**3. Results**

**Figure 1.** Schematic of vibratory cavitation test apparatus.

occurring on alloy surfaces using seawater through time-lapse imaging as recorded by the scanning electron microscopy (SEM). It is not by any means the purpose of this section to provide an exhaustive cavitation analysis for a big list of available alloys in the market but to only focus on the three alloy microstructures mentioned above. Therefore, the main objectives of this chapter are to consider how these three alloys react and to summarize the role of

Cavitation erosion phenomenon is very complicated, and materials having different properties will react in different ways [1–4]. The main mechanism of a material's reaction to attack centers around mechanical stressing seems clear from available information. Cavity collapse will cause typical stresses at the material surface. Shear stresses within the material which arise from the nonuniformity of the normal pressure caused by the cavitation action can lead

The ultrasonically induced cavitation technique used is made of a solid-state generator and a piezoceramic transducer which is designed to resonate at a frequency of 20 kHz at amplitude of 25 μm [5]. The transducer transmits the energy to the specimen tip through a velocity

materials properties with respect to cavitation erosion in seawater.

to plastic deformation [4].

**2. Experimental**

110 Cavitation - Selected Issues

**2.1. Apparatus**

#### **3.1. NCI assessment of surface damage**

Metallographic examination of polished and etched NCI specimen revealed the structure of nodular cast iron of ferritic matrix. During the very early stages of cavitation testing in seawater, there was no damage observed on NCI sample surfaces. Micro-galvanic activity was detected at the graphite nodule and the ferrite matrix after 30 s of cavitation. This microgalvanic activity allows the ferrite matrix to dissolve for being anodic to the graphite nodule, which is cathodic. Therefore, the first attacked area was the graphite/ferrite interface. SEM examinations of cavitated specimens for different periods of time were carried out to determine the morphology of cavitation damage as shown in **Figure 2**. After 15 min of cavitation testing in seawater, localized areas suffered from surface damage: some graphite nodules were partially fragmented (**Figure 2a**), and others were totally removed (**Figure 2b**). After 30 min of cavitation (**Figure 2c**), total removal of the graphite nodules was dominant. The average size of these cavities was generally 30–80 μm in diameter which to a certain extent similar to that of the graphite nodules. The presence of other micro-cavities (2 μm) in the matrix of this alloy that are not related to the removal of the graphite nodules was also observed. Surface deformations with an increasing number of cavities and pitting were observed on the attacked areas of the specimen after 70 min of testing (**Figure 2d**). The cavitation damage has extended to other areas on the alloy's sample causing the formation of large cavities after 120 min (**Figure 2e**). After 240 min of cavitation (**Figure 2f**), the ductile removal of material in the ferrite matrix leads to the coalescence of these pits with time forming deep craters on the

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The cavitation action has led to the fragmentation of a graphitic nodule and partial removal of another. In addition to the cavity pit that formed as a result of the removal of graphite nodule,

**Figure 3.** SEM micrographs of the surface of NAB after cavitation testing in seawater for various lengths of time: (a) 3 h,

several micropits were also formed on the surface of the ferrite matrix.

surface of NCI specimens.

(b) 13 h, (c) 25 h, and (d) 40 h.

**Figure 2.** SEM micrographs of the gradual destruction and fragmentation of graphite nodules and plastic deformation of the ferrite matrix as a function of cavitation testing in seawater: (a) 15 min, (b) 20 min, (c) 30 min, (d) 70 min, (e) 120 min, and (f) 240 min.

of this alloy that are not related to the removal of the graphite nodules was also observed. Surface deformations with an increasing number of cavities and pitting were observed on the attacked areas of the specimen after 70 min of testing (**Figure 2d**). The cavitation damage has extended to other areas on the alloy's sample causing the formation of large cavities after 120 min (**Figure 2e**). After 240 min of cavitation (**Figure 2f**), the ductile removal of material in the ferrite matrix leads to the coalescence of these pits with time forming deep craters on the surface of NCI specimens.

The cavitation action has led to the fragmentation of a graphitic nodule and partial removal of another. In addition to the cavity pit that formed as a result of the removal of graphite nodule, several micropits were also formed on the surface of the ferrite matrix.

**Figure 3.** SEM micrographs of the surface of NAB after cavitation testing in seawater for various lengths of time: (a) 3 h, (b) 13 h, (c) 25 h, and (d) 40 h.

**Figure 2.** SEM micrographs of the gradual destruction and fragmentation of graphite nodules and plastic deformation of the ferrite matrix as a function of cavitation testing in seawater: (a) 15 min, (b) 20 min, (c) 30 min, (d) 70 min, (e) 120

partially fragmented (**Figure 2a**), and others were totally removed (**Figure 2b**). After 30 min of cavitation (**Figure 2c**), total removal of the graphite nodules was dominant. The average size of these cavities was generally 30–80 μm in diameter which to a certain extent similar to that of the graphite nodules. The presence of other micro-cavities (2 μm) in the matrix

min, and (f) 240 min.

112 Cavitation - Selected Issues

## **3.2. NAB assessment of surface damage**

In order to understand the performance of NAB against cavitation erosion, it is of interest to understand its complex microstructure. There are many constituent phases that make up the microstructure of NAB which includes the following: α phase which is a *fcc* copper-rich solid solution, eutectoid phases of "β phase" or retain β, and four intermetallic κ phases designated as κI , κII, κIII, and κVI [6–9]. The κI , κII, and κIV phases are all iron-rich precipitates based on the structure of Ni-Al [9]. The κII and κIV precipitates were found to be 10 μm in size and <0.5 μm in thickness, respectively. The microstructure also contains a precipitate free zone at the α grain periphery.

