Basic Knowledge of Corrosion Prevention in Petrochemical Industry

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I. Basic Concepts 1. Definition of corrosion: Corrosion is the deterioration or degradation of materials caused by their reaction with the environment. More broadly, it refers to all types of damage to a material other than simple mechanical destruction. According to this definition, phenomena such as rubber aging, high-temperature oxidation of metals, swelling or cracking of plastics, wood decay, weathering of granite, and peeling of cement all fall under the category of corrosion. In the petrochemical industry, corrosion is more concentrated in metal equipment and pipelines. 2. Classification of corrosion. There are many methods for classifying corrosion, which are briefly described as follows: 2.1 Classification based on the corrosion failure process: It can be divided into chemical corrosion (such as the high-temperature oxidation of metals, and the aging and cracking of plastics and rubbers under sunlight) and electrochemical corrosion (including corrosion and perforation of heat exchangers, as well as corrosion and leakage in buried pipelines). 2.2 Classified by the corrosion environment, it can be divided into chemical medium corrosion, atmospheric corrosion, soil corrosion, seawater corrosion, etc. 2.3 Classified by the mode of corrosion damage, it can be divided into two main categories: uniform corrosion and local corrosion. Local corrosion is further divided into the following 10 categories: ① Pitting corrosion (also known as spot corrosion) ； ② Pitting corrosion ； ③ copper corrosion ； ④ stress corrosion ； ⑤ intergranular corrosion ； ⑥ Selective corrosion ； ⑦ hydrogen embrittlement ； ⑧ wear corrosion ； ⑨ Bacterial corrosion ； ⑩ Other forms of local corrosion (including: crevice corrosion, under-deposit corrosion, concentration cell corrosion, filiform corrosion, etc.). II. Basic methods of corrosion prevention Based on the definition and mechanisms of corrosion, methods for preventing corrosion should also focus on both materials and the environment. It mainly includes three types of approaches: ① Rational selection of materials (including composite materials) and structural design ； ② Changing the environment (control and treatment of corrosive media, also known as process anti-corrosion measures) ； ③ Electrochemical protection. 1. Proper material selection and structural design: Proper material selection and structural design are the most important methods for corrosion control. The material selection must take into account the actual environment and working conditions; the material with the best corrosion resistance is not necessarily the best choice. The best material selection is a rational combination based on the optimal corrosion resistance between the material and the corrosive medium, as well as the lowest cost and safety performance. For example, stainless steel–nitric acid, carbon steel–concentrated sulfuric acid, Monel–hydrofluoric acid, lead–dilute sulfuric acid, hard rubber–hydrochloric acid, etc., are all optimal combinations. Sometimes, choosing the wrong materials can lead to catastrophic consequences. For example, when exposed to corrosive media such as halide ions and polythionic acids, austenitic stainless steels that generally exhibit good corrosion resistance under normal conditions are very likely to suffer from pitting or stress corrosion, leading to sudden perforation or rupture of equipment. Another cost-effective material selection method is to choose materials with composite structures. For example, anti-corrosion coatings on the surface of steel equipment (including anti-corrosion paints, electroplated layers, chemically plated layers, aluminum infiltration layers, zinc infiltration layers, metal spraying, composite steel plates, rubber linings, fiberglass-reinforced plastic linings, etc.) all exhibit good performance in practical applications. A reasonable structural design is also very important. For example, during design it is necessary to consider ease of cleaning to prevent the formation of dead zones; components that are prone to corrosion should be easy to replace or have their material upgraded. Stress concentration, direct erosion, uneven local temperatures, localized vaporization, and connections between different metals must all be avoided. 2. Modifying the environment (control and treatment of corrosive media, also known as process-based corrosion prevention measures). Common methods for modifying the environment include: ① Lowering the temperature ； ② Reduce flow rate ； ③ Remove corrosive agents or oxidizers (including electrodialysis, deacidification, and addition of neutralizing agents) ； ④ Changing the medium concentration (including the blending of high-sulfur, high-acidity crude oils) ； ⑤ Use corrosion inhibitors (including water treatment chemicals, process corrosion inhibitors, etc.). 