**5. Conclusions**

In this work, the corrosion behavior of a Cu5Zn5Al1Sn alloy was examined in a 3.5 wt.% NaCl solution in the absence and presence of cysteine. Potentiodynamic polarization studies showed that cysteine acts as a mixed-type inhibitor during the corrosion process. The inhibition efficiency increased with an increase in the concentration of cysteine and reached 97.2% in the presence of 10−<sup>2</sup> M cysteine. Copper was the main corrosion product formed on the Cu5Zn5Al1Sn alloy in 3.5 wt.% NaCl solution. The outermost surface also contained to a small extent Zn- and Sn-rich constituents, as confirmed by XPS investigation. The content of alloy constituents other than Cu decreased in the presence of cysteine and became negligible at a cysteine concentration of 10−<sup>2</sup> M. The inhibition mechanism relies on the formation of a binding Cu(I)-Cys film and/or Cu(I)-Cys-Cu(II)-Cys film on the alloy surface, which protect the alloy from corrosion attack.

**Author Contributions:** Conceptualization, K.W.S., F.H., and Y.J.; data curation, K.W.S. and F.H.; formal analysis, K.W.S. and F.H.; investigation, K.W.S. and F.H.; methodology, K.W.S., F.H., and Y.X.; project administration, Y.J.; resources, Y.J.; software, K.W.S. and F.H.; supervision, Y.X., L.W., and Y.J.; writing—original draft, K.W.S.; writing—review & editing, F.H., Y.X., and Y.J.

**Funding:** This research was funded by the Development and Reform Committee of PRC, (no. YYXM-1412-0001), Fundamental Research Funds for the Central Universities (no. FRF-TP-16-040A1) and 111 Project, (no. B12012).

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
