Under certain conditions, when a certain corrosive medium acts at the same time, the stainless steel plate will produce stress corrosion. When the tensile stress is lower than the strength limit, brittle cracking will occur, which is called stress corrosion. The stress corrosion time is very short, but it is extremely destructive. First of all, due to the effect of corrosive medium in the surrounding area, the corrosion-sensitive parts of the stainless steel plate will form tiny pits.
Then small pits are formed under the action of residual stress. Fine cracks are formed, and the cracks spread rapidly, eventually leading to corrosion cracks. Such corrosive media include nitric acid, ammonium nitrate, calcium bromide, hydrochloric acid, hydrofluoric acid, potassium hydroxide and solutions containing chloride ions. The corrosion-sensitive part refers to the transition zone between activation and passivation, the incomplete potential range of the passivation film.
The initial stage of stress corrosion formation and pitting corrosion is the same as crevice corrosion, both of which are carried out in convective, closed micro-zones, also known as closed pore corrosion. Once the micro-cracks are formed, there will be a narrow moving channel inside the stainless steel plate. Under the action of tensile stress, the film at the front end of the movable channel is repeatedly broken intermittently, and corrosion proceeds along a path perpendicular to the tensile stress.
In the occluded area (crack tip), hydrogen is released due to anion corrosion, and a part of the hydrogen may diffuse into the tip metal, resulting in embrittlement and brittle fracture. Under the repeated action of corrosion and brittle fracture, cracks develop rapidly. There are two main types of cracks. One is cracking along the grain boundary, called intergranular cracking, and the other is passing through the grain boundary, called penetrating crystal cracking.