The unique feature of stress corrosion cracking is that there must be a phenomenon of metal cracking caused by the simultaneous action of a tensile stress and a specific corrosion (medium) environment. It has three main characteristics:
(1) There must be tensile stress, the greater the tensile stress, the shorter the time required for rupture. The stress required for rupture is generally lower than the yield strength of the material.
(2) The corrosion medium is specific. Only certain metal-medium combinations will undergo stress corrosion cracking.
(3) The rupture rate is within a certain range, which is much greater than the corrosion rate when there is no stress, and is far less than the rupture rate caused by purely mechanical factors.
When stress corrosion occurs, the uniform corrosion rate of the metal is very small. There must be corrosion before stress corrosion can occur. Therefore, stress corrosion is localized and is limited to the bottom of the notch or the root of the crack. In the typical corrosive environment of various metals, there is no obvious relationship between the harmful anions and their uniform corrosion rate to the metal. Under the action of tensile stress, it is easy to cause stress corrosion cracking in special corrosive environments.
Under the effect of tensile stress, when the heat-treated steel is in the boiling magnesium solution, although there are obvious corrosion channels along the grain boundary, it is actually suffered from the damage of the transcrystalline form due to stress corrosion cracking. Because there are not many studies on stress corrosion cracking, the mechanism that can best explain stress corrosion cracking is the anode dissolution mechanism. That is, when the alloy under tensile stress is exposed to corrosion in the corrosion environment, local dissolution occurs in the metal. Cracks, in addition, the applied stress effectively ruptures the brittle oxide film at the crack tip, bringing the newly exposed anode into contact with the corrosive medium, thereby causing corrosion. It is known from this theory that in the stress corrosion system, the more important role of stress is to destroy the protective film on the metal surface and form a local anode area. The anode area of ​​the metal surface directly contacts the corrosive medium to cause stress corrosion cracking.
Most of the corrosion media such as hydrogen sulfide, carbon dioxide, salt, formation water, etc. are electrochemical corrosion. When the buried steel gas pipeline under tensile stress is in contact with the electrolyte solution, due to uneven surface or buried steel The type, concentration and temperature of the electrolyte solution in different parts of the pipeline are different, so that the anode area appears in the metal exposed part, and the anode area is closed to each other through the buried steel pipe to form many corrosion batteries.
In actual production, stress corrosion cracking generally starts on the outer surface of the pipeline. The most common is where the coating is peeled off. Most of the soil in China is neutral, with a pH between 6 and 8. According to reports, stress corrosion The presence of NaZCq / NaHc field solution, or sometimes sodium bicarbonate crystals, underneath the cracked Xu layer is the easiest to induce stress corrosion cracking. When most engineering gas pipelines are buried, they are generally under cathodic protection to prevent uniform corrosion. Because cathodic protection steel is polarized, cathodic protection of underground pipelines can cause the accumulation of alkaline substances on the surface of the pipeline. The substance consists of NaoH, Nazeo3 / NaHeo3 solution. The hydrogen ions, metal cations and water containing dissolved oxygen in the solution enter the pipe surface through the gaps or defects in the coating. It can be seen that, to prevent stress corrosion cracking, the potential range must be controlled.
According to the above mechanism analysis, it can be seen that the stress corrosion cracking of buried gas pipelines mainly has the following three factors: stress action: the pipeline is subjected to external stresses such as its own gravity stress, oil and gas flow stress, thermal expansion and contraction deformation stress due to temperature changes, and Corrosion products and the effect of concentrated stress caused by the corrosion process. The combination of these types of stresses constitutes the tensile stress of the pipeline and strengthens the local cracks of the pipeline, resulting in stress corrosion cracking. Corrosion of metals: due to uneven distribution of alloy elements or impurities in the metal structure, it may also be caused by the difference in metal composition between the metal pipe and the configured pipe, or the difference in concentration and temperature of the electrolyte solution, etc. A potential difference is generated between the regions, which causes corrosion of the battery, and the combination of metal and corrosive medium causes stress corrosion cracking. The reason of anti-corrosion layer: the anti-corrosion layer of the pipeline is one of the effective measures to prevent corrosion of the pipeline. Its purpose is to separate the metal surface from the corrosive medium and cut off the corrosion circuit between the metal and the medium. However, because the buried pipeline is in the corrosive medium for a long time In the environment, it causes the aging and cracking of the anticorrosive layer, which exposes local metals, and eventually leads to stress corrosion cracking.
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