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Crevice corrosion is related to small amounts of stagnant solution present in crevices such as under gaskets, at lap joints, in surface deposits, and beneath nuts and rivet heads. Corrosive liquids in the environment can enter these gaps, but they cannot flow freely; as a result, this particular form of corrosion occurs. Gaps are the main design defects that cause corrosion, and they are difficult to avoid in the design of many pressure vessels, especially at the joints and supports of the structure. 1. Definition of crevice corrosion: Crevice corrosion, also known as interstice corrosion, is a form of localized corrosion that occurs in electrolyte solutions when the movement of the medium is hindered within narrow crevices formed between metals or between metal and non-metal surfaces. Gap corrosion is a type of localized corrosion that occurs in metals within gaps formed on their surfaces due to the presence of foreign substances or structural factors, which makes it difficult for substances to migrate through those gaps. A type of localized corrosion that occurs in metals within gaps is collectively referred to as crevice corrosion. Crevices include gaps formed between metals, gaps created where metal comes into contact with non-metallic materials such as plastic, rubber, glass, fiberboard, etc., which are used as gaskets for flange connections; as well as gaps resulting from the deposition or adhesion of corrosion products, sand particles, dust, dirt, marine organisms, etc., on metal surfaces. In a given electrolyte solution, severe corrosion occurs locally within these crevices. 2. The hazards of crevice corrosion to pressure vessels: Since crevice corrosion occurs in electrolyte solutions (especially those containing Cl-), it is a form of localized corrosion that takes place on the surfaces of narrow gaps between metals or between metals and non-metals. This type of corrosion can damage the integrity of mechanical connections and the sealing properties of equipment, leading to serious malfunctions or failures in its normal operation and thus causing destructive accidents. Crevice corrosion often occurs in crevices formed at mechanically joined components such as bolts, gaskets, and flange surfaces of equipment and nozzles. The joints between the tube sheets and tube bundles in shell-and-tube heat exchangers, as well as welding defects in equipment like lack of penetration and undercutting, are also common sites for crevice corrosion. These areas are typically subject to high service stresses or welding residual stresses. Under the influence of these stresses, crevice corrosion can create a condition in which the local medium becomes sensitive to SCC, thereby increasing the susceptibility to SCC and accelerating the degradation of the structure. Additionally, crevice corrosion can also occur in the gaps formed on metal surfaces by corrosion products, deposits, debris, sludge, and scale that are generated during equipment operation. Crevice corrosion is not limited to stainless steel; it can occur in many alloy series, including titanium, aluminum, copper, and nickel-based alloys. For instance, crevice corrosion frequently occurs in titanium equipment used in the chlor-alkali industry. Similar to the damage caused by pitting, crevice corrosion can also lead to corrosion penetration of equipment and material leakage. One of the main failure modes of shell-and-tube heat exchangers is tube perforation, which can lead to contamination of the fluid within the heat exchanger, as well as instability in its temperature and pressure. These issues can result in equipment failure, shutdowns, environmental pollution, and threats to safe production. For example, an important form of damage to the tubes in nuclear power plant steam generators is \"denting corrosion\". This phenomenon occurs when an annular gap forms between the heat transfer tubes made of nickel-based alloys such as Inconel 600 in the steam generator and the carbon steel plates (or support plates) that were originally used; the carbon steel plates suffer from crevice corrosion in high-temperature water, and the corrosion products, which are about twice as large as the base metal, push against the walls of the heat transfer tubes, resulting in a reduction in the tube diameter and the formation of dents. Due to stress, it can also easily cause the pipes to rupture, severely affecting the safe operation of nuclear power plants. The crack corrosion rate is generally more than one order of magnitude faster than that of general corrosion; it often leads to perforation at the corroded areas, and it is one of the common failure modes in pressure vessels such as heat exchangers. Crevice corrosion will reduce the effective geometric dimensions of the components, thereby decreasing the degree of fit. The increase in the volume of corrosion products within the seam generates local stress and makes assembly difficult. 