## 3 Results and Discussion  
### Corrosion Inhibition According to the Nitrite Dosage  
Figure 2 shows the surface morphologies of the test coupons after immersion in the chloride-free solutions containing various inhibitor dosages (3000-17,000 ppm) at 50 \({}^{\circ}\)C and 80 \({}^{\circ}\)C for 24 h. In regard to the uninhibited condition, general corrosion and visible rust were observed. This trend was more pronounced at 80 \({}^{\circ}\)C. At lower inhibitor dosages (3000-5000 ppm), corrosion was partially inhibited; however, localized rust (highlighted in the red box) was observed on the surface, suggesting incomplete inhibition of the corrosion damage. At higher inhibitor dosages above 5000 ppm, the coupons exhibited corrosion-free surfaces at both temperatures, demonstrating excellent inhibition performance against general and localized corrosion. These results indicate that an inhibitor dosage of approximately 5000 ppm is required to achieve effective passivation in the absence of a chloride-induced attack. Figure 2: Surface morphology of FCD-500 after immersion tests in chloride-free solutions containing various inhibitor dosages at both temperatures, 50 \({}^{\circ}\)C and 80 \({}^{\circ}\)C, for 24 h.  
Figure 3 presents the surface morphologies of the test coupons after immersion in solutions containing 100 ppm chloride and various inhibitor dosages (3000-17,000 ppm) at 50 \({}^{\circ}\)C and 80 \({}^{\circ}\)C for 24 h. The corrosion patterns were similar to those shown in Figure 2 under chloride-free conditions. The corrosion morphology shifted from uniform corrosion in regard to the uninhibited condition to localized corrosion upon dosing using the inhibitor, indicating a change in the surface passivation behavior. Under 100 ppm Cl\({}^{-}\), surface protection was achieved at both temperatures when the inhibitor dosage exceeded 13,000 ppm. This suggests that the presence of Cl\({}^{-}\) deteriorates the effectiveness of the inhibitor and destabilizes the passive film. Furthermore, as the Cl\({}^{-}\) level increases, higher inhibitor dosages are required to achieve and maintain surface stability against corrosion. Figure 4 shows the weight loss of the immersion test coupons as a function of the inhibitor dosage and chloride ions (0 ppm, 100 ppm). Weight loss serves as an indicator of the extent of metal corrosion, with lower values indicating more effective inhibition. The graph clearly illustrates the overall trends in weight loss, showing a decrease with an increasing inhibitor dosage and an increase in the presence of chloride ions and at elevated temperatures. Specifically, when subject to the solution condition containing 100 ppm of chloride ions at 80 \({}^{\circ}\)C (yellow line), the weight loss was measured to be 6.7 mg in the absence of an inhibitor. However, even with the addition of 3000-7000 ppm of inhibitor, a greater weight loss was observed of 13 mg, 9.9 mg, and 6.3 mg, respectively. These results suggest that the range of low dosages was insufficient to prevent chloride-induced corrosion. Although localized corrosion affects only a small surface area, it can induce a significantly greater mass loss due to accelerated anodic dissolution and a deep penetration depth at active sites. An insufficient inhibitor dosage may lead to increased localized surface activity, indicating the necessity of exceeding the threshold concentration to achieve corrosion protection. In this study, when the inhibitor concentration exceeded 13,000 ppm, all the test conditions showed stable corrosion inhibition, with weight loss consistently maintained below 1.0 mg. In actual field conditions, corrosion develops over serval years; SEM provides a practical approach to estimating long-term corrosion behavior from short-term experimental data. Figure 5 presents SEM images of immersion test specimens, aimed at evaluating the microstructural corrosion under selected solution conditions.