Industrial Corrosion and Protection--01 Main Types of Metal Corrosion
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Corrosion covers a wide range of fields and intersects with many disciplines. As human understanding of materials and corrosive environments continues to deepen, different definitions of corrosion have been proposed from various perspectives. For example: when characterizing the essential difference between corrosion and simple mechanical damage, corrosion is defined as \"damage or deterioration of a material resulting from a reaction with its environment\" or \"any form of damage other than simple mechanical damage\"; when indicating that corrosion depends on both the material and the environment, it is defined as \"a harmful reaction between the material and its environment\"; and when aiming to explain that the metal corrosion process is thermodynamically spontaneous and that the corrosion products are similar to corresponding natural minerals, it is defined as \"the reverse process of metallurgy\". The currently widely accepted definition of metal corrosion is: the degradation or deterioration of a metal resulting from chemical or electrochemical reactions between the metal and its surrounding environment (medium). Metal corrosion 1. Chemical corrosion. Chemical corrosion refers to the direct reaction between a metal and a corrosive medium, with no electric current generated during the reaction process. This type of corrosion process is a pure redox chemical reaction, in which metal atoms with valence electrons interact directly with the molecules of reactants such as oxygen. Therefore, the transformation of the metal into an ionic state and the reduction of the oxidizing components in the medium occur simultaneously and at the same location. The most important form of chemical corrosion is gas corrosion, such as the oxidation of metals or the chemical reaction of metals at high temperatures with substances like SO2 and water vapor. The corrosion products of chemical corrosion form a surface film on the metal surface, and the properties of this surface film determine the rate of chemical corrosion. If the membrane has good integrity, strength, and plasticity, if its coefficient of thermal expansion is similar to that of the metal, and if there is a strong affinity between the membrane and the metal, this helps to protect the metal and reduce the corrosion rate. Chemical corrosion can be divided into: corrosion in dry gases (usually referring to the corrosion of metals under the action of high-temperature dry gases). For example, the formation of thick iron oxide scales during steel rolling, and the corrosion of gas turbine blades while in operation): Corrosion in non-electrolyte solutions (referring to the corrosion that occurs in metals within non-conductive non-electrolyte solutions). For example, the corrosion of AI in CCl4, CHCl3, or CH3CH2OH, and the corrosion of magnesium and titanium in CH3OH, etc.). In fact, pure chemical corrosion is rare; electrochemical corrosion is more common. 2. Electrochemical corrosion. Electrochemical corrosion refers to the corrosion that occurs as a result of an electrochemical reaction between a metal and an electrolyte solution (usually an aqueous solution). Its characteristic is that during the corrosion process, two relatively independent reaction processes occur simultaneously – the anodic reaction and the cathodic reaction – and electricity is generated as a result of these reactions. The corrosion of metals in acids, bases, and salts is electrochemical corrosion. There is a fundamental difference between the mechanisms of electrochemical corrosion and chemical corrosion, but further research shows that some types of corrosion often develop gradually from chemical corrosion to electrochemical corrosion. Electrochemical corrosion is the most common form of corrosion. Under natural conditions, such as in humid atmospheres, seawater, soil, groundwater, as well as in chemical and metallurgical processes, the corrosion of metals in the vast majority of media is typically of an electrochemical nature. 3. Physical corrosion. Physical corrosion refers to the degradation of metals caused by pure physical dissolution. Corrosion in molten metal refers to the dissolution or cracking of metal that occurs when solid metal comes into contact with molten liquid metals such as lead, sodium, mercury, etc. This corrosion is caused by the formation of an alloy through physical dissolution, or by liquid metal infiltrating the grain boundaries. For example, iron pots used for hot-dip galvanizing are quickly corroded and thinned due to the dissolving action of liquid zinc. Furthermore, the corrosion of solid metals in high-temperature molten salts and molten alkalis also falls under physical corrosion. 4. Biological corrosion. Biocorrosion refers to the corrosion that occurs on metal surfaces under the influence of certain microorganisms. This type of corrosion is difficult to occur on its own, but it creates the necessary conditions for chemical and electrochemical corrosion, thereby accelerating the corrosion of metals. When microorganisms carry out their metabolic activities, they produce various chemical substances. In oil field production systems, sulfur-containing bacteria can oxidize sulfur or sulfides under aerobic conditions; the reaction ultimately produces sulfuric acid. The acid generated by these bacterial metabolic activities can cause severe corrosion of mechanical equipment and pipelines such as water pumps, oil well casings, and production pipes.
