What are the causes of pitting corrosion in 316L stainless steel pipes under passivation conditions

Stainless steel can rust under certain conditions, even highly corrosion-resistant 316L stainless steel pipes are no exception. 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 the reasons why pitting corrosion occurs in 316L stainless steel pipes under passivation conditions.

First. The Concept of Pitting Corrosion in 316L 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.

Second. Causes of Pitting Corrosion in 316L Stainless Steel Pipes

The cause of pitting corrosion in 316L stainless steel pipes is that active anions preferentially adsorb onto the passivation film on the surface of the 316L stainless steel pipe, displacing the original oxygen atoms. These anions then combine with the 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, with a pore size mainly of 20-30 μm, 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 316L stainless steel pipe decreases.

In practical applications, when the anodic potential in the environmental medium reaches a certain value, the current density suddenly decreases. This indicates that a stable passivation film has begun to form on the surface of the 316L 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 environmental medium increases, the critical current density increases, the primary passivation potential also increases, and the passivation zone of the 316L stainless steel tube decreases.

The explanation for this characteristic of 316L stainless steel tube is that within the passivation potential zone, chloride ions compete with oxides and enter the thin film, leading to lattice defects and reducing the resistivity of the oxides. Therefore, in an environment containing chloride ions, passivation is neither easily formed nor easily maintained.

Although the local passivation film of the 316L stainless steel tube 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 passivated state. The electrolyte solution meets the thermodynamic conditions for electrochemical corrosion. The activated 304 stainless steel tube becomes the anode, and the passive 316L stainless steel tube acts as the cathode.

Pitting corrosion only involves a small portion of the metal; the surface of the 316L stainless steel tube will be a large cathode area. In electrochemical reactions, the cathodic and anodic reactions proceed at the same rate. Therefore, the corrosion rate concentrated at the anodic corrosion points will be very fast, and the penetration effect will be very obvious, resulting in pitting corrosion.

The above explains the reason why pitting corrosion occurs in 316L stainless steel pipes under passivation conditions. Because chloride ions are adsorbed on the passivation film on the surface of the 316L stainless steel pipe, the original oxygen atoms are removed. This allows the chloride ions to combine with cations in the passivation film to form soluble chlorides, leading to pitting corrosion. Furthermore, pitting corrosion damages the localized passivation film of the 316L stainless steel pipe, while other protective films remain intact, which intensifies the pitting corrosion.