Insights into the corrosion mechanisms in a multiphase medium-manganese steel in marine environment: Experimental study and density functional theory simulations

The current study examines the complex interactions between the microstructural characteristics and corrosion behavior of cold rolled and annealed (973K-1273K) Ni-containing medium manganese steel (Ni-MMS) in 3.5 wt% NaCl solution. The primary corrosion mechanism in the specimens is driven by micro-galvanic corrosion, attributed to the distinct volta potential of the constituent phases. The specimen cold rolled and annealed at 1073 K (CR1073K) exhibits the best corrosion resistance due to minimal volta potential differences between ferrite and austenite . In contrast, the specimen cold rolled and annealed at 1273 K (CR1273K) has the highest corrosion rate due to the greater volta potential among the phases (especially between austenite and ferrite as well as martensite and ferrite). More importantly, the specimen cold-rolled and annealed at 973 K (CR973K) exhibits a higher corrosion rate than the one annealed at 1173 K (CR1173K), despite having a lower volta potential difference between the ferrite and B2 precipitate. This increased corrosion rate in CR973K specimen is attributed to the smaller grain size of the ferrite phase in the former specimen compared to that in the CR1173K specimen. Additionally, density functional theory calculations reveal that Cl atoms have a higher tendency to get adsorbed onto MnO and Fe 2 O 3 surfaces, resulting in higher corrosion of the product layer enriched in these oxides .