Microstructure-dependent corrosion and passivation of iron with a high carbon content in weak alkaline solution

•Corrosion of Fe-3.8 wt%C and Fe-4.4 wt%C in borate buffer solution was observed.•Quenched Fe-C alloys showed higher open circuit potential than slow-cooled alloys.•The quenched alloys were passivated in a solution containing chloride ions at pH 9.•Corrosion of the Fe-C alloys in solution containing steelmaking slag was examined. The corrosion of metallic Fe grains present in steelmaking slag used in road construction is a cause for concern. In this study, we examined the behaviors of Fe-3.8 wt%C and Fe-4.4 wt%C alloys in borate buffer solutions (pH 9) and compared them with those of two types of commercial steel bars for a better understanding of unrefined Fe corrosion in weak alkaline environments. The influences of the alloy microstructures and Cl− ions were evaluated via open circuit potential measurements and linear sweep voltammetry. These measurements revealed that Fe with high C content exhibited greater corrosion resistance after quenching and was passivated in a solution containing Cl−, which corroded the commercial steel bars and slow-cooled alloys tested. In a solution containing real steelmaking slag, the Fe-C alloys and commercial steel bars did not corrode for at least 3 months when decarburization slag was used due to its high basicity. The Fe-C alloys exhibited either passivation or corrosion when dephosphorization slag was used. Although the inhomogeneity of actual steelmaking slag affected the reproducibility of these tests, the quenched alloys showed higher tendency toward passivation compared with the commercial steel bars and slow-cooled alloys.