# Effect of Marine Coolant Additives on Cavitation Erosion-Corrosion of Diesel Engine Cylinder Liner
## 3 Results and Discussion
### Cavitation Erosion-Corrosion Mechanism  
Over the course of the cavitation erosion test, DCI exhibited a tendency for continuous increases in weight loss (Figure 4a), surface roughness, and maximum surface damage depth (Figure 9) due to surface damage caused by micro jets and shock wave attacks resulting from cavitation bubble collapse. In the initial stage, within approximately 5 min of the cavitation erosion test, a similar trend of cavitation erosion damage was observed across all experimental conditions, regardless of the presence of the nitrite-based additive. The graphite exposed on the DCI surface was initially eroded and removed by cavitation erosion (Figure 6). The small craters formed at this stage serve as areas of concentrated cavitation erosion attacks, causing depth-oriented surface damage accompanied by fatigue cracks. As the cavitation erosion test continued, surface damage trends varied depending on the presence of the nitrite-based additive. In the presence of a nitrite-based additive, cavitation erosion damage occurred predominantly, whereas in the absence of nitrite-based  
\begin{table}
\begin{tabular}{c c c c} \hline
**Analysis** & **DW + A** & **DW + C** & **DW + A + C** \\ \hline Damaged area ratio [\%] & 44.10 & 46.05 & 45.66 \\ Number of craters & 22 & 29 & 23 \\ Maximum crater volume [\(\mu\)m\({}^{3}\)] & 627,505.5 & 990,943.6 & 329,334.9 \\ Minimum crater volume [\(\mu\)m\({}^{3}\)] & 678.9 & 198.2 & 193.9 \\ Total volume of craters [\(\mu\)m\({}^{3}\)] & 1,886,238.9 & 2,322,721.3 & 1,968,446.5 \\ \hline \end{tabular}
\end{table}
Table 6: Results of 2D image analysis after cavitation erosion test during 90 min.  
Figure 11: Two-dimensional images after cavitation erosion test during 90 min; (**a**) DW+A, (**b**) DW+C, and (**c**) DW + A + C.  
Figure 10: Three-dimensional image and profile after cavitation erosion test during 90 min.  
additive, cavitation erosion-corrosion damage occurred. In the case where the nitrite-based additive was present, passivation behavior appeared on the surface, effectively protecting it from corrosion damage, and physical cavitation erosion damage predominated (Figures 2 and 3). Cavitation erosion preferentially occurred inside the craters, where repeated stress concentration led to fatigue failure accompanied by cracks. These small craters exhibited a damage mechanism in which adjacent craters coalesced and grew into larger craters (Figure 7). As the test time progresses, cavitation erosion damage increases, leading to the formation of numerous craters with micro tunnels observed in the depth direction on the surface (Figures 8 and 10). On the other hand, in the absence of nitrite-based additives, the material is not protected by passivation behavior, making it susceptible to chemical corrosion attacks. As a result, cavitation erosion is accompanied by corrosion, leading to the occurrence of cavitation erosion-corrosion (Figures 2 and 3). Due to the synergistic effect of the interaction between cavitation erosion and corrosion, the surface damage rate of DCI progressed rapidly, causing a relatively large increase in weight loss over test time (Figure 4a). The accompanying corrosion damage primarily appeared as micro pits near the craters, which facilitated the growth of the craters (Figure 7). Due to this corrosion effect, the craters tend to grow not only in the depth direction but also in the lateral direction. As a result, while surface roughness and weight loss continued to increase over test time, the maximum surface damage depth did not increase significantly after 30 min of test time (Figures 4 and 10).