What the hell is stress corrosion? (one)

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This post was last edited by B0SS on 2020-1-31 12:47 Corrosion is the most common phenomenon in the chemical industry. Let’s introduce its composition below: Local corrosion and total corrosion. Localized corrosion is further divided into stress corrosion cracking, pitting corrosion, intergranular corrosion, corrosion fatigue and crevice corrosion. Pitting corrosion: Pitting corrosion, referred to as pitting corrosion, refers to an extremely local corrosion form in which small holes are formed on the metal surface in the corrosive medium, also known as pitting corrosion. Corrosion holes exist in isolation, and some are compactly together. Pitting corrosion causes little weight loss in the metal, but can perforate and damage the equipment. Piting corrosion is a unique form of anode reaction in electrochemical reactions. If the metal is in a passive state in this medium, the active anions (such as chloride ions) contained in the medium can still be preferentially and selectively adsorbed on the passivation film, and combine with the cations in the passivation film at specific points (such as defects and impurities) to form soluble compounds. Small corrosion spots will occur there and become pitting corrosion nuclei (about 20~30pm). If it continues to develop, it may become a corrosion hole. intergranular corrosion: Intergranular corrosion is a type of localized corrosion. Corrosion that extends inward along the interface between metal grains. Mainly due to the difference in chemical composition between the surface and interior of the grain and the existence of grain boundary impurities or internal stress. Intergranular corrosion destroys the bond between grains, * * Reduce the mechanical strength of metal. Moreover, after corrosion occurs, the surfaces of metals and alloys still maintain a certain metallic luster, and no signs of damage can be seen. However, the bonding force between grains is significantly weakened, the mechanical properties deteriorate, and cannot withstand knocking, so it is a very dangerous corrosion. Usually found in brass, duralumin alloys and some stainless steels and nickel-based alloys. Corrosion fatigue is the phenomenon of crack formation and expansion under the interaction between alternating loads and corrosive media. The reduction of fatigue resistance due to the action of corrosive media (fatigue strength refers to the maximum stress that a material can exert without causing damage under infinitely many alternating loads, which is called fatigue strength or fatigue limit. In fact, it is impossible for metal materials to undergo infinite number of alternating load tests). Under alternating loads, fatigue damage first occurs on the surface, and eventually fracture or leakage occurs under the action of continuous corrosion rings. Corrosion fatigue can occur in materials that are either sensitive or insensitive to stress corrosion, so no metal or alloy is immune to corrosion fatigue. Corrosion fatigue cracks are usually transgranular. One difference from stress corrosion is the stress intensity factor of the crack (the physical quantity that reflects the strength of the elastic stress field at the crack tip is called the stress intensity factor). Cracks will propagate with time even if they are less than the critical stress intensity factor value (KISCC) for simple stress corrosion. The final fracture stage of corrosion fatigue is purely mechanical and independent of the medium. Crevice corrosion refers to localized corrosion that occurs in crevices and other hidden areas on metal surfaces in corrosive media. Crevice corrosion often occurs in holes, gasket contact surfaces, lap joints, under sediments, and in fastener gaps. Metals that rely on oxide films or passivation layers to resist corrosion are particularly susceptible to this kind of corrosion. Crevice corrosion occurs in many media, especially oxygen-containing media. Crevice corrosion is also a type of electrochemical corrosion. General corrosion is the most common form of corrosion, which is characterized by corrosion being distributed over the entire surface of the metal, causing the metal to become thinner overall. The conditions for comprehensive corrosion to occur are: The corrosive medium can reach all parts of the metal surface evenly, and the composition and structure of the metal are relatively uniform. For example, the dissolution of carbon steel or zinc plates in dilute sulfuric acid and the corrosion of certain materials in the atmosphere are typical general corrosions. What the hell is stress corrosion? Stress corrosion refers to the failure phenomenon of materials, mechanical parts or components under the combined action of static stress (mainly tensile stress) and corrosion. It refers to the damage caused by metal in corrosive media under the action of tensile stress. This kind of corrosion generally passes through the crystal grains (metal grains refer to a number of crystals formed by metal atoms that are continuously arranged according to certain rules with crystal nuclei as the core during the cooling and solidification process of liquid metal. ), the so-called transgranular corrosion. Stress corrosion is a material destruction process caused by the combined action of strain and corrosion caused by residual or applied stress. The breakage of materials caused by stress corrosion is called stress corrosion fracture. Characteristics of stress corrosion cracking: (1) The stress resistance caused by the external load borne by the component under working conditions ； (2) Internal stress caused by processing, manufacturing and heat treatment ； (3) Internal stress caused by assembly and installation ； (4) Thermal stress caused by temperature difference ； (5) The wedging effect within the crack caused by the volume effect of corrosion products can also generate the stress required for crack expansion. Material-medium combinations that produce stress corrosion cracking. As shown in the table below. Alloy media aluminum alloy chloride, humid industrial atmosphere, marine atmosphere copper alloy ammonium ions, amine nickel-based alloy hot concentrated sodium hydroxide, hydrofluoric acid vapor titanium alloy chloride, methanol, solid chloride at a temperature greater than 290°C low carbon steel boiling sodium hydroxide, boiling nitrate Steels for oil fields hydrogen sulfide, carbon dioxide low alloy high strength steel chloride austenitic stainless steel 300 series boiling chloride, boiling sodium hydroxide, polythionate ferritic and martensitic stainless steel (400 series) chloride, reactor cooling water maraging steel chloride