What is the main deterioration mechanism caused by chloride ions in reinforced concrete?

The main deterioration mechanism caused by chloride ions in reinforced concrete is the corrosion of the reinforcing steel. Reinforced concrete relies on the embedded steel bars to provide tensile strength. Under normal conditions, the high alkalinity of the concrete (pH around 12.5 to 13.5) creates a passive layer of iron oxide on the surface of the steel, protecting it from corrosion. This passive layer is stable in a high-pH environment. However, when chloride ions penetrate the concrete and reach the steel, they disrupt this passive layer. Chloride ions are very small and mobile and can migrate through the concrete's pore structure. Once the chloride concentration exceeds a certain threshold at the steel surface, the passive layer breaks down, and the steel begins to corrode. The corrosion process is an electrochemical reaction that produces rust (iron oxide). Rust occupies a significantly larger volume than the original steel. This expansion creates internal pressure within the concrete, leading to cracking, spalling, and delamination. The loss of steel cross-section due to corrosion reduces the load-carrying capacity of the reinforced concrete structure. Chloride ions can enter the concrete from various sources, including deicing salts, seawater, and chloride-containing admixtures. For example, bridges exposed to deicing salts in winter are highly susceptible to chloride-induced corrosion. To mitigate this, strategies include using low-permeability concrete, applying protective coatings, using corrosion inhibitors, and employing stainless steel or epoxy-coated rebar.