【Daily Question】12.26: Determine the mechanism of stress corrosion cracking (3 rewards for replies; +3 to –8 rewards for correct answers)

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Stress corrosion fracture is caused by the combination of mechanical failure of metals under stress and corrosive damage under the action of chemical media (Wrong)

Stress corrosion fracture is caused by the combination of mechanical failure of metals under stress and corrosive damage under the action of chemical media (Wrong)

Stress corrosion fracture is caused by the combination of mechanical failure of metals under stress and corrosive damage under the action of chemical media (True)

Stress corrosion fracture is caused by the combination of mechanical failure of metals under stress and corrosive damage under the action of chemical media (True)

Error: Stress corrosion fracture is not caused by the combination of mechanical failure of a metal under stress and corrosive damage under the action of chemical agents; rather, it is a type of fracture that occurs as a result of the combined effect of stress and chemical agents, following specific mechanisms.

Hide stress corrosion fracture is a failure phenomenon resulting from the combination of mechanical degradation of metals under stress and corrosive degradation under the action of chemical media. The mechanism of common stress corrosion is as follows: under the action of stress (mainly tensile stress; compressive stress has little effect on stress corrosion) and a corrosive medium, the oxide film on the surface of the part is corroded and damaged. The damaged surface and the undamaged surface act as the anode and cathode respectively; the metal at the anode is dissolved into ions, generating an electric current that flows toward the cathode. Since the anode area is much smaller than that of the cathode, the current density at the anode is high, further corroding the already damaged surface. With the effect of tensile stress, cracks gradually form at the failure site, and these cracks expand over time until fracture occurs. Such cracks can not only propagate along the grain boundaries of metals, but also through the grains. Experimental studies on the stress corrosion process show that applying an anodic current to a metal can exacerbate stress corrosion, whereas applying a cathodic current can stop it. /hide

Stress corrosion fracture results from the combination of mechanical failure due to stress and corrosive damage caused by chemical agents acting on metals (correct). The characteristics of stress corrosion in metal materials can be explained from four aspects: 1. Stress. The stress applied in this context is mainly the static component; it can be caused by external loads or assembly forces (such as the force required to tighten bolts or expansion forces), or it can be internal stresses generated during processes such as machining, heat treatment, and welding. Regardless of the source, the stress that causes stress corrosion cracking must contain a component of tensile stress; compressive stress does not cause stress corrosion cracking. Furthermore, this stress is usually relatively mild. In an environment free from corrosion, such low stresses will not cause mechanical failure of the components. The stress value that causes failure must be determined based on specific conditions such as the material and the corrosive medium. 2. Corrosive media: The materials and media that cause stress corrosion are not arbitrary; stress corrosion occurs only when there is a certain combination of these two elements. The corrosive agents that cause stress corrosion in ordinary steel include: hydrogen oxide solutions ； Aqueous solution containing *ao acids, carbonates, and hydrogen sulfide ； Seawater, sulfuric acid-*ao acid mixture ； Melted zinc, lithium ； Hot ferric chloride solution ； Liquid ammonia. The media that cause stress corrosion in austenitic stainless steels include acidic and neutral chloride solutions ； seawater ； molten chloride ； Hot fluoride solution ； Hydroxide solution of hydrogen on the day. 3. Materials: It is generally believed that extremely pure metals do not suffer from stress corrosion failure; it occurs only in alloys or metals containing impurities. 4. Destruction process a. Incubation stage. This is a period prior to the formation of stress corrosion cracks, preparing for crack nucleation. b. Stage of stable crack propagation. Under the combined action of stress and corrosive media, the crack expands slowly. c. Crack instability expansion stage. This is the final mechanical damage. Additionally, stress corrosion cracking in metallic materials is characterized by the fact that the cracking of the metal is independent of the metal’s thickness. The common situation of large thickness and slow corrosion (uniform corrosion) does not apply here. Therefore, delaying stress corrosion cracking by increasing the metal thickness is almost ineffective.

Stress corrosion fracture is caused by the combination of mechanical failure of metals under stress and corrosive degradation under the action of chemical media (V)