Oxygen corrosion, acid corrosion, alkali corrosion, and soda corrosion of thermal equipment

(1) Oxygen corrosion
Oxygen corrosion is a kind of electrochemical corrosion caused by dissolved oxygen in the medium. It is a common form of corrosion in thermal equipment. Oxygen corrosion may occur when thermal equipment is in operation and out of service. Oxygen corrosion during operation mainly occurs in the water supply system with higher water temperature, as well as the drainage system with higher dissolved oxygen content and the internal cooling water system of the generator. Due to the large amount of air entering the system when the equipment is out of service, oxygen corrosion is usually common, mostly under normal temperature and high humidity. If proper shutdown protection is not carried out, serious oxygen may occur in all parts of the water vapor system of the entire unit. corrosion.

(2) Acid corrosion
Acid corrosion is a form of corrosion caused by hydrogen ions in the medium, that is, hydrogen evolution corrosion. The acid corrosion that may occur in thermal equipment mainly includes the acid corrosion of the water treatment system outside the furnace, the free CO2 corrosion of the condensate system and the drainage system, and the acid corrosion of the initial condensate part of the low pressure cylinder of the steam turbine.

(3) Alkali corrosion
This kind of corrosion is common in boiler water wall tubes. When there are loose deposits on the water wall tube to the fire side and the boiler water contains free NaOH, the boiler water can penetrate into the interface between the deposit and the tube wall, and it will be concentrated at the interface to form a high concentration. In the alkaline solution (pH>13), the metal oxide on the surface of the pipe wall reacts with the concentrated alkali as follows:

As a result, the protective film on the surface of the water wall tube (steel for boiler) is destroyed, and the following electrode reactions continue to occur:

Fe3O4+4NaOH→2NaFeO2+ Na2 FO2+2H2O

Anode reaction:3Fe→3Fe2+ +6e

Cathodic reaction:6H2O+6e→6OH+6H

The resulting Fe2+ and OH- react further:3Fe2+ +6OH→Fe3O4+2H2O+H2

The reaction product Fe3O4is loose and has almost no protective effect on the base metal, and the corrosion reaction will continue. The anode reaction proceeds at the interface between the metal and the metal oxide. The Fe2+ generated diffuses through the oxide layer, and reacts with OH- at the interface between the oxide and the furnace water to form Fe3O4. The electrons also pass through the oxide layer at the same time in the oxide and furnace water. The interface reacts with water to generate hydrogen atoms. Since hydrogen atoms are formed at the interface between the oxide and the furnace water, the hydrogen atoms recombine into hydrogen and quickly enter the steam-water mixture and be taken away, so the hydrogen atoms will not diffuse into the metal, that is, it will not cause hydrogen embrittlement of the furnace tube.

(4) Soda water corrosion
When the temperature of the superheated steam exceeds 450℃ and the wall temperature of the superheated steam pipe exceeds 500℃, the steam can directly react chemically with the iron in the carbon steel between 450-570℃ to form Fe3O4, namely:

3Fe+4H2O →Fe3O4+4H2

When the temperature reaches above 570℃, the reaction product is Fe2O3, namely:

Fe+H2O →FeO+H2
2FeO+H2O →Fe2O3+H2

When this kind of corrosion occurs, the pipe wall will be thinned uniformly, and the corrosion products will be scaly or powdery, most of which are Fe3O4. This chemical corrosion is called soda corrosion. Steam water corrosion generally occurs in the superheater or reheater tube. It may be uniform corrosion or local corrosion. Uniform corrosion usually occurs in the parts where the metal temperature exceeds the allowable temperature, and dense iron oxide scales are formed at the overheated parts of the metal. Local corrosion appears in the form of ulcers, groove marks and cracks. Ulcer-like soda corrosion often occurs in the parts where the metal alternately contacts steam and water. The temperature of the metal in these parts often reaches or exceeds 70℃, which will accelerate the partial rupture of the protective film, so that the steam can repeatedly contact the exposed local metal surface. , And finally accelerate the corrosion rate of the local surface, the formed ulcer is often covered by Fe3O4.