## 3 Results and Discussion
### Corrosion Behavior Under Simulated Marine CWS Conditions  
In marine engines, the failure to properly manage the CW system can lead to a rapid increase in the chloride ion concentration, caused by either a depletion of corrosion inhibitor additives or seawater penetration. In this study, four representative CW conditions were defined under the optimal engine operating temperature (80 \({}^{\circ}\)C) to evaluate the corrosion behavior of FCD-500 in relation to varying inhibitor concentrations and levels of chloride ions. The four conditions are as follows:  
* Condition 1: well-maintained CW, with optimal additives and a negligible chloride level (Cl\({}^{-}\) free);
* Condition 2: adequate inhibitor concentration maintained, but chloride concentration has reached the recommended limit (Cl\({}^{-}\) 100 ppm);
* Condition 3: inhibitor concentration maintained, but the chloride concentration has exceeded the recommended limit due to insufficient management (Cl\({}^{-}\) 200 ppm);
* Condition 4: inhibitor concentration maintained, but with seawater intrusion, equivalent to 2% of the total coolant volume (Cl\({}^{-}\) 500 ppm). Figure 6 presents the weight loss from the immersion coupon tests conducted under different chloride ion concentrations (0, 100, 200, and 500 ppm) and inhibitor dosages (11,000-17,000 ppm) at 80 \({}^{\circ}\)C. Under 500 ppm of Cl\({}^{-}\), the average weight loss reached 6.8 mg, which is attributed to severe localized corrosion. As the inhibitor dosage increased, the weight loss was significantly reduced; however, visible rust was still observed. Under all the other conditions, the weight loss was less than 1 mg, and corrosion evidence was not observed. These results indicate that when the chloride concentration exceeds the threshold limit of 100 ppm, an adequate inhibitor dosage (11,000-17,000) can still effectively inhibit corrosion. Nevertheless, at high chloride concentrations (above 500 ppm), such as due to seawater penetration, visible rust on the surface was observed, even at 17,000 ppm, which exceeds the manufacturer's recommended dosage. Figure 7 presents the potentiodynamic polarization curves of FCD-500 in solutions containing various concentrations of inhibitors and chloride ions (Cl\({}^{-}\)) at 80 \({}^{\circ}\)C. As shown in Figure 7a, the FCD-500 in the uninhibited conditions exhibits a typical Tafel behavior in both the anodic and cathodic regions, characterized by a gradual increase in the current density with increasing potential [29]. This indicates that the dissolution process is dominated by activation-controlled kinetics. Under this condition, the \(E_{corr}\), and \(i_{corr}\), were measured  
Figure 6: Weight loss of immersion coupons with varying inhibitor dosage (11,000–17,000 ppm) and chloride ion concentrations (0–500 ppm) at 80 \({}^{\circ}\)C.  
to be \(-660\) mV and 13 \(\upmu\)A/cm\({}^{2}\), respectively. In contrast, the corrosion current density increased markedly with the addition of chloride ions. This effect is attributed to the specific adsorption of Cl\({}^{-}\) on the metal surface, which weakens the passive film and facilitates charge transfer [30]. Figure 7b presents the polarization curves of FCD-500 in the presence of 9000 ppm of corrosion inhibitor, with varying chloride ion concentrations. In all conditions involving the inhibitor, a well-defined passive region was observed in the range from \(10^{-6}\) to \(10^{-5}\) A/cm\({}^{2}\), indicating the formation of a stable passive film on the metal surface. Furthermore, with the addition of the anodic inhibitor, NaNO\({}_{2}\), the corrosion potential was observed at approximately \(-240\) to \(-270\) mV, suggesting that nitrite ions contribute to the enhancement of the passive layer. However, at 500 ppm of Cl\({}^{-}\), a sharp increase in the anodic current was observed as the potential increased, suggesting the breakdown of the passive film and the initiation of localized corrosion.