Mechanistic Understanding of Cu‐Driven Inclusion Transformation and Pitting Suppression in Carbon Steel

It is investigated how the addition of Cu influences the pitting corrosion behavior of carbon steel induced by (Ca, Mn)S‐(Al, Mg)O complex inclusions. While the overall microstructure of the steel matrix remained unchanged, Cu markedly altered the morphology and elemental distribution of these inclusions. First‐principles calculations revealed that Cu preferentially adsorbed on the CaS interface due to its lower adsorption energy and stronger Cu‐S orbital hybridization. This preferential adsorption facilitated the phase separation of MnS and CaS and resulted in localized Cu enrichment within CaS regions. At higher Cu contents, a dense and continuous Cu‐rich layer developed around the inclusions. This layer served as a physical barrier, effectively impeding the ingress of corrosive species, retarding the dissolution of sulfide inclusions, and suppressing pit propagation. Immersion tests demonstrated that increasing Cu content resulted in smaller and shallower pits and reduced localized corrosion rates. Electrochemical analyzes further validated the enhanced corrosion resistance at higher Cu content. The synergistic action of interfacial adsorption and barrier‐layer formation accounts for the reduced pitting susceptibility of Cu‐alloyed carbon steel.