Cavitation seawater

Titanium resists erosion—corrosion by fast-moving sand-laden water. In a high velocity, sand-laden seawater test (8.2 m/s) for a 60-d period, titanium performed more than 100 times better than 18 Cr—8 Ni stainless steel. Monel, or 70 Cu—30 Ni. Resistance to cavitation, ie, corrosion on surfaces exposed to high velocity Hquids, is better than by most other stmctural metals (34,35). 

This type of damage is dealt with comprehensively in Section 8.8. It can be particularly severe in seawater giving rise to cavitation corrosion or cavitation erosion mechanisms, and hence can be a considerable problem in marine and offshore engineering. Components that may suffer in this way include the suction faces of propellers, the suction areas of pump impellers and casings, diffusers, shaft brackets, rudders and diesel-engine cylinder liners. There is also evidence that cavitation conditions can develop in seawater, drilling mud and produced oil/gas waterlines with turbulent high rates of flow. 

Fig 33, from Ref 17, is a schematic representation of the effects of a nearby surface on pressure pulse shapes at various distances below the water surface. It also shows the expected pulse shapes for acoustic rather than shock waves A shock wave in water will be reflected as a rarefaction wave when it encounters another medium less dense than water, eg, a water/air boundary. The rarefaction wave, generated by the reflection of the primary shock wave from the surface, propagates downward and relieves the pressure behind the primary shock wave. If the shock wave is treated as a weak (acoustic) wave, this interaction instantaneously decreases the pressure in the primary shock wave to a negative value, as shown by the broken line in Fig 33 (Ref 17), Point A. Cavitation occurs in seawater when its pressure decreases to a value somewhat above its vapor pressure. The pressure of the primary shock wave is, therefore, reduced to a value which is usually so close to ambient water pressure that the shock wave pulse appears to have been truncated... 

Tinbronze CuSnlO From C90200 Seawater-, corrosion- and cavitation-resistant, tough alloy with good wear resistance. Well suited for pumps and turbine components, fittings for high mechanical loads, and thin-walled castings. 

Seawater- and cavitation-resistant, hard and tough alloy with relatively high strength. 

Seawater-, corrosion- and cavitation-resistant alloy with relatively very high strength. 

Typical layout of a PRO-based power plant is represented in Figure 9.5. Seawater from the offshore intake is first moved to a sedimentation basin. Then, an electric pump supplies a proper head to overcome the pressure drop of the filtration system and avoid cavitation in the main pump. A complex filtration system is required both on seawater and freshwater to prevent membrane fouling that negatively affects its permeation. After filtration, pressure of the seawater stream is increased to some bar (stream S2) before it is addressed to the permeate side of the osmotic membrane module. 

The standard steels of the type SAE 316 (DIN-Mat. No. 1.4401, X5CrNiMol7-12-2) are not suitable for seawater-exposed pipes and fail as a result of pitting and crevice corrosion [155, 156]. The sensitivity to pitting corrosion of these standard steels can be further increased by deposits of maritime bacterial films [157]. Despite these facts, these steels are frequently used as materials for pump parts and have worked well as such because they are cathodically protected by contact with other parts made of less noble materials, e.g. pump casing made of cast iron [130]. [158] reports on tests of the cavitation behaviour of the pump materials GX5CrNiMol9-ll-2 (DIN-Mat. No. 1.4408) in 3% NaQ solution. 

Untersudiungen der Kavitationskorrosion an Pumpenwerkstoffen in Meerwasser (Tests of the cavitation corrosion in pump materials in seawater) (in German) Werkst. Korros. 33 (1982) 5, p. 288... 

Aluminum bronzes containing 7% Al, 2% Ni, show an outstanding resistance to de-alloying and cavitation corrosion in most fluids and seawater, because of nickel addition which is highly resistant to corrosion. Aluminum bronze, such as 76 Cu-22 Zn-2 Al, are used for marine heat exchangers and condenser because of its excellent corrosion resistance. Aluminum is responsible for increased corrosion resistance. But the velocity must not exceed a safe threshold to avoid erosion-corrosion. 

Effect of Precsure. Generally, pressure has not been found to significantly alter the corrosion resistance of aluminum alloys. An increase in pressure can minimize cavitation damage in special cases. The exposure of aluminum at great depths in seawater under high pressure has not shown consistent trends. 

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