Cavitation damage locations

In general, cavitation damage can be anticipated wherever an unstable state of fluid flow exists or where substantial pressure changes are encountered. Susceptible locations include sharp discontinuities on metal surfaces, areas where flow direction is suddenly altered (Fig. 12.5), and regions where the cross-sectional areas of the flow passages are changed. 

Cavitation damage is a fairly common form of erosion-corrosion of pumps, impellers or hydrauhc turbine internals. Cavitation is caused by collapsing gas bubbles at high-pressure locations adjacent metal surfaces are damaged by the resultant hydraulic shock waves. Cavitation damage is usually in the form of loosely spaced pits that produce a roughened surface area. 

Typically, affected surfaces are highly localized to specific regions, although if cavitation is severe and widespread, the area affected may be extensive (Fig. 12.7). On S3mimetrical components having repeated elements, the pattern of damage may repeat itself at identical locations on each element (Fig. 12.8). 

As we have mentioned before, acoustic streaming, cavitation and other effects derived from them, microjetting and shock waves take also relevance when the ultrasound field interacts with solid walls. On the other hand, an electrochemical process is a heterogeneous electron transfer which takes place in the interphase electrode-solution, it means, in a very located zone of the electrochemical system. Therefore, a carefully and comprehensive read reveals that all these phenomena can provide opposite effects in an electrochemical process. For example, shock waves can avoid the passivation of the electrode or damage the electrode surface depending on the electrode process and/or strength of the electrode materials [29]. 

Low-pressure zones in a fluid system under turbulent flow conditions are at the origin of cavitational wear. When the hydrodynamic pressure at some location temporarily drops below the vapor pressure of the fluid a fraction of the hquid is transformed into gas bubbles. The recurrent formation and implosion of these bubbles leads to material damage by local fatigue. If simultaneously corrosion takes place we speak of cavitation corrosion. 

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