**4. Conclusions**

In this study, we investigated the e ffect of zinc thermal spray coated carbon steel (Zn-coated) and copper thermal spray coated carbon steel (Cu-coated) on iron corrosion in air or an aquatic environmental condition. We also explored which factor(s) such as biofilm formation (MIC) and ionization tendency (electrochemical corrosion) worked dominantly in iron corrosion. We expected that the Zn-coated and Cu-coated could inhibit iron corrosion due to the sacrificial corrosion of zinc and contact killing of biofilm-forming bacteria, respectively. In both air and water environments, the Zn-coated inhibited iron corrosion, but the Cu-coated accelerated iron corrosion, which was negatively correlated to ionization tendency (i.e., Zn > Fe > Cu). The dominant factor of iron corrosion will be electrochemical corrosion in the air environment and MIC in the water environment, however, it is probable that biofilm formation plays an important role in iron corrosion. In an air environment, biofilms can store water (moisture) that makes galvanic cells elute metallic ions according to ionization tendency. In a water environment, biofilms can accelerate iron corrosion caused by bacterial metabolites [66]. MIC-related bacteria have been found in specific environments such as oil tanks and water systems of nuclear power plants. In this study, MIC-related bacteria were barely detected in the biofilms formed on the surface of the Zn-coated, Cu-coated, and carbon steel. This implies that biofilm formation is an essential factor for iron corrosion, but MIC-related bacteria are not always necessary for it. Additionally, iron ions can predominantly attract bacteria more than zinc ions

and copper ions, therefore, inhibiting the elution of iron (ions) will be an effective approach to regulate biofilm formation as well as iron corrosion.

**Author Contributions:** Conceptualization, A.O. and H.K.; methodology, A.O.; formal analysis, A.O.; investigation, A.O. and H.K.; resources, K.S.; data curation, A.O., K.T., H.K., D.K., T.K. and K.S.; writing—original draft preparation, A.O. and D.K.; writing—review and editing, S.T.; visualization, A.O., D.K. and H.K.; supervision, N.H.; project administration, A.O.; funding acquisition, H.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding

**Acknowledgments:** We had financial support from the Suga Weathering Technology Foundation (SWTF: 5-4-14, Shinjyuku, shinjyuku-ku, Tokyo, 160-0022, Japan) through a recommendation by the Japan Thermal Spray Society (JTSS: 2-2-29, Eiwa, Higashi Osaka, Osaka Pref. 577-0809, Japan) to carry out the experiments. We would like to take this opportunity to extend our special thanks to them all.

**Conflicts of Interest:** The authors declare no conflict of interest.