After 3 h of cavitation testing, the NAB surface became a bit rough as shown by SEM micrographs in **Figure 3a**. The surface damage increased, and several micro-cavities were observed on NAB surface after 13 h of cavitation (**Figure 3b**).

The NAB surface contained large-size cavities after 25 h of testing (**Figure 3c**). Severe surface roughness was observed, and the amount of cavities has increased after 40 h of testing period (**Figure 3d**). Later on and after 40 h of cavitation testing, ductile tearing and grain boundary attack were detected.

When NAB-polished samples were immersed in stagnant seawater for 48 h, examinations by SEM indicated that the α phase was preferentially attacked at the α/κIV interfaces (**Figure 4a** and **b**).

#### **3.3. Monel 400 assessment of surface damage**

**Figure 5a**–**d** shows SEM micrographs of a standard specimen of Monel 400 before and after cavitation testing revealing its solid solution binary structure. **Figure 5a** shows the microstructure of this alloy in the as-received solution heat-treated condition. The microstructure

consists essentially of a single face-centered cubic (FCC) phase with some of annealing twins.

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Small second-phase particles of possibly manganese sulfide and silicon carbide are also present in the microstructure of **Figure 5a**. **Figure 5b**–**d** shows SEM micrographs of the same region of this specimen after 0.66, 1.10, and 1.42 h of cavitation in seawater, respectively. Minimal surface attack was observed after 0.66 h of cavitation. However, after 1.10–1.42 h of cavitation,

The presence of cavities and ductile tearing is readily explainable in terms of the known dev-

**Figure 5.** SEM micrograph of the surface of UNS N04400 alloy at high magnifications after (a) 0.0 h, (b) 0.66 h, (c) 1.10 h, and (d) 1.42 h of cavitation testing in seawater at 25°C showing the damage at grain boundaries, annealing twins, and

attack is visible along grain boundaries, twins, and plastic deformation of the matrix.

The grain size varied from 25 to 100 μm in **Figure 5a**.

astating effects of cavitation.

the matrix.

**Figure 4.** SEM micrographs of NAB after exposure to quiescent seawater for 48 h, showing preferential attack of the α phase at the α/κIII interfaces. The precipitate free zone did not suffer from corrosion attack (3620×).

consists essentially of a single face-centered cubic (FCC) phase with some of annealing twins. The grain size varied from 25 to 100 μm in **Figure 5a**.

Small second-phase particles of possibly manganese sulfide and silicon carbide are also present in the microstructure of **Figure 5a**. **Figure 5b**–**d** shows SEM micrographs of the same region of this specimen after 0.66, 1.10, and 1.42 h of cavitation in seawater, respectively. Minimal surface attack was observed after 0.66 h of cavitation. However, after 1.10–1.42 h of cavitation, attack is visible along grain boundaries, twins, and plastic deformation of the matrix.

The presence of cavities and ductile tearing is readily explainable in terms of the known devastating effects of cavitation.

**Figure 4.** SEM micrographs of NAB after exposure to quiescent seawater for 48 h, showing preferential attack of the α

In order to understand the performance of NAB against cavitation erosion, it is of interest to understand its complex microstructure. There are many constituent phases that make up the microstructure of NAB which includes the following: α phase which is a *fcc* copper-rich solid solution, eutectoid phases of "β phase" or retain β, and four intermetallic κ phases designated

structure of Ni-Al [9]. The κII and κIV precipitates were found to be 10 μm in size and <0.5 μm in thickness, respectively. The microstructure also contains a precipitate free zone at the α

After 3 h of cavitation testing, the NAB surface became a bit rough as shown by SEM micrographs in **Figure 3a**. The surface damage increased, and several micro-cavities were observed

The NAB surface contained large-size cavities after 25 h of testing (**Figure 3c**). Severe surface roughness was observed, and the amount of cavities has increased after 40 h of testing period (**Figure 3d**). Later on and after 40 h of cavitation testing, ductile tearing and grain boundary

When NAB-polished samples were immersed in stagnant seawater for 48 h, examinations by SEM indicated that the α phase was preferentially attacked at the α/κIV interfaces (**Figure 4a**

**Figure 5a**–**d** shows SEM micrographs of a standard specimen of Monel 400 before and after cavitation testing revealing its solid solution binary structure. **Figure 5a** shows the microstructure of this alloy in the as-received solution heat-treated condition. The microstructure

, κII, and κIV phases are all iron-rich precipitates based on the

phase at the α/κIII interfaces. The precipitate free zone did not suffer from corrosion attack (3620×).

**3.2. NAB assessment of surface damage**

, κII, κIII, and κVI [6–9]. The κI

on NAB surface after 13 h of cavitation (**Figure 3b**).

**3.3. Monel 400 assessment of surface damage**

as κI

grain periphery.

114 Cavitation - Selected Issues

attack were detected.

and **b**).

**Figure 5.** SEM micrograph of the surface of UNS N04400 alloy at high magnifications after (a) 0.0 h, (b) 0.66 h, (c) 1.10 h, and (d) 1.42 h of cavitation testing in seawater at 25°C showing the damage at grain boundaries, annealing twins, and the matrix.
