# Effect of Marine Coolant Additives on Cavitation Erosion-Corrosion of Diesel Engine Cylinder Liner
## 3 Results and Discussion
### Corrosion Behavior  
Figure 2 presents the curve of the OCP of DCI measured in the experimental solution with immersion time. For DW + C without additives, the potential started at approximately \(-0.49\) V and gradually decreased over immersion time, reaching about \(-0.56\) V at the end of the experiment. Frequent potential fluctuations were observed throughout the experiment. It is believed that the oxide film formed on the DCI surface in the atmosphere was destroyed by the highly corrosive Cl\({}^{-}\) ions present in the experimental solution, leading to a decrease in potential and fluctuations in potential associated with surface corrosion reactions. In particular, it is known that exposure of DCI to a corrosive electrolyte solution containing Cl\({}^{-}\) ions, as in the present study, leads to graphite corrosion [38]. Graphite corrosion can be explained by a galvanic corrosion mechanism. As shown in the microstructure in Figure 1, graphite, which possesses a noble electrochemical potential, promotes anodic dissolution through galvanic contact with the adjacent matrix, which has an active potential. As a result, the matrix surrounding the graphite corroes preferentially, leaving only the graphite structure remaining. Meanwhile, the OCP of DW + A and DW +  
Figure 1: Microstructure of ductile cast iron (GCD-500). A + C, both containing additives, started at a higher potential of approximately \(-\)0.34 V compared to DW + C, and gradually increased, reaching approximately \(-\)0.25 V and \(-\)0.27 V, respectively, at the end of the experiment. The relatively higher OCP of DW + A and DW + A + C compared to DW + C is attributed to the effective passive behavior exhibited by the nitrite-based additive on the DCI surface [39; 40]. This is consistent with previously reported similar corrosion behavior of carbon steel in neutral solutions in the presence of nitrite ions [41; 42]. To interpret this behavior, it is useful to explain the inhibition mechanism of nitrite ions. This process occurs through the adsorption of nitrite ions onto the metal surface [43], followed by a reduction reaction that produces oxygen ions, as shown in the following reaction described by Equation (1) [18; 42]. \[2 NO_{2}^{-}+4e^{-}\ \rightarrow\ N_{2}O+3O^{2-} \tag{1}\]  
In fact, the reduction in nitrite ions generates oxygen in the form of \(O^{2-}\) ions, which promotes the oxidation of ferrous ions to ferric ions [44]. In alkaline solutions, a stable passive film of maghemite forms on the metal surface, which can be summarized by the following reaction described by Equation (2) [45; 46]. \[Fe^{2+}+2 OH^{-}+2 NO_{2}^{-}\ \rightarrow 2 NO+\gamma-Fe_{2}O_{3}+H_{2}O \tag{2}\]  
According to this reaction equation, it can be seen that not only nitrite ions but also hydroxide ions play a crucial role in the passivation of iron [22]. In contrast to DW + C, the OCP of DW + A and DW + A + C exhibited little voltage fluctuation, indicating that the passive behavior induced by the additive was established quite stably. The relatively lower OCP of DW + A + C compared to DW + A is believed to result from the partial negative influence of the Cl\({}^{-}\) ions present in DW + A + C on passive behavior. This OCP behavior trend is consistent with the findings of other researchers, who reported that the OCP behavior changes with varying nitrite concentrations [39]. To quantitatively assess the corrosion rate of DCI depending on the experimental solution, the potentiodynamic polarization experiments were performed, followed by [PERSON]'s analysis, as shown in Figure 3. The resulting corrosion potential and corrosion current density are presented in Table 5. The corrosion potential of DW + C was \(-\)0.24 V lower than that of DW + A + C, and the corrosion current density was about 31.7 times higher, confirming its relatively poor corrosion resistance.