Unraveling the influence of Mo on the corrosion mechanism of Ni-advanced weathering steel in harsh marine atmospheric environments

Ni-advanced weathering steel holds paramount importance in marine atmospheric environments, especially those with heightened Cl– concentrations. The meticulous compositional design plays a crucial role in establishing a rust layer capable of withstanding intrusion by Cl–, making it imperative for the viability of coating-free weathering steel in marine atmospheric environments. This study explores the corrosion evolution and corrosion-resistant mechanisms within a steady-state rust layer in 3Ni weathering steel, with a particular focus on the role of Mo in challenging marine atmospheric conditions. The findings unequivocally demonstrate that the augmentation of the protective properties of the rust layer is directly correlated with an increase in Mo content, transitioning from 0.5 to 1.5 wt.%. This transition is most evident in the reduction of the corrosion rate for the 3Ni-Mo steel, dropping from an initial 1.74 mm y–1 to a robust 1.31 mm y–1 after 768 h of corrosion exposure. The heightened Mo content expedites the formation of a stable and durable rust layer, significantly enriching the proportion of α-FeOOH within this protective layer. The stabilized rust layer of 3Ni-Mo weathering steel exhibits a distinct three-layer structure, comprising an outer layer primarily of γ-FeOOH, an intermediate layer mainly composed of Fe2O3/Fe3O4, and an inner layer predominantly composed of α-FeOOH and β-FeOOH. Additionally, an alkaline interface enriched with NiFe2O4 and CuFe2O4 develops between the inner layer and the substrate. Firstly, Mo promotes the deposition of MoO2, MoO3, and molybdate on both the inner layer and alkaline steel-rust interface to repair corrosion pits and fill cracks. Secondly, Mo facilitates the generation of compounds such as NiFe2O4 and CuFe2O4, which heightens the electronegativity of the intermediate rust layer and the steel-rust interface, preventing Cl–-induced interface acidification and pitting corrosion. The higher Mo content expedites the formation of this alkaline interface and promotes inner layer densification. Most significantly, Mo creates additional nucleation sites for hydroxide oxides through oxide formation, leading to the formation of nano-sized α-FeOOH and β-FeOOH within the inner layer thereby enhancing the stability and compactness of the inner layer. These synergistic effects fortify the resilience of 3Ni-Mo advanced weathering steel in corrosive environments, ultimately strengthening its capacity to withstand environmental challenges.