Classification of metal corrosion patterns I. General corrosion or uniform corrosion. The characteristic of general corrosion or uniform corrosion is that the entire surface of the metal exposed to the corrosive environment corrodes at more or less the same rate. The degree of corrosion can be expressed as the weight loss per unit area or the average corrosion depth. The latter can be determined by direct measurement or by calculating it from the weight loss per unit area when the density of the material is known. Uniform corrosion usually occurs through the action of a corrosion cell without separate anode and cathode surfaces. This is an ideal form of corrosion, a controllable state. II. Localized corrosion. It is the most common type we encounter in daily life, and it is also the focus of our research. Local corrosion presents various forms of damage, and its threat to metal structures is much greater than that of general corrosion. There are mainly the following types. A. Galvanic corrosion. When two metals or alloys with different electrode potentials come into contact in an electrolyte solution, it can be observed that the corrosion of the metal with the lower potential accelerates, while the corrosion of the metal with the higher potential slows down (it is protected). This type of electrochemical corrosion that occurs under certain conditions (such as in electrolyte solutions or the atmosphere) – that is, a phenomenon in which the corrosion rate of a metal or alloy increases due to contact with another metal that has a higher electrode potential – is known as galvanic corrosion or bimetallic corrosion; it is also referred to as contact corrosion. B. Pitting corrosion. Pitting, also known as spot corrosion, involves the corrosion of very small areas on a metal surface, resulting in small, deep holes; the depth of these holes is generally greater than their diameter. Severe pitting can lead to the penetration of equipment. The distribution of the erosion holes varies; some exist isolatedly, while others are clustered together. The upper part of pits is often covered with corrosion products. Pitting is a typical form of corrosion for stainless steel and aluminum alloys in the presence of chloride ions. C. Crevice corrosion. Metal components are generally assembled using methods such as riveting, welding, or screw connection, and seams may appear at the joints. The metal within the gaps suffers severe selective degradation in the corrosive medium, causing the metal structure to be damaged prematurely. Crevice corrosion occurs in various electrolyte solutions, and passivated metals such as stainless steel, aluminum alloys, and titanium are the most susceptible to it. Filamentary corrosion and exfoliation corrosion are both specific forms of crevice corrosion under certain environmental conditions. Filiform corrosion is a type of atmospheric corrosion commonly seen on coated metal products. This type of corrosion has been found on nickel-plated steel plates, as well as on chrome-plated or enamel-coated steel components. On the metal beneath varnish or enamel, this type of corrosion develops more severely. Since it occurs mostly beneath the paint film, it is also known as underfilm corrosion. Exfoliation corrosion, also known as exfoliation. This type of corrosion occurs at individual points on the surface, then spreads further beneath it along grain boundaries parallel to the surface. Since the volume of the corrosion products is larger than that of the original metal, this causes the metal to swell or delaminate. This type of corrosion can occur on the surfaces of certain alloys and stainless steel profiles or sheets, as well as on metal surfaces protected by a metal coating. D. Intergranular corrosion. Corrosive damage develops along the boundaries between metal grains, causing the loss of cohesion between them; as a result, the metal can lose its mechanical properties significantly even if its shape remains largely unchanged. E. Selective corrosion. In a multi-component alloy, one component dissolves preferentially in the corrosive medium, resulting in the accumulation of other components on the surface of the alloy. Zinc removal from brass is a typical example of such corrosion; due to the preferential corrosion of zinc, copper accumulates on the surface of the alloy, giving it a red color. Local corrosion under stress A. Stress corrosion cracking. When metal equipment and components are subjected to the combined effects of stress and specific corrosive environments, they experience brittle cracking that occurs below the material’s strength limit; this phenomenon is known as stress corrosion cracking or stress corrosion failure, abbreviated as SCC. B. Hydrogen embrittlement and hydrogen-induced cracking. The brittle deterioration and failure of metals and alloys under tensile stress, caused by the absorption of hydrogen (including hydrogen generated from corrosion reactions), is known as hydrogen embrittlement. Within the metal, hydrogen accumulates, leading to a decrease in local strength due to the formation of voids or reduced bonding strength. The propagation of cracks through mechanical fracture along these weak, low-strength regions is referred to as hydrogen-induced cracking. C. Corrosion fatigue. Under the combined action of alternating stress and a corrosive medium, the fatigue strength or fatigue life of metals is reduced compared to when they are exposed to air alone; this phenomenon is known as corrosion fatigue. D. Erosion corrosion. It refers to the surface damage of a material caused by both wear and corrosion when the abrasive and the material move relative to each other at high speeds. This type of damage is much greater than the sum of the damages caused by wear or corrosion alone. Due to the sudden nature of stress corrosion and hydrogen embrittlement, they are the most hazardous, often leading to catastrophic accidents; therefore, sufficient attention should be paid to them in actual production and application.
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