Corrosion of pressure vessels and its prevention

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3.jpg Corrosion and anti-corrosion of pressure vessels. Classification of metal corrosion. Main factors affecting metal corrosion. Prevention of corrosion. Classification of metal corrosion. Operating conditions of pressure vessels: high temperature, high pressure, wear, and corrosion by the medium. Among these, corrosion poses significant hazards, and its mechanisms are complex. Even for the same material in the same medium, different forms of corrosion can occur due to changes in internal or external conditions such as the material’s microstructure, the stresses it is subjected to, as well as the temperature, concentration, and pressure of the medium. 1.1 Classification of metal corrosion: A. By temperature: low-temperature corrosion and high-temperature corrosion; B. By corrosive environment: corrosion in chemical media, seawater corrosion, soil corrosion ； C Wet corrosion: Occurs in the presence of liquid, such as in aqueous solutions or electrolytes ； D Dry corrosion: no liquid phase or above the dew point; the corrosive agents are steam and gases ； Classified by the mode of metal corrosion and failure: ① General corrosion ② Galvanic corrosion ③ Pitting corrosion ④ Crevice corrosion ⑤ Selective corrosion; Classification of metal corrosion ⑥ Intergranular corrosion ⑦ Wear corrosion ⑧ Stress corrosion ⑨ Corrosion fatigue ⑩ Hydrogen damage. Classified by the principle of corrosion: Chemical corrosion and electrochemical corrosion; 1.2 Chemical corrosion of metals 1.2.1 Chemical corrosion: It is a type of metal corrosion that occurs as a result of a chemical reaction between the metal and its surrounding environment. No electric current is generated during this corrosion process, and it mainly includes the corrosion of metals in dry or high-temperature gases, as well as in non-electrolyte solutions. Examples include high-temperature oxidation, high-temperature sulfidation, carburization and decarburization of steel, and hydrogen corrosion ； 1.2.2 Example: In petroleum refining, conversion furnaces and cracking furnaces – high-temperature oxidation acting on the outer walls of the furnace tubes ； B The inner wall of the tubes in the cracking furnace is affected by carburization caused by hydrocarbons. C The sulfur trioxide generator used in sulfuric acid production is subjected to sulfidation due to high-temperature sulfur trioxide. Classification of metal corrosion. D The absorption tower suffers severe corrosion from high-temperature hydrogen sulfide, resulting in hydrogen embrittlement and hydrogen bulging ； 1.3 Electrochemical Corrosion 1.3.1 Electrochemical corrosion refers to the corrosion of metals in an electrolyte solution, which occurs as a result of electrochemical reactions taking place on the metal surface. For example, when a carbon steel specimen and a graphite specimen are placed in a container containing seawater, and wires are used to connect them to a milliammeter, an electrochemical cell is formed. Current flows through the milliammeter; the carbon steel has a lower potential and acts as the anode, where oxidation reactions occur and electrons are lost. Graphite has a higher potential and acts as the cathode, where reduction reactions occur and electrons are gained, causing iron to be converted into iron ions and thus leading to corrosion. Various steels contain different types of impurities and carbides; when these come into contact with an electrolyte, the impurities have a higher potential and act as the cathode, while iron has a lower potential and acts as the anode, resulting in microcellular corrosion ； Classification of metal corrosion 1.3.2 Examples: A When the metal surface is scratched, the scratched area acts as the anode; B When there is uneven stress, the area with higher stress acts as the anode; C When there are pores on the surface, the metal inside those pores acts as the anode; D At grain boundaries, the grain boundary itself acts as the anode; E At welds and the base material, the weld area acts as the anode; 1.4 High-temperature oxidation: The process in which metal reacts with its surrounding environment at high temperatures to form metal oxides. Carbon steel develops an oxide film at air temperatures of 200–300°C, and the oxidation rate increases significantly at 800–900°C ； 1.5 Carburization and Decarburization of Steel 1.5.1 Carburization: This process occurs when certain carbides (such as CO and hydrocarbons) come into contact with steel at high temperatures; they decompose to release free carbon, which penetrates through the steel’s surface oxide layer to form carbides. The products of