Critical Threshold of Chromium Content and Early-Stage Corrosion Mechanism of Weathering Bridge Steel in Simulated Acid Rain Environment

High-performance weathering steel is a critical material for the sustainability of bridge infrastructure in aggressive acidic atmospheric environments. However, the precise alloy design criterion for Chromium (Cr) content to balance manufacturing cost and corrosion resistance remains ambiguous. This study investigates the corrosion evolution of 420 MPa-grade bridge steels with varying Cr contents (0.5%–1.8%) under simulated acid rain conditions. Particular attention is paid to the initial 10-day nucleation kinetics, which is identified as the governing factor for the subsequent adhesion and stability of the protective patina. Electrochemical analysis utilizing Distribution of Relaxation Times (DRT) and gravimetric tests reveal a distinct resistance threshold between 1.5% and 1.8% Cr. The mechanism is attributed to a "Structural Determinism" effect: the 1.8% Cr addition promotes a uniform lath bainite microstructure, eliminating the micro-galvanic corrosion observed in ferrite/bainite mixed matrix. This homogeneity facilitates the rapid "early anchoring" of Cr and Mo, promoting the formation of a dense, cation-selective inner layer enriched with Fe₂CrO₄ and MoO₃. Consequently, the 1.8% Cr content is proposed as a critical alloy design criterion for next-generation weathering bridge steels, providing a quantitative guideline for engineering material selection in industrial-marine atmospheres.