Research on the initial corrosion behavior of A100 steel in salt fog-SO₂ environment

To investigate the service performance of A100 steel with the chemical composition 23Co₁₄Ni₁₂Cr₃Mo in actual ship deck environments, this study innovatively adopts the ASTM G85 "salt spray-SO₂ modified salt spray test" to simulate the weakly acidic marine atmosphere containing engine exhaust pollutants. This approach addresses the limitation of domestic studies that primarily rely on neutral salt spray tests following GJB150.11A, which fail to reflect complex service conditions. The corrosion behavior, product composition and electrochemical kinetics of A100 steel were systematically characterized using multiple techniques including SEM, XRD, XPS, FT-IR, EDS, electrochemical workstation and EBSD. Results show that the corrosion process exhibits triphasic kinetics: rapid initial corrosion from 2 to 4 days due to direct contact between the metal and corrosive medium, with charge transfer resistance (R _ct) decreasing from 2860 Ω cm² to 1029 Ω cm² and corrosion current density (I _corr) increasing from 5.09 µA cm^-2 to 7.35 µA cm^-2; decelerated degradation from 4 to 8 days by the formation of a dual-layer rust structure consisting of a dense inner layer of Fe₃O₄ and γ-FeOOH as well as a porous outer layer of Fe₂O₃, with R _ct peaking at 5421 Ω cm² and I _corr dropping to the minimum of 1.73 µA cm^-2; and renewed acceleration from 8 to 10 days caused by the synergistic damage of Cl^- and HSO₃ ^- to the rust layer, with R _ct plummeting to 426 Ω cm² and I _corr surging to 23.44 µA cm^-2. Alloying elements including Cr, Co and Ni regulate corrosion resistance by modifying the rust layer structure and electrochemical properties. Microstructurally, the initial passivation of austenite delays corrosion, while the subsequent dissolution of martensite and the establishment of a dynamic phase equilibrium ultimately expose austenite, accelerating degradation beyond 10 days. This work clarifies the corrosion mechanism of A100 steel under salt spray-SO₂ coupling, providing a reliable experimental basis and theoretical support for its safe application in marine aviation equipment.