Electrochemical Corrosion Mechanisms of Reinforcement in Subway Shield Tunnels Under Coupled Sulfate Corrosion and Stray Current Effects

Shield tunnels are the primary structural form of subway tunnels. Under the influence of sulfate ions and stray currents, the internal bars in shield tunnels will corrode rapidly, affecting the safe operation of the subway. In this paper, a numerical calculation model was developed to simulate rebar corrosion in shield tunnels under coupled electric and chemical field effects, the accuracy of the numerical calculation model was verified using a model test. Based on this model, the migration patterns of sulfate ions within the tunnel under stray current conditions are investigated, as well as the corrosion mechanisms of tunnel rebars. Results show that under the coupled effects of stray currents and sulfate ions, the corrosion area in a segment rebar gradually spreads from the arch waist area of a tunnel. Compared to bilateral leakage, both the migration rate of sulfate ions and the corrosion rate of rebar were higher than those in the unilateral leakage condition. Changes in the input voltage at the top of the ballast did not alter the shape of the distribution curves for the voltage and corrosion current density of segmental rebar. When the input voltage increased from 1 V to 5 V, the voltage at the mid-section of the rebar increased by 5.2 times under a bilateral leakage condition, and the voltage at the mid-section of the rebar increased by 8.2 times under a unilateral leakage condition. Compared to the bilateral leakage case, the amplitude of fluctuations in the curve was significantly reduced under unilateral leakage, but the corrosion current density increased proportionally with increasing leakage voltage in both cases. The research conclusions provide a theoretical basis for the safety assessment and disease treatment of shield tunnel structures.