Stress corrosion mechanism of brazed plate heat exchanger

Stress corrosion cracking of metal materials refers to the cracking of metal under the combined action of tensile stress and specific corrosive medium. At present, it has become the main form of corrosion damage of stainless steel parts in brazed plate heat exchanger. In recent 30 years, due to many factors affecting stress corrosion cracking, scholars at home and abroad have carried out extensive research on it and achieved many important research results. Although scholars have put forward many theories, they have not yet obtained a complete mechanism. The slip dissolution mechanism introduced here is widely supported.

We know that the alloy surface is usually covered with a protective film, which can be as thin as atomic layer or visible thickness. Under the action of active ions, metallurgical defects and tensile stress in the medium, the protective film breaks locally and forms small corrosion holes or cracks. When the metal is exposed to the film crack, its potential is negative and becomes an anode. At the same time, if there is no tensile stress, the blocking battery can only form pitting corrosion or crevice corrosion. With the existence of tensile stress, the protective film produced by the exposed part will continue to crack and the crack will continue to develop. If the exposed metal still cracks after cracking, the corroded surface will no longer be passivated and the corroded surface will continue to expand. The decrease of tensile stress concentration leads to slow crack development. If the bare metal surface is passivated immediately after fracture, stress corrosion cracks will not appear. However, when the surface film breaks and the bare metal surface corrodes to a certain extent, it will be passivated again, that is, the passivation ability of the bare metal is within an appropriate range. Both sides of the crack will be passivated again to form a protective film and inhibit the corrosion of the extended surface area. Although the crack can be passivated again, the film will continue to crack and the crack will develop further due to the action of tensile stress.