E-book material ~ Corrosion Prevention and Protection in Petrochemical Refineries (Chapter 1)

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Chapter 1 Introduction to Corrosion Section 1 Basic Concepts of Corrosion Metal corrosion science is a discipline that studies the degradation of metal materials under the influence of their surrounding environment, as well as ways to mitigate or prevent such degradation. As is well known, metal materials are widely used engineering materials, but during use they are subject to various forms of direct or indirect damage. The most important and common forms of damage are fracture, wear, and corrosion. These three main forms of degradation have each developed into three separate interdisciplinary fields. Fracture refers to the failure of a metal component that occurs when the forces acting on it exceed its elastic limit and plastic limit. It can be classified from different perspectives as brittle fracture, plastic fracture, intergranular fracture, transgranular fracture, mechanical fracture, and so on. As a result of the fracture, the component becomes ineffective, but the metal material itself can be remelted and reused. Wear refers to the loss or damage that occurs on a metal surface due to friction, resulting from relative motion between that surface and the objects in contact with it or its surrounding environment. It is a gradual process. Sometimes worn parts can still be repaired. For example, brush plating can be used to repair slightly worn shafts. Corrosion refers to the degradation or deterioration of a metal caused by its interaction with its surrounding environment. From different perspectives, various definitions of corrosion have been proposed, such as: (1) “Damage or deterioration of a material resulting from its reaction with the environment”” ； (2) “All forms of damage other than simple mechanical destruction”” ； (3) “The reverse process of metallurgy”” ； (4) “Harmful reactions between materials and the environment”. Definitions (1) and (2) are used to distinguish simple mechanical damage, such as mechanical fracture and wear, but include stress corrosion fracture, hydrogen-induced delayed fracture, and abrasion. Definition (3) indicates that the corrosion products are similar in composition to the ore from which the metal was smelted, thereby showing the thermodynamic spontaneity of the corrosion process. Definition (4) indicates that in some cases, corrosion has not yet reached a level that causes damage to the material, but it is still sufficient to reduce the material’s performance, leading to problems and losses, such as rusting that results in a loss of luster or discoloration of the metal. These definitions, with the exception of (3), actually cover all materials, including metals and non-metals. Indeed, non-metals also face corrosion problems; for example, the weathering of brick and stone, the decay of wood, and the aging of paint, plastics, and rubber are all forms of corrosion that require research and solutions as well. Since there are significant differences in the principles of corrosion between metallic and non-metallic materials, this textbook covers only metal corrosion. Considering the nature of metal corrosion, it is generally defined as the degradation or deterioration of a metal resulting from chemical or electrochemical reactions between the metal and its surrounding environment (medium). In other words, metal corrosion occurs at the interface between the metal and the medium. Due to chemical or electrochemical multiphase reactions between the metal and the dielectric, the metal is converted into an oxidized (ionic) state. It can be seen that the corrosion system formed by the metal and its environment, as well as the chemical and electrochemical reactions that occur within this system, are the main subjects of study in metal corrosion science. Metal corrosion science is an interdisciplinary field that has developed on the basis of disciplines such as metallurgy, metal physics, physical chemistry, electrochemistry, and mechanics. The main purposes and contents of studying and researching metal corrosion are: (1) to study and understand the general laws governing the interaction between metal materials and environmental media. It is not only necessary to study the possibility of metal corrosion from a thermodynamic perspective, but more importantly, to examine the rate and mechanism of corrosion from a kinetic perspective. (2) Study and understand the causes of metal corrosion under various conditions, as well as various measures to control or prevent it. (3) Research and master testing methods and techniques for measuring metal corrosion rates, establish various standards for corrosion assessment methods and protective measures, and develop on-site monitoring technologies for corrosion. Section 2: A Brief History of the Development of Corrosion Science and Technology. It is no exaggeration to say