Discussion on oxygen corrosion of low temperature

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Brief discussion on oxygen corrosion and preventive measures of low-temperature hot water boilers

abstract the mechanism and influencing factors of oxygen corrosion of boilers in the advantageous field of low-temperature hot water are analyzed, and preventive measures are put forward in combination with practice

keywords low temperature hot water boiler oxygen corrosion mechanism factors measures

in recent years, the comprehensive statistics of the overhaul of the main pressure parts of the low temperature hot water boiler in our city show that the low temperature hot water boiler mainly has the following three forms of damage: serious corrosion of the pressure parts of the boiler; Cracks or bulges occur at the bottom of the boiler shell; After high temperature, the tube sheet produces penetrating cracks. Among the three forms of damage, about 70% of the boilers have been overhauled due to serious corrosion, especially the flue pipes of low-temperature hot water boilers. In 1998 alone, more than 20 boilers in our city were forced to overhaul due to serious oxygen corrosion, at a cost of 900000 yuan. One kxl1 of one unit All smoke pipes of 7/95/70 boiler have been replaced three times in five years. It can be seen that the problem of oxygen corrosion in low-temperature hot water boilers should be paid great attention to by relevant departments

1 mechanism of oxygen corrosion

oxygen corrosion is essentially an electrochemical corrosion. Because pot water is a polar electrolyte, the result of the action of the electrolyte is that iron is hydrated, and the hydration energy exceeds the binding energy between ion atoms. Therefore, water molecules suck iron ions out of the steel surface and make it in the electrolyte layer on the metal surface. The negative charge (electrons) with the same amount of charge as these ions remain in the metal, forming a negative charge layer, which is in an electrostatic state, keeping the hydrated ions of the metal at the boundary between the metal and the liquid. When there is a certain potential difference, electrons will transfer from the iron pole to the position with high electrode potential, which will cause the iron ions corresponding to its charge to transfer to the pot water, so that the steel will gradually produce corrosion, and the dissolved oxygen in the pot water will intensify the release of electrons in the iron pole, play the role of depolarization, and intensify the corrosion process. The main characteristic of oxygen corrosion is ulcer corrosion. When steel is corroded by oxygen in water, many small blisters are often formed on its surface. At present, we do not directly produce graphene blisters. The front color of the surface varies from yellowish brown to brick red. The secondary layer is black powder, which are corrosion products. When these corrosion products are removed, pits caused by corrosion will appear

2 main factors affecting oxygen corrosion

2.1 concentration of dissolved oxygen in boiler water

the oxygen corrosion rate of metal accelerates with the increase of dissolved oxygen concentration in boiler water, and the two are in direct proportion. Generally speaking, the conditions for oxygen corrosion are met when the dissolved oxygen content in boiler water exceeds the requirements of dissolved oxygen index in the water quality standard of low-pressure boiler. The dissolved oxygen in the boiler water mainly comes from the raw water injected into the boiler. At one atmospheric pressure, the solubility of oxygen in water at 15 ℃ is 10mg/1. The solubility of oxygen in water decreases with the increase of temperature and increases with the increase of pressure. In water with absolute pressure of 0.17mpa and temperature of 90 ℃, the solubility of oxygen still reaches 3.5mg/1. It can be seen that when the water without deaeration enters the boiler, some oxygen will be separated out due to the reduced solubility. These precipitated oxygen is collected and discharged by the gas gathering device on the boiler or in the system, but the remaining dissolved oxygen in the boiler water still far exceeds the requirements for dissolved oxygen (0.1mg/1) in the low-pressure boiler water quality standard, and the reverse applied oxygen is supplemented by a large amount of daily water replenishment, resulting in continuous oxygen corrosion

2.2 pH value of pot water

the pH value of pot water has a great impact on the corrosion of metals. Low pH value means high h+ concentration in the solution. As h+ is a depolarizer in electrochemical corrosion, it will cause corrosion of steel. Generally speaking, the lower the pH value of the solution, the more likely the steel in contact with the pot water will corrode

2.3 effect of metal temperature in various parts of the boiler on the rate of oxygen corrosion

the higher the temperature of the inner surface of steel, the higher the temperature of the boiler water in contact with it, and the faster the diffusion rate of various substances in the aqueous solution. The resistance of electrolyte aqueous solution decreases, which accelerates the corrosion rate. However, when the steel surface temperature reaches a certain value, the boiler water in contact with it will form steam bubbles on the metal surface due to intense heating. Thus, the dissolved oxygen near the steel is taken away by the rising steam, isolating the contact between oxygen and metal. Therefore, in the operating state, the rising pipe and the lower part of the boiler shell will not suffer from oxygen corrosion due to intense heating. Only under the condition of fire suppression or furnace shutdown, the metal in this part will have a certain degree of oxygen corrosion. The parts of the boiler that are less heated and the parts that are not heated will accelerate corrosion because the boiler water