Pitting is one of the most destructive forms of corrosion as it will potential cause equipment failures due to perforation or penetration. pitting generally occurs on metal surfaces protected by oxide film such as Stainless steel, aluminum, etc. Typically for boiler and feed water system, pitting corrosion rate increase dramatically with the increase of oxygen content in the fluid. This corrosion can occurs in any metal surfaces.
Carbon dioxide combines with water to form carbonic acid which is very aggressive to steel (H2O + CO2 --> H2CO3; so pH is lower than water); and results in large areas of rapid metal loss that can completely erode sucker rods and couplings. The corrosion severity increases with increasing CO2 partial pressure and temperature. CO2 corrosion pits are round based, deep with steep walls and sharp edges. The pitting is usually interconnected in long lines but will occasionally be singular and isolated. The pit bases will be filled with iron carbonate scale, a loosely adhering gray deposit generated from CO2.
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