Metallic structures can undergo stresses due to various physical forces which can damage and shorten their expected life.
What is Cavitation?
Cavitation damages metals and other substrates by the force of implosion of air or gas bubbles in a liquid at a solid-liquid interface. It is typically seen in equipment operating with high velocity flow of liquids and sudden changes in pressure. The cavitation process is responsible for the mechanical breakdown of the protective
(passive) surface film on the metal. Upon this de-passivation, accelerated corrosion will take place. Severe cavitation can also result in the mechanical removal of the base metal due to stresses induced on the surface that are in excess of the material’s mechanical strength.
Cavitation may be slowed down, but cannot be totally eliminated through the use of protective coatings. The cause of cavitation must be identified and removed in order to prevent this type of attack.
Key Properties of Cavitation
● Is caused by imploding vapour bubbles.
● The appearance is similar to that of pitting.
● Pressures may be as high as 4200 bar (60,000 psi).
● The force is high enough to tear away passive oxide layers as well as underlying metal.
● It is progressive in nature.
● Cavitation is usually a process or design related flaw.
Where does it occur?
- Pump impellers and casings
- Turbine blades
- Propeller blades
- Pipe walls near restrictions
How to Control Cavitation?
The effects of cavitation may be controlled in a variety of ways, including:
- Equipment or system design
- Selection of materials
- Coatings
- Use of smooth surfaces
Source: Chesterton ARC Manuals
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