Propellers cavitation corrosion

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. 

Cavitation corrosion is caused by repeated pounding resulting from rapid collapse of vapor bubbles. On a metal surface where there is a violent flow of water, such as that of a ship s propeller, the pressure at some area may be reduced to where localized boiling forms bubbles of vapor. At another site, these bubbles suddenly collapse. 

Figure 7.50 a) Cavitation corrosion on the propeller of a high-speed passenger ship, h) Close np of the damages. (The material is nickel-aluminium hronze.) (Photos T.E. Hammervold and O Sastre.)... 

Particularly susceptible to damage from cavitation corrosion are, for example, ships propellers, condenser pipes, pump impellers, widened segments in pipelines or flow channels. 

Fig. 10.4 (a) Cavitation corrosion of a water-cooled cylinder casing of a diesel engine. Corrosion holes have penetrated the wall, (b) The water pump propeller in the same diesel engine corroded by cavitation... 

In some instances, cathodic protection has been successful in reducing or preventing cavitation damage, but because cavitation damage usually involves physical as well as electrochemical processes, it cannot always be prevented by this means. In some cases, inhibitors have been used successfully to limit cavitation corrosion, as in the water side of diesel engine cylinder liners. Cavitation is a problem with ship propellers, hydraulic pumps and turbines, valves, orifice... 

Cavitation corrosion occurs at fluid dynamic conditions, causing large pressure variations due to high velocities, as often is the case for water turbines, propellers, pump rotors and external... 

It is often localized at areas where water changes direction. Cavitation (damage due to the formation and coUapse of bubbles in high velocity turbines, propellers, etc) is a form of erosion corrosion. Its appearance is similar to closely spaced pits, although the surface is usually rough. 

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. 

Whenever corrosion resistance results from the accumulation of layers of insoluble corrosion products on the metallic surface, the effect of high velocity may be either to prevent their normal formation or to remove them after they have been formed. Either effect allows corrosion to proceed unhindered. This occurs frequently in smaU-diameter tubes or pipes through which corrosive liquids may be circulated at high velocities (e.g., condenser and evaporator tubes), in the vicinity of oends in pipe hnes, and on propellers, agitators, and cen-trifiig pumps. Similar effects are associated with cavitation and impingement corrosion. 

At very high water speeds cavitation-damage (Section 8.8) is sustained by any meti high-speed bronze propellers, for instance, may suffer seriously. This form of attack is mainly mechanical, although an element of true corrosion may be present, and is not specifically associated with sea water. 

Cavitation-erosion is the loss of material caused by exposure to cavitation, which is the formation and collapse of vapor bubbles at a dynamic metal-liquid interface—for example, in rotors of pumps or on traihng faces of propellers. This type of corrosion causes a sequence of pits, sometimes appearing as a honeycomb of small relatively deep fissures (see Uhlig s Corrosion Handbook, 2nd edition, R. W. Revie, editor, Wiley, New York, 2000, Fig. 12, p. 261). 

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... 

General Description. Cavitation is a form of erosion-corrosion that is caused by the formation and collapse of vapor bubbles in a liquid against a metal surface. Cavitation occurs in hydraulic turbines, on pump impellers, on ship propellers, and on many surfaces in contact with high-velocity liquids subject to changes in pressure. The appearance of cavitation is similar to pitting except that surfaces in the pits are usually much rougher. The affected region is free of deposits and accumulated corrosion products if cavitation has been recent. 

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