Stainless steel can rust under certain conditions, even highly corrosion-resistant 316L stainless steel pipes. To improve the corrosion and oxidation resistance of the pipes, effective passivation treatment is necessary. However, even in a passivated state, the steel pipe still retains some reactivity. Let's understand why pitting corrosion occurs in 316L stainless steel pipes under passivated conditions.

First, the concept of pitting corrosion in stainless steel pipes: The excellent corrosion resistance of 316L stainless steel pipes is due to the formation of an invisible oxide film on the steel surface, making it passive. This passivation film forms due to the stainless steel pipe reacting with oxygen when exposed to the atmosphere, or due to contact with other oxygen-containing environments. If the passivation film is damaged, the stainless steel pipe will continue to corrode. In many cases, the passivation film is only damaged in localized areas on the metal surface.

Secondly, the causes of pitting corrosion in 316L stainless steel pipes: The cause of pitting corrosion in 316L stainless steel pipes is that active anions (such as chloride ions) preferentially adsorb onto the passivation film on the surface of the stainless steel pipe, displacing the original oxygen atoms, and then combine with anions in the passivation film to form soluble chlorides. Therefore, small corrosion pits form at specific points on the newly exposed base metal. These pits have a pore size of 20-30 μm and are called pitting nuclei, or active centers for pit formation. The presence of chloride ions directly damages the passivation film on the surface of the 316L stainless steel pipe. Generally, as the chloride ion concentration increases, the passivation zone on the surface of the stainless steel pipe decreases. In practical applications, when the anodic potential in the ambient medium reaches a certain value, the current density suddenly decreases, indicating that a stable passivation film has begun to form on the surface of the stainless steel pipe, and the corresponding resistance will be relatively high, remaining within a certain potential range for a long time. However, as the chloride ion concentration in the ambient medium increases, the critical current density increases, the primary passivation potential also increases, and the passivation zone decreases.

The explanation for this characteristic is as follows: Within the passivation potential region, chloride ions compete with oxidants and enter the thin film, leading to lattice defects and reducing the resistivity of the oxide. Therefore, in an environment containing chloride ions, passivation is neither easily formed nor easily maintained in stainless steel pipes. Although the local passivation film of the 316L stainless steel pipe is damaged, the remaining protective film remains intact, allowing pitting corrosion conditions to be realized and intensified. According to the electrochemical formation mechanism, the electrode potential of the activated state is much higher than that of the passivation state. The electrolyte solution meets the thermodynamic conditions for electrochemical corrosion. The activated 304 stainless steel pipe becomes the anode, and the passive stainless steel pipe acts as the cathode. Pitting corrosion only involves a small portion of the metal, and the surface of the stainless steel pipe becomes a large cathode area. In the electrochemical reaction, the cathodic and anodic reactions proceed at the same rate, so the corrosion rate concentrated at the anodic corrosion point will be very fast, and the penetration effect will be very obvious, thus producing the phenomenon of pitting corrosion.

The above explains the reason why pitting corrosion occurs in 316L stainless steel pipes under the passivation state. Because chloride ions adsorb onto the passivation film on the surface of stainless steel pipes, they eliminate the original oxygen atoms, causing the chloride ions to combine with cations in the passivation film to form soluble chlorides, leading to pitting corrosion in 316L stainless steel pipes. Furthermore, pitting corrosion damages the passivation film in certain areas, while other protective films remain intact, which intensifies the pitting corrosion.