【Daily Question】1.13 Differences in the conditions for stress corrosion and corrosion fatigue (3 wealth points for a reply; +3 to -8 wealth points for a correct answer)

Stress corrosion requires tensile stress and specific media； Corrosion fatigue requires variable stress and a corrosive medium.

Both stress corrosion and corrosion fatigue occur in the presence of both stress and a corrosive environment. Stress corrosion requires tensile stress and a specific medium; corrosion fatigue requires varying stress and a corrosive medium. Tensile stress is a necessary condition for the occurrence of stress corrosion cracking. Fracture of metal parts caused by the combined effect of alternating stress and corrosive media is known as corrosion fatigue fracture.

When stress corrosion occurs, a specific corrosive environment and tensile stress act on the metal. When corrosion fatigue occurs, the medium acting on the metal is not specific. Corrosion fatigue can occur when there are media that corrode metal materials along with cyclic alternating stresses, making it even more widespread.

The differences in the conditions for stress corrosion and corrosion fatigue are as follows: 1. Stress corrosion cracking: Failure of a material under the combined action of stress and a corrosive environment; abbreviated as SCC. a. Three necessary conditions – stress (usually tensile stress), a corrosive environment, and a material that is susceptible to such effects. b. Important influencing factors – temperature, composition of the medium, material composition, microstructural state, and stress levels. c. Sources of stress – working loads, residual welding stresses, cold deformation stresses, thermal stresses, etc. d. Characteristics of cracking – the cracks occur perpendicular to the direction of the main stress source, and branching often occurs during the crack propagation process. 2. Corrosion fatigue: Failure that occurs under the combined action of alternating stress and a corrosive environment. a. It mainly occurs in vibrating components such as pump shafts, rods, propeller shafts, oil and gas well pipes, as well as heat exchanger tubes and boiler tubes that are subjected to alternating thermal stresses. b. Characteristics of the fracture surface: The macroscopic fracture surface shows some similarities to that of fatigue fractures, but obvious corrosion products can be seen on the fracture surface. The deeper the crack and the more severe the notch effect, the higher the stress level at the tip, the higher the corrosion potential, and the more severe the corrosion becomes. c. Stainless steel can suffer from corrosion fatigue in any corrosive medium. As the strength of the steel increases, fatigue fracture in stainless steel either disappears or its service life is extended, which indicates that the original fracture was due to mechanical fatigue ； d. If the corrosion resistance of the steel is improved or the effect of corrosive agents is eliminated, resulting in the disappearance of fatigue fracture or an extension of the service life, it can be concluded that the original fracture was caused by corrosion fatigue. e. Corrosion fatigue can involve either a single crack or multiple cracks existing simultaneously (multiple nucleation sites). f. Stress corrosion can be accurately distinguished from corrosion fatigue based on the characteristics of the fracture surface. g. The fracture surfaces are usually serrated and stepped in shape ； Microscopically, cracks generally have no branches and their tips are blunt

Stress corrosion requires tensile stress and specific media； Corrosion fatigue requires variable stress and a corrosive medium.

What is the difference between the conditions for stress corrosion and corrosion fatigue? Stress corrosion refers to the phenomenon in which metals develop cracks under various tensile loads due to the influence of specific corrosive agents; during this process, no obvious corrosion products are formed in the material, and brittle fracture occurs. Corrosion fatigue refers to the phenomenon of low deformation and predominantly intergranular fracture in metals caused by the interaction between mechanical cyclic loading and corrosion.

Stress corrosion requires tensile stress and specific media； Corrosion fatigue requires variable stress and a corrosive medium.

Stress corrosion: It is a form of failure that occurs in stress-sensitive materials under the combined influence of stress and corrosive conditions. There are residual stresses in the welds, stresses generated locally by the inherent loads during equipment operation, as well as assembly forces; these are mainly tensile stresses. CL ions, H2S, ammonia, and alkaline environments cause corrosion of the equipment materials, leading to rapid failure in areas where stresses exist. Corrosion fatigue: It can occur in all materials under a sufficient number of sufficiently large alternating loads, independent of the environmental medium; fatigue failure will occur as long as the amplitude and frequency of the alternating loads exceed certain limits. The fracture pattern is different from that of stress corrosion fracture.

What is the difference between the conditions for stress corrosion and corrosion fatigue? Answer: The difference in conditions between stress corrosion and corrosion fatigue is that stress corrosion requires tensile stress and a specific medium ； Corrosion fatigue requires variable stress and a corrosive medium.

Stress corrosion cracking: Fracture of a material under the combined action of stress and a corrosive environment; corrosion fatigue: Failure that occurs under the combined effect of alternating stress and a corrosive environment. Fatigue can involve either a single crack or multiple cracks simultaneously, while stress corrosion requires tensile stress along with the presence of a specific corrosive agent； Corrosion fatigue requires variable stress and a corrosive medium.