that the history of humanity’s effective use of metals is essentially the history of fighting against metal corrosion. As early as the Shang Dynasty, our country had already begun to smelt bronze, that is, tin was used to improve the corrosion resistance of copper. Judging from the weapons unearthed from the Spring and Autumn and Warring States periods, the blue steel swords from the era of Qin Shi Huang, and the large number of arrowheads, some of which remain completely free of rust to this day, analysis has shown that these arrowheads have a layer of chromium-containing oxide on their surface ； And the matrix does not contain chromium. It is very likely that this surface protective layer is obtained by artificially oxidizing chromium compounds and subjecting them to high-temperature treatment. This protection technology, created two thousand years ago and similar to modern chromate passivation, is undoubtedly a miracle in the history of Chinese civilization. Although the history of metal corrosion protection is long, it has long remained at an empirical stage. It was not until the mid-18th century that research and explanations on corrosion phenomena began to appear gradually. Among them, Lomonosov explained the phenomenon of metal oxidation in 1748. In 1790, Keil described the passivation of iron in nitric acid. In 1830, de La Rive proposed the microcell concept for metal corrosion. Between 1833 and 1834, Faraday formulated the law of electrolysis. These are of great significance for the further development of corrosion science. However, metal corrosion as an independent discipline began to take shape only in the early 20th century. Since the 20th century, the rapid development of industries such as petroleum and chemicals has promoted research and application in corrosion theory and corrosion-resistant materials. Through the diligent efforts of many electrochemists and metallurgists, as well as extensive in-depth and systematic research, the basic laws of metal corrosion and oxidation were gradually established, laying the foundation for corrosion theory. Among the scientists who made significant contributions were: Evans and Hall from the UK, Ullig and Fantner from the US, Haufe and Wagner from Germany, Boubay from Belgium, and Akimov, Frumkin, and Tomashov from the Soviet Union. Since the 1950s, metal corrosion has developed into an independent, interdisciplinary field. Over the past 30-plus years, with the rapid development of modern industry, the previously widely used high-strength steel and high-strength alloy components have begun to suffer from severe corrosion problems. This has prompted scholars from various related fields such as modern electrochemistry, solid-state physics, fracture mechanics, materials science, engineering, and microbiology to conduct comprehensive research on corrosion issues, giving rise to many specialized subfields within this area, including corrosion electrochemistry, corrosion metallurgy, corrosion engineering mechanics, biological corrosion, and protective system engineering. The scientific research on corrosion and protection in our country has seen significant progress since the founding of the People’s Republic of China. As early as the early days of the founding of the country, the **Science and Technology Commission established a corrosion and protection subgroup within the mechanical sciences division. After 1958, some institutions of higher education in our country established specialized programs in corrosion protection and carried out related teaching and research activities. In 1961, in order to strengthen research in the field of corrosion and protection, the **Science and Technology Committee decided to establish a separate group for corrosion science** under its direct oversight. National academic conferences on corrosion and protection have been held on many occasions, and a national development plan for corrosion science has been formulated, which has led to significant progress in China’s research and technology in this field. In December 1978, the **Science and Technology Committee resumed the work of the corrosion science subgroup. The Chinese Society for Corrosion and Protection was established in December 1979. From then on, the field of corrosion and protection science in our country entered a new phase of development. Thanks to the diligent efforts of numerous professionals in the field of corrosion and protection technology, China has now largely resolved corrosion problems in modern industries such as oil and gas exploration, petrochemicals, the chemical industry, shipbuilding, aerospace, and nuclear energy. Many corrosion-resistant metallic and non-metallic materials have been developed, which basically meet the needs of industrial production. These advancements have contributed to the development of the national economy and the strengthening of national defense, while also fostering a team of skilled professionals