Corrosion mechanism of stress in brazed plate heat exchangers　

Stress corrosion cracking of metal materials refers to the cracking of metals under the combined action of tensile stress and specific corrosive media. At present, it has become the main form of corrosion damage to stainless steel components in brazed plate heat exchangers. In the past 30 years, due to the numerous factors affecting stress corrosion cracking, scholars at home and abroad have conducted extensive research on it and achieved many important research results. Although scholars have proposed many theories, the complete mechanism has not yet been obtained. The slip dissolution mechanism introduced here has received widespread support.

We know that the surface of alloys is usually covered with a protective film, which can be as thin as an atomic layer or visible thickness. Under the action of active ions, metallurgical defects, and tensile stress in the medium, the protective film locally fractures, forming small corrosion holes or cracks. Metal exposed at the crack of the film has a negative potential and becomes an anode. At the same time, if there is no tensile stress, the blocking effect of the battery can only form pitting or crevice corrosion. With the presence of tensile stress, the protective film generated by the exposed part will continue to crack, and the cracks will continue to develop. If the exposed metal still cracks after cracking, the corroded surface will no longer be passivated and will continue to expand. The concentration of tensile stress decreases, resulting in slow crack development. If the bare metal surface is passivated immediately after rupture, stress corrosion cracking will not occur. However, when the surface film ruptures and the exposed metal surface corrodes to a certain extent, it will be passivated again, that is, the passivation ability of the exposed metal is within an appropriate range. Both sides of the crack will be passivated again, forming a protective film and suppressing corrosion on the expanded surface area. Although the crack can be passivated again, due to the effect of tensile stress, the film will continue to crack and the crack will further develop.