**4. Discussion**

## **4.1. NCI**

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 of matrix where a cluster of cavities existed.

**5. Conclusions**

causes of cracking.

**Acknowledgements**

**Author details**

**References**

phase particles leading to metal loss.

for its partial financial support of this work.

Abdulhameed Al-Hashem\*, Abdulmajeed Abdullah and Wafa Riad

Petroleum Research Center, Kuwait Institute for Scientific Research, Safat, Kuwait

Corrosion'96, Paper No. 415. Houston, TX: NACE International; 1996

erosion behaviour of materials. Materials Characterization. 1998;**41**:193-200

[1] McCaul C. An advanced cavitation resistant austenitic stainless steel for pumps.

[2] Marques PV, Trevisan RE. An SEM-based method for the evaluation of the cavitation

[3] Kim K-H, Chahine G, Franc J-P, Karimi A, editors. Advanced Experimental and Numerical Techniques for Cavitation Erosion Prediction. Dordrecht: Springer Publishing; 2014

\*Address all correspondence to: ahashem@kisr.edu.kw

1. The cavitation erosion of NCI, NAB, and Monel 400 alloys under ultrasonically induced

The Effect of Alloy Microstructure on Their Cavitation Erosion Behavior in Seawater

http://dx.doi.org/10.5772/intechopen.79531

117

**2.** Graphite nodule fragmentation, ductile tearing, and micro-galvanic activities at graphite/ ferrite interface are believed to be the main mechanisms of metal loss of NCI in seawater.

**3.** For NAB, α phase was selectively attacked at the interfaces with the intermetallic κ precipitates in quiescent seawater. The κ precipitates and the precipitate free zones did not suffer from corrosion. Selective phase corrosion and cavitation stresses were considered to be the

**4.** The formation of slightly rough surfaces for this alloy with attacks along grain boundaries, annealing twins, as well as plastically deformed matrix regions. Corrosion of Monel 400 alloy mainly initiated in and around the grain boundaries, annealing twins, and second-

The authors would like to thank Kuwait Foundation for the Advancement of Science (KFAS)

cavitation testing in seawater was attributed mainly to the mechanical factors.

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 plastic deformation and metal loss [14–16].

## **4.2. NAB**

**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 the intermetallic particle interfaces.

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

#### **4.3. Monel 400**

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 plastic deformation of its FCC matrix.

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].
