The corrosion mechanism of copper and copper alloy is reviewed. A number of scientific papers have been investigated to determine the corrosion mechanism and protection techniques of copper and copper alloy corrosion. Results have shown that copper can be corroded in an acidic or an alkaline environment, and oxide formation is the corrosion initiation process. The use of corrosion inhibitors is one of several ways of controlling metal corrosion. There are inorganic (toxic) and organic (green) corrosion inhibitors invented so far. Nowadays, environmental issue is a concern of several scientists in the world. From the results of recent scientific papers, green corrosion inhibitors can be used for copper corrosion protection and they are both economical and environmentally safe. Furthermore, future researches are needed to determine more efficient, environmentally friendly corrosion inhibitors for copper and copper alloys.

In this paper, the principle and application of atomic force microscopy (AFM) are reviewed and discussed in detail. Several scientific papers are used to find out data about AFM. The obtained scientific results are summarized to get a better understanding of the method and its application. The application of AFM for corrosion study is discussed in detail, and the possible conclusion is made based on the results of several articles. It is summarized that AFM is an important method to determine the surface phenomena of metal corrosion.

In this work, the corrosion mechanism of a Cu-5Zn-5Al-1Sn alloy was examined in a 3.5 wt.% NaCl solution. At the same time, the effect of a cysteine inhibitor was also investigated through a multi-analytical approach. Electrochemical results suggested that inhibition efficiency increased with the increase of cysteine concentration. From potentiodynamic polarization (PD) analysis, a decrease in corrosion current and corrosion potential shift toward a more negative direction was observed. The potential difference between the blank and inhibited surface was found to be 46 mV, which is less than 85 mV, revealing a mixed type inhibition effect of cysteine for the Cu-5Zn-5Al-1Sn alloy. The inhibition mechanism of cysteine (Cys) and the effect of alloying elements were investigated by fitting experimental impedance data according to a projected equivalent circuit for the alloy/electrolyte interface. A Langmuir adsorption isotherm was proposed to explain the inhibition phenomenon of cysteine on the Cu-5Zn-5Al-1Sn alloy surface. Surface morphology observation confirmed that the Cu-5Zn-5Al-1Sn alloy was damaged in 3.5 wt.% NaCl solution and could be inhibited by using the cysteine inhibitor. The impact of alloying elements on the corrosion mechanism was further examined by surface analysis techniques such as X-Ray photoelectron spectroscopy (XPS)/Auger spectra, the results of which indicated that the corrosion inhibition was realized by the adsorption of the inhibitor molecules at the alloy/solution interface. Appl. Sci. 2019, 9, 3896 2 of 17 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...

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The effect of cysteine on the corrosion characteristics of Cu5Zn5Al1Sn alloy in 3.5 wt% NaCl solution has been studied by electrochemical and surface characterization techniques in various immersion times. The results of electrochemical impedance spectroscopy (EIS) revealed that the degradation of Cu5Zn5Al1Sn alloy occurred in 3.5 wt% NaCl and was aggravated with increasing immersion time. The results of inhibition efficiency calculated from EIS data showed that cysteine can act as an effective anti-corrosion substance, which was also proved by the less eroded morphology of the alloy surface observed on scanning electron microscopy (SEM). Furthermore, the elemental analysis of alloy surfaces was investigated by Raman, electron dispersion spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS), which confirmed the presence of S and N species. An adequate adsorption isotherm and inhibition mechanism was also suggested based on EIS results.