in corrosion protection technology. Section 3 Classification of Corrosion I. Classification Methods for Corrosion Due to the broad and diverse nature of corrosion, there are various classification methods. The most common way is to classify them from the following different perspectives: (1) Corrosion environment ； (2) Corrosion mechanism ； (3) Types of corrosion morphology ； (4) Metal materials ； (5) Scope of application or industrial sector ； (6) Protection methods. From the perspective of corrosion classification, it is most appropriate to classify first according to the corrosion environment. It can be divided into humid environments, dry gases, molten salts, etc. This also means that, based on the mechanism of action, it is an electrochemical mechanism in humid environments, while it is a chemical mechanism in dry gases. Moreover, the various corrosion testing methods mainly depend on the corrosive environment. Different types of corrosion patterns, such as pitting and stress corrosion cracking, belong to further classifications. Classifying by various metal materials is common and practical in manuals, but from a taxonomic perspective, it is not effective. Classified by application scope or industrial sector, they are actually special applications classified by environment ； Classified by protection methods, it is based on corrosion prevention, and is categorized according to the nature and limitations of the measures taken, such as: (1) modifying the metal material itself. Corrosion-resistant alloys are developed by altering the composition or microstructure of the material. (2) Change the corrosive medium. Such as adding corrosion inhibitors or changing the pH value of the medium. (3) Changing the electrode potential of the metal/medium system. Such as cathodic protection and anodic protection, etc. (4) Use a surface coating to separate the metal from corrosion and media. II. Classification of corrosion environments: Based on the corrosion environment, corrosion can be classified into the following categories. (1) Dry corrosion: 1. The metal loses its luster as it corrodes (oxidizes) in dry gas at room temperature above the dew point, resulting in the formation of a very thin layer of surface corrosion products; this is a mechanism of chemical corrosion. 2. High-temperature oxidized metals corrode (oxidize) in high-temperature gases, sometimes forming very thick oxide scales. Oxide scale peeling can be caused by thermal stress or mechanical stress. It belongs to high-temperature corrosion. (II) Wet corrosion: Wet corrosion mainly refers to corrosion in humid environments and aqueous media. The vast majority of corrosion at room temperature falls into this category. It is the mechanism of electrochemical corrosion. Wet corrosion can be further divided into the following two categories. (1) Corrosion in natural environments: atmospheric corrosion, soil corrosion, seawater corrosion, microbial corrosion. (2) Corrosion in industrial media: Corrosion in acid, alkali, and salt solutions ； Corrosion in industrial water ； Corrosion in high-temperature and high-pressure water. (III) Corrosion in anhydrous organic liquids and gases (chemical corrosion mechanism) (1) Corrosion in halogenated hydrocarbons, such as the corrosion of Al in CCl4 and CHCl3. (2) Corrosion in alcohols, such as AI in ethanol, and Mg and Ti in methanol. These types of corrosive media are all non-electrolytes; whether they are in liquid or gas form, the corrosion reaction remains the same. In these reactions, water actually acts as a corrosion inhibitor. However, corrosion in organic liquids such as oil is, in the vast majority of cases, caused by the presence of trace amounts of water, which often contains salts and acids; thus, this type of corrosion is actually of electrochemical origin. (IV) Corrosion in molten salts and slags (electrochemical corrosion) (V) Corrosion in molten metals (physical corrosion mechanisms) III. Classification by corrosion mechanism (I) Chemical corrosion Chemical corrosion refers to the degradation that occurs as a result of direct pure chemical reactions between the metal surface and a non-electrolyte. The reaction mechanism is characterized by a redox reaction occurring directly between the atoms on the metal surface and the oxidants in the non-electrolyte, resulting in corrosion products. During the corrosion process, the transfer of electrons occurs directly between the metal and the oxidizer, so no electric current is generated. Pure chemical corrosion is not common. It mainly refers to the corrosion of metals in anhydrous organic liquids and gases, as well as in dry gases. High-temperature oxidation of metals was considered a typical example of chemical corrosion until the 1950s; however, in 1952 Wagner proposed, based on the modern view of oxide films, that although the oxidation of metals in high-temperature gases initially occurs through chemical reactions, the subsequent