The corrosion of thermal equipment is directly related to the quality of water vapor.
(1) Impurities in water
Although boiler feed water usually undergoes strict water purification treatment, a small amount of impurities will still be mixed into the water vapor circulation system during the operation of thermal equipment. For drum boilers, due to the large amount of evaporation, the boiler water concentration ratio is very high. For example, for units of 300MW and above, the boiler water concentration ratio is generally several + times to several hundred times. Under high temperature and high pressure conditions, it is easy to cause corrosion. In addition to using individual indicators (such as iron, copper, silicon, sodium, etc.) to limit impurities in water, conductivity indicators are also used to limit the total salt content or hydrogen conductivity is used to characterize the content of anions other than hydroxide.
(2) Dissolved gas in water
Mainly refers to the dissolved oxygen and carbon dioxide in the water, which can be caused by the intake of the make-up water, the leakage of the vacuum system of the condenser, and the decomposition of trace impurities in the furnace. Dissolved oxygen is the most common oxidizing substance that causes metal corrosion. After carbon dioxide gas is dissolved in water, it will lower the pH of the water or increase the driving force of the metal to corrode the galvanic battery. Changes in these factors will promote the corrosion of the metal.
(3) The influence of pH
The corrosion of metals in water has a great relationship with the pH value. Carbon steel is more resistant to corrosion in water with a pH of 9.6 to 11.0, and copper alloys are more resistant to corrosion in water with a pH of 8.8 to 9.1. For systems where the above two metals coexist, a compromise method is usually adopted. For example, when the water supply system contains copper alloys, the pH value control range is specified to be 8.8~9.3. Under high temperature and high pressure conditions, the water conductivity, dissolved oxygen content and The pH value is a key factor affecting metal corrosion.
(4) Temperature, pressure and stress
Too high temperature will cause metal creep; too high pressure will cause blasting of weak metal parts; metal in a special corrosive environment is prone to stress corrosion cracking under the action of tensile stress, of which alloy steel and stainless steel are particularly sensitive.
(5) The influence of water velocity
When the water is in a reducing atmosphere, increasing the flow rate tends to accelerate the occurrence of corrosion. This phenomenon is most obvious in the hydrophobic system of a high-pressure heater; when the water is in an oxidizing atmosphere, the flow rate has little effect on corrosion. In still water, because the local oxygen supply is not uniform, it is easy to produce oxygen concentration cell corrosion effect; in flowing water, because the oxygen concentration of each part of the metal in contact with the water is basically the same, it is prone to general corrosion.
(6) Corrosion during equipment outage
Corrosion during the outage of thermal equipment tends to occur more easily. When the thermal equipment is shut down, the steam in the water vapor system condenses, the temperature and pressure gradually drop, and even a negative pressure is formed. As a result, the air enters the water vapor system from the place with poor equipment sealing or overhaul, and the oxygen in the air dissolves Into the water. At the same time, it is impossible to ensure that every part is emptied when the water is stopped. Some parts still have some water. On the one hand, some metals are immersed in the water. On the other hand, due to the evaporation of the water, the internal humidity of the water vapor system is very high, forming oxygen. Corrosion battery, metal corrodes and rusts quickly.