Cavitation Corrosion
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Under high velocity flow conditions, particularly when the flow is turbulent, areas of high and low pressure will be induced. In areas of low pressure, gas and vapor bubbles will be produced. When these bubbles move to an area of higher pressure, they collapse and their implosion creates a pressure wave that can remove protective films and cause increased corrosion.
Definition.
Cavitation corrosion is corrosion that is enhanced through the formation and collapse of gas or vapor bubbles at or near the metal surface.
Mechanism.
As described above, the formation and collapse of gas or vapor bubbles in a liquid can cause localized damage to the films responsible for limiting corrosion. Once this localized corrosion is established, the local roughening can often serve as a new site for further turbulence and more cavitation attack.
Examples.
Cavitation is commonly encountered in pumps and in high speed propellers. It is enhanced when entrained air is present in systems with high velocity flow. Cavitation can also occur on or near high intensity sound generators. Stainless steels, some nickel alloys, and titanium alloys are highly resistant to cavitation damage but even these will be attacked under severe conditions.
Appearance.
Cavitation corrosion is similar to erosion corrosion and pitting is usually encountered. Cavitation can often be verified by a hydrodynamic analysis that can be used to locate and minimize bubble formation or move the area of bubble collapse to an area where the attack will have a minimal effect.
Significant Measurements.
There are no standard tests for cavitation attack. Only through actual full scale tests or from experience can failure due to cavitation be avoided. High velocity flow should be avoided in the design of all systems.