3. Electrochemical protection: Electrochemical protection involves using an external current (including impressed current and sacrificial anodes) to keep the corrosion potential of the equipment being protected in a depolarized region, thereby providing protection. It shows significant effectiveness in areas such as anti-corrosion for port facilities, buried pipelines, and oil storage tanks. III. Common corrosion phenomena in petrochemical enterprises and countermeasures 1. Atmospheric corrosion in chemical plants: Due to the large amount of corrosive gases emitted by petrochemical enterprises, corrosion in the plant areas is generally relatively severe. For atmospheric corrosion in the chemical industry, protection is generally achieved by using galvanized metal components (including galvanized iron sheets, galvanized cable trays, galvanized lamp posts, etc.) or coating treatments for corrosion resistance. Commonly used coatings include epoxy resins, alkyd resins, chlorosulfonated polyethylene, chlorinated rubber, vinyl chloride, polyurethanes, fluorocarbon coatings, zinc-rich coatings, and coatings containing aluminum powder. The selection of coating types should be based on a comprehensive consideration of factors such as environmental conditions, application conditions, durability, and cost. 2. Soil corrosion: Due to differences in geological conditions and regions, as well as the interference of stray currents, the corrosion rate can vary by dozens of times. Common effective protection methods include anti-corrosion coatings (including traditional petroleum asphalt, epoxy coal tar pitch, coal tar enamel coatings for glass steel structures, as well as structural anti-corrosion methods such as hot-melt epoxy powders, polyethylene tape, three-layer polyethylene, and rigid polyurethane foam developed in recent years). A detailed comparison of the performance of various anti-corrosion structures is provided in the attached table. Cathodic protection technology can also be applied to key long-distance pipelines. A special lecture on cathodic protection technology. 3. Corrosion in the HCl-H2S-H2O system: This type of corrosion is primarily caused by the hydrolysis of salts contained in crude oil (such as CaCl2 and MgCl2), which produces hydrochloric acid; together with H2S, which is a product of the decomposition of sulfur-containing substances in crude oil, these factors create an acidic environment that facilitates corrosion. It mainly occurs at the top of atmospheric pressure columns and the vaporization line, air coolers, tower top coolers, etc., and is a typical case of uniform corrosion in the liquid phase region. Common anti-corrosion measures include process-related anti-corrosion measures such as “one desalting and three injections” (electrostatic desalting, water injection into the overhead vapor line, ammonia injection, and corrosion inhibitor injection), as well as coating applications on heat exchangers for anti-corrosion purposes. Composite-lined stainless steel (carbon steel + 0Cr13) is also sometimes used. The effectiveness of electrodesalination is the most critical factor determining the anti-corrosion performance. It is generally required that the salt content after secondary electrodialysis be below 2.5 mg/L. 4. Wet hydrogen sulfide corrosion: In secondary processing units and storage and transportation systems, the decomposition of sulfides in crude oil generates large amounts of H2S. In the presence of water, this can easily lead to hydrogen blistering of equipment materials as well as stress corrosion cracking caused by hydrogen sulfide under ambient temperature conditions. The areas prone to problems are the absorption and stabilization system of the catalytic unit and the liquefied gas storage tanks. In particular, high-strength steel materials are more prone to cracking in such environments. Common protective measures include: ① Keeping the hydrogen sulfide concentration below 50 ppm ； ② Control pH value to be greater than 7 ； ③ Remove moisture ； ④ Raise the temperature to above 65°C ； ⑤ Control the concentrations of harmful ions such as Cl-, CO32-, CN- in the medium ； ⑥ Eliminate residual stresses in materials, including avoiding cold working and controlling welding residual stresses ； ⑦ Control the composition and hardness of metal materials, including selecting steel with a low sulfur content and performing post-weld heat treatment to ensure a hardness below HB200. 5. HCN-H2S-H2O corrosion system: This type of corrosion primarily occurs in the reaction systems of catalytic units. Sulfides and nitrogen-containing substances in the feedstock decompose to form HCN and H2S, leading to uniform corrosion of steel, hydrogen blistering, and sulfide stress corrosion cracking. This type of corrosion is characterized by the appearance of Prussian blue (ferricyanide of iron) on the surface of the material. Common protective measures include using water washing to reduce concentration, as well as employing chromium-molybdenum steel, ferritic stainless steel, etc. The use of austenitic stainless steel must be strictly restricted. 