3. Morphology of crevice corrosion: Based on its manifestations, crevice corrosion can be divided into three common specific forms: through-thickness corrosion, filiform corrosion, and deposition corrosion. In most cases, intergranular corrosion is a form of macroscopic cell corrosion. The forms of corrosion can range from pitting, intergranular corrosion, SCC, and corrosion fatigue within the metal gaps to general corrosion. Generally speaking, materials with good corrosion resistance tend to suffer from local forms of corrosion such as pitting, while those with poor corrosion resistance are more prone to general corrosion in an activated state. When acidification in the gap is severe, general corrosion predominates; when the acidification is mild, local corrosion is more likely to occur. The presence of gaps or deposits often promotes pitting corrosion in stainless steel. When gap corrosion occurs, accelerated corrosion takes place inside the gaps, while the corrosion outside the gaps is less severe. However, in copper and copper alloys, corrosion caused by concentration differences occurs on the exposed surface near the gap, rather than inside the gap; that is, the gap opening acts as the anode, while the interior of the gap can become the cathode. Sometimes, copper deposition can also be observed. Under-deposit corrosion occurs because the blocked areas are subject to acidic corrosion, with hydrogen generation as a result; this leads to the formation of hemispherical bulges on the surface of the deposits. Under the deposits, large corrosion pits appear, and in severe cases the wall thickness is penetrated, as shown in the figure. Similar to pitting, influenced by gravity, surface corrosion on the upward-facing side is more severe than on the vertical and downward-facing surfaces. 4. Mechanism of crevice corrosion Crevice corrosion occurs when gaps are formed on the metal surface due to the presence of foreign substances or structural factors; the width of these gaps allows liquid to flow in while also preventing it from flowing away. Such gaps are common in practice, and this is one of the main reasons why metal intergranular corrosion is a frequent form of localized corrosion. Fibrous materials (such as gasket connectors) can draw solutions into the gaps between the gasket and the metal through capillary action, thus making gap corrosion particularly likely. (1) Mechanism of crevice corrosion. An important characteristic of crevice corrosion is that, due to the special geometry or the accumulation of corrosion products at the entrances of crevices, pits, or cracks, the passages become blocked. This restricts the diffusion of corrosive agents, resulting in significant differences in the composition, concentration, and pH value of the medium within the cavity compared to the overall medium, thereby giving rise to closed-cell corrosion. Cathodic reactants (such as dissolved oxygen) can easily reach the metal surface outside the gap through convection (natural convection and forced convection) and diffusion. Since they can only enter the gap by diffusion through its narrow opening, there is very little oxygen in the stagnant solution inside the gap. Therefore, most early theories held that crevice corrosion was caused by a concentration gradient cell formed due to the uneven distribution of metal ions and dissolved gases in the media inside and outside the crevice. Like the earlier two theories: one is the concentration cell of metal ions proposed in the 1920s, and the other is the uneven gas supply cell proposed by Evans, that is, the oxygen concentration cell. The mechanism of crevice corrosion that is now widely accepted is the result of the combined action of the oxygen concentration cell and the autocatalytic effect of the closed-cell. At the onset of corrosion, the oxygen concentration difference inside and outside the crack increases; the potential of the metal inside the crack becomes more negative, which accelerates the anodic dissolution rate there. As a result, the concentration of Men+ increases, and Cl- migrates into the crack. 5. Main factors affecting crevice corrosion in pressure vessels. The standard methods for evaluating a material’s resistance to crevice corrosion are primarily immersion tests and electrochemical tests. For the immersion test method, various types of artificial cracks are designed and a range of corrosive media are used; the test results are generally evaluated based on the degree of corrosion or the depth of corrosion. The electrochemical testing method uses certain electrochemical parameters as criteria to compare the relative sensitivity of metal materials to crevice corrosion. Generally speaking, electrochemical testing can shorten the induction period for crevice corrosion, thereby accelerating the corrosion test process. Immersion testing methods include the ferric chloride test, accelerated testing methods for crevice corrosion, multi-crevice corrosion tests, the MTI test, the