Causes of pitting corrosion in stainless steel pipes in passivated state

The stainless steel pipe in the passivated state will still have a certain degree of reactivity because its passivation film is always in a dynamic equilibrium state of dissolution, repair and re-passivation. When the environmental medium in which the stainless steel pipe is located contains active anions, the equilibrium state is easily destroyed, and the dissolved state is more dominant at this time.
The reason why this happens is that active anions such as chloride ions will preferentially adsorb on the passivation film on the surface of the stainless steel pipe, crowd out the original oxygen atoms, and then combine with the cations in the passivation film to form soluble chloride. , the result is that small corrosion pits will be formed at specific points of the newly exposed base metal. The pore diameter of such small corrosion pits is mainly around 20-30μm. These small corrosion pits are called pitting corrosion nuclei, and they can also be It is understood as the active center for the formation of corrosion holes. The presence of chloride ions will directly destroy the passivity on the surface of stainless steel pipes. Generally, the scope of the passivation zone on the surface of stainless steel pipes will decrease as the chloride ion concentration increases.
In practical applications, when the anode potential in the environmental medium reaches a certain value, the current density will suddenly become smaller, which indicates that a stable passivation film has begun to form on the surface of the stainless steel pipe, and the corresponding resistance will be relatively high, and at a certain potential long-term maintenance within the region. However, as the concentration of chloride ions in the environmental medium increases, its critical current density will increase, the primary passivation potential will also increase, and the scope of the passivation zone will be reduced. The explanation for this characteristic is that in the passivation potential region, chloride ions compete with oxidizing substances and enter the film, thereby forming lattice defects and reducing the resistivity of the oxide. Therefore, in the environmental medium where chloride ions exist, it is neither easy to form passivation nor easy to maintain passivation.
While the local passivation film of the stainless steel pipe is destroyed, the rest of the protective film remains intact, which enables the conditions for pitting corrosion to be realized and strengthened. According to the electrochemical formation mechanism, the electrode potential of stainless steel in the activated state is much higher than that of stainless steel in the passivated state. The electrolyte solution reaches the thermodynamic conditions of electrochemical corrosion. The activated stainless steel becomes the anode and the passivated stainless steel serves as the cathode. The corrosion point involves only a small portion of the metal, and the rest of the surface will be a large cathodic area. In the electrochemical reaction, the cathodic reaction and the anode reaction are carried out at the same speed, so the corrosion rate concentrated on the anode corrosion point will be very fast and the penetration effect will be obvious, so that pitting corrosion will occur.