growth of the film follows an electrochemical mechanism. This is because the dielectric on the metal surface has changed at this point from a gas phase to a semiconductor oxide film that can conduct electricity both through electrons and ions. After the metal dissociates at the anode (metal/film interface), electrons can be transferred through the film to the oxygen on the film surface, causing it to be reduced to oxygen ions (O2-). Oxygen ions and metal ions can then conduct electricity within the film; that is, the oxygen ions migrate toward the anode (metal/film interface) while the metal ions migrate toward the cathode (film/gas phase interface), or they may combine again somewhere within the film. All of these fall under the category of electrochemical corrosion mechanisms; therefore, high-temperature oxidation of metals is no longer considered to be simple chemical corrosion. (II) Electrochemical corrosion: Electrochemical corrosion refers to the degradation that occurs as a result of electrochemical reactions between the metal surface and an ion-conducting medium (electrolyte). Any corrosion reaction that occurs through electrochemical mechanisms involves at least one anodic reaction and one cathodic reaction, with an electric current flowing through the metal and ion currents in the medium forming a circuit. The anodic reaction is an oxidation process, in which metal ions are transferred from the metal to the medium while electrons are released; the cathodic reaction is a reduction process, in which the oxidizing components in the medium absorb electrons from the anode. For example, when carbon steel corrodes in acid, iron in the anodic area is oxidized to Fe2+ ions. The electrons released flow from the anode (Fe) to the cathode (Fe3C) within the steel, where they are absorbed by H+ ions and reduced to hydrogen gas. Thus, the anodic reaction is: Fe ⇔ Fe2+ + 2e-, while the cathodic reaction is: 2H+ + 2e- ⇔ H2. The overall reaction is: Fe + 2H+ ⇔ Fe2+ + H2. It can be seen that, unlike chemical corrosion, electrochemical corrosion is characterized in that its corrosion process can be divided into two relatively independent processes that can occur simultaneously. Since there are anodic and cathodic regions separated in space or time on the corroded metal surface, the transfer of electrons during the corrosion reaction can occur as electrons flow from the anodic region to the cathodic region through the metal, which inevitably results in the generation of an electric current. The current generated by electrochemical corrosion and the transfer of reactants can be quantitatively related through Faraday’s law. Based on the aforementioned electrochemical mechanism, the electrochemical corrosion of metals is essentially the result of the action of a short-circuited galvanic cell. This type of galvanic cell is called a corrosion cell. Electrochemical corrosion is the most widespread and common type of corrosion. The corrosion of metals in the atmosphere, seawater, soil, and various electrolyte solutions falls into this category. Electrochemical action can cause metal corrosion on its own, or it can work together with mechanical and biological actions to lead to metal corrosion. When a metal is subjected to both tensile stress and electrochemical effects, stress corrosion fracture can occur. Metals can suffer from corrosion fatigue under the combined action of alternating stress and electrochemistry. If a metal is subjected to both mechanical wear and chemical action, it can lead to wear corrosion. The metabolism of microorganisms can create conditions for electrochemical corrosion, participating in or facilitating the electrochemical corrosion of metals; this is known as microbial corrosion, or bacterial corrosion. (III) Physical corrosion: Physical corrosion refers to the damage to metals caused by pure physical dissolution. Corrosion in molten metal refers to the dissolution or cracking of solid metal that occurs as a result of its contact with molten liquid metals such as lead, zinc, sodium, mercury, etc. This corrosion is not caused by chemical reactions, but rather by physical dissolution, the formation of alloys, or the penetration of liquid metal into the grain boundaries. For example, iron pots used for hot-dip galvanizing corrode rapidly due to the dissolving action of liquid zinc. IV. Classification by corrosion pattern: Based on the pattern (characteristics) of corrosion, it can be divided into three main categories: uniform corrosion, local corrosion (crevice corrosion, intergranular corrosion), and stress corrosion. (l) Uniform corrosion: It is characterized by corrosion occurring evenly across the entire metal surface. Most chemical corrosions fall into this category. Uniform corrosion is the least hazardous type of corrosion; in engineering, providing an adequate corrosion allowance is usually sufficient to ensure the mechanical strength and service life of the