6. High-temperature sulfur corrosion and naphthenic acid corrosion mainly occur in areas such as heating furnaces, the bottoms of fractionation towers, transfer lines, heat exchangers, etc., in coking, vacuum distillation, and catalytic cracking units that process high-sulfur crude oil. The corrosive agents include elemental sulfur, hydrogen sulfide, thiols, naphthenic acids, etc. The corrosion rate is highest at 380°C. The control measures mainly involve raising the material grade by using chromium-molybdenum steel, austenitic stainless steel, etc. Reducing the medium flow rate is also very effective. 7. Sulfuric acid dew point corrosion occurs mainly in the low-temperature areas of the flue gases in heaters and boilers that use sulfur-containing fuels. It is liquid-phase acid corrosion that occurs when the sulfurous acid and sulfuric acid, which are formed as a result of burning sulfur-containing fuels, come into contact with metal surfaces at lower temperatures. This is a very severe form of corrosion; it not only corrodes metal components such as furnace tubes, preheaters, heat pipes, tube sheets, and insulation pins, but also has a strong corrosive effect on refractory castables and fibrous materials. It is a highly destructive type of corrosion. The most fundamental control method is to desulfurize the fuel, ensuring that its sulfur content does not exceed the specified limits, thereby ultimately keeping the sulfur content in the flue gases within acceptable levels. Secondly, increase the flue gas temperature so that the wall temperature of the furnace tubes or furnace walls is higher than the dew point temperature of the flue gas. However, this will sacrifice some thermal efficiency. Thirdly, steel grades resistant to sulfuric acid dew point corrosion should be selected, including ND steel and others. 8. Pyrosulfuric acid corrosion: Pyrosulfuric acid (H2SxO6) is formed as a result of the reaction between FeS adhering to the surface of equipment during shutdown periods, along with water and oxygen. It is a medium that promotes stress corrosion in austenitic stainless steel. It most commonly occurs on austenitic stainless steel used in the processing of sulfur-containing crude oil, and frequently appears on stainless steel equipment and furnace tubes in units such as hydrorefining, hydrocracking, and catalytic reforming. This type of corrosion mainly occurs during the downtime for equipment maintenance. Corrosion cracking is characterized by intergranular cracking, and it most frequently occurs near welds or in areas that have been exposed to stress at elevated temperatures. The control method follows RP0170—97 \"Protection of Austenitic Stainless Steels and Other Austenitic Alloys Used in Refining Equipment Against Sulfenic Acid Stress Corrosion During Downtime\", issued by the National Association of Chemical Engineers (NACE). During downtime, nitrogen purging and drying as well as alkaline cleaning are carried out; meanwhile, austenitic stainless steels that contain stabilizing elements such as Ti and Nb, or those belonging to the ultra-low carbon category, must be used. 9. Circulating water corrosion: The corrosion in circulating water systems is a problem with wide-ranging implications. The most important factor affecting the corrosion of circulating water is water quality. Factors such as high salt content, high hardness, high oxygen levels, and high microbial counts all increase corrosivity. Controlling the corrosion of equipment and pipelines in circulating water systems mainly involves two aspects: water quality control and anti-corrosion measures for equipment and pipelines. Water quality control primarily involves controlling indicators such as the hardness, temperature, pH value, and microorganisms of the circulating water by adding various chemical reagents and performing necessary wastewater discharge, thereby keeping corrosion within acceptable limits. Practice has shown that depending on the quality of water quality control, the corrosion rate can vary by more than a hundredfold. Measures for equipment corrosion prevention include using corrosion-resistant materials, applying anti-corrosion coatings and platings for isolation, and utilizing sacrificial anodes for electrochemical protection. IV. Anti-corrosion standards related to petrochemical enterprises 1. Technical specifications for anti-corrosion coating of petroleum and chemical industry equipment and pipelines SH 3022 2. Specifications for construction and acceptance of anti-corrosion projects for industrial equipment and pipelines HGJ 229 3. Specifications for construction and acceptance of polyethylene adhesive tape anti-corrosion coatings on buried steel pipelines SY 4014 4. Specifications for construction and acceptance of epoxy coal tar pitch anti-corrosion coatings on buried steel pipelines SY/T 0447 5. Technical standards for polyethylene anti-corrosion coatings on buried steel pipelines SYT 0413 6. Technical standards for liquid epoxy coatings as internal anti-corrosion layers in steel storage tanks SYT 0319 7. Technical standards for chlorosulfonated polyethylene external anti-corrosion layers in steel storage tanks SYT 0320 8. Technical standards for coal tar enamel external anti-corrosion layers on buried steel pipelines SYT 0379 9. Technical specifications for anti-corrosion engineering of steel oil storage tanks GB 50393