stress corrosion cracking

The failure of a material caused by the combined effect of stress and corrosive environments is called stress corrosion. It should be emphasized here the combined effect of stress and corrosion. 　　Failure caused by stress corrosion in materials typically exhibits the following characteristics: (1) The stress that leads to stress corrosion failure is a static stress, one that is much lower than the material’s yield strength; it is generally a tensile stress (although in recent years it has also been found that compressive stress can cause stress corrosion in stainless steels). This stress can be an applied stress, or it can be residual tensile stress resulting from welding, cold working, or heat treatment. The corrosion damage of the first cold-worked brass **shells** observed in a medium containing humid ammonia gas was caused by the residual tensile stress resulting from cold working. If stress-relief annealing is performed, such accidents can be avoided. 　　(2) Damage caused by stress corrosion is brittle fracture, with no significant plastic deformation. 　　(3) Stress corrosion occurs only when specific alloy compositions are combined with specific media. For example, α-brass will only corrode and be damaged in ammonia solution, whereas β-brass can crack in water. 　　(4) The crack growth rate in stress corrosion is generally between 10-9 and 10-6 m/s; it is somewhat similar to fatigue, being gradual and slow. This subcritical growth continues until a certain critical size is reached, at which point the remaining cross-section can no longer withstand external loads, leading to sudden fracture. 　　(5) Cracks caused by stress corrosion usually originate from surface pits, and the propagation path of these cracks is often perpendicular to the tensile axis. 　　(6) The fracture surface of stress corrosion failure is dull in color, with corrosion products often present on its surface, whereas the surface of a fatigue fracture is usually smoother and shinier, especially in the case of fresh fractures. 　　(7) The main crack propagation in stress corrosion often exhibits branching. But avoid forming absolute concepts; stress corrosion cracks are not always branched. 　　(8) Fracture caused by stress corrosion can be transgranular or intergranular. In the case of transgranular fracture, the fracture surface is cleaved or sub-cleaved, and the cracks exhibit herringbone or feather-like patterns. 　　The aforementioned characteristics of stress corrosion failure can help us determine whether a failure incident is due to stress corrosion. However, a comprehensive consideration is necessary; one should not draw simple conclusions based on just one characteristic.

By definition, stress corrosion can occur only in the presence of stress; generally, this is tensile stress (although compressive stress cannot be ruled out in some cases). There must also be a corrosive medium, and then there’s the material itself, which must be sensitive to that corrosive medium. In other words, a specific combination of the corrosive medium and the material is necessary for stress corrosion to take place. Of course, many such specific combinations have been identified to date. Since it is called stress corrosion cracking, the obvious result is that the material cracks. Of course, this type of cracking is different from pure stress cracking and corrosion fatigue cracking.

Stress corrosion cracking is a form of failure that occurs in metals under the combined effect of stress (tensile stress) and corrosion, along with certain temperature conditions. Stress corrosion is complex; in the absence of stress, corrosion is minimal ； Under stress, metals can crack even when corrosion is not severe. Since the cracking is brittle and occurs without any obvious warning signs, it can lead to catastrophic accidents. The main combinations of metal materials susceptible to stress corrosion failure and their environments are as follows: 1. Carbon steel and low-alloy steel: media such as alkaline solutions, nitrate solutions, anhydrous liquid ammonia, wet hydrogen sulfide, acetic acid, etc. 2. Austenitic stainless steel: chloride ions, chlorides + steam, wet hydrogen sulfide, alkaline solutions, etc. 3. Molybdenum-containing austenitic stainless steel: alkaline solutions, aqueous chloride solutions, solutions of sulfuric acid + copper sulfate, etc. 4. Brass: ammonia gas and its solutions, ferric chloride, wet sulfur dioxide, etc. 5. Titanium: methanol or ethanol containing hydrochloric acid, molten sodium chloride, etc. 6. Aluminum: wet hydrogen sulfide, hydrogen sulfide-containing substances, seawater, etc

Harm of stress corrosion cracking: Stress corrosion cracking is a common form of failure caused by the environment. DuPont Corporation in the United States analyzed 685 cases of damage to metal pipes and equipment that occurred over a period of 4 years, and found that nearly 60% of these cases were caused by corrosion. Among the damages resulting from corrosion, stress corrosion cracking accounted for 13.7%. According to extensive statistics from various countries, in cases of wet corrosion damage to stainless steel, stress corrosion cracking accounts for as much as 60%, making it the most common type of corrosion-related damage. The frequent occurrence of stress corrosion cracking and the severe damage it causes have drawn people’s attention.

Stress corrosion cracking: The brittle cracking that occurs in metal materials under the combined action of tensile stress and a corrosive environment is known as stress corrosion cracking. Stresses include the effect of external loads, thermal stresses, as well as residual stresses generated after hot or cold processing or welding. It is characterized by the appearance of cracks or even fractures, with the origin of these cracks often being at the bottom of pitting corrosion holes or corrosion pits; crack propagation can occur along grain boundaries, through the grains, or in a mixed manner ； The main crack is usually perpendicular to the stress direction and often has branches; its ends are sharp, the degree of corrosion on the inner walls of the crack and on the metal surface is generally mild, the expansion rate at the crack ends is fast, and the fracture surface exhibits characteristics of brittle fracture. Stress corrosion cracking can occur over a very wide range of materials; aside from pure metals, almost all alloys will experience stress corrosion cracking as long as they are in a certain environment. Chlorides, alkalis, and hydrogen sulfide are the three main agents that cause stress corrosion cracking. Equipment such as liquid ammonia storage tanks made of low-alloy steel, and carbon dioxide regeneration towers (using thermal potassium carbonate as a medium) in 300,000 tons/year ammonia synthesis plants may also suffer from stress corrosion failure.

Thank you all; I’ve learned something new again.

Thank you all for the introductions; it’s been very informative.:victory: