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Erosion corrosion

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Albert David 4/5/2014 Lib-100 Mrs. Van Hoeck Erosion-Corrosion First Daft

History
Erosion-corrosion is the combination of physical erosion and a corrosive solution that can have a great effect on pipes and other metal parts. Erosion is the disturbance of the surface film of a material, caused by high velocity flows passed in the given material, and the protective film that is thinning or removed is done by corrosion. Erosion and corrosion have a synergetic effect on each other. This type is often accelerated at pipe elbows, tube constriction, and anywhere fluid flows are altered, which will increase the velocity of flow. Higher pressures and temperatures of fluids have had the effects to increase erosion-corrosion. The earliest published case would be in 1947. The common feature is use of carbon steel in locations of high water flow or the decrease in pressure. High turbulence with a water chemistry of modest alkalinity and free of oxidizing agents are also common features. The most recent case occurred at Mihama 3, a PWR power plant in Japan. The mechanism for erosion corrosion is not completely known, due to the fact that the models have certain flaws. A good model is the platelet mechanism of erosion, which starts with a small turbulences in the flow of fluid that causes a small divot in the passive film of the metal surface. The small divot increases as the turbulence increases in the local point, which in turn cuts into the protective oxide film on the surface, removing material. The divot takes different forms of the local point that may look like grooves, wave gullies, and tear dropped shaped.

Types of Erosion-Corrosion

 * A Flow-Accelerated Corrosion, or maybe referred as Flow-Assisted Corrosion are a type of erosion-corrosion. This has to do with the high velocity that a solution in a pipe could occur due to a burr in the copper pipe. The thinning of the pipe is the mechanical wear by erosion, and the solution and inner pipe rubbing can be a chemical attack by corrosion. The continued reaction will lead to thermal stresses that will form cracks where the solution will erode under the deposits where impurities can concentrate.
 * Another type is cavitation erosion, which is the implosion of gas bubbles on a metal surface. Cavitation erosion can be referred as liner pitting. This occurs by the rapid change in pressure letting open air pockets to be part of the flow . These pockets collapse when they reach a region of higher pressure, creating a micro jet of fluid. There is some debate over the face the micro jet or the shock wave of the imploding pocket causes the erosion.

Ways to avoid Erosion-Corrosion
By avoiding the change in pressure in the pipes as well as keeping a constant velocity in the pipes. Having spare parts that can be easily replaced of the part that will be subjected to erosion-corrosion is a great way to go, but can be very expensive. The use of plastic or rubber coatings between pipe connections can have an upside and down side. The upside is that the plastic or rubber will allow the flow to be even when the flow is passing the pipe connections. The downside is that if the placement of the plastic or rubber is incorrect the flow in the pipes will get turbulence and start the erosion-corrosion process. Another way to minimize corrosion is to have the plant design to include an air removal system in the condenser, which is called a Deaerator or DA for short. This Deaerator would remove the additional air and non-condensable gas ahead of the high-pressure Feedwater Heaters, and oxygen scavenger injection to Feedwater to remove any traces of oxygen that might escape the DA. If a designer is able to redesign the pipes flow of the fluid to make sure there is less turbulence that will decrease the chances of erosion-corrosion The obvious indication that erosion-corrosion is the cause of metal loss can be seen of clear flow-related markings on the pipes affected surface. The transition from metal loss to deposition is also demarcated by a color change from the black magnetite in the zone of metal loss to the red oxide hematite, Fe2O3 in the deposition zone.