Discussion on the Analysis of Corrosion Causes in Petrochemical Spiral Wet Gas Cylinders

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This post was last edited by Wang Wei2 on 2020-6-4 at 16:00. Let’s discuss the analysis of the causes of corrosion in petrochemical spiral wet gas holders. Based on the condition of corrosion in these gas holders, it can be seen that the steel plates in the middle section are severely corroded, resulting in localized corrosion and perforations, while the steel plates at the top of the dome are less corroded; this indicates that the degree of corrosion varies depending on whether there is a gas phase or a gas-liquid mixture present. Cracks generally occur at the fusion line and heat-affected zone adjacent to the weld, indicating that stress levels are high in those areas, making them prone to stress corrosion cracking. Corrosion of gas cylinders involves both stress corrosion cracking and uniform corrosion as well as pitting. The originally used corrosion-resistant materials such as chlorosulfonated polyethylene had poor corrosion resistance and a short service life. 1. Damage to the paint film surface: When the surface of the equipment to be protected is coated with alkyd or chlorosulfonated polyethylene coatings, due to the breathability of these coatings, corrosive substances can penetrate through the coating and spread at the metal interface, thereby damaging the adhesion between the coating and the metal surface. At the same time, under sunlight exposure, the paint film, whose film-forming substance is oil, ages; the coating becomes soft and brittle, losing its original mechanical and physical properties, which leads to the destruction of the paint film. When the middle section and bell cover move up and down, and the original coating surface is soft with a high porosity, water seeps through the coating. This water reaches the interface between the paint film and the substrate, reducing the bonding strength between them; as a result, the paint film peels off or bubbles form, which in turn leads to metal corrosion. 2. Metal corrosion under the paint film: Metal corrosion under the paint film is caused by electrochemical processes. At the cathode, oxygen has a depolarizing effect; the reaction is as follows: O2 + H2 + 2e = 2OH-. As a result, the solution inside the bubbles beneath the membrane is slightly alkaline, with the pH value at the cathode area exceeding 13. Once the surface becomes highly alkaline, further alkaline dissolution of the matrix oxide film and alkaline decomposition of the paint film occur. At the same time, the following reaction occurs at the anode: Fe → Fe2+ + 2e-. Fe2+ reacts with oxygen, water, and OH- to form corrosion products such as Fe(OH)2, Fe(OH)3, and Fe2O3·XH2O. The volume of these products increases several times, causing the paint film to bulge and eventually crack. At this time, the solution inside the bubble is acidic, with a pH value of only 2–4. From the cathodic and anodic reactions that occur at the areas where the paint film peels off, the OH- ions generated by the cathodic reaction raise the pH value at the interface, leading to the hydrolysis of Fe2+ ions: Fe2+ + 2H2O = Fe(OH)2 + 2H+. This in turn lowers the pH value at the interface, thereby accelerating the anodic reaction and expanding the area of corrosion as well as the extent to which the paint film peels off. Some surface coatings crack and peel off within less than half a year, causing corrosion of the metal surface. Additionally, due to the repeated lifting and lowering of the gas holder during operation, the surface of the equipment experiences alternating dry and wet conditions. When a metal surface is exposed to air, and a rust layer forms on it, this rust layer acts as a reservoir for water and oxygen. The ionization of the iron in the base material beneath the rust layer functions as a strong oxidizing agent. Under certain conditions, corrosion products can affect the electrode reactions in atmospheric corrosion. Evans believes that the rust layer resulting from atmospheric corrosion, being in a wet condition, can act as a strong oxidizing agent. Based on the Evans model within the rust layer, it can be seen that the anodic reaction occurs at the metal/Fe3O4 interface, while the cathodic reaction takes place at the Fe3O4/FeOOH interface; in other words, a reduction reaction of Fe3+ → Fe2+ occurs within the rust layer, indicating that the rust layer is involved in the cathodic process. Due to changes in weather, the temperature and humidity on the metal surface also change; as a result, under chemical and electrochemical effects, the corrosion of the metal surface accelerates. 3. Corrosion by acidic water: Analysis shows that the gas contains 1% (V/V) of CO2 and H2S gases, with the H2S concentration being greater than 1.08%. After entering the gas tank, the gas continues to be cooled. At night or in winter, saturated gas will cause condensate containing acidic gases such as H2S and CO2 to precipitate on the cold walls, resulting in electrochemical corrosion in the form of an H2S-CO2-H2O liquid film. In addition to causing pitting, it will also lead to stress corrosion cracking. As corrosive gases such as H2S in the gas along with dust continuously enter the water in the sink and water seal, the water becomes an electrolyte solution with strong corrosivity; this corrosiveness increases over time if the water is not replaced. The surface of the water-soaked wall panels is actually covered with a thin film of liquid (H2S-CO2+O2-H2O), allowing gaseous substances to diffuse easily to the steel surface. At the same time, due to the alternation of wet and dry conditions and water evaporation, the corrosive agents in the liquid film become concentrated, thereby exacerbating corrosion on the metal surface. Furthermore, the erosive effect of water causing alternating wet and dry conditions on the coating leads to its premature damage. Therefore, it can be seen that the corrosion is most severe near the water line of the water seal groove. The concentration of H2S dissolved in water is greater than 0.08%, which is almost at saturation. A viscous polymer (dense and airtight) 8–12 mm thick floats on the water’s surface, sealing the tank and preventing H2S from escaping, resulting in an extremely strong odor. The corrosion products on the tower wall are dark brown in color and fall off at the slightest touch. X-ray diffraction analysis shows that its main components are a mixture of sulfides and oxides; Fe9S8 + FeS ≥ 70%, Fe2O3·3H2O ≥ 20%. Analysis of the corrosion products showed that H2S is the main cause of corrosion, as it can induce pitting and stress corrosion cracks. 4. Bacterial corrosion: The upper part of the water surface is in contact with the atmosphere, so the corrosion is mild. Due to the water in the sink not being changed for a long time, the stagnant water at 30–40°C is ideal for the growth of sulfate-reducing bacteria, with cell counts reaching 107–108 per unit. It can reduce sulfides to produce H2S, increasing the concentration of H2S in water and thereby accelerating corrosion on the metal surface. The water at the bottom within the rings under the sink and water seal is actually a layer of stagnant water, containing large amounts of sulfides, organic matter, and dust. Under hypoxic conditions, bacteria reduce sulfides and organic substances, producing large amounts of H2S; this leads to severe pitting and ulcerative corrosion at the bottom of tanks and water seals, with an annual corrosion rate of around 1 mm. Therefore, selecting appropriate protective coatings and using suitable water corrosion inhibitors to improve water quality are issues that need to be addressed urgently.