Wear caused by cavitation is referred to as cavitation erosion. Microjets developing during cavitation cause violent pressure surges when they impinge on solid walls. The concentration of forces on a small surface area can lead to the material's destruction.
Pitting becomes apparent after a certain incubation period, initially in areas of reduced resistance to erosion. Very fine dimples or cavities develop as a result and these are exposed to attack. The material exhibits pitting due to erosion with a porous, spongy structure.
Assessing loss of material by erosion on internal pump components
- By measurement
- By weight loss
- By the amount of build-up metal deposited by repair welding
- By the amount of time needed to carry out repairs
If it proves impossible to limit the effects of cavitation by design or operational measures (e.g. through the use of more gradual transition sections, changes in the inlet conditions), or if it proves impossible to shift the collapse of the vapour bubbles away from the wall towards the centre of the flow path, then the erosion caused by cavitation can only be reduced by selecting appropriate materials.
Materials resistant to cavitation combine high fatigue strength with high levels of corrosion resistance. If, for instance, cast iron (JL 1040) is given a cavitation-induced weight loss index of 1.0, the following grading applies for other materials. See Fig. 1 Cavitation erosion
|Cast chrome steel|
|Aluminium multi-alloy bronze|
|Cast chrome nickel steel|
|Duplex stainless steel||GX3CrNiMoCuN24-6-2-3|
Fig. 1 Cavitation erosion: Materials graded by increasing cavitation resistance; weight loss index for typical cast metals (based on grey cast iron JL 1040 with an index of 1.0)
The index values specified represent average values established on the basis of international literature and by means of experimental studies with devices allowing the systematic generation of cavitation, taking into account a number of uncertainty factors. It is not possible to consider them to be absolute values, as the individual index numbers are strongly influenced by the type of cavitation. Another important factor for cavitation erosion is the chemical and electrochemical behaviour of the substances involved, i.e. the fluid and the base material.