carburization are low-strength, brittle materials such as oxides, carbides, and graphite. For example, in the petrochemical industry, the inner surfaces of cracking furnace tubes are carburized by methane at temperatures above 800°C. Classification of metal corrosion 1.5.2 Decarburization: This occurs when cementite (Fe3C) in steel reacts with gases at high temperatures, such as oxygen and hydrogen, resulting in the decomposition of cementite and thereby a decrease in the hardness and strength of the steel surface ； 1.6 Hydrogen Corrosion 1.6.1 Hydrogen corrosion: When steel is exposed to high-temperature, high-pressure hydrogen, its microstructure changes, which results in a decrease in the steel’s strength and ductility, with the fracture pattern exhibiting brittle characteristics ； 1.6.2 Mechanism: It is mainly due to hydrogen molecules diffusing to the high-temperature surface of steel, where they decompose into atoms that are then absorbed. Hydrogen atoms, with their small diameter, can easily diffuse into the interior of the steel, where methane is formed. Methane has poor diffusion capabilities, so it accumulates over time, leading to local high pressures and stress concentrations that can result in cracks. The conditions for hydrogen corrosion include a starting temperature of 220°C and a starting hydrogen partial pressure of around 1.4 MPa. Below these values, the rate of hydrogen corrosion is extremely slow; and below the starting hydrogen partial pressure, no serious hydrogen corrosion occurs, even at high temperatures, with only surface decarburization taking place ； Main factors affecting metal corrosion 2.1 Influence of metal and alloy composition: Inclusions in metals can accelerate their corrosion; impurities such as sulfur, oxygen, and hydrogen can damage the protective layer on the metal surface. The corrosion rate of an alloy is closely related to its composition – for example, in Fe-Cr alloys, an increase in the Cr content significantly enhances the alloy’s corrosion resistance ； 2.2 Effects of deformation and stress: The high internal stresses resulting from cold and hot working accelerate the corrosion process; in the presence of substances such as hydrogen sulfide, this can even lead to stress corrosion cracking. 2.3 Effects of medium composition and concentration: Most metal materials have a certain range of compatibility with corrosive media. For example, carbon steel dissolves rapidly in dilute sulfuric acid but remains stable in concentrated sulfuric acid; stainless steel is resistant to corrosion in medium- and low-concentration acids, but not in concentrated acids ； 2.4 Effect of temperature: An increase in temperature accelerates electrochemical corrosion; 2.5 Effect of pressure: An increase in pressure leads to a higher gas concentration, which in turn speeds up metal corrosion; 2.6 Effect of flow rate: An increase in flow rate causes the corrosion products to detach or erodes the protective film, and very high flow rates can also lead to wear corrosion, all of which accelerate the rate of metal corrosion. Prevention of Corrosion 3.1 General Corrosion of Metals and Its Prevention 3.1.1 General corrosion (uniform corrosion): A relatively uniform chemical or electrochemical reaction that occurs on the entire surface or most of the surface of a metal, such as the rusting of steel in the atmosphere; 3.1.2 Methods of prevention: (1) Selecting appropriate metals for specific corrosive environments; (2) Adding corrosion inhibitors to the corrosive medium, but this is applicable only to certain metals, environments, temperatures, and concentrations ； ⑶Cathodic protection, such as: preventing corrosion of heat exchanger cooling water and underground pipelines by soil. ⑷ Anodic protection: such as: protecting equipment like concentrated sulfuric acid coolers. ⑸ Applying corrosion-resistant coating layers ； Prevention of corrosion 3.2 Crevice corrosion and its prevention 3.2.1 Crevice corrosion: Local corrosion occurs in crevices and concealed areas due to the accumulation of small amounts of stagnant solution; it often appears in holes, the bottom surfaces of gaskets, as well as in the gaps under joints, screws, and rivet heads ； Typically, the width of the narrow slit is 0.1 mm; 3.2.2 Prevention measures: ① Ensure that all liquid is removed to prevent dirt from accumulating; ② For the connection between the heat exchanger tube sheet and the tubes, use a butt welding technique to reduce gap corrosion between the tubes and the tube sheet; ③ Prefer a fully welded structure for the container to avoid the formation of internal pores and gaps; 3.3 Pitting corrosion and its prevention 3.3.1 Pitting corrosion: Spot-like