Evolution of rust layers and degradation of mechanical properties of Q420B steel in a simulated wet/dry cyclic coastal atmosphere

Abstract

The evolution of rust layers and degradation of mechanical properties of Q420B steel used for the construction of ultra-high voltage (UHV) transmission towers were investigated through accelerated wet/dry cyclic corrosion test (CCT). After 135 cycles of CCT, the corrosion of Q420B low alloy steel was divided into three distinct stages. Initially, the corrosion rate rose, then declined, and finally stabilized, with turning points of 45CCT and 105CCT respectively. Corrosion rates were influenced by the morphology of the rust layer and the evolution of its phases. The internal rust layer containing β-FeOOH, a strong oxidative corrosion product with a tunneling structure, significantly accelerated the corrosion rate of Q420B steel in coastal environments. In the initial corrosion stage, strong oxidizing products facilitated the cathodic process. As corrosion proceeded, the thickening rust layer and increasing α-FeOOH content in the middle to late stages impeded the cathodic/anodic process. Furthermore, the mechanical properties of the specimens showed a significant decline after wet/dry cyclic corrosion. The material failure occurred at the turning point between the second and third stages of corrosion evolution, 105CCT, marked by a 7.55 % weight loss. Elongation was the most susceptible to corrosion, with the degradation patterns of plasticity and strength following linear and quadratic functions, respectively. The inhomogeneous corrosion component had a more pronounced effect on the mechanical properties of the specimen during the CCT. However, the protective rust layer served to moderately slow down the deterioration of mechanical properties. This study provides a reference for predicting the performance degradation of UHV transmission tower components in coastal environments.