The interdependent roles of chloride, moisture and dissolved oxygen in macro-voids at the steel-concrete interface in corrosion

Corrosion of steel in concrete is a key durability issue for reinforced concrete structures, leading to significant economic costs and safety risks. Research suggests that moisture within voids at the steel-concrete interface (SCI) plays a critical role in corrosion. However, experimental evidence clarifying this role remains limited. Here, we systematically investigated the influence of macro-voids at the SCI by exposing reinforced mortar specimens to different wet-dry cycles and to continuous submersion in chloride solution, thereby generating distinct moisture conditions within the interfacial voids. X-ray computed tomography (XCT) was used to monitor the evolution of the gas-liquid configuration inside these voids along with steel corrosion over 9 months. Under wet/dry cycles, corrosion initiated at lower chloride levels than under continuous immersion. XCT revealed that interfacial voids became fully saturated after prolonged immersion but remained partially saturated during wet/dry exposure. These findings are interpreted through the lens of corrosion science and gas-liquid interactions in porous materials. We suggest a mechanism in which the dissolution of pressurized gas bubbles trapped within interfacial voids leads to an increase in dissolved oxygen concentration near the void, thereby locally elevating the steel corrosion potential ( E corr ). Together with chloride accumulation, which gradually lowers the pitting potential, this shift in E corr enhances the likelihood for local corrosion initiation under wet-dry cycles. Overall, this study contributes to understanding how different water exposure conditions affect the local environment at the SCI, providing insight into the mechanistic links between interfacial void gas-liquid content and corrosion in reinforced concrete. • X-ray imaging tracks steel corrosion and moisture evolution in concrete voids. • Cyclic exposure maintains partially saturated voids at the steel-concrete interface. • Wet/dry cycling starts corrosion at lower chloride conc. than continuous immersion. • Interfacial voids promote the formation of conditions favorable for corrosion. • Dissolution of pressurized gas bubbles in voids plays a critical role in corrosion.