Increasing chromium redirects rust-layer evolution in Cu-bearing weathering steels during tropical marine exposure

Weathering steels are vulnerable to sustained chloride deposition when their rust layers fail to stabilize. To clarify how a Cr gradient regulates rust formation in Cu-bearing steels, Q235 and four Cr-Cu steels containing 0.5–3.5 wt.% Cr with relatively stable Cu contents were exposed for two years in the tropical marine atmosphere of Wenchang, China. Mass-loss measurements, rust-layer characterization, and electrochemical tests show that Q235 retains a porous, chloride-permeable rust layer. With increasing Cr content, the relative accumulation of α-FeOOH is favored and a more continuous Cr-enriched inner layer develops, while Cu-containing products preferentially enrich near defects. These concurrent spatial changes are associated with improved resistance of the rust layer to chloride ingress. From Q235 to the highest-Cr steel, the corrosion current density decreases from 43 to 16 μA·cm -2 , and the combined rust-layer and charge-transfer resistance increases from 139.0 to 540.3 Ω·cm 2 . These results indicate that increasing Cr content progressively stabilizes the rust layer in Cu-bearing steels during tropical marine exposure, while Cu-containing species remain preferentially associated with defective regions of the rust layer.