# Effect of Marine Coolant Additives on Cavitation Erosion-Corrosion of Diesel Engine Cylinder Liner
## 3 Results and Discussion
### Cavitation Erosion Behavior
In the high magnification images (Figure 6b,d), it was observed that cavitation erosion damage tended to propagate radially from the center of the damaged area of the hemispherical crater surface, with fatigue cracks and micro pits. This is because the crater surface damage forms a notch-like structure with high-stress concentration, creating a favorable environment for cavitation erosion to occur [54]. Additionally, pearlite, which has a higher strength than ferrite, acts as a protective framework against surface damage. The relatively softer ferrite acts as a starting point for fatigue crack initiation before the pearlite structure is affected by strain hardening and fatigue. This is related to the collapse characteristics of cavitation bubbles. The shock waves and micro-jets generated during the collapse of cavitation bubbles create significant localized heating and high-pressure regions. Cavitation erosion is considered a material fatigue phenomenon because surface damage, such as elastic or plastic deformation and cracks, occurs after repeated impacts from these bubble collapse shock forces [55; 56]. For this reason, pits or expanded craters with significant stress concentration areas appear in the regions where the graphite has been removed, and radial micro-crack formation is further promoted. This implies that the collapse characteristics of cavitation bubbles and the microstructural characteristics of DCI have a combined effect on cavitation erosion damage. As shown in Figure 7, after 10 min of cavitation erosion tests, both the DW + A and DW + C specimens showed growth of crater damage, with the coalescence of adjacent craters leading to a further increase in surface damage (Figure 7b,d). Additionally, in the early stages of the cavitation erosion test, crater damage progressed more significantly in the depth direction rather than in the width direction. This surface damage trend was more pronounced in the DW + C specimen than in the DW + A specimen, with the observation of micro tunnels. This is believed to be related to the localized material destruction process caused by micro-jets generated during cavitation bubble collapse [56; 57]. Meanwhile, several micro pits were also observed around the craters in the DW + C specimen surface. The reason for the concentrated formation of micro pits around the craters, unlike in the DW + A specimen, is that the DW + C specimen is exposed to the attack of C1\({}^{-}\) ions without the passivation protection of the nitrite-based additive. At the same time, the DW + C specimen is exposed to the attack of C1\({}^{-}\) ions without the passivation protection of the nitrite-based additive. it is believed that the cavitation bubble collapse phenomenon concentrates at the crater damage site, leading to the formation of a high-temperature and high-pressure environment that promotes corrosion reactions. This phenomenon is believed to be the result of such interactions. As shown in Figures 5-7, the microstructure of DCI was the main factor in determining the cavitation erosion damage tendency during the 10 min cavitation erosion test. In particular, it can be understood that the preferential cavitation erosion attack of the spheroidal graphite exposed on the surface of DCI induced the unstable cavitation erosion rate at the beginning of the test, as shown in Figure 4b. Subsequently, as shown in Figure 8, by the end of the cavitation erosion test, the size of the crater damage increased and the coalescence process intensified, resulting in the formation of deeper and larger craters. The DW + A specimen mainly shows crater damage progressing in the depth direction, while the DW + C specimen shows development in both depth and width directions due to the interaction with the corrosion reaction. As a result, in the 200\(\times\) magnification images (Figure 8a,c), the area of the undamaged flat surface is observed to be smaller in the DW + C specimen than in the DW + A specimen.