Causes and processes of corrosion in concrete equipment and structures in industry
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Concrete is made from cement, coarse aggregate, fine aggregate, water, and other materials. In composite materials, Ca(OH)2 in the cement paste is the most susceptible part to corrosion in concrete. When acidic substances cause corrosion, water-soluble reactants are produced, and the set cement begins to dissolve from the surface, resulting in soluble corrosion. During corrosion, a large amount of practically insoluble reaction products are generated within the hardened cement, causing volume expansion and internal stresses in the concrete, thereby leading to expansive corrosion of the concrete. Acidic gases penetrate into concrete and react with free Ca(OH)2 in the cement to form salts. These salts then react with calcium aluminate hydrate to produce hydrated salts, which have a volume much larger than that of the original compounds. This leads to the expansion and deterioration of the concrete. Furthermore, in concrete, the decrease in the concentration of Ca(OH)2 solution in the cement paste (i.e., the reduction in the alkalinity of the concrete) leads to the destruction of the protective layer surrounding the rebar, causing the rebar to rust. After rusting, steel bars expand in volume, generating outward destructive stresses on the concrete cover from within, causing the cover to blister, peel off, and powder.
Concrete in industrial environments often corrodes due to chemical and physical factors. Firstly, when external acidic substances (such as carbon dioxide, sulfides, etc.) react with the free calcium hydroxide in concrete, soluble salts are formed. These salts can then react with other components to produce hydrated salts of larger volume, thereby causing the concrete to expand and crack. Secondly, as the concentration of calcium hydroxide decreases, the alkalinity of the concrete drops, and the passivation film that originally protected the rebar is destroyed, causing the rebar to start rusting. Rusted rebar expands in volume, exerting pressure on the interior of the concrete, which ultimately leads to cracking, spalling, or pulverization of the concrete. Therefore, the durability of concrete is closely related to its environment and maintenance methods. .