Corrosion evolution and mechanical property deterioration of Q355NH weathering steel in long-term neutral salt spray environment

To investigate the morphology and mechanical behavior of Q355NH steel after long-term corrosion, neutral salt spray accelerated test were conducted for up to 180 days, with Q355 steel as the control group. By macro and micro observations, the evolution of rust layers and cracking mechanisms was elucidated. The corrosion thickness loss was divided into depth of uniform corrosion and pitting depth, and their relationships with corrosion time were established, respectively. Corrosion kinetic equations were also established, revealing no significant differences between two types of steel when the corrosion period was less than 65 days. However, as the corrosion period exceeded the critical time, Q355NH steel gradually exhibited better corrosion resistance. Additionally, corrosion rate for Q355NH and Q355 steel in various environments were compared, the average thickness loss rate of Q355 steel was revealed higher in intermittent neutral salt spray than continuous neutral salt spray. However, Q355NH steel exhibited the opposite trend due to the formation of the protective rust layer. Degradation equations were developed for the relationships between strengths, strains and mass loss rate in both Q355NH and Q355 steel, and constitutive models were also established for these corroded steels. Furthermore, based on the pitting evolution model, a numerical analysis method for evaluating the mechanical properties of corroded weathering steel was proposed and validated. Through parametric analysis, strength degradation formulas for double-sided non-uniform corrosion were established. It is demonstrated that the pit aspect ratio did not affect the strength. However, the strength significantly decreased when the initial thickness was small. For engineering applications involving uncoated weathering steel, a minimum thickness of 6 mm is recommended, preferably greater than 10 mm.