**5. Conclusions**

**4. Discussion**

116 Cavitation - Selected Issues

of matrix where a cluster of cavities existed.

plastic deformation and metal loss [14–16].

the intermetallic particle interfaces.

plastic deformation of its FCC matrix.

In the early stages of NCI cavitation testing, the damage appeared as fragmentation of some graphite nodules and the total removal of others leaving surface cavities. The cavitation damage of nodular cast iron is initiated both at the ferrite matrix and graphite nodules [10–13]. The large removal rate of material from the NCI surface was attributed to the fragmentation of graphite nodules for being brittle and ductile tearing of the ferrite matrix and brittle in areas

The presence of subsurface cracks deep into the ferrite matrix is possible due to little ductility and the brittle nature of NCI, which is exhibited during the vibratory cavitation conditions. The mechanical impact of the cavitation action on the surface of a body-centered cubic (BCC) metal or alloy such as NCI would lead to a transition from ductile to brittle behavior causing

**Figure 3** shows that the cavitation action on the surface of NAB sample has created large-size cavities causing the surface to be rough. Grain boundary attack and ductile tearing were also detected under stagnant condition. NAB surface damage and metal loss can also be attributed to surface and interfacial defects as well as the electrochemical dissolution of the matrix along

Microcracks (5–10 μm) were detected at the α phase and right next to κ precipitates as shown in **Figure 4**. It is believed [6–9] that the intermetallic κ precipitates are known to be cathodic to the α matrix, thus causing the areas adjacent to them to dissolve electrochemically when exposed to seawater. It is only the mechanical action of the collapsing air bubbles of the cavitation process that plays a major role in the removal of the κ precipitates (**Figure 4**). Therefore, metal loss of NAB under cavitation conditions in seawater may be attributed to mechanical

The morphological investigation revealed that corrosion initiated locally at grain boundaries and twin lines of Monel 400 alloy. It is believed that both the mechanical action of the attacking vapor bubbles and the micro-galvanic activity between the matrix and the manganese sulfide and silicon carbide second-phase particles lead to metal loss from the FCC matrix. The cyclic mechanical contact of the bubbles on the surface of this alloy has led to slight ductile

Therefore, the cavitation-corrosion resistance of the Monel 400 alloy can be due to its inherent matrix corrosion resistance and its high strength. It is also believed that the slight plastic

deformation of the matrix (grains) is probably due to cyclic stresses [19–25].

and galvanic factors as was reported by other authors [13, 17–19].

**4.1. NCI**

**4.2. NAB**

**4.3. Monel 400**