corrosion that occurs dispersedly on the metal surface and at considerable depths; 3.3.2 Prevention measures: ① Choose materials resistant to pitting corrosion, such as titanium, nickel, chromium, molybdenum alloys, etc ； ②Add a corrosion inhibitor ； Prevention of Corrosion 3.4 Intergranular Corrosion and Its Prevention 3.4.1 Intergranular corrosion: It occurs at and near grain boundaries, and is caused by impurities on those boundaries or excessive or insufficient amounts of certain alloying elements. It leads to a significant reduction in mechanical strength and elongation. For example, it occurs in chromium-nickel austenitic stainless steels; when cooled slowly at high temperatures or held at sensitization temperatures (450–850°C), the chromium content in the areas near the grain boundaries decreases, resulting in chromium depletion. When the chromium content drops below 12%, the chromium-depleted areas become active, and the grains form a galvanic cell, with the grain boundaries acting as the anode and the grains as the cathode, thereby causing corrosion in those areas. Therefore, when welding austenitic stainless steels, it is necessary to strictly control the welding current and the number of reweldings in order to minimize heat input, as the heat-affected zone of the weld often falls within the sensitization temperature range. Prevention of Corrosion 3.4.2 Preventive measures: ① High-temperature solution treatment; ② Use of stabilizing alloying elements (Ti, Nb, etc.) ； ③Ultra-low carbon alloys are used, with a carbon content in the steel of less than 0.03%; 3.5 Wear and Corrosion and Their Prevention 3.5.1 Wear and corrosion: A phenomenon in which the relative movement between the corrosive medium and the metal surface accelerates the corrosion process. 3.5.2 Prevention methods: ① Use materials resistant to corrosion; ② Avoid sudden changes in the cross-section of flow channels and in the direction of flow; ③ Change the corrosive environment: deoxidize the fluid medium, add corrosion inhibitors, and remove solid particles from the medium; 3.6 Stress Corrosion and Its Prevention 3.6.1 Stress corrosion: A brittle failure that occurs when a material is subjected to tensile stress along with the action of a specific corrosive medium. It is the result of a combination of electrochemical corrosion and mechanical effects. Ferritic steels develop intergranular cracks in alkaline media, while austenitic stainless steels develop transgranular cracks in chloride-containing media. 3.6.2 Prevention methods: ① Stress-relief heat treatment: Heat the steel below AC1 and hold it at that temperature for a period of time before cooling it slowly (cooling in the furnace); ② Surface shot blasting: This creates compressive stress on the surface of the container; ③ Choose appropriate materials – seawater tends to cause stress corrosion in stainless steel but has little effect on low-carbon steel; ④ Apply cathodic protection using external currents or sacrificial anodes; 3.7 Corrosion Fatigue and Its Prevention 3.7.1 Corrosion fatigue: Fracture that occurs in metals under the combined action of a corrosive medium and alternating stresses. The difference between this and ordinary fatigue failure is that there are corrosion products in the area where fatigue cracks propagate. 3.7.2 Prevention methods: ① Choose materials with strong resistance to pitting corrosion; ② Improve the fatigue strength of metal materials; ③ Use surface treatments such as shot blasting, sandblasting, and nitriding to create compressive stress on the surface ； ④Cathodic protection is employed ； 3.8 Hydrogen damage: The phenomenon in which hydrogen penetrates into the metal, leading to a deterioration of its mechanical properties. There are four forms of damage associated with this: ① Hydrogen embrittlement: Under certain conditions, hydrogen enters the metal and accumulates at dislocations and tiny gaps, reaching a supersaturated state; this prevents dislocations from moving and hinders slip, resulting in brittleness in the metal. ② Hydrogen bulging: Hydrogen atoms enter the voids and interlayers within the metal, where they form molecular hydrogen; the resulting high pressure causes those interlayers to bulge ； ③Decarbonization ； ④Hydrogen corrosion ； Prevention of corrosion: For example, high-strength steel in a wet H2S environment may suffer from hydrogen embrittlement and hydrogen blistering ； 3.9 Preventing corrosion of pressure vessels: ① Proper selection of materials; ② Reasonable structural design: avoiding gaps, erosion, stress corrosion, etc ； ③Manufacturing and installation quality control: cold working forming, welding process, etc ； ④Maintenance management ；