As a material basis for the survival and development of human beings, the corrosion of metal materials has brought huge property losses to human society every year, and also caused a large number of corrosion failure accidents. There are many forms of corrosion failure. According to relevant statistics, the proportion of metal material failure caused by stress corrosion is the largest.
Stress corrosion cracking is a kind of "catastrophic corrosion". Bridge collapses, plane crashes, oil tank explosions, and pipeline leaks related to stress corrosion have all brought huge losses of life and property to human beings.
When metal materials serve in various environments, corrosion will occur even without load, resulting in weight loss. When stress exists, corrosion in a specific environment will cause crack nucleation and expansion, resulting in hysteresis cracking, which is called stress corrosion.
Stress corrosion cracking (SCC) is different from cracking caused by simple stress, it can also crack under very low load stress; it is also different from simple corrosion, even a weakly corrosive medium can cause stress corrosion cracking.
Stress corrosion cracking is an "insidious" form of corrosion that produces a significant reduction in mechanical strength with minimal metal loss. Damage so subtle that it is difficult to detect by casual inspection, stress corrosion cracking can trigger rapid mechanical fracture and even catastrophic failure of components and structures.
In the process of stress corrosion, if microcracks are formed, their growth rate is several orders of magnitude faster than that of other types of localized corrosion.
Conditions for stress corrosion to occur
The occurrence of stress corrosion cracking needs to meet three necessary conditions at the same time:
a) Sensitive materials. As the saying goes, flies do not bite seamless eggs, and stress corrosion cracking must first be made of sensitive materials. The sensitivity of the material gives us a hint that in some working conditions where stress corrosion cracking may occur, the material selection should be cautious. Austenitic stainless steel such as 304 is in service in an environment containing chlorine, and stress corrosion cracking is a problem that should be paid special attention to. Austenitic stainless steels are face-centered cubic metals, and face-centered cubic metals are especially prone to stress corrosion cracking, which is determined by the crystal structure.
b) Environments prone to stress corrosion cracking. Even if the material is sensitive, there is no medium that causes stress corrosion cracking, and stress corrosion cracking will not occur. It is like the A/B side of a coin. Environmental medium is also an important condition for stress corrosion cracking.
c) Sufficient tensile stress. It is generally considered that static tensile stress is a necessary condition for stress corrosion cracking. Some people may ask, what about alternating loads? I think it may be due to corrosion fatigue. Why is it said that sufficient static tensile stress is required for stress corrosion cracking? Because it needs to meet the critical stress intensity factor KISCC in the corrosion state.
Crack morphology mainly includes the following aspects:
1. Cracks usually originate from the metal surface. The crack direction is "dendritic", and the trunk is the main crack, pointing to the source of the crack along the convergence direction
2. Crack growth may be transgranular, intergranular, or mixed; there are often corrosion products on the fracture surface
3. Transgranular extended fractures are mostly cleavage, quasi-cleavage, sometimes mixed with intergranular or dimples
4. The intergranular extended fracture is in the shape of rock sugar, sometimes mixed with a small amount of quasi-cleavage or intergranular and dimples
