Rust layer evolution and corrosion performance of Q500qENH weathering steel welded joints in salt-spray conditions

Abstract

In this study, the microstructure, chemical composition, electrochemical properties, and evolution mechanism of the weld and base-metal rust layers formed on the Q500qENH weathering steel welded joints under salt-spray corrosion conditions were investigated. Through X-ray diffraction, scanning electron microscopy, Raman spectroscopy, and energy-dispersive X-ray spectroscopy, the composition, microstructure, and elemental distribution of the rust layers were systematically characterized. Furthermore, electrochemical impedance spectroscopy and Tafel polarization methods were used to evaluate the corrosion resistance of the rust layers for different corrosion periods. Results showed that with prolonged corrosion time, the α-FeOOH content in the rust layer of the Q500qENH weathering steel welded joints increases significantly, while the γ-FeOOH content gradually decreases. The α-FeOOH/γ-FeOOH ratio in the weld rust layer remains consistently higher than that in the base-metal rust layer, exhibiting superior compactness and corrosion resistance. While the rust layer of the base metal displays a well-defined inner–outer stratification, the outer rust layer in the weld region is more complex, featuring cotton-ball-like, fibrous, and layered morphologies. Trace metal elements such as Cr, Ni, and Cu are enriched in the inner rust layers of both the weld and the base metal, forming compounds characterized by compact structures and corrosion-element adsorption effects. Electrochemical analyses indicate that the charge transfer resistance of the weld region is significantly higher than that of base metal.