material. Uniform corrosion is commonly evaluated using the depth of corrosion of the metal material by the corrosive agent per unit time, or the amount of thinning of the wall thickness of the metal component (referred to as the corrosion rate). Standard SH 3059 specifies that materials with a corrosion rate of no more than 0.05 mm/year are considered fully corrosion-resistant; materials with a corrosion rate of 0.05–0.1 mm/year are considered corrosion-resistant ； Materials with a corrosion rate of 0.1–0.5 mm/year are considered moderately corrosion-resistant materials ； Materials with an annual corrosion rate of over 0.5 mm/year are considered non-corrosion-resistant materials. (2) Local corrosion: Local corrosion, also known as non-uniform corrosion, is characterized by the occurrence of corrosion in specific areas of the metal material. Although local corrosion does not cause as much metal loss as uniform corrosion, its hazards are far greater than those of uniform corrosion. This is because uniform corrosion is easy to detect and prevent, whereas local corrosion is difficult to predict and stop; it often causes sudden failure of metal components without any warning, leading to serious accidents. When selecting materials, care is taken to avoid the occurrence of local corrosion. Depending on the conditions and patterns under which corrosion occurs, local corrosion can be classified into the following types: 1) Galvanic corrosion. When two metals or alloys with different electrode potentials come into contact in an electrolytic solution, the metal with the more negative potential will corrode more rapidly, while the metal with the more positive potential will experience slower corrosion; this type of corrosion is known as galvanic corrosion. When carbon steel comes into contact with stainless steel in an electrolytic environment, the carbon steel will corrode more rapidly, while the stainless steel remains protected. In engineering, contact between metals with different potentials in a corrosive environment should be avoided as much as possible. 2) Pitting: In metals with a passivation film or protective coating on their surface, when defects such as mechanical cracks, scratches, or inclusions exist on the surface of this passivation film, resulting in uneven thickness of the film or even exposure of the underlying metal, an activation-passivation corrosion cell is formed, leading to localized corrosion. This type of corrosion generally progresses in a vertical direction, forming pits or corrosion holes. Such metals include aluminum and aluminum alloys, stainless steel, heat-resistant steel, titanium alloys, etc. Such corrosive environments are often media containing chloride ions or chlorides. Pitting is highly destructive and insidious; it often serves as a catalyst for intergranular corrosion, exfoliation, stress corrosion, corrosion fatigue, and other forms of corrosion. Engineering measures to prevent pitting include selecting materials that are not sensitive to pitting, and controlling the chloride ion content in the medium. 3) Crevice corrosion: The corrosion of metal in gaps that occur on its surface due to the presence of foreign substances or structural reasons (usually between 0.025 and 0.1 mm). This corrosion is caused by the difficulty of migration of corrosive agents, leading to damage to the metal within those gaps. Crevice corrosion occurs quite frequently, but every effort should be made to prevent it from becoming a cause of other types of corrosion such as pitting. In engineering, for media that may suffer from severe crevice corrosion, structural design should avoid the presence of any crevices. 4) Intergranular corrosion: Intergranular corrosion is a type of localized degradation caused by the action of microcells; it occurs in metal materials within specific corrosive environments along the grain boundaries of the material. Its characteristic is that the bonding force between the grains has been lost even before any damage can be seen on the surface. It is a highly harmful form of local corrosion. The occurrence of intergranular corrosion requires two conditions: first, the physicochemical state of the material at the grain boundaries differs from that of the grains themselves; second, a specific corrosive environment is present. There are mainly three engineering measures to prevent intergranular corrosion (in austenitic stainless steels). The first is to reduce the carbon content in the stainless steel to below its room-temperature solubility in the austenite phase (0.02–0.03%). The second is to carry out solution heat treatment. Third is the use of austenitic stainless steel containing stabilizing elements (mainly titanium and niobium). (3) Corrosion under stress: Like localized corrosion, corrosion under stress can also suddenly lead to the failure of metal components without any prior signs, making it another type of highly destructive corrosion. Unlike