**4. Conclusions**

The development of new and ecologically friendly strategies to protect outdoor iron surfaces has a clear economical, as well as ecological, interest. We demonstrated the potential of bacteria as an alternative for the development of innovative and green methods to protect iron artifacts from detrimental corrosion [24–28]. Here, the obtained results allow us to better understand how reactive corrosion layers are converted into biogenic iron phosphates with *Desulfitobacterium hafniense*. These biogenic minerals covered almost all of the remaining original corrosion layer. This layer could act as a barrier isolating the unstable iron corrosion products that would eventually still be

present underneath from the exposure to atmospheric oxygen and moisture that could lead to further corrosion. Vivianite is in fact a stable, poorly soluble, and non-oxidizing Fe2<sup>+</sup> mineral [22]. However, grea<sup>t</sup> attention has to be drawn to the composition of the bacterial solution applied, which not only drives the type of biogenic minerals produced, but also potentially contaminates the corrosion layer with undesired compounds that are able to instigate further corrosion, such as sulfur-containing compounds. In fact, through the stratigraphy study carried out here, a sulfur-rich layer was detected below the biogenic iron phosphates. This layer was not detected with surface analyses of the coupons, and only stratigraphic investigations allowed us to conclude that careful attention has to be paid to the culture medium composition in order to produce a stable vivianite layer that would passivate the iron surface. Our study on cross-sectioned samples further improved the evaluation of the depth e fficiency of the proposed bacterial treatment, as well as demonstrated the formation of biogenic vivianite as an adherent, even, and uniform layer. These features are important criteria when designing new protective systems to provide long-term inhibition of corrosion on iron surfaces.

**Author Contributions:** Conceptualization, E.J.; methodology, L.C., M.A. and W.K.; validation, L.C., J.M., T.L., P.J. and E.J.; investigation, L.C., M.A., W.K., and T.L.; resources, J.M.; data curation, L.C.; writing—original draft preparation, L.C. and E.J.; writing—review and editing, L.C., M.A., W.K., J.M., T.L., P.J. and E.J.; visualization, L.C.; supervision, P.J. and E.J.; project administration, E.J.; funding acquisition, E.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Swiss National Science Foundation, Ambizione, gran<sup>t</sup> number PZ00P2\_142514, 2013–2016 and professorship gran<sup>t</sup> number PP00P2\_163653, 2016–2020.

**Acknowledgments:** The authors are also grateful to the research conservation laboratory of the Swiss National Museum for providing the steel plate used in the experiments.

**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.
