After the pretreatment and ion exchange process, suspended solids, colloids, and dissolved salts in the water supply of high pressure boilers have been removed, but a small amount of oxygen and carbon dioxide dissolved in water can cause the corrosion of the boiler water supply system.
- Reasons for water side corrosion of high pressure boiler
2.1 Dissolved oxygen corrosion
Dissolved oxygen corrosion is a kind of electrochemical corrosion, iron and oxygen as the two electrodes, which form a corrosion cell. There are two reasons for it. On one hand, iron oxide cap produces an acid solution, accelerating the dissolution of iron; on the other hand producing iron oxide cap, the corrosion products prevents the proliferation of oxygen. In the corrosion products formed under the anode area of oxygen, the external will form a rich oxygen cathode area, thus, a concentration difference cell is formed. The two part of the differences accelerates the corrosion reaction. With the increase of the amount of dissolved oxygen in water and the water temperature, the corrosion becomes stronger.
2.2 Sediment corrosion
Under normal operating conditions, the pH value is kept between 9-11，at this time, a dense Fe2O3 protective film is formed on the metal surface, So there is no serious corrosion. However, when the existence of sediment on boiler heating surface, the temperature rise of the metal pipe wall under the sediment due to the bad heating surface, and the water evaporation of the sediment, it will produce corrosion under sediment. From the boiler equipment inspection, scale, salt accumulation of the shape, color, composition analysis, corrosion status analysis, prone to corrosion of deposition site is the main drum, two stage superheater, low temperature reheater.
2.3 Sour corrosion
In acidic water，iron reacts with acid to produce iron ions and releases hydrogen，meanwhile hydrogen atoms react with carbon steel to produce the CH4. Decarburization will make metal cracks along the grain boundary. When Boiler pressure in 3.5MPa, hydrogen ions can quickly diffuse into the metal and cause the metal to become brittle and damaged. CO2 and steam flows through saturated steam and superheater pipe, steam turbine, then a part of the air pump is taken away, and the other part is integrated into the water, Formatting H2CO3 weak acid. H2CO3 partially decomposed to produce H+ and cause corrosion. When the surface water is used as the water source for the boiler,it can produce inorganic acid corrosion.The cooling water into the boiler, the pH value will decline and format the acid corrosion. Especially in the treatment of volatile water, corrosion will be more serious.
2.4 Alkaline corrosion
Free alkali can be concentrated between porous sediments and metal surfaces of furnace tubes, Excessive concentration of alkali dissolved metal protective film and the formation of a mixture of Ferrous iron acid radical and acid radical ion. This kind of alkali corrosion is especially easy to occur in the place where the heat is larger and have the sediment. Carbon steel and water contact surface pH value in the 8.5-10.3, the general it will not occur alkaline corrosion.
2.5 Impact corrosion
① In the condensed water pipe, steam mixed flow velocity changes and wash wall, cause scour wall thinning and rupture.
② The flow velocity is high, the flow is turbulent and the local position of the turbulence zone is formed continuously.
③ The deaerator position is too low, high temperature water through the pump will instantly vaporize, and bubbles will produce cavitation pits in water pump.
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