**1. Introduction**

In recent years, research on copper alloy corrosion have garnered more attention in the industrial sector since copper and its alloys form a number of industrially important materials [1]. Based on alloying elements, the important properties range from good thermal properties, electrical conductivities, formability, and visual appearance, to good corrosion resistance [2–6]. Cu-5Zn-5Al-1Sn (Cu5Zn5Al1Sn) consists of 89 wt.% copper, 5 wt.% aluminum, 5 wt.% zinc, and 1 wt.% tin, with the latter usually used for the front covering due to its shiny and golden appearance [7]. The features of the microstructure of the Cu5Zn5Al1Sn alloy have been studied by comparing it with pure copper [8]. The microstructure of Cu5Zn5Al1Sn owns numerous properties that are promising from a corrosion resistance perspective. These properties consist of meaningfully smaller grain size (2.1 μm versus 10.4 μm) and a higher segmen<sup>t</sup> of coherent twin boundaries [8]. Smaller grain size and a closer, well-ordered grain boundary illustrate the relatively high corrosion resistance of the alloy [8–10]. The Cu5Zn5Al1Sn alloy forms relatively poorly soluble oxide films with its constituent metals in the presence of chlorides when exposed to long-term outdoor and short-term laboratory conditions [11]. Even though corrosion resistant behavior of this golden alloy is much better than that of other comparable alloys, it is found to be easily corroded in acidic media. Similarly to Cu metal, Cu2O and Cu2(OH)3Cl are the principal corrosion products produced from the Cu5Zn5Al1Sn alloy in marine outdoor conditions, with the

latter largely present at the outmost surface of both Cu and the Cu5Zn5Al1Sn alloy [7,11]. Although a big difference exists in their corrosion rates, the corrosion process is highly administered by the molecular diffusion between the metal surface and the electrolyte [12].

One approach to prevent corrosion of Cu and its alloy is the use of corrosion inhibitors [13,14]. Among the corrosion inhibitors used in practice, cysteine (Cys) is found to be widely employed for Cu protection in various media [15,16]. Cysteine is an amino acid that contains a –SH group in addition to the amino group; this mercaptan group is strongly attracted to copper. A. A. Nazeer et al. have studied the inhibitive effect of cysteine on a Cu10Ni alloy in sulfide containing atmospheres [16]. Based on this study, cysteine can act as a mixed-type inhibitor and cysteine molecules are adsorbed on the alloy surface. The inhibition mechanism of cysteine with copper-based materials is characterized by the formation of a stable Cu (I)–cysteine complex [17]. As confirmed by many researchers, the better corrosion inhibition effect of cysteine relies on its surface adsorption through sulfur atoms [18,19]. I. Milošev et al. on the other hand have studied different amino acids as inhibitors for copper in acidic environments and confirmed that cysteine has higher efficiency. Based on their molecular dynamic simulation results, it has been suggested that the –SH group is in charge of the good protective effect of cysteine [20]. G. M. Abd El-Hafez et al. have also investigated the protective action of methionine, N-acetyl cysteine, and cysteine for a Cu-10Al-5Ni alloy in 3.5 wt.% NaCl solution and they concluded that cysteine showed higher efficiency ascribed to the existence of the mercaptan moiety [21]. On the other hand, our previous study regarding the effect of cysteine on copper metal in corrosive environments also showed its effectiveness on corrosion inhibition.

It can be seen from the above investigated results cysteine can be a very promising corrosion inhibitor for copper and its alloys, and no information has been found on the action of cysteine for the inhibition of Cu5Zn5Al1Sn alloy corrosion. Furthermore, the interaction between the cysteine molecule and the alloy surface in chloride-containing solution has not ye<sup>t</sup> been investigated. In the present paper, the corrosion and corrosion inhibition of the Cu5Zn5Al1Sn alloy in chloride solutions is investigated. The influence of various cysteine concentrations is also taken into account. The performance of the studied inhibitor is evaluated using various electrochemical and surface analysis techniques. The inhibition efficiency, surface behavior, and adsorption mechanism are clearly analyzed and summarized using the obtained experimental results.

#### **2. Materials and Methods**

A commercial Cu5Zn5Al1Sn alloy in ther form of a 1 mm thick sheet (89 wt.% Cu, 5 wt.% Zn, 5 wt.% Al, and 1 wt.% Sn, equivalent to 84 at.% Cu, 11 at.% Al, 4.5 at.% Zn, and 0.5 at.% Sn) was obtained from Aurubis (Hamburg, Germany). The specimens were wrapped in epoxy resin, exposing a 1 cm<sup>2</sup> surface area. Before any test, the alloy was ground by silicon carbide paper from #800 to #2000 and then successively polished with diamond paste to 2.5, 1.5, and 0.5 μm, respectively. The diamond polished specimens were rinsed with analytical grade ethanol and dried in air.

3.5 wt.% sodium chloride (NaCl) was used as the corrosive solution. It was made from an analytical grade reagen<sup>t</sup> of NaCl and deionized water. Cysteine (KC90277-100gm, Suzhou tianke Co. Ltd., Suzhou, China) was used as the inhibitor and stock solution of 1 × 10−<sup>2</sup> M was prepared by mixing a suitable amount of cysteine in distilled water. Solutions with cysteine concentrations of 1 × 10−<sup>3</sup> to 1 × 10−<sup>5</sup> M were set from the stock solution using a dilution method.

The electrochemical cell used was a three-electrode glass cell with a capacity of 400 mL. A silver/silver chloride electrode (Ag/AgCl) was used as the reference electrode. All potential values in the manuscript were referred to using Ag/AgCl. A flat platinum sheet (4 cm2) was used as the counter electrode. An electrochemical workstation Gamry (Reference 600) with a high input resistance of 10<sup>14</sup> Ω, a current detection limit of 60 pA, and a current resolution of 20 aA was used to conduct electrochemical measurements. Prior to the tests of electrochemical impedance spectroscopy (EIS), the specimen was deepened in the test solution for 60 min. EIS was carried out under potentiostatic conditions in the frequency range of 100 kHz to 0.01 Hz at open circuit potential (OCP) with an amplitude of 5 mV. The EIS data was analyzed by Zsimpwin software to determine a simulated circuit and respective parameters. Tafel curves were obtained from potentiodynamic polarization study which was performed by varying the electrode potential automatically from −800 mV to +400 mV with respect to OCP at a rate of 1 mV/s.

An FEI Quanta 250 scanning electron microscope was used to observe the surface morphology of the Cu5Zn5Al1Sn alloy specimen with and without the cysteine inhibitor in the 3.5 wt.% NaCl solution. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) measurements were performed on an ESCALAB 250Xi spectrometer with Al Kα X-ray (1486.6 eV) irradiation as the photo source. Binding energy (BE) was calibrated against the C1s line of the aliphatic carbon contamination set at 284.8 eV. Xpspeak41 software was used for data fitting, which was carried out graphically within the constraints of Gaussian peak shapes.
