# The Protection Role of Cysteine for Cu-5 Zn-5 Al-1 Sn Alloy Corrosion in 3.5 wt.% NaCl Solution
## 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; 3; 4; 5; 6]. Cu-5 Zn-5 Al-1 Sn (Cu5 Zn5 Al1 Sn) 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 Cu5 Zn5 Al1 Sn alloy have been studied by comparing it with pure copper [8]. The microstructure of Cu5 Zn5 Al1 Sn owns numerous properties that are promising from a corrosion resistance perspective. These properties consist of meaningfully smaller grain size (2.1 \(\upmu\)m versus 10.4 \(\upmu\)m) and a higher segment 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; 9; 10]. The Cu5 Zn5 Al1 Sn 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, Cu\({}_{2}\)O and Cu\({}_{2}\)(OH)\({}_{3}\)Cl are the principal corrosion products produced from the Cu5 Zn5 Al1 Sn alloy in marine outdoor conditions, with thelatter largely present at the outmost surface of both Cu and the Cu5 Zn5 Al1 Sn 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. [PERSON] et al. have studied the inhibitive effect of cysteine on a Cu10 Ni 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]. [PERSON] 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]. [PERSON] et al. have also investigated the protective action of methionine, N-acetyl cysteine, and cysteine for a Cu-10 Al-5 Ni 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 Cu5 Zn5 Al1 Sn alloy corrosion. Furthermore, the interaction between the cysteine molecule and the alloy surface in chloride-containing solution has not yet been investigated.