## 3 Results and Discussion
### Electrochemical Analysis
#### 3.3.2 Polarization Curves  
Figure 8 displays the N80 steel polarization curves that were obtained at different immersion periods. The rate of corrosion for the N80 steel sample was determined using the Tafel extrapolation method. Specifically, a linear fit was applied within a linear interval where either the anode or cathode curve exceeded or fell below the corrosion potential by 50 mV. The rate of corrosion was calculated by converting the corrosion current, which was determined at the point of intersection between the straight line and the potential for corrosion. The results of these calculations are provided in Tables 3 and 4. The calculated values revealed that the corrosion rate gradually decreased within the immersion period of 3.5 days to 14 days, and then accelerated at 28 days (Figure 9). Specifically, the corrosion rates after 3.5 days, 7 days, and 14 days of immersion were 4.31 \(\times\) 10\({}^{-3}\) mm/y, 8.78 \(\times\) 10\({}^{-5}\) mm/y, and 7.54 \(\times\) 10\({}^{-5}\) mm/y, respectively. As a result, the rate of corrosion for the sample declined as the immersion duration increased. One possible explanation for this decrease might be the formation of corrosion products on the material's surface, which provide protection. Electrochemically, this phenomenon could be explained through resistance polarization. However, by the 28 th day, the corrosion rate had slightly increased. There were several factors that could have contributed to this, such as secondary dissolution of the coating of corrosion products and the replacement of Ca and Mg ions, leading to structural damage of the corrosion product layer. As the immersion time extended, the cathodic polarization curve exhibited a general leftward and downward shift, indicating changes in the cathodic reaction behavior. On the other hand, the changes observed in the anodic polarization curve were more intricate. When the electrostatic current density of the electrochemical polarization reached zero, the corresponding polarization potential corresponded to the self-corrosion potential (E\({}_{\text{corr}}\)), which progressively shifted in the forward direction with increasing mineralization. Notably, this trend was largely consistent with the observed changes in the OCP with respect to the immersion time. The examination of the Tafel curve indicated that with an increase in temperature, the rate of corrosion for N80 steel followed a pattern of initial reduction and subsequent elevation. This pattern suggested that the protective layer of corrosion products established at higher temperatures did not have an effective safeguarding impact on the structure. Within the temperature range of 40 \({}^{\circ}\)C to 80 \({}^{\circ}\)C, the corrosion products formed on the electrode surface exhibited increased density, thereby enhancing their ability to protect the matrix. However, as the temperature continued to rise, the diffusion of ions in the solution and their penetration through the corrosion product film became more prominent. This effect surpassed the film's compactness, resulting in a weakening of its protective effect on the matrix between 80 \({}^{\circ}\)C and 120 \({}^{\circ}\)C.  
The polarization curves depicted in Figure 8 were processed and analyzed using the CHI660E Electrochemical Workstation software to perform fitting. This fitting process facilitated the determination of corrosion-related parameters for N80 steel samples exposed to different immersion time and temperatures. This analysis allowed us to obtain specific parameters, including the corrosion current density (I\({}_{\text{corr}}\)), the corrosion potential (E\({}_{\text{corr}}\)), and the Tafel constants \(\beta_{\text{a}}\) (anode slope) and \(\beta_{\text{c}}\) (cathode slope). Furthermore, we calculated the polarization resistance (R\({}_{\text{p}}\)) using the Stein-Geary equation [29; 30].