Destruction of the passivation film of stainless steel by chloride ions:
The metal in the passive state still has a certain reactivity, that is, the dissolution and repair (repassivation) of the passivation film are in a dynamic equilibrium state. When the medium contains active anions (commonly such as chloride ions), the equilibrium is disrupted and dissolution prevails. The reason is that chloride ions can preferentially and selectively adsorb on the passivation film, expel oxygen atoms, and then combine with the cations in the passivation film to form soluble chloride, which results in the formation of specific points on the newly exposed base metal. Small corrosion pits (the pore size is mostly 20~30μm), these small corrosion pits are called pit corrosion nuclei, which can also be understood as the active center of pit formation.
The presence of chloride ions directly destroys the passive state of stainless steel. The metal passivation area decreases with the increase of chloride ion concentration. A-no chloride ion; B-low concentration of chloride ion; C-high concentration of chloride ion For metals showing passivation, the effect of chloride ion on the anodic polarization curve: from the stainless steel in solutions containing different concentrations of chloride ion It can be seen from the potential-current relationship curve of the sample measured by the potentiostatic method that the anode potential reaches a certain value, and the current density suddenly decreases, indicating that a stable passivation film begins to form, and its resistance is relatively high, and in a certain potential region ( passivation zone).
The figure shows that with the increase of chloride ion concentration, the critical current density increases, the primary passivation potential also increases, and the range of the passivation region is narrowed. The explanation for this characteristic is that in the passivation potential region, chloride ions compete with oxidizing species and enter the film, thus generating lattice defects and reducing the resistivity of the oxide. Therefore, in the presence of chloride ions, it is not easy to generate passivation, nor to maintain passivation.
