Erosion Corrosion and Cavitation Corrosion

Erosion Corrosion, also known as flow-enhanced corrosion or impingement attack, takes place in flowing systems where turbulence occurs, typically in pipe bends (elbows), tube constrictions, and other structures that alter flow direction or velocity. The mechanism for this type of corrosion is the continual flow of water, which removes any protective film or metal oxide from the metal surface. The exposed surface quickly corrodes and the resulting oxide is in turn eroded away. Erosion corrosion usually leads to rapid failure.

Erosion corrosion can occur both in the presence and in the absence of suspended matter in the flow stream. In the presence of suspended matter, the effect is very similar to sandblasting, and even strong films can be removed at relatively low velocities.

Cavitation is similar to erosion corrosion in that they both occur in piping systems and also that protective films are removed from the metal surface during fluid flow. Low pressure, which could be caused by changes in velocity due to discontinuity in the flow path, causes gas or vapor bubbles to form in the fluid stream. When these bubbles hit the metal surface, they rapidly collapse or implode, producing a shock wave sufficiently strong to remove protective films.

Erosion Corrosion

Controlling Erosion Corrosion and Cavitation

Some prevention methods for erosion corrosion and cavitation include improving the flow lines within the pipe, by deburring (smoothing out irregularities), allowing bends to have larger angles, and changing pipe diameters gradually rather than abrupt changes. Other methods include slowing the flow rate (reducing turbulence), reducing the amount of dissolved oxygen, changing the pH, and changing the pipe material to a different metal or alloy.

Example: The pipe shown below is leaking due to erosion corrosion. A) Indicate the most probable locations for leaks. B) This is one example of a way to redesign the pipe to eliminate or minimize erosion corrosion.