CCT method, etc.; electrochemical testing methods include ASTM standard test methods, potentiostatic testing, potentiodynamic polarization testing, and remote crevice device tests, etc. Generally, an increase in temperature accelerates the anodic reaction, but the effect of temperature changes on crevice corrosion is relatively complex. Because temperature has different, or even opposite, effects on various related factors. On the one hand, an increase in temperature accelerates the transport process and reaction kinetics, thereby increasing the rate of the anodic reaction; however, the types of reactions at the anode and cathode may change. On the other hand, in solutions of open systems, the concentration of dissolved oxygen decreases as temperature rises, and at around 80°C, crevice corrosion in stainless steel reaches its maximum level. In a closed system, an increase in temperature usually accelerates the rate of crevice corrosion. Furthermore, temperature can also affect pitting potential, hydrolysis equilibrium, as well as the composition, structure, and properties of surface films, thereby influencing crevice corrosion. As the temperature increases, the degree of crevice corrosion in titanium worsens, and the incubation period shortens. When the temperature is below 85°C, the Ti-Pd alloy did not suffer from crevice corrosion during the 120-hour experiment under conditions of high chloride concentrations. The results of synchronous monitoring of the potential within the gap also show that as the temperature increases, the potential within the gap becomes more negative during gap corrosion. ④ As long as the metal outside the gap can remain in a passive state, a decrease in pH leads to an increase in gap corrosion. The critical pH values at which different materials maintain passivation vary significantly, and are highly dependent on the composition of the medium and temperature. Ordinary 18-8 austenitic stainless steel at room temperature, 6. Control of crevice corrosion in pressure vessels: The main measures to prevent or reduce crevice corrosion include optimizing the structural design, selecting appropriate materials, and controlling the medium. (1) Improve structural design and operating conditions. Avoiding the creation of crevice structures in the design and manufacturing processes of pressure vessels is the most effective way to prevent crevice corrosion. Try to avoid the use of metal and non-metal connections; in the design, prefer full-welded structures, using welding instead of riveting or bolted connections. Use butt welding rather than lap welding; any gaps resulting from lap welding should be sealed using continuous welding, brazing, or crimping methods. During welding, defects such as pores, lack of fusion, and cracks on the side of the weld that is in contact with the solution should be avoided as much as possible. It must be ensured that no residual solution remains in the container when it is emptied. The design should avoid sharp corners and dead zones so that the structure can allow complete drainage, preventing the accumulation of organisms or debris in such areas, as well as enabling timely removal of any deposits that may form. Non-corrosive sealants or welding should be used to seal existing gaps. Gaskets on the flanges of connecting components should be made of non-absorbent materials (such as polytetrafluoroethylene where possible). Before assembly, the joint surfaces should be coated or covered with a layer, or clad with materials resistant to gap corrosion. (a) For bolted connections, sealant is used to fill the gaps and eliminate spaces around the fasteners; (b) In shell-and-tube exchangers, the expansion joints between the tube sheet and the tube bundle should be as uniform in thickness as possible with that of the tube sheet; (c) Tubes should be joined by butt welding rather than socket welding; (d) Gaps between the jacket welds and the shell should be avoided; (e) Vessels should be supported by brackets, and if a vessel with a flat bottom sits directly on the ground, measures must be taken to fill the gap between the bottom and the ground. Improving operating conditions is particularly important for enhancing the resistance of pressure vessels to crevice corrosion. Pressure vessels should be cleaned periodically, the mixing and flow of solutions should be improved to prevent the accumulation of organisms or precipitates, which also helps to prevent or reduce crevice corrosion. When the process conditions permit, the addition of a certain corrosion inhibitor can be used to prevent crevice corrosion. A mixture of sufficient amounts of phosphates, chromates, and nitrates is effective for steel, brass, and Zn alloys. However, since the corrosion inhibitor is often hindered from entering the gaps, it is consumed in large quantities; if used improperly, it can actually accelerate corrosion. (2) Material selection: When gaps are unavoidable, use materials resistant to gap corrosion.