**4. Conclusions**

*S. putrefaciens* was observed to be biofilm forming bacteria. The highest coverage to EPS material as well as reduced iron/oxyhydroxides were observed in the IRB biofilm in the presence of yeas<sup>t</sup> extract in the Postgate C medium. The IRB contained a higher carbohydrate portion in the biofilm than the abiotic corrosion product film.

The build-up of organic material and reduced minerals did not provide any significant protection to the metal and active corrosion occurred when external potential was applied. Dissolution of Fe(III) oxyhydroxides occurred in the presence of the IRB which led to a greater dissolution of Fe2+ ions into the solution than under abiotic conditions. The IRB were also able to set up anaerobic conditions in solution by the removal of residual O2 present in solution.

Undercutting pitting corrosion morphology was observed in the presence of the IRB in the nutrient-rich media, and to a greater extent, in nutrient-limited conditions. Deep pits on carbon steel were observed when IRB were cultured in the inorganic medium. From the results of this study, it can be concluded that the IRB biofilm does not protect carbon steel from corrosion and was observed to accelerate abiotic corrosion processes in a localised manner.

**Author Contributions:** S.W.; Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Software, Validation, Visualization, Writing—review & editing. S.A.-S.; Investigation, Methodology, Formal analysis, Validation, Writing—review & editing. W.P.G.; Conceptualization, Investigation, Methodology, Formal analysis, Validation, Writing—review & editing. R.K.S.R.; Conceptualization, Investigation, Methodology, Formal analysis, Validation, Writing—review & editing. C.P.; Investigation, Methodology, Writing—review & editing. All authors have read and agreed to the published version of the manuscript.

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data is contained within the article.

**Acknowledgments:** Authors acknowledge the use of microscopic facilities at Monash Centre for Electron Microscopy (MCEM).

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