Early Rust-Layer Evolution of Q355 Carbon Steel in the Pingtan Marine Atmosphere

Q355 carbon steel was exposed for 0.5 and 1 year in Pingtan Strait to examine how surface-wetness changes affect early rust-layer evolution and localized corrosion. Environmental monitoring, corrosion-rate measurements, rust-phase analysis, and electrochemical characterization were combined to characterize corrosion in this humid, salt-laden marine atmosphere. The exposure regime shifted from sustained wetness during the first half-year to frequent wet-dry cycling during the second. Although the average corrosion rate remained nearly unchanged, localized corrosion intensified and adjacent pits became interconnected. This change was closely associated with the evolution of the rust-phase assemblage and its spatial distribution. After 0.5 years, β-FeOOH was dominant, consistent with a long time of wetness (TOW) and Cl^- enrichment in surface electrolyte films, and pits remained largely isolated. After 1 year, Fe₃O₄ increased substantially and became the dominant phase, possibly because frequent wet-dry cycling altered oxygen transport within the rust and repeatedly produced locally oxygen-deficient conditions. Fe₃O₄ enrichment promoted continued pit deepening, followed by pit expansion and coalescence. Meanwhile, local α-FeOOH enrichment developed in relatively oxygen-rich regions near the rust surface and inhibited lateral pit propagation. Thus, shifts in the wetting regime of the humid, salt-rich Pingtan atmosphere markedly regulate localized corrosion of Q355 steel by controlling rust-phase evolution and spatial distribution.