A brief discussion on the causes of damage to anti-corrosion coatings in concrete structures exposed to industrial wastewater

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A brief discussion on the causes of damage to anti-corrosion coatings on concrete structures used in industrial wastewater treatment 1. Causes of damage to fiberglass-reinforced plastic materials: Initially, for anti-corrosion protection in concrete tanks (in outdoor, open environments), epoxy or epoxy-coated fiberglass-reinforced plastic materials were commonly used. When these materials are used alone over time, their surfaces age, crack, and separate from the underlying substrate, resulting in a loss of their anti-corrosion functionality. This is mainly because epoxy resins contain ether bonds, and the paint film tends to degrade and break down when exposed to sunlight and ultraviolet rays. As a result, fiberglass products made with epoxy resins cannot withstand outdoor exposure to sunlight; the paint film loses its luster, becomes dusty, and loses its functionality. 2. Damage due to the use of a composite protective layer: To address the aging issue of epoxy fiberglass, three coats of chlorosulfonated polyethylene paint were applied to its surface. Although this approach temporarily solved the aging problem of the fiberglass, problems still arose after some time of use. The main issues were blistering and cracking of the chlorosulfonated polyethylene layer, followed by large-scale peeling of the surface layer, which meant that it could no longer serve as an anti-aging layer. Since chlorosulfonated polyethylene paint is of the solvent-type variety, and its base material is a thermoplastic (non-transforming) resin, the solvents contained in the subsequent coats would dissolve the surface layer of the previous coat, causing the layers to merge together. Once the solvents evaporated, the two layers became one, resulting in good adhesion between them. However, the epoxy-based fiberglass layer is of the swelling type; that is, the binder of the underlying paint film is thermosetting (conversion-type), and after the paint film dries, a three-dimensional structural framework is formed that is insoluble in the original solvent. The solvent used in the second coating cannot dissolve the surface layer, but only cause it to swell. For this material itself, with proper construction, it can fully become a three-dimensional network structure. Although the two aforementioned materials each have their own advantages, when combined – with the fiberglass body serving as the impermeability barrier and chlorosulfonation acting as the anti-aging agent – they offer even better performance. It seems reasonable on the surface, but there are many problems in actual use. The reason is that the former is of the swelling type, while the latter is of the melting type. However, there are certain issues at the interface between the two; it does not belong to either the swelling type nor the melting type. So, in a sense, they are simply combined together without forming a tight three-dimensional network structure. Therefore, the chlorosulfonated layer finds it difficult to penetrate between the molecules in the surface layer of the epoxy paint, resulting in poor adhesion between the layers. Furthermore, although polyvinyl chloride sulfonate coatings are stated in their product specifications to have good water resistance, their performance in sewage is poor; in other words, they suffer from the problem of \"swelling due to water absorption\". When this material operates under conditions of prolonged exposure to water, it absorbs water and swells; the resulting stress exceeds the adhesion force, leading to bubbling. At this point, the epoxy fiberglass layer does not suffer from corrosion or any changes, and this phenomenon is attributed to the fact that the stress caused by volume expansion is greater than the interlayer adhesion force, resulting in bubbling. Therefore, when using swollen-type and melted-type coatings together, it is important to be careful in certain situations and to choose the combination wisely. 3. Damage to the fiberglass body and concrete surface: When the anti-aging layer and the fiberglass body are partially damaged, wastewater comes into direct contact with the concrete surface, remaining between the concrete and the fiberglass body. Due to the presence of wastewater, the pH value of most water is acidic; whereas concrete, containing Ca(OH)2, is highly alkaline. However, when water contains harmful impurities such as CO2 and SO2, it neutralizes the alkalinity of concrete, turning it into water-soluble aragonite, which is gradually lost due to water erosion. This lowers the pH value of the concrete, renders the surface pulverization effect ineffective, and increases its volume, resulting in extensive damage to the surface of the fiberglass composite. Therefore, choosing an appropriate protective layer to address the aging issue of the fiberglass surface is a complex task. The anti-corrosion coating that should be chosen must be able to resist corrosion from sewage as well as atmospheric aging – this is the coating we need to select.