What is stress corrosion?

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The cracking of materials or parts caused by the combined action of stress and corrosion environment is called stress corrosion cracking, which is the result of the combined action of stress and corrosion. If there is only one aspect, stress or medium, damage will not occur, but when the two act together, cracking can occur quickly. Therefore, when stress corrosion occurs, the stress is very low and the corrosiveness of the medium is also very weak. Because of this, stress corrosion is often ignored, resulting in "unexpected" accidents that continue to occur, causing huge harm and losses. ◆Category 1. Pitting corrosion is a form of localized corrosion that leads to corrosion. 2. Intergranular corrosion The grain boundaries are disordered and misaligned boundaries between grains with different crystallographic orientations. Therefore, they are favorable areas for the segregation of various solute elements in steel or the precipitation of metal compounds (such as carbides and δ phases). Therefore, it is not surprising that in some corrosive media, the grain boundaries may be corroded first. This type of corrosion is called intergranular corrosion, and most metals and alloys may exhibit intergranular corrosion in certain corrosive media. 3. Crevice corrosion is a form of local corrosion, which may occur in the gaps where the solution stagnates or in the shielded surface. Such gaps can form at metal-to-metal or metal-to-nonmetal joints, for example, where they come into contact with rivets, bolts, gaskets, valve seats, loose surface deposits, and marine growth. 4. General corrosion is a term used to describe the corrosion phenomenon that occurs in a relatively uniform manner on the entire alloy surface. When comprehensive corrosion occurs, the material gradually becomes thinner due to corrosion, or even the material fails due to corrosion. Stainless steel may show general corrosion in strong acids and alkalis. Failure due to general corrosion is less of a concern because it can usually be predicted by a simple immersion test or by reviewing the corrosion literature. ◆Features 1. What causes stress corrosion damage is static stress, which is far lower than the yield strength of the material, and is generally tensile stress. 2. The damage caused by stress corrosion is brittle fracture without obvious plastic deformation. 3. Stress corrosion will only occur when a specific alloy composition is combined with a specific medium. 4. The crack growth rate of stress corrosion is generally 10-9~10-6m/s. It is a bit like fatigue and is gradual and slow. This subcritical expansion condition reaches a certain critical size, and when the remaining section cannot bear the external load, a sudden fracture occurs. 5. Stress corrosion cracks mostly originate from surface corrosion pits, and the crack propagation path is often perpendicular to the tensile axis. 6. The fracture surface damaged by stress corrosion is dark in color and there are often corrosion products on the surface. 7. The main crack of stress corrosion often expands with branches. 8. Fracture caused by stress corrosion can be transgranular fracture or intergranular fracture. ◆Measures 1. Reasonable selection of materials. It is a basic principle to select materials that are resistant to stress corrosion according to the stress and usage conditions of the parts. For example, copper is highly susceptible to stress corrosion by ammonia. Therefore, copper alloys should be avoided for parts that come into contact with ammonia. ； For another example, in high-concentration chloride media, low-carbon high-chromium ferritic stainless steel containing no nickel or copper or only trace amounts of nickel or copper, or chromium-nickel stainless steel with higher silicon content can generally be used. Nickel-based and iron-nickel-based corrosion-resistant alloys can also be used. In pressure pipeline engineering, the following medium combinations are prone to stress corrosion and should be avoided: Picture 2. Reduce or eliminate residual tensile stress in parts. Residual tensile stress is an important condition for stress corrosion. For this reason, the design should minimize the stress concentration on the parts. From a process perspective, heating and cooling should be uniform, and if necessary, an annealing process should be used to eliminate internal stress. Alternatively, shot peening or surface heat treatment can be used to generate a certain residual compressive stress on the surface of the part, which is also effective in preventing stress corrosion. 3. Improving medium conditions can be considered from two aspects: On the one hand, try to reduce or eliminate harmful chemical ions that promote stress corrosion cracking. For example, through water purification treatment, reducing the chloride ion content in cooling water and steam is very effective in preventing chlorine embrittlement of austenitic stainless steel. ； On the other hand, corrosion inhibitors can also be added to the corrosive medium. For example, adding 300×10-6mol/L phosphate to high-temperature water can improve the stress corrosion resistance of chromium-nickel austenitic stainless steel. * * improve. 4. Use electrochemical protection. Since metal in the medium will only produce stress corrosion within a certain electrode potential range, the method of external potential is used to keep the potential of the metal in the medium away from the stress corrosion sensitive potential area. This is also a measure to prevent stress corrosion. Generally, the cathodic protection method is used. However, this protection method cannot be used for high-strength steel and other materials that are sensitive to hydrogen embrittlement. Sometimes it is also very effective to use the sacrificial anode method for electrochemical protection.