Clarifying the Nonlinear Evolution Mechanism of Rust Layers on a Weathering Steel in a Simulated Tropical Marine Atmosphere

This study investigates the nonlinear evolution of rust layers on weathering steel in a simulated tropical marine atmosphere. Seven critical time points (24, 72, 168, 288, 360, 480, and 720 hours) were identified using electrochemical impedance spectroscopy, and changes in rust layer morphology, composition, and distribution were analyzed. In the early stage (0-72 h), a single-layer rust formed, consisting of a mixture of five phases: α-FeOOH, γ-FeOOH, β-FeOOH, γ-Fe 2 O 3 , and Fe 3 O 4 . During the metastable stage (168-288 h), the rust layer began to stratify, forming an inner layer adjacent to the steel substrate, composed of α-FeOOH and Fe 3 O 4 . Between 360 and 480 h, the rust layer further differentiated into four distinct zones: an inner rust layer, a valley zone rich in all five phases, an outer rust layer, and an outermost layer consisting of γ-FeOOH. After 480 h, the stable stage, a stable, protective rust structure was established. The improved weatherability at later stages is attributed to the valley’s disappearance and the inner layer’s thickening enriched in stable α-FeOOH. ● Using electrochemical impedance spectroscopy to track rust layers evolution. ● Rust layer differentiation plays a key role in forming a protective barrier. ● Inner rust layers of α-FeOOH and Fe 3 O 4 are key to rust layer protectiveness. ● Analysis of rust layer evolution in a simulated tropical marine atmosphere.