Time-Dependent Corrosion Behaviors of Al-Si Coated Steel Sheet Under a Chlorine-Containing Wet–Dry Cycling Environment

The corrosion behavior and time-dependent mechanism of 22MnB5 steel featuring a thinned Al-Si coating (60 g/m2) were systematically investigated in a chloride ion wet–dry cyclic environment, motivated by the demand for thinning and toughening development of aluminum-silicon coatings. A periodic immersion accelerated corrosion test using 3.5% NaCl solution was conducted, together with macro/microscopic morphology observation (SEM/EDS), phase analysis (XRD, FTIR), and electrochemical measurements (polarization curves, EIS). The Al-Si coated steel was studied over corrosion periods of 1, 8, 10, and 20 days to elucidate its corrosion behavior, interfacial evolution, and failure mechanism. The results indicated that the corrosion process exhibited a three-stage evolution: stable protection, rapid failure, and dynamic equilibrium. At the initial stage (1 day), a dense Al2O3 passive film formed on the coating surface, providing excellent substrate protection, with a corrosion current density of only 1.77 µA/cm2 and a maximum charge-transfer resistance (R2) of 652 Ω·cm2. In the middle stage (8 days), Cl− permeated through the cracked film, triggering selective dissolution of Al, while Si was enriched in situ to form a porous residual layer; the corrosion current density (Icorr) sharply increased to 13.25 µA/cm2, and R2 dropped to its minimum of 156.6 Ω·cm2. Corrosion products at this stage were mainly Al2O3 and SiO2, accompanied by small amounts of iron oxyhydroxides and hydroxides, and local coating failure began to appear. During the later stage (10–20 days), the corrosion products evolved into γ-FeOOH, α-FeOOH, and Fe2O3, which, together with an amorphous SiO2 gel network enriched at the interface, formed a dual-layer composite rust layer. R2 consequently recovered from 156.6 Ω·cm2 at 8 days to 424 Ω·cm2 at 20 days, indicating a reduced corrosion rate and entry into a stable inhibition stage. The critical failure mechanism is that Cl− preferentially penetrates the surface of the Al2O3 passive film, disrupting the metastable state of the coating and thereby creating pathways for corrosive media intrusion. The findings of this study can provide technical support for the safe application of such as-received coatings in non-load-bearing components with heat and corrosion resistance requirements.