The mechanism of chloride ion corrosion to stainless steel: In chemical production, corrosion commonly occurs during the use of pressure vessels, which is one of the main factors that cause various defects in pressure vessels. Ordinary steel has poor corrosion resistance, while stainless steel has excellent mechanical properties and good corrosion resistance. Cr and Ni are the most important alloying elements for stainless steel to obtain corrosion resistance. Cr and Ni make stainless steel form a very dense oxide film in oxidizing medium, passivate stainless steel, reduce the corrosion rate of stainless steel in oxidizing medium, and improve the corrosion resistance of stainless steel. The activation of chloride ions plays an important role in the establishment and destruction of stainless steel oxide film.
Although the mechanism of how chloride ions transform the passivating metal into the activated state has not been concluded, it can be roughly divided into two viewpoints. According to the theory of phase-forming film, due to the small radius of chloride ions and strong penetrating ability, it is most likely to penetrate the extremely small pores in the oxide film, reach the metal surface, and interact with the metal to form soluble compounds, making the oxide film. The structure changes and the metal corrodes. The adsorption theory holds that the fundamental reason why chloride ions destroy the oxide film is that chloride ions have a strong ability to be adsorbed by metals. They are preferentially adsorbed by metals and discharge oxygen from the metal surface. Because oxygen determines the passivation state of the metal, chloride ions and oxygen compete for adsorption sites on the metal surface, and can even replace the passivation ions in the adsorption to form chlorides with metals. The adsorption of chlorides and metal surfaces is not stable, forming Soluble substances are removed, which leads to accelerated corrosion.
The electrochemical method to study the passivation state of stainless steel shows that the activation effect of chloride ions on the metal surface only occurs in a certain range, and there is a specific potential value at which the stainless steel begins to activate. This potential is the breakdown potential of the film. The higher the breakdown potential, the more stable the passivation of the metal. Therefore, the stability of the passivation state of stainless steel and the corrosion resistance in various media can be measured by the breakdown potential value.