local corrosion, it usually results from the combined effect of stress and corrosion. In practical engineering applications, metal components usually operate under stress. Therefore, it is also a common type of corrosion. Depending on the conditions and patterns under which corrosion occurs, it can be classified into the following types: 1) Stress corrosion refers to the degradation of metal components resulting from the combined effect of tensile stress and a corrosive environment. Three conditions must be met for stress corrosion fracture to occur; the first is the presence of a specific corrosive environment (including the composition, concentration, impurities, and temperature of the corrosive medium) ； The second is to have a sufficiently high tensile stress (above a certain limit value) ； Thirdly, metal materials have specific alloy compositions and microstructures (including grain size, grain orientation, morphology, phase structure, and various defects). The engineering measures to prevent stress corrosion cracking include the following: first, reducing stress levels, avoiding or minimizing local stress concentrations, and eliminating residual stresses from machining and welding. The second is to control sensitive environments, such as by adding corrosion inhibitors, raising the pH value of the medium, and employing electrochemical protection measures. Thirdly, the right materials should be selected to avoid material-environment combinations that are prone to stress corrosion cracking. 2) Hydrogen damage: The degradation of metal components caused by the presence of hydrogen or reactions with hydrogen is referred to as hydrogen damage in metals. Based on the different conditions, mechanisms, and forms of metal degradation caused by hydrogen, hydrogen-induced damage can be classified into four main categories: hydrogen embrittlement, hydrogen blistering, surface decarburization, and hydrogen corrosion (internal decarburization). Hydrogen embrittlement refers to the phenomenon where hydrogen, regardless of the way it enters the steel, causes the steel to become brittle, resulting in a significant decrease in elongation and reduction of area; this effect is particularly severe in high-strength steels. If the hydrogen in the steel is released, the mechanical properties of the steel can still be restored. Hydrogen embrittlement is reversible. Hydrogen bubbling mainly occurs in wet hydrogen sulfide environments; as hydrogen atoms diffuse into the steel, they encounter defects such as cracks, delaminations, voids, and inclusions within the steel. There, they gather together to form hydrogen molecules, resulting in volume expansion and generating extremely high pressures inside the steel. If these defects are near the surface of the steel, bubbles form. Surface decarburization refers to the reaction of cementite in steel with hydrogen at high temperatures to form methane; as a result of this reaction, the amount of cementite in the surface layer decreases. Carbon then gradually diffuses from the adjacent, unreacted metal layers into this reaction zone, causing a layer of metal of certain thickness to turn into ferrite due to a lack of carbon. Decarburization results in a decrease in the surface strength and fatigue limit of steel. Hydrogen corrosion refers to the phenomenon in which, when steel is exposed to hydrogen under high temperature and pressure, its mechanical properties deteriorate, with a significant decrease in strength and toughness; this process is irreversible, and it is known as hydrogen corrosion. 3) Corrosion fatigue is also very common in the petrochemical production process; it occurs wherever alternating stresses and the environment are present together. It is a special case of stress corrosion; therefore, its hazards are no less severe than those of stress corrosion cracking. There are generally two measures used in engineering to prevent corrosion fatigue: one is to apply cathodic protection to metal components or add corrosion inhibitors to the medium, in order to eliminate or mitigate the corrosive environment. The second is to reduce the stress level or peak stress of metal components. 4) Wear corrosion refers to the accelerated degradation of metal caused by the relative movement between a corrosive fluid and the metal surface. It often occurs in devices where fluids are in motion, such as process equipment (especially at elbows), centrifuge impellers, heat exchanger tubes, steam equipment, etc. Wear corrosion can be further divided into several types, such as turbulent erosion, cavitation erosion, and microvibration erosion, depending on the manner of wear. Engineering measures to prevent wear and corrosion include: using materials with good corrosion resistance ； Structural design measures are taken, such as installing protective plates at the elbows in the equipment where abrasion is severe ； For cavitation corrosion, it is advisable to use a