Stainless steel pipe in the passivated state will still have a certain ability to react because the passive film is always in a dynamic equilibrium state of dissolution and repair and passivation. When the stainless steel pipe contains the active anion in the environmental media, the equilibrium state is easily destroyed, and the dissolved state at this time is more dominant.
The reason for this happening is that the active anions such as chloride ions are preferentially adsorbed on the passive film on the surface of the stainless steel pipe, and squeeze out the original oxygen atoms, and then combine with the cations in the passive film to form soluble chlorides. The resulting result is that small etch pits will be formed at specific spots of newly exposed base metal. Such small pits have pore diameters of about 20-30 μm. These small pits are called pit nucleuses. Understand the formation of active centers for pitting. The presence of chloride ions can directly destroy the passive surface of the stainless steel pipe. Usually, the passivation area of the surface of the stainless steel pipe decreases with increasing chloride ion concentration.
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 started to form a stable passive film, 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 media increases, the critical current density increases, the primary passivation potential also increases, and the passivation zone is reduced. The explanation for this characteristic is that in the passivation potential region, the chloride ions compete with the oxidizing species and enter the thin film, thereby forming lattice defects and reducing the resistivity of the oxide. Therefore, in the presence of chloride ion in an environmental medium, neither passivation nor passivation is easily maintained.
While the partial passivation film of the stainless steel pipe is damaged, the rest of the 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. The electrolyte solution achieves 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 part of the metal, while 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 at the anode etching point will be very fast, and the penetrating effect will be obvious, so that pitting corrosion will be generated.
Source: China Stainless Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)