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Erosion and cavitation corrosion Print E-mail


Erosion corrosion is the corrosion of a metal which is caused or accelerated by the relative motion of the environment and the metal surface. It is characterized by surface features with a directional pattern (comet tails, horseshoe marks, etc) which are a direct result of the flowing media. Erosion corrosion is most prevalent in soft alloys (i.e. copper, aluminum and lead alloys).

Image of erosion corrosion 1Alloys which form a surface film in a corrosive environment commonly show a limiting velocity above which corrosion rapidly accelerates. Other factors such as turbulence, cavitation, impingement or galvanic effects can add to the severity of attack. Erosion Corrosion are also known as flow-enhanced corrosion or impingement attack. 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.

So erosion corrosion is the result of a combination of an aggressive chemical environment and high fluid-surface velocities. This can be the result of fast fluid flow past a stationary object, such as the case with the oil-field check valve shown on the left below, or it can result from the quick motion of an object in a stationary fluid, such as happens when a ship's propeller churns the ocean.

Image of Erosion corrosion2
"Horseshoe" type erosion-corrosion damage in a copper pipeline.

Surfaces which have undergone erosion corrosion are generally fairly clean, unlike the surfaces from many other forms of corrosion.

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.

Image of Cavitation
Cavitation of a nickel alloy pump impeller blade exposed to a hydrochloric acid medium.

Prevention of Erosion Corrosion and Cavitation
  • selection of alloys with greater corrosion resistance and/or higher strength. Erosion corrosion can be controlled by the use of harder alloys (including flame-sprayed or welded hard facings) or by using a more corrosion resistant alloy. Alterations in fluid velocity and changes in flow patterns can also reduce the effects of erosion corrosion.
  • Re-design of the system to reduce the flow velocity, turbulence, cavitation or impingement of the environment.
    Many people assume that erosion corrosion is associated with turbulent flow, it is true. Because all practical piping systems require turbulent flow-the fluid would not flow fast enough if lamellar (nonturbulent) flow were maintained. Most, if not all, erosion corrosion can be attributed to multiphase fluid flow. The check valve on the left above failed due to sand and other particles in an otherwise noncorrosive fluid. The tubing on the right failed due to the pressure differences caused when gas bubbles collapsed against the pipe wall and destroyed the protective mineral scale that was limiting corrosion.
  • Reduction in the corrosive severity of the environment.
  • Use of corrosion resistant and/or abrasion resistant coatings.
  • Cathodic protection.
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.

Erosion corrosion is often the result of the wearing away of a protective scale or coating on the metal surface. The oil field production tubing shown above on the right corroded when the pressure on the well became low enough to cause multiphase fluid flow. The impact of collapsing gas bubbles caused the damage at joints where the tubing was connected and turbulence was greater.
 
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