Cavitation Corrosion

July 12, 2016

Cavitation occurs when vapor cavities, or bubbles, form in a liquid. When the bubbles eventually implode, a shock wave causes cyclic stress and fatigue on the inside of pipes and vessels, dramatically shortening its lifetime.

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Although the collapse of small cavities (above) is a relatively low-energy event, highly localized collapses can erode metals, such as steel, over time. The pitting caused by the collapse of cavities produces great wear on components and can dramatically shorten a propeller or pump's lifetime.

Cavitation occurs when vapor cavities, or bubbles, form in a liquid. Vapor forms easily when the liquid undergoes a rapid pressure loss, which causes a phase change by lowering the pressure to below the vapor pressure. When the pressure rises once again, the bubbles quickly implode and generate a shock wave. These shock waves cause cyclic stress and fatigue on the inside of pipes and vessels through repeated implosions.

Types of Cavitation

Cavitation can be divided into inertial (transient) cavitation and non-inertial cavitation. Inertial cavitation is the most frequent form and occurs when voids and bubbles in a liquid implode and form a shock wave. Non-inertial cavitation is less common and occurs when a bubble oscillates in either size or shape due to an energy input. This type of cavitation is used in ultrasonic cleaning baths.

A Few Upsides of Cavitation

While cavitation is generally avoided due to its tendency to cause damage to machinery, it can also be used in many applications. For example, cavitation is often used to mix colloidal compounds like paint or milk and many industrial mixers also use this principle in their design. Devices have also been designed which employ cavitation to purify water because cavitation conditions are so extreme that they are able to break down pollutants and organic molecules.

Bigger Downsides

However, in most cases, cavitation is undesirable and can cause extreme damage if left unaddressed. Moving parts such as impellors and pumps are at the most risk for impellor damage, because their movement is the most likely to create local low pressure areas that could lead to cavitation. Pitting caused by cavitation on impellors or pumps can greatly reduce efficiency and eventually cause the machinery to fail completely. Once cavitation wear has begun, it tends to increase exponentially because pits serve both to increase flow turbulence and to provide a nucleation site for more cavitation bubbles to form. Additionally, pitting causes residual stress and makes the material more prone to stress. Cavitation can be minimized by ensuring a well-developed flow enters pumps to allow for maximum performance. This is usually accomplished by allowing for 10 diameters of straight pipe prior to the pump inlet.

Where Else Can Cavitation Occur

Less common places for cavitation to occur are control valves, spillways, and engines. Cavitation in control valves occurs when the actual pressure drop across the valve is larger than the sizing allows, causing flashing and cavitation. If the pressure at the smallest point of the valve is below the fluid’s vapor pressure, then cavitation will occur, and if the pressure recovers to a point above the vapor pressure, bubbles will implode and potentially cause damage. In spillways, surface irregularities cause areas of flow separation in high speed flows, and therefore, low pressure which leads to cavitation. In larger diesel engines, cavitation is caused by high compression and undersized cylinder walls. When the cylinder walls vibrate, they cause areas of low and high pressure in the coolant, leading to cavitation pitting which can eventually allow coolant to leak into the cylinder and combustion gases to leak into the coolant.

While cavitation can be useful in select applications, it must always be carefully designed for and in many cases is downright harmful. It is important to understand that corrosion comes not only from the environment, but can also come from inside of your pipes. Therefore, it is important to consider all sources of potential corrosion when designing a process.

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