**4. Conclusions**

(1) Defects such as pores and shrinkage generated in the process of producing ductile iron easily become a gathering place for aggressive ions, which induce the dissolution of graphite spheres and the matrix interface, and then induce serious local corrosion. After surface mechanical treatment to eliminate holes and shrinkage, SEM observations and CLSM statistics of the pits show that localized corrosion pits on the material surface are significantly suppressed. The average icorr in the different solutions is lower and the corrosion kinetic processes are inhibited. All of the results prove that surface treatment techniques can improve the corrosion resistance of ductile iron, and inhibit localized corrosion.

(2) Simulations revealed that the solution around graphite spheres for defects such as pores and shrinkage loosening is highly susceptible to acidification, whereby an acidification autocatalytic cell effect is formed, accelerating the dissolution of the substrate and reducing the corrosion resistance of the material.

(3) Electrochemical test results showed that changes in water quality have a greater impact on the corrosion kinetic process of ductile iron. For unsurfaced ductile iron, the order of the relative weight of the environmental factors that affect corrosion resistance in all environments is oxygen solubility > temperature > water hardness > immersion time > pH > Cl− concentration. Under oxyfuel conditions, the order of the relative weight of the environmental factors that affect corrosion resistance is Cl− concentration > water hardness > immersion time > pH > temperature.

(4) For surface-treated ductile iron, with the removal of obvious defects from the ductile iron's surface, oxygen concentration is no longer the main factor in determining the icorr value. Under oxyfuel conditions, the immersion time becomes the largest factor affecting the icorr contribution, not Cl− or pH. This indicates that uniform corrosion emerges as the dominant type of erosion as immersion time increases.

**Author Contributions:** Methodology, T.L., K.T. and X.Y.; Software, X.Y.; Validation, T.L.; Formal analysis, T.L. and K.T.; Investigation, L.Z.; Resources, C.L.; Data curation, B.W. and C.L.; Writing— original draft, B.W.; Writing—review and editing, C.L. and X.C.; Project administration, L.Z. and X.C. Funding acquisition, C.L. and X.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** The National Natural Science Foundation of China (No. 52104319); the National Science and Technology Resources Investigation Program of China (No. 2019FY101400).

**Data Availability Statement:** The raw/processed data required to reproduce these findings cannot be shared at this time as the data are related to an ongoing study.

**Acknowledgments:** The authors acknowledge Qinglin Li and Yunyang Wu in University of Science and Technology Beijing for their assistance in the experiment.

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