ABSTRACT
Aluminum-based, iron-based and nickel-based alloys of known microstructures were exposed to cavitation erosion conditions using a vibratory cavitation testing apparatus in seawater as a testing medium. The cavitation tests were made at a frequency of 20 KHz as per ASTM-G30-90 and at a temperature of 25°C. Cavitation made the surface of these alloys very rough, exhibiting large cavity pits in the middle region of the attacked area as revealed by the scanning electron microscope (SEM). The cavitation damage initiated at interfaces of high energy such as grain boundaries, twins, Stacking faults and second phase particles. Hydrodynamic, electrochemical and metallurgical factors played a major role in loss of metals for these alloys as a result of the cavitation action.
The cavitation erosion behavior of Aluminum-based, iron-based and nickel-based alloys was investigated in seawater. This work stemmed from a previous failure investigation that was carried out on nodular cast iron globe valves of different sizes 3 in (7.62 cm), 4 in (10.16 cm), and 8 in (20.32 cm).1 The valves were used in the return lines of seawater heat exchangers of refineries and petrochemical plants in Kuwait.2–5 Several of these valves failed within six months of service. The small size valves 3 in (7.62 cm) suffered from cavitation corrosion and erosion cracks, while the larger size valves failed due to intensified localized attack under voluminous scale and galvanic corrosion with the seat rings. Therefore, it became of interest to study the cavitation-corrosion behavior of UNS A96061, UNS G10950 and UNS N04400 alloys which may be used in the manufacturing of seawater regulating valves for the Kuwaiti refining and petrochemical industries.
Cavitation damage is a form of a localized attack found on many types of materials exposed to turbulent flow rates of liquids. Although mainly mechanical in nature, this type of damage is more severe in mediums where the cavitation mechanism acts synergistically with corrosion.6–8 The mechanical action of cavitation usually attacks protective surfaces, exposing unprotected surfaces to corrosion. Cavitation is often defined as the growth and collapse of vapor bubbles because of local pressure fluctuations in a liquid. If the pressure suddenly falls below the vapor pressure, these bubbles then collapse violently when they are submitted to a higher pressure. This collapse is accompanied by the sudden flow of liquid, which imposes stress pulses capable of causing plastic deformation on solid surfaces.9
