Corrosion and Protection of Refinery Circulating Water Towers

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Corrosion and Protection of Circulating Water Towers in Refineries 1. Corrosion Conditions and Cause Analysis of Cooling Towers A concrete cooling tower at a certain refinery was put into use in the 1970s; due to the effects of chemical-rich air and water vapor, the surface of this cooling tower suffered severe weathering, cracks appeared on the concrete surface, and in some areas of the tower columns, the reinforcing bars became exposed. Some surfaces are covered with microorganisms. It affects the service life of the equipment. Based on years of observation, the causes of corrosion in cooling towers are as follows: (1) The acidic substances present in industrial air react with tricalcium aluminate and calcium hydroxide, which are present in the solidified products resulting from cement hydration, to form salts that are somewhat soluble. As these corrosive substances continue to be supplied and water is lost through evaporation, it leads to the destruction of the structural components. ⑵ When it rains, CO2 in the air reacts with water to form H2CO3, which gets trapped in the pores of the concrete. However, H2CO3 in water can also react further with CaCO3 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 Ca2CO3, thereby causing damage to the concrete. ⑶ The circulating water has a calcium carbonate composition; when the cooling tower is in operation, some water vapor is carried away by the fans and sprayed onto the outer wall at the top of the tower. When CaCO3 in water is at saturation, there is a capacity to continue dissolving CaCO3 as well as the carbonates in concrete, resulting in a decrease in the strength of the concrete. ⑷ Algae cover the inner surface of the cooling tower and the lower surface of the cooling tower columns. Due to the bacteria produced within algal cells, anaerobic sulfate-reducing bacteria often parasitize beneath aerobic sulfur bacteria. Their reproduction leads to the corrosion and damage of concrete structures. 2. Criteria for selecting anti-corrosion materials: Based on the degree of corrosion and damage in the cooling tower, a selection is made through evaluation. Treatment is carried out using a composite layer of cyanide cement and chlorosulfonated polyethylene coating. Cyanopolymer cement is a chemical slurry composed of polyurethane (prepolymer) along with various additives such as solvents, plasticizers, catalysts, surfactants, and fillers. A prominent feature of this coating is that it undergoes a chemical reaction as soon as it comes into contact with water, resulting in the formation of an insoluble solid. It boasts high stability, resistance to acids and alkalis, resistance to mold, wear resistance, aging resistance, and a long service life. Due to the high humidity of the air around water towers, film-forming cyanide-based coatings offer good film-forming properties, a short curing time, high-quality coatings, a brief construction period, and excellent anti-corrosion and leak-sealing effects. Chlorosulfonated polyethylene coatings exhibit good resistance to corrosion in chemical atmospheres, excellent film-forming properties, high density, and strong adhesion to cyanide-based coatings. Therefore, it is feasible to use cyaninated fiberglass reinforced plastic as a reinforcement layer for the outer wall of the cooling tower, and chlorosulfonated polyethylene coating as a sealing layer, as materials for preventing corrosion and sealing leaks in concrete cooling towers. 3. Corrosion protection construction of the tower: Before construction, the concrete surface is cleaned to remove oil, mud, dust, and other contaminants, followed by washing with water. The crack opening should be enlarged so that the entire crack exposes the fresh concrete surface; it should be washed with water, dried, and then the crack filled with cyanacrylate putty. All the pulverized layer that had detached from the tower structure was removed to expose the concrete surface; thereafter, the exposed rebar was cleaned and smoothed with cement mortar. Construction is carried out after the tower surface is dry. The construction process is as follows: surface preparation—application of cyanate-based primer—application of two layers of fiberglass cloth—two coats of cyanate-based paint—three coats of chlorosulfonated polyethylene paint. When cyanide-based coatings come into contact with water, secondary penetration occurs, increasing the porosity and affecting the quality of the construction. Therefore, during construction, avoid contact with moisture; work should not be carried out in high humidity, or measures must be taken before working. 4. Condition of use: The 4 concrete cooling towers at the refinery have been in use since anti-corrosion and leak-sealing work was carried out on them in 1987; to date, there is no signs of peeling or bulging, and they are in good working condition. Practice has shown that the combined use of cyanide cement and chlorosulfonated polyethylene coatings has the following characteristics: ① Strong adhesion to concrete surfaces and high strength. ②The coating has good sealing properties. ③It has good durability. ④The combination of cyanate curing and chlorosulfonated polyethylene coatings enhances the rigidity of the entire concrete surface. It compensates for the microporosity of cyanic curing coatings.