How does chloride ion corrosion in concrete tanks affect cement or lime?

　Cl- corrosion in concrete is the main cause of steel bar rusting, especially in coastal areas. The Code for Design of Concrete Structures (GB 50010-2002) requires that the maximum Cl- content in concrete be 0.06% (as a percentage of the cement content). The reason is that Cl- has a small radius and high reactivity, allowing it to easily penetrate the passivation layer of concrete and cause corrosion of the steel bars. The resulting Fe(OH)2 decomposes into H2O and FeO with crystalline water, leading to volume expansion and reduced durability. Therefore, detecting the Cl- content in concrete is an important measure to ensure the durability of structures.

Carbonation of concrete is a type of chemical corrosion that affects concrete. The process by which CO2 in the air penetrates into concrete, reacts chemically with its alkaline substances to form carbonates and water, thereby reducing the alkalinity of the concrete, is known as concrete carbonation, or neutralization. The chemical reaction involved is: Ca(OH)2 + CO2 = CaCO3 + H2O. During the hydration process, cement produces large amounts of calcium hydroxide, filling the pores in the concrete with a saturated solution of calcium hydroxide. This alkaline environment provides good protection for the steel rebar, as it causes insoluble compounds such as Fe2O3 and Fe3O4 to form on the surface of the rebar; this layer is referred to as a protective film. Carbonation reduces the alkalinity of concrete; when carbonation penetrates beyond the concrete’s protective layer, in the presence of water and air, the concrete loses its ability to protect the rebar, causing the rebar to start rusting.