1. The effect of Cl- on metal corrosion is manifested in two aspects: one is to reduce the possibility of forming a passivation film on the surface of the material or to accelerate the destruction of the passivation film, thereby promoting local corrosion; on the other hand, it reduces the solubility of CO2 in aqueous solution. , So as to alleviate the corrosion of the material.
Cl- has the characteristics of small ion radius, strong penetrating ability, and strong adsorption by metal surface. The higher the concentration of Cl-, the stronger the conductivity of the aqueous solution and the lower the resistance of the electrolyte. The easier it is for Cl- to reach the metal surface and accelerate the process of local corrosion; the presence of Cl- in an acidic environment will form chlorides on the metal surface Salt layer, and replace FeCO3 film with protective properties, resulting in high pitting corrosion rate. During the corrosion process, Clˉ not only accumulates in pitting pits, but also accumulates in areas where pitting pits are not produced. This may be the early process of pitting pit formation. It reflects that the electric double layer structure at the interface between the matrix iron and the corrosion product film is easy to preferentially adsorb Clˉ, which makes the concentration of Clˉ at the interface increase. In some areas, Clˉ will accumulate and form nuclei, leading to accelerated anodic dissolution in this area. In this way, the metal matrix will be corroded by deep digging down, forming pitting pits. The dissolution of anode metal will accelerate the diffusion of Clˉ through the corrosion product film into the pitting pits, and further increase the concentration of Clˉ in the pitting pits. This process belongs to Clˉ The catalytic mechanism is that when the Clˉ concentration exceeds a certain critical value, the anode metal will always be in an activated state and will not be passivated. Therefore, under the catalysis of Clˉ, the pitting pits will continue to expand and deepen. Although the Na content in the solution is relatively high, the energy spectrum analysis of the corrosion product film did not find the existence of Na element, indicating that the corrosion product film has a certain role in the diffusion of cations to the metal direction; while the anion is relatively easy to penetrate. The over-corrosion product film reaches the interface between the substrate and the film. This indicates that the corrosion product film has ion selectivity, leading to an increase in the anion concentration at the interface.
2. The corrosion of austenitic stainless steel by chloride ions mainly causes pitting corrosion.
Mechanism: Chloride ions are easily adsorbed on the passivation film, squeezing out the oxygen atoms, and then combine with the cations in the passivation film to form soluble chlorides. As a result, a small pit is corroded on the exposed body metal. These small pits are called pitting nuclei. These chlorides are easily hydrolyzed, so that the pH value of the solution in the small pit will drop, and the solution will become acidic, dissolving a part of the oxide film, resulting in excess metal ions. In order to corrode the electrical neutrality in the pit, the external Cl- ions continue to go into the air. Internal migration, the metal in the void is further hydrolyzed. In this cycle, the austenitic stainless steel continues to corrode faster and faster, and develops toward the depth of the hole until a perforation is formed.
3. Cl- has a catalytic effect on crevice corrosion. When corrosion begins, the iron loses electrons at the anode. With the continuous progress of the reaction, iron continuously loses electrons, a large amount of Fe2 accumulates in the gap, and oxygen outside the gap is not easy to enter. The highly mobile Cl- enters the gap and forms high-concentration, highly conductive FeCl2 with Fe2, and FeCl2 is hydrolyzed The generation of H causes the pH value in the crevice to drop to 3 to 4, thereby intensifying corrosion.
Post time: Aug-12-2021