The stainless steel pipe in the passivated state still has a certain reaction ability because its passivation film is always in a state of dynamic equilibrium of dissolution and repair and repassivation. When the stainless steel pipe is in an environmental medium containing active anions, the equilibrium state is easily destroyed, and the dissolved state at this time is more dominant.
The reason for this is that a living anion such as chloride ion preferentially adsorbs on the passivation film on the surface of the stainless steel pipe, and discharges the original oxygen atom, and then combines with the cation in the passivation film to form a soluble chloride. The result is that small etch pits are formed at specific points of the newly exposed base metal. The pore size of such small etch pits is mainly around 20-30 μm. These small etch pits are called pitting cores. Understand the active center formed by the etched holes. The presence of chloride ions acts as a direct destructive effect on the passive state of the stainless steel pipe surface. Generally, the range of the passivation zone on the surface of the stainless steel pipe decreases 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 the surface of the stainless steel pipe has begun to form a stable passivation film, and the corresponding resistance will be relatively high, and at a certain potential Long-term maintenance in the region. However, as the concentration of chloride ions in the environmental medium increases, the critical current density increases, the primary passivation potential also increases, and the range of the passivation region is reduced. The explanation for this characteristic is that in the passivation potential region, chloride ions compete with the oxidizing species and enter the film, thereby forming lattice defects and reducing the resistivity of the oxide. Therefore, in an environmental medium in which chloride ions are present, it is neither easy to form passivation nor easy to maintain passivation.
When the local passivation film of the stainless steel pipe is damaged, the remaining protective film remains intact, which enables the pitting conditions to be achieved and strengthened. According to the electrochemical formation mechanism, the stainless steel in the activated state has a much higher electrode potential than the passivated stainless steel, and the electrolyte solution reaches the thermodynamic condition of electrochemical corrosion, the activated stainless steel becomes the anode, and the passivated stainless steel acts as the cathode. The corrosion point involves only a small portion of the metal, and the other surface will be a large cathode area. In the electrochemical reaction, the cathode reaction and the anode reaction are carried out at the same speed, so the corrosion rate concentrated on the corrosion point of the anode is fast, and the penetration effect is remarkable, so that pitting corrosion is generated.
Source: China Stainless Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)