Corrosion Behavior of Q420qNH Plate and Welded Joint in Deicing Salt Medium by Cyclic Immersion + Infrared Drying

The corrosion behavior of Q420qNH steel plate and welded joint in deicing salt medium is investigated by cyclic immersion + infrared drying accelerated corrosion experiments. The results show that the corrosion products are mainly Fe2O3 and FeOCl under the influence of Cl− concentration differential corrosion microcells in the immersion corrosion anoxic stage, and the corrosion products are mainly β-FeOOH, γ-FeOOH, and Fe3O4 under the influence of oxygen concentration differential corrosion microcells catalyzed in the infrared drying stage. Throughout the corrosion process, the corrosion weight gain and average corrosion rate of the welded sample are always lower than that of the plate sample. The Iα-FeOOH/Iβ-FeOOH + FeOCl peak intensity ratio of the welded sample is always higher than that of the plate sample and the heat-affected zone (HAZ) sample. The corrosion types of the three samples are inhomogeneous total corrosion + pitting, and the depth of the pits in the plate is much larger than that in the weld and the HAZ. In the late stage of corrosion, the self-corrosion current density is: plate > HAZ > weld, and the resistance of the rust layer of the weld and HAZ samples is 1.66 times and 1.31 times that of the plate sample, respectively. This is because the microstructure of the weld and HAZ are composed of a large number of fine acicular ferrite and lath-shaped bainite generated inside the original austenite grains, and the small angle grain boundary is not conducive to the initiation and expansion of pitting corrosion. In addition, the higher content of Cu, Cr, Ni, and other elements in the weld can refine the rust layer products and effectively impede the intrusion of Cl−, contributing to corrosion resistance improvement. As the combined effect of Cl− and immersion anoxic environment inhibits the formation of α-FeOOH and promotes the formation of a large number of loose and unstable β-FeOOH and soluble FeOCl, the weathering steel is not well protected by the rust layer during cyclic immersion corrosion in deicing salt medium.