1. Humidity: Determines the "foundation" of rust layer formation
Moderate humidity (40%–70%): Ideal for rust layer stabilization. A thin, intermittent water film promotes slow corrosion, allowing alloying elements (Cu, Cr, Ni, P) in weathering steel to gradually migrate to the rust layer. These elements form insoluble compounds (e.g., Cu₂O, Cr(OH)₃) that fill pores, transforming loose, porous rust (γ-FeOOH, Fe₃O₄) into dense α-FeOOH.
Excessively high humidity (>80%, e.g., tropical rainforests): A thick, persistent water film accelerates aggressive corrosion. The rust layer grows too quickly to trap alloying elements, remaining loose and porous-unable to block further water/oxygen penetration, leading to continuous matrix corrosion.
Excessively low humidity (<30%, e.g., arid deserts): No continuous water film forms, so corrosion is almost stagnant. The rust layer fails to develop or remains thin and discontinuous, lacking protective capability.
2. Aggressive Ions (Cl⁻, SO₂): Key "disruptors" or conditional "regulators"
(1) Chloride ions (Cl⁻): The most destructive factor
High Cl⁻ environments (coastal areas, snow-melting salt regions): Cl⁻ accumulates at the rust layer-matrix interface, accelerating anodic dissolution of the steel matrix. It also inhibits the transformation of γ-FeOOH to α-FeOOH, keeping the rust layer loose. In severe cases, it causes "pitting corrosion"-localized rust layer breakdown that leads to deep matrix damage.
Low Cl⁻ environments (inland rural areas): Minimal Cl⁻ interference allows alloying elements to function normally, promoting the formation of a dense, protective α-FeOOH layer.
(2) Sulfur dioxide (SO₂): Dual effects depending on concentration
Low SO₂ concentration (<0.1 ppm): Mild corrosion promotes uniform rust layer growth. Sulfate ions (SO₄²⁻) formed by SO₂ oxidation can react with Fe³⁺ to form temporary precipitates, which later dissolve and facilitate the redistribution of alloying elements, indirectly aiding α-FeOOH formation.
High SO₂ concentration (>1 ppm): Excessive SO₂ accelerates corrosion, forming thick, loose rust layers rich in FeSO₄·7H₂O (water-soluble). These layers are porous and easily washed away by rain, preventing the stabilization process entirely.
3. Light & Ventilation: Accelerate "maturation" of the rust layer
Sufficient light & good ventilation (e.g., open-air bridges, south-facing surfaces):
Light increases surface temperature, accelerating the evaporation of the water film-creating a repeated "wet-dry cycle" that concentrates alloying elements in the rust layer.
Good ventilation replenishes oxygen (O₂) for redox reactions (critical for α-FeOOH formation) and removes accumulated corrosive gases (e.g., SO₂) or moisture, avoiding localized over-corrosion.
Result: Faster formation of a uniform, dense α-FeOOH layer.
Lack of light & poor ventilation (e.g., shaded underpasses, enclosed spaces):
Stagnant air and low light slow water evaporation, maintaining a persistent wet environment.
Oxygen depletion inhibits the transformation of unstable rust phases to α-FeOOH, leading to a loose, dark rust layer with weak protection.
4. Temperature: Adjusts the "speed" of the stabilization process
Moderate temperature (15–30°C): Optimizes reaction rates. Electrochemical corrosion (rust formation) and the diffusion of alloying elements proceed steadily, enabling the gradual transformation of loose rust to dense α-FeOOH.
Extremely low temperature (<0°C): Water freezes, stopping electrochemical reactions. Rust layer formation stagnates, and existing rust may crack due to freeze-thaw cycles, losing protection.
Extremely high temperature (>40°C): Accelerates water evaporation, leading to an overly dry surface. Corrosion slows, and the rust layer becomes thin and brittle. High temperatures may also cause thermal expansion of the rust layer, creating microcracks that allow corrosive media to penetrate.