machining surface with a high surface finish in order to reduce or even prevent the formation of nuclei for bubbles. Figure 1-1-1 shows a schematic diagram of the corrosion patterns, while Figure 1-1-2 presents the results of a survey conducted in the United States on corrosion incidents between 1968 and 1971. As can be seen from the graph, corrosion fatigue, general corrosion, and stress corrosion failure account for a high proportion. The unexpectedly high number of general corrosion incidents indicates improper material selection, as well as a lack of knowledge regarding corrosion protection among the relevant personnel. Under normal circumstances, localized corrosion is much more dangerous than general corrosion. Due to the suddenness of stress corrosion and hydrogen embrittlement, they are the most hazardous, often causing catastrophic accidents, which has led to extensive and in-depth research over the past 20 years. Section 4: The Hazards of Corrosion. The damage and losses caused by corrosion to human life and industrial activities exceed the total losses resulting from natural disasters such as fires, wind disasters, floods, and earthquakes; it can be said that corrosion is also a major disaster. I. Loss of metallic materials: Metallic materials can corrode in various environments, causing a large amount of steel to turn into rust. Out of the world’s 10T of steel, 3T is wasted due to corrosion, of which 1T turns completely into useless rust that cannot be recycled. II. Causing significant economic losses: Corrosion not only consumes large amounts of steel, but it also leads to premature failure of equipment, and at times it poses considerable risks as well. This will not only cause huge economic losses but also lead to severe casualties. III. Damage to social benefits: Corrosion can cause damage to machinery and factory buildings, leading to accidents. Especially in petrochemical production, corrosion problems are more severe; they can even lead to fires and explosions, posing a threat to people’s health and safety. Corrosion is a major safety hazard that directly undermines societal benefits. At the same time, corrosion issues also directly affect the implementation of many new technologies and processes; especially in the development of certain chemical products, production cannot begin if the problem of preventing corrosion in the equipment cannot be resolved. Proper corrosion prevention not only helps to save a large amount of money and metal materials, avoiding significant economic losses, but it also prevents many serious accidents. It is also essential for promoting the development of new technologies and processes. By applying existing corrosion prevention techniques appropriately, a large number of corrosion problems can be resolved, thereby **reducing the losses caused by corrosion**. The damage caused by corrosion to various industries: (1) Steel industry – Corrosion leads to the loss of large amounts of steel: Each year, approximately 10–20% of the annual metal production is lost due to corrosion. (2) Transportation: Corrosion and wear of railway tracks and locomotive components ； Ø High-temperature corrosion and stress corrosion of aircraft components ； Ø Corrosion of ships in marine environments ； Ø Rust and wear of the rails Ø Locomotive frame: Local corrosion has penetrated the heads of the rivets on the side beams Ø Locomotive cylinders: Cavity corrosion in the cylinder liner walls – 9 mm, 5 mm; in some cases penetration occurs, posing a serious risk of accidents! Ø In marine environments: seawater corrosion media, marine atmospheric corrosion media ； Sea water erosion, complex fluctuating loads, etc ； Ø Ship corrosion: hull rust, electrode corrosion of propeller blades, waterline corrosion, stress corrosion, etc. The amount of steel corroded by seawater in our country each year is roughly equivalent to the annual production of Baosteel over one and a half years. In 2010, the losses due to seawater corrosion amounted to 1.2 trillion, accounting for 3% of GDP; this is equivalent to each Chinese person having to pay 1,000 yuan as a consequence. (3) Energy and electricity: Ø In hydroelectric power, there is corrosion of turbine units as well as blade cavitation ； Ø In thermal power generation, there is corrosion of boilers and pipelines ； Ø Corrosion in nuclear power plants due to high temperatures, radiation, and liquid metals ； Ø Coal mine safety ； The underground environment is humid and the water quality varies greatly; the corrosion rate of equipment in coal mine shafts is 0.17–0.25 mm/year, with an average service life of 15 years ； Ø Oil and gas extraction and transportation cause corrosion of equipment and pipelines. The extraction, gathering, and transportation of oil and gas lead to metal corrosion, which can result in leaks from equipment; such leaks can cause the release of large amounts of toxic substances, polluting the environment and posing a