**2. Experimental**

#### **2.1. Apparatus**

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

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


**Table 1.** Chemical composition of Arabian Gulf seawater (Doha plant).

transformer, otherwise known as a horn. The main purpose of the velocity transformer is to increase the amplitude from that at the transducer face to the amplitude required at the specimen tip. The alloy samples were fixed using a special holder and being 0.125 cm way from the vibrating horn. Sample's surface area in these experiments was kept at 1 cm2 . The testing medium is Gulf seawater with high salinity (47,000 TDS) and was kept in special glass containers cooled with a water bath and maintained at 25 ± 1°C (**Figure 1**). **Table 1** shows the constituents of the testing medium.

### **2.2. Materials**

The chemical composition of NCI consisted of 3.54 C, 2.26 Si, 0.40 Mn, 0.05 P, and the rest Fe. The microstructure of NCI after polishing and etching in 4% nital solution consisted of spheroidal graphite in a ferrite matrix. NAB specimen composition in weight percent was found to be 80 Cu, 9 Al, 4.9 Fe, 4.9 Ni, and 1.2 Mn. The percent (%) chemical composition of the UNS N04400 alloy includes 63.0 Ni, 28.0–34.0 Cu, 2.5 Fe, 2.0 Mn, 0.3 C, 0.024 S, 0.05 C, and 0.5 Si.

ASTM G32-G92 standard was followed for the preparation of all tested samples in terms of dimensions and surface preparation which then were etched before testing. Etching was made for all samples in order to be able to follow the mode of attack for the different constituent alloy's phases through time-lapse scanning electron microscopy SEM images.
