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The forms of metal corrosion can be divided into two main categories: general (uniform) corrosion and local corrosion. The former occurs more evenly across the entire surface, while the latter occurs only locally. Such as pitting corrosion, crevice corrosion, intergranular corrosion, stress corrosion cracking, corrosion fatigue, hydrogen-induced cracking, selective corrosion, wear corrosion, delamination corrosion, etc. Generally, local corrosion is much more harmful than general corrosion, and some forms of local corrosion can be sudden and catastrophic. Accidents such as fires, explosions, and environmental pollution can occur if equipment and pipelines perforate or rupture, resulting in the leakage of flammable, explosive, or toxic fluids. According to some statistical data, local corrosion accounts for about 70% of the corrosion of chemical processing equipment. Although uniform corrosion poses a low level of risk, a large amount of metal is exposed to the gases and water that cause it, resulting in severe economic losses as well. General (uniform) corrosion: refers to the corrosion of all or most of the metal surface, with the degree of corrosion being roughly uniform. Generally, a layer of corrosion product film on the surface can slow down corrosion; high-temperature oxidation is an example of this. Similarly, easily passivated metals such as stainless steel, titanium, and aluminum form an extremely thin passivation film in an oxidizing environment, which provides excellent protection and essentially stops corrosion. The oxide film (rust) formed on iron in the atmosphere and water provides very low protection. Generally, uniform corrosion is quite severe. Pitting: It is a highly localized form of corrosion. Most of the metal surface does not corrode or corrodes only slightly, with one or several holes appearing only in localized areas. Holes vary in size; generally, the diameter of the hole’s surface is equal to or less than its depth, but there are also pit-like, disc-shaped shallow holes. Small but deep holes can penetrate metal sheets, leading to accidents such as fluid leakage, fires, and explosions; it is one of the most destructive and hazardous forms of corrosion. Crevice corrosion: It is a special form of pitting that occurs in crevices (such as those under welds, rivets, gaskets, or deposits). The damage takes the form of grooves, and in severe cases, it can lead to penetration. The gap contains an oxygen-deficient area that is also blocked; the pH value inside the gap decreases, while the chloride ion concentration increases. There is often a relatively long incubation period; once the pH value inside the crack drops to a critical level, accelerated corrosion occurs, similar to what happens in pores. It generally occurs most easily in solutions containing chloride ions, and an effective preventive measure is to eliminate gaps. Delamination corrosion: Corrosion occurs between the various layers of a metal structure; it first develops vertically inward, then changes direction to selectively corrode the materials that are parallel to the surface. The expansion force of the corrosion products causes the uncorroded surface layer to peel off in layers. Intergranular corrosion: Corrosion progresses from the surface along the grain boundaries inward; there are no signs of corrosion on the outer surface, but loose corrosion products accumulate at the grain boundaries. Under a metallographic microscope, network-like corrosion can be observed at the grain boundaries. Severe intergranular corrosion can cause the metal to lose its strength and ductility, leading to fracture under normal loads. Selective corrosion: Industrial alloys contain different components and impurities, have varying structures, and thus exhibit different corrosion resistance. In a certain solution, some active components dissolve, leaving behind loose, inactive components that lose all of their strength and ductility. A common example of this type of selective corrosion is zinc loss in brass. Wear corrosion: When a metal surface is subjected to the impact of high-flow, turbulent fluids, it suffers both wear and corrosion damage, a phenomenon known as wear corrosion. Impact corrosion is the main form of wear corrosion; other forms include cavitation corrosion and fatigue vibration corrosion. Stress corrosion cracking: The cracking of an alloy under the combined action of corrosion and tensile stress in a specific direction is known as stress corrosion cracking. There are two types of crack morphology: along the grain boundary direction, known as intergranular fracture ； Cracking that passes through the grains is called transgranular fracture ； There are also mixed types, such as those where the main crack is intergranular while the secondary cracks or tips are transgranular; this is one of the most dangerous forms of corrosion and can lead to sudden failures. Corrosion fatigue: Fracture caused by the combined effect of corrosion and alternating stress (stress that changes direction periodically, also known as cyclic stress) is referred to as corrosion fatigue. Corrosion fatigue occurs most easily in environments where pitting can take place. Hydrogen corrosion: includes hydrogen blistering and hydrogen embrittlement. Hydrogen bubbling: In low-strength steels, especially those containing a large amount of non-metallic inclusions, the hydrogen atoms generated in the solution can easily diffuse into the metal. Most of this hydrogen combines to form H2 and escapes through the wall of the container, but a small amount of it remains in the voids within the steel, where it also combines to form H2. Since hydrogen molecules cannot diffuse, they accumulate, creating high internal pressures that cause the surface of the steel to bubble up or even rupture ； Hydrogen embrittlement: In high-strength steels, the lattice undergoes significant deformation; when hydrogen enters, the lattice structure changes further, reducing toughness and ductility and leading to embrittlement, which can result in fracture under external forces.