threat to human health. (4) In the chemical industry (petrochemicals, paper manufacturing, etc.), metal corrosion can also lead to leaks, with similar consequences in terms of environmental pollution and health risks. Chemical processing equipment: pipes, boilers, storage systems, valves, condensers ； (5) Mechatronics ； (6) People’s livelihood: Rust on water supply pipes ； (7) Environmental pollution ； Section 5: Methods for Assessing Corrosion I. Methods for Assessing Corrosion of Metal Materials When metal materials are corroded, their appearance, quality, dimensions, mechanical strength, and microstructure all undergo changes. Based on these changes in physical and mechanical properties, the degree of corrosion of the metal can be assessed. Due to the various forms of corrosion damage, there are many methods for assessing the degree of corrosion. (1) Assessment of general corrosion: The average corrosion rate is usually expressed by the change in the mass or thickness of the metal before and after corrosion. 1. Evaluate, based on changes in mass, whether, during the corrosion process, the mass of the metal decreases or increases due to the dissolution of the metal or the accumulation of corrosion products on its surface. Based on this change, the degree of corrosion is evaluated by the change in mass (g) caused by corrosion per unit area (m3) of the corroded metal per unit time (h). Its corrosion rate can be calculated using the following formula: 2. Assessment based on thickness changes – After metal corrodes, its dimensional properties change, usually resulting in a reduction in thickness. We can assess the degree of corrosion based on the amount by which the metal thins due to corrosion per unit time (a) (corrosion depth, in mm). This assessment method can more intuitively reflect the severity of general corrosion, thus having greater practical significance. Because it allows for the direct estimation of the equipment’s service life, while also enabling a direct comparison of the degree of corrosion in metals with different densities. Its corrosion rate can be calculated using the following formula. To compare the corrosion resistance of various metal materials and to facilitate material selection, the corrosion resistance of metal materials can be divided into four grades based on the magnitude of their corrosion rate (K-depth), as listed in Table 1-1-1. (II) Assessment of localized corrosion: Since there are many forms of damage caused by localized corrosion in metals, the changes in their physical and mechanical properties vary as well. For example, pitting corrosion only reflects changes in corrosion depth at the pitted areas, while there are basically no changes in other areas. Another example is intergranular corrosion: although there are no obvious changes in the metal’s quality or dimensional properties, its mechanical strength can change significantly. However, for localized corrosion, it is not possible to assess it using the simple changes in mass or dimensional properties mentioned above; instead, it is necessary to use appropriate indicators that can truly reflect the changes in its physical and mechanical properties, depending on the specific form of corrosion. For intergranular corrosion and stress corrosion, it can be assessed by measuring the change in the mechanical strength of the metal before and after corrosion. II. Evaluation methods for the corrosion of non-metallic materials. The corrosion of non-metallic materials differs fundamentally from that of metals. It mainly involves the penetration and diffusion of environmental media into the material, leading to damage in the form of chemical reactions, swelling, dissolution, and stress cracking. At present, there is no good method for assessing the degree of corrosion in non-metallic materials. It cannot have its corrosion resistance evaluated using the corrosion rate as a standard, like metal materials do. Typically, indicators for comprehensive evaluation include the loss of strength of the material (%), changes in mass (%), and descriptions of shape damage. Generally, the following three-level criteria are used to evaluate the corrosion resistance of non-metallic materials (other than graphite, glass, and ceramics). Level 1: Good, with slight corrosion or virtually no corrosion. Grade 2: Serviceable, with obvious corrosion, such as mild deformation, discoloration, loss of strength, or changes in mass. Level 3: Not applicable, with severe deformation damage or loss of strength. The aforementioned three-level standards are mainly established based on production experience, and they possess considerable reliability. However, in practical application, it should also be used flexibly according to specific circumstances. For some polymer materials (such as plastics, rubbers, fiberglass, and adhesives), the following standards can be referred to to determine their suitability. (1) Decrease in bending strength