Mechanism of formation and transformation of rust layer on weather-resistant steel through spheroidization treatment of inclusions in industrial polluted atmospheric environment

Cyclic immersion corrosion tests were conducted to simulate the industrial atmospheric corrosion behavior of Q345qENH weathering steel and Q345qD steel. Corrosion kinetics were evaluated via mass-loss measurements, and microstructural evolution and corrosion mechanisms were investigated using metallography, confocal laser scanning microscopy, SEM-EDS, SKPFM, XRD, and electrochemical tests. The results indicate that Q345qENH steel exhibits superior corrosion resistance. Following spheroidization, its inclusions are uniformly distributed with reduced porosity, forming a composite structure of (Al, Mg)O x enveloped by CaS-CaO/MnS. This homogeneous microstructure promotes uniform corrosion, enabling the development of a protective rust layer. As the test progressed, the rust layer thickened and eventually developed a double-layer structure: Cr enriched in the inner layer, with Cu concentrated in discrete regions. Both steels produced corrosion products consisting of α-FeOOH, Fe 3 O 4 , γ-FeOOH, and Fe 2 O 3 . Semi-quantitative analysis confirmed that the α*/γ* ratio of Q345qENH increased continuously over time. The corrosion mechanism comprises three stages: (1) Initiation, in which dissolution of CaS/CaO at inclusions lowers the local pH, accelerating the dissolution of Cr, Fe, and Cu and forming Cr-/Cu-rich products; (2) Rapid formation of a wet-dry cyclic rust layer, where Cr and Cu participate in reactions to generate protective α-(Fe 1-x Cr x )OOH, FeCr 2 O 4 , CuFeO 2 , and metallic Cu, inhibiting corrosive medium penetration and repairing defects in the rust layer; (3) Dynamic stabilization, during which continued corrosion leads to further thickening of the rust layer, resulting in a loose outer layer and a dense inner layer that primarily contributes to the protective performance of Q345qENH steel.