Wear corrosion, galvanic corrosion and high and low temperature corrosion of thermal equipment

Wear corrosion

Wear corrosion is a kind of localized corrosion caused by the combination of electrochemical erosion and mechanical wear of the medium when there is relative movement between the corrosive medium and the metal surface. For example, on the water side of the condenser tube, especially the inlet end, the scouring corrosion caused by the turbulent cooling water or the scouring action of suspended matter in the water is a typical wear corrosion, and the corrosion part often has obvious characteristics of fluid scouring marks. Under the condition of deoxygenated alkaline fully volatile treated water, the turbulent flow zone (such as pipeline elbow, tee, variable diameter, etc.) of the water supply system, especially the economizer pipeline, often occurs accelerated corrosion due to the impact of turbulent flow, which is called flow accelerated corrosion. In addition, steam bubbles are constantly formed and broken in the liquid on the surface of the high-speed rotating feed pump impeller. The shock wave generated when the bubble bursts will destroy the protective film on the metal surface, thereby accelerating the corrosion of the metal. This wear corrosion is called cavitation corrosion.

Galvanic corrosion

When two metals with different corrosion potentials are in contact with each other in a corrosive medium, the metal with a relatively negative corrosion potential will undergo local accelerated corrosion near the contact part, which is called galvanic corrosion. For example, at the connection between the carbon steel tube plate of the condenser and the stainless steel (or titanium, copper alloy, etc.) tube, due to the negative electrode potential of carbon steel in the corrosive medium, galvanic corrosion occurs.

High temperature corrosion on the smoke side of the boiler

The high-temperature corrosion on the smoke side of the boiler mainly refers to a type of corrosion that occurs on the outer surfaces of the boiler water-cooled fireplace tubes, superheater tubes, and reheater tubes, as well as on the surface of the suspension parts in the boiler furnace. Including high temperature oxidation caused by flue gas and molten salt corrosion caused by boiler fuel combustion products, the latter is more serious in comparison. Among them, the molten salt of water-cooled fireplace tubes is mainly sulfide or sulfate; the molten salt of superheater and reheater tubes is mainly Na3Fe-(SO4)3 and K3Fe(SO4)3 and other double salts.

Low temperature corrosion of heating surface at boiler tail

Due to the reaction of SO3 in the flue gas and the moisture in the flue gas to form H2SO4, the surface of the smoke side of the air preheater in the flue at the tail of the boiler is corroded.

In large thermal power generating units, regardless of whether it is a subcritical unit using a drum boiler or a once-through furnace, or a supercritical and ultra-supercritical thermal power unit that can only use a once-through boiler due to the characteristics of the parameters itself, the thermal equipment of the thermal equipment has the above types of corrosion. Among them, oxygen corrosion and acid corrosion are the most prominent and common, and the harm is more serious, which are the main reasons for the corrosion of condensate and water supply systems. Corrosion of condensate water and water supply system not only directly causes corrosion and damage of thermal equipment, but also may produce a large amount of corrosion products and be carried to the boiler by the water supply and deposit in the water-cooled fireplace pipes, which can cause local overheating, corrosion, and even tube bursting of the furnace tube, which seriously endangers the safe and economical operation of the power plant.