The inner wall of the concrete tank should be fully coated with an anti-corrosion layer

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This post was last edited by Wang Wei2 on 2019-8-26 at 10:00. Concrete tanks need to have an anti-corrosion coating applied to their entire surface. Recently, a colleague in equipment management discussed with me the idea that in some industrial concrete sewage tanks, corrosion occurs mainly in the gas phase; therefore, it is sufficient to apply an anti-corrosion coating only to that area. However, through analysis, I believe that overall corrosion prevention can achieve the desired results. Our equipment managers often hold such a misconception regarding industrial wastewater tanks, especially those with concrete structures; they think that for such a tank, as long as proper corrosion protection is provided in the gas phase area and in the zone where the gas phase meets the liquid phase, it will be sufficient to protect it from the corrosion caused by industrial wastewater. Actual use has shown that such a method has problems. The reasons are as follows: 1. For concrete tanks used for industrial wastewater, corrosion occurs in both the gas phase and the liquid phase. 1.1 Causes of corrosion of concrete by gases ⑴ In humid industrial atmospheres, acidic substances react with the free tricalcium aluminate and calcium hydroxide hydrates in cement to form soluble salts. Due to the continuous erosion by these corrosive substances, along with the loss of water, the components are damaged. ⑵CO2 in the air reacts with water to form H2CO3, which gets trapped in the pores of concrete. H2CO3 and CaCO3 in the water then react further to produce Ca(HCO3)2, which is soluble in water; this substance gradually dissolves and carries away the useful components within the concrete. The Ca(HCO3)2 that is dissolved is replaced by solid CaCO3, thereby causing damage to the concrete. ⑶The sulfates of sodium, calcium, and magnesium present in water undergo chemical reactions with the aqueous phase of concrete that contains hydrated lime and hydrated alumina, thereby producing calcium sulfate and calcium aluminosulfate respectively. Volume expansion after the reaction leads to the destruction of the concrete. The above-mentioned factors interact and accumulate with one another, significantly accelerating the rate of corrosion and leading to more severe damage. 1.2 Causes of surface corrosion in liquid-phase concrete: Sewage containing sulfur and harmful substances has a large amount of organic matter; when sulfides come into contact with oxygen or ozone, oxidation reactions occur. The oxidation of sodium salts and ammonia salts produces acidic thiosulfates, which cause severe corrosion of concrete. Several phenomena can be observed: (1) Colloid dissolution: When sewage comes into contact with concrete, the colloids in the concrete dissolve. First, calcium hydroxide is dissolved and carried away, resulting in cracks; as a result, the concrete’s water resistance gradually decreases until it eventually gets damaged. ⑵ Crystalline expansion: When subjected to the erosion caused by wastewater, the cement paste and aggregates in concrete undergo chemical reactions. Some of the products of these reactions dissolve in water, while other crystalline substances fill the pores that are created as a result. This leads to stress on the inner walls of those pores, thereby causing the concrete to crack rapidly. 　　 ⑶ Expansion of rust: In the case of concrete cover layers that are saturated with water, although the thickness of such cover layers is small, corrosion causes the surface to peel and flake off. The corrosive water comes into contact with the rebar, triggering an electrochemical reaction that results in the formation of large amounts of rust. The increasing volume of rust causes the concrete to swell continuously, thereby leading to more severe damage to the concrete. 2. The siphoning effect of concrete can damage the anti-corrosion coating. In the case of concrete tanks, protection is only provided against gaseous substances, not against liquid substances, which can lead to localized damage of the anti-corrosion coating. The reason is that when there is no isolation layer in the liquid-filled area, concrete being a porous material results in many pores throughout its structure. If water is used to wet the concrete surface, more water will penetrate into the concrete, increasing its moisture content. It is as if, during use of the concrete tank, some wastewater seeps into the surface of the concrete, and under siphonic action, it moves toward the concrete surface that has an anti-corrosion coating. When the anti-corrosion coating is subjected to erosion by sewage, it leads to a decrease in the adhesion of the coating, bulging of the coating, and ultimately its failure. Therefore, for concrete tanks, not only the gas-liquid transition zone but also the liquid part needs to be protected against corrosion, and the entire structure must be properly sealed.