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Surface Finishing Tutorial

#062 Erosion & Corrosion

Category : Corrosion - Corrosion Protection
November12, 2010

 "Erosion" is a wear phenomenon that occurs mechanically, and "Corrosion" is a decomposition phenomenon. The term corrosion represents a "wear due to decomposition".

 As with water flowing in a plumbing pipe, flowing fluid would rub against the inner wall surface. Slow flowing fluid may not cause any wear, but high flow velocity would result in wear. Furthermore, any inclusion of solid particles in the fluid will cause vigorous abrasions and mechanical wear.

 Regarding carbon steels, they are likely to corrode in water and other solutions resulting in rust forming on their surfaces. Unlike the rusts on stainless steels, carbon steel rust is not as corrosion resistant as to prevent any further rusting. But the rust will slow down the speed of further corrosions compared to not having any at all.

 However, if any fluid abrasion effects exist in this state such corrosion byproducts would be scraped away and fresh carbon steel surface would be exposed. If dissolved oxygen in water acts with the fresh surface, new corrosion will progress, naturally. The "Erosion" effect removes the rust and new "Corrosion" effect advances. This compounded effect increases the speed of corrosion, compared to having either one or the other only.

 This can be evidenced by immersing a small piece of carbon steel in salt water solution, and scrape off rust in a specific location 2~3 time per day. In a few months, scraped portion will become indented.

 Actual well known cases of Erosion & Corrosion can be seen on high speed flow water/salts plumbing where turbulences are like to occur. Even on relatively slow flowing plumbing can be subject to this such as with pipe bends and heat exchanger inlets. These locations are likely to be affected by Erosion & Corrosion phenomenon.

 When solids are included in the flowing fluid, such as with coal transporting pipe lines, Erosion & Corrosion is highly like to occur. The phenomenon is also likely to occur with gas-liquid mixtures and gas-mist mixtures.

 Rust resistant stainless steels are also resistant to Erosion & Corrosion phenomenon. The passive surface layer of stainless steel can quickly reform even after being eroded away. However, there are some aggressive Erosion & Corrosion instances of stainless steel where used for salt water pump impellers.