Corrosion of carbon steel and stainless steel by chloride ions

It seems to be potential difference corrosion; I’m not exactly sure either. Additionally, when carbon steel and stainless steel are placed together, which one will corrode? Some say it corrodes carbon steel; so if carbon steel is damaged in that case, what’s there to worry about? @ylb913 @Fish in the Desert

Place carbon steel and stainless steel together, and it will contaminate the stainless steel

It is closely related to temperature, pressure, and concentration: Metals in a passive state still possess a certain degree of reactivity, meaning that 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. The reason is that 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 regarded as 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 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 causing 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. In pressure vessels made of austenitic stainless steel, stress corrosion can also occur in the presence of chloride solutions. This is because the chloride ions in the solution damage the passivation film on the surface of stainless steel; under tensile stress, cracks appear in the areas where the passivation film has been damaged, which become the anodic regions of corrosion cells. Continuous electrochemical corrosion can ultimately lead to the fracture of the metal. This type of corrosion is not greatly related to the concentration of chloride ions; even trace amounts of chloride ions can cause stress corrosion. In actual production, some equipment is not corroded and damaged under normal operating conditions, but rather develops stress corrosion cracks during shutdown due to low-concentration (5%) chloride condensate remaining in the containers.

The corrosion of stainless steel by chloride ions is a type of electrochemical corrosion; that is, if there are pits or gaps on the surface of the stainless steel, chloride ions will accumulate in those pits or gaps, while the positively charged ions inside the gaps move outward, thereby causing corrosion. Similarly, chloride ions also cause corrosion in carbon steel. The corrosion rate is related to the concentration of chloride ions.

Austenitic stainless steels can be corroded at levels above 25PPM.

It mainly corrodes stainless steel; generally, it doesn’t exceed 300

Stainless steel corrodes in chloride environments; when the chloride concentration is above 25 ppm, the steel is subject to stress corrosion, pitting corrosion, and intergranular corrosion. In fact, in engineering applications where there are high concentrations of chloride ions, ZS-711 inorganic anti-corrosion coating from Zhisheng Weihua is used to effectively prevent the corrosion of stainless steel caused by chloride ions. It also prevents corrosion resulting from chlorides containing dissolved oxygen; when the oxygen content is 1.0×10-6 or higher, such chlorides can cause corrosion of stainless steel. The inorganic anti-corrosion coating provides excellent protection against the corrosive effects of chloride ions. The corrosion of stainless steel by high concentrations of chloride ions causes stress corrosion in an environment of corrosive media containing oxygen and chloride ions. Stress corrosion failure accounts for about 45% of cases. Preventive measures should be taken by applying inorganic anti-corrosion coatings to extend the service life of stainless steel and protect it from corrosion caused by high concentrations of chloride ions.