Regarding the process of chloride ion corrosion

The last edit to this post was made by xyz2009xyz on 2010-1-21 at 14:43. Explanation 1 for why stainless steel is not resistant to chloride ion corrosion----------------- The mechanism by which chloride ions damage the passivation film of stainless steel: Metals in a passive state still possess a certain degree of reactivity; that is, the dissolution and restoration (re-passivation) of the passivation film are in dynamic equilibrium. When the medium contains active anions (such as chloride ions), the equilibrium is disrupted and dissolution becomes dominant. Chloride ions can selectively adsorb preferentially on the passivation film, displacing oxygen atoms, and then combine with the cations in the passivation film to form soluble chlorides. As a result, small pits (with pore sizes ranging from 20 to 30 μm) are formed at specific points on the newly exposed base metal. These small pits are known as pitting nuclei, and can also be considered the active centers where pitting occurs. 　　The presence of chloride ions has a direct destructive effect on the passive state of stainless steel. By examining the potential-current relationship curve obtained using the potentiostatic method on stainless steel specimens in solutions containing different concentrations of chloride ions, it can be seen that when the anodic potential reaches a certain value, the current density drops suddenly, indicating the onset of the formation of a stable passivation film. This film has a relatively high resistance and remains within a specific potential range (the passivation region). As the chloride ion concentration increases, the critical current density rises, the primary passivation potential also increases, and the size of the passivation region decreases. The explanation for this property is that within the passivation potential region, chloride ions compete with oxidizing agents and penetrate into the film, thereby creating lattice defects and reducing the resistivity of the oxide. Therefore, in the presence of chloride ions, it is neither easy to achieve passivation nor to maintain it. 　　While the local passivation film is damaged, the remaining protective films remain intact, which enables and enhances the conditions for pitting. According to the electrochemical generation mechanism, stainless steel in the activated state has a much higher electrode potential than that in the passivated state; the electrolyte solution thus meets the thermodynamic conditions for electrochemical corrosion, with the activated stainless steel acting as the anode and the passivated stainless steel serving as the cathode. The corrosion site affects only a small portion of the metal, while the remaining surface constitutes a large cathodic area. In electrochemical reactions, the cathodic and anodic reactions occur at the same rate; as a result, the corrosion rate at the anodic corrosion site is extremely high, leading to significant penetration, which in turn results in pitting corrosion. Explanation 2----------------- The corrosion mechanism of CL- on austenitic stainless steel: It can form complexes with Cr in the stainless steel at the intergranular regions of the austenite structure. This results in chromium-depleted areas at those intergranular sites, causing corrosion and damage to the stainless steel starting from those areas. This is what is known as intergranular corrosion. Therefore, austenitic stainless steel should not be used in environments containing CL-. Alternatives to consider include: in neutral environments – HC, duplex stainless steel, titanium; in acidic environments – HB is better than HC, while titanium seems unsuitable (e.g., in hydrochloric acid). This is just a preliminary suggestion; I hope those with more accurate information can supplement it

You can refer to stainless steel surface treatment techniques