inhibition mechanism of cysteine of the Cu5Zn5Al1Sn alloy in 3.5 wt.% NaCl with different concentrations of cysteine was generated using the data obtained by PDP and EIS experiments. In this paper, the Langmuir and Tamkin adsorption isotherms (Figure 4) have been used to find out the most appropriate model to describe the inhibition mechanism of cysteine for the copper alloy. The Langmuir adsorption isotherm is proposed as a suitable model to describe the inhibition mechanism of cysteine on the Cu5Zn5Al1Sn alloy surface, which can be accredited to the adsorption of inhibitor particles on the active site of the Cu5Zn5Al1Sn alloy substrate [68,69].

In the blank solution, the appearance of Warburg impedance in the simulated circuit of the EIS result (Figure 3) reflects the diffusion process of the ionized alloy molecules from the alloy surface to the bulk solution, or the diffusion process of dissolved oxygen from the wholesale solution to the superficial of the alloy [70,71]. Composition analysis does help to study the corrosion phenomena of the Cu5Zn5Al1Sn alloy in 3.5 wt.% NaCl solution. A Cu2p 1/2 peak at 932.79 eV and Auger CuLMM peak at 570.11 eV with band broadening to low BE direction are observed and may be attributed to the presence of both metallic and ionic copper as corrosion products. There are other alloying elements present on the alloy surface which illustrate their involvement in the corrosion process. As can be seen from Table 3, the ionic ratio of Zn and Sn ions in the blank solution are higher than that for the inhibited surface. This can be ascribed to the ionization of Zn and Sn species in blank solution; they become bounded by the inhibiter film after the addition of cysteine. The damaged surface can also be observed from the SEM image (Figure 5b), which strengthens the results of the electrochemical and XPS experiments.

A relatively protected surface appears after the addition of a small amount of cysteine. The electrochemical experiments (PDP and EIS) show the inhibitive effect of cysteine (Figures 1 and 2), which is confirmed by the decrease in current density and an increase in impedance in the potentiodaynamic polarization and EIS curves, respectively. On the other hand, the inhibition efficiency obtained by electrochemical parameters of both potentiodynamic polarization and EIS is seen to increase with the increase in inhibitor concentration. There are S and N constituents in the inhibited surfaces, indicating the presence of the inhibitor film. However, the atomic ratio of this species is lower compared to a surface which has a high cysteine concentration. As for the alloying elements, the atomic ratio of Cu2p increases and the atomic ratio of Zn and Sn decreases in the presence of 10−<sup>4</sup> M cysteine, inferring the bounding of the alloy surface by the Cu-inhibitor film.

The atomic ratio of alloying elements other than Cu decreases with the addition of cysteine and becomes negligible at higher cysteine concentration (10−<sup>2</sup> M), which shows that the inhibitor film is mainly formed by the dominant alloy constituent (Cu) [8] and the inhibitor. In addition, the surface inhibited with 10−<sup>2</sup> M cysteine concentration shows an almost similar morphology, with an unexposed polished surface surmising a good and almost full coverage of the alloy by cysteine molecules by forming a metal/inhibitor film. It is difficult to obtain XPS data for Al due to peak overlap with Cu3s and Cu3p. Schematic representation of the probable corrosion mechanism of Cu5Z5Al1Sn alloy in 3.5 wt.% NaCl solution with and without the Cys inhibitor is illustrated in Figure 9. As mentioned above, the existence of alloying elements on the blank surface can be attributed to the ionization of these metals in 3.5 wt.% NaCl forming the respective metal oxide and hydroxide (Figure 9a) [11]. On the other hand, the removal of some alloying elements, especially Zn and Sn in the presence of low concentration of the inhibitor, indicates the alloy surface is partially protected by an adsorption film between the dominant metal ion (Cu<sup>+</sup>, Cu2+) and inhibitor molecules (Cu(I)-Cys, Cu(I)-Cys-Cu(II)-Cys). With the increase of Cys concentration to 10−<sup>2</sup> M, the main alloying element existing in the surface film is copper (Table 3), and it is believed that the continuous surface inhibited film formed between the copper ion and Cys molecules has existed on the Cu5Zn5Al1Sn alloy surface (Figure 9c). Therefore, better corrosion efficiency is obtained when 10−<sup>2</sup> M Cys is added into the 3.5 wt.% NaCl solution.

**Figure 9.** Schematic representation of the corrosion mechanism of the Cu5Zn5Al1Sn alloy in 3.5 wt.% NaCl solution before (**a**) and after addition of 10−<sup>4</sup> M (**b**) and 10−<sup>2</sup> M (**c**) cysteine.
