Abstract
UNS S32550 alloy was exposed to cavitation conditions using a vibratory cavitation testing apparatus in seawater. The work also included measurements of free corrosion potentials, and mass loss in the presence and absence of cavitation. The cavitation tests were made at a frequency of 20 KHz and at temperatures of 25°C. Cavitation conditions caused a noble shift in the free corrosion potential for this alloy. Cavitation also increased the rate of mass loss of this alloy by several orders of magnitude with respect to stagnant conditions. Another set of cavitation experiments was also carried out for this alloy in a distilled water in order to distinguish between the mechanical and electrochemical factors that contribute to metal loss. Results indicated that the mechanical factor has an over-riding role in metal loss of this alloy. Cavitation made the surface of this alloy very rough, exhibiting large cavity pits in the middle region of the attacked area as revealed by the scanning electron microscope (SEM). Mechanical, electrochemical and metallurgical factors were determined to be the leading cause of metal loss.
Introduction
The cavitation erosion behavior of commercial duplex stainless steel alloy UNS S32550 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 S32550 alloy 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 UNS S32550 is a duplex stainless steel alloy with high strength and excellent corrosion resistance in a range of media including sea water. In contrast to the austenitic grade steels, the duplex alloys are formulated and thermodynamically processed to produce a two-phase microstructure containing nearly equal proportions of ferrite and austenite. It is also used for marine engineering, chemical and hydrocarbon processing equipment, valves, pumps, shafts and fittings.
