## 2 Corrosion Mechanisms and Simulation Schemes for Corroded Liner  
### Corrosion Mechanism of the Liner  
The liner is generally secured to the entire inner wall of the NCS by anchorages to provide the required airtightness of the structure. Liner corrosion can be categorized into two types: inside corrosion (occurring on the side facing the internal environment of the NCS) and outside corrosion (occurring on the side in contact with the concrete wall) [21]. Figure 1 provides a diagram of the corrosion mechanism of the liner. The inside corrosion is primarily caused by the age-related failure of the antirust coating or waterproof layer on the surface of the liner, subsequently exposing the liner to the internal environment of the NCS. In this case, the corrosion mechanism of the liner is similar to the electrochemical corrosion mechanism of carbon steels in atmospheric conditions. Electrochemical corrosion is essentially a redox reaction, including both anodic and cathodic reactions. The anodic reaction involves the transfer of metal ions from the metal to the medium, releasing electrons and causing the dissolution of the liner. The cathodic reaction involves a reduction process of the oxidant (e.g., oxygen) component of the medium, absorbing electrons from the anode. Conversely, the cathodic reaction entails the oxidant (e.g., oxygen) component of the medium absorbing electrons from the anode. Additionally, the rate of inside corrosion is influenced by the humidity and temperature within the NCS.  
The outside corrosion mechanism of the liner is the same as that on the inside, but there are differences in the corrosion conditions and processes. Upon contact with the alkaline environment within the concrete, a stable passivation film will form on the surface of the liner. This film can greatly reduce the anodic reaction rate and thus provide a protective barrier against the corrosion of the liner. The outside corrosion of the liner initiates with the destruction of the passivation film, and there are two main aspects that contribute to such destruction. On the one hand, the liner typically serves as an inner mold for the concrete pouring process. During the construction of the NCS, the liner was exposed to the ocean environment for an extended period of time, resulting in localized corrosion on the outer surface of the liner. After the concrete is poured, the local area on the outer surface of the liner fails to develop a dense passivation film. Simultaneously, chloride ions remaining on the surface of the liner will intensify the destruction of the passivation film [22]. On the other hand, some acidic foreign materials (e.g., wood, gloves, etc.) are inadvertently left in the concrete during the construction process. When these materials come into contact with the outer surface of the liner, the pH in the contact area will decrease. When the pH drops to the threshold value (e.g., pH < 10), the passivation film will be destroyed [23]. Once the  
Figure 1: Diagram of the corrosion mechanism of the liner.  
passivation film is destroyed, the liner becomes vulnerable to electrochemical corrosion in environments where there is an adequate presence of water and oxygen.