Cavitation damage resistance

Saetre [102] has reported cavitation erosion in GRP pipe bends guiding sea water at around 10ms and at pressures of less than 1 bar. Hammond et aL [103] examined the cavitation damage resistance of FRP before and after sea water immersion using a modified ASTM G32 method. A stationary specimen was placed beneath a Ti-6A1-4V tip oscillated at 20kFIz with an amplitude of 25 pm. The materials studied were ... 

Weld overlays of stainless steel or cobalt-based wear-resistant and hard-facing alloys such as Stellite may salvage damaged equipment. In addition, weld overlays incorporated into susceptible zones of new equipment may provide cost-effective resistance to cavitation damage. 

Being less plentiful and more expensive than nickel, cobalt is usually alloyed with chromium for applications where the alloys have practical advantages over similar nickel- or iron-base alloys. The cobalt-base alloys, for example, are better resistant to fretting corrosion, to erosion by high-velocity hquids, and to cavitation damage. 

Selection of materials. Materials, like 18-8 steels and titanium, are resistant to cavitation damage. 

Under certain conditions, for example, in pumps and at the back of propellers, a partial vacuum is created. The water boils at ambient temperature at this reduced pressure, in the form of bubbles which collapse when they move to a position of increased pressure. The pressure exerted by the collapsing bubbles are very intense. Collapsing bubbles can produce pressure as high as 60 000 psi. If the process is repeated too often, the protective oxide layer is completely destroyed by the implosion of bubbles. The resistance of the metal to cavitation depends on its ability to form a compact, dense and adherent film. Brittle films are not normally protective and are subjected to cavitation damage. 

Using materials resistant to cavitation damage (Table 4.17) [116]. 

Make the eroding surface resistant to corrosion. Examples include the use of cobalt-base hardfacing alloys to resist liquid erosion, cavitation, and slurry erosion aluminum bronze hardfacing alloys to prevent cavitation damage on marine propellers or to repair props that have... 

Areas of struetural metals affected by cavitation can be surfaced with welding wire or strip, overlay welding, or eoating with dense high-tensUe materials that resist cavitation damage (e.g. chromium stainless steel 18-8). 

Depositing filler metal on a smface by welding, spraying, or braze welding to increase resistance to abrasion, erosion, wear, galling, impact, or cavitation damage. 

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. 

Cavitation Formation of transient voids or vacuum bubbles in a liquid stream passing over a surface is called cavitation. This is often encountered arouna propellers, rudders, and struts and in pumps. When these bubbles collapse on a metal surface, there is a severe impact or explosive effec t that can cause considerable mechanical damage, and corrosion can be greatly accelerated because of the destruction of protective films. Redesign or a more resistant metal is generally required to avoid this problem. 

Cavitation is more or less continuous, but the cavitation intensity is so low that only the coating of corrosion products is dislodged. Resistance of the corrosion products to cavitation relative to the resistance of the metal to cavitation is low, making the corrosion products more susceptible to damage. 

The theoretical maximum suction lift at sea level for water (14.7 psi) (2.31 fi/psi) = 34 ft. However, due to flow resistance, this value is never attainable. For safety, 15 feet is considered the practical limit, although some pumps will lift somewhat higher columns of water. WTen sealing a vacuum condition above a pump, or the pump pumps from a vessel, a seal allowance to atmosphere is almost always taken as 34 feet of water. High suction lift causes a reduction in pump capacity, noisy operation due to release of air and vapor bubbles, vibration and erosion, or pitting (cavitation) of the impeller and some parts of the casing. (The extent of the damage depends on the materials of construction.)... 

In practice it is expensive, and therefore uneconomic, to produce a pump which operates completely free from cavitation. As a result it is usual for commercial pumps to operate in the NPSH range between inception and a point where erosion damage is unacceptable. The extent of this range may be increased by using impellers made from the more resistant materials shown in Fig. 8.77. The subject of cavitation in pumps has been dealt with extensively by Pearsall and Grist... 

We can divide the surface contact properties of polymers into two categories those in which the surface remains intact when it comes into contact with another surface and the polymer surface s resistance to damage. Friction and cling fall into the first category. Surface damage can be caused by erosion, abrasion, or cavitation. 

Aluminum bronzes, copper-nickel alloy (69 Cu-30 Ni-1 Fe and copper-nickel alloys containing added manganese are resistant to impingement and cavitation-induced damage under flowing conditions. 

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