Morphological evolution and mechanical property degradation of Q355NH weathering steel and Q355 steel under stress corrosion

Steel structures in coastal environments are typically subject to the combined effects of stress and corrosion, which lead to more severe degradation of both morphology and mechanical properties, resulting in a reduced service life compared to corrosion under stress-free conditions. To thoroughly investigate the performance of Q355NH weathering steel under stress corrosion, accelerated neutral salt spray tests were conducted, with stress ratios ranging from 0 to 0.8 and corrosion periods up to 100 days, and Q355 steel was chosen as the control group. The evolution of pitting morphology was observed microscopically, leading to the development of evolution equations for pit size and mass loss rate for both steels. Mechanical performance tests established quantitative relationships between mass loss rate and the elastic modulus, strength, and corresponding strain. When the mass loss rate (η) < 21 %, the failure mode exhibits ductile fracture, but when η > 21 %, it transitions to brittle fracture. When η exceeds 13.46 % for Q355NH steel and 14.44 % for Q355 steel, the elongation no longer meets standard requirements. Furthermore, when η reaches 10.44 % for Q355NH steel and 8.11 % for Q355 steel, the ultimate strength decreases by 9.94 % and 16.01 % respectively, failing to satisfy strength specifications. Using mechanical-chemical theory and Faraday’s law, evolution formulas for mass loss rate and pit depth were derived and validated. It was found that η prediction error falls within 15 % for 95 % of Q355NH steel and 90 % of Q355 steel samples. Based on this theoretical foundation, a numerical simulation method of in-situ pit evolution enabled extensive parametric analysis, providing supplementary insights into mechanical performance under various stress ratios and corrosion periods. A general strength prediction model for Q355NH and Q355 steel under stress corrosion was subsequently developed, offering a theoretical and experimental basis for evaluating the post-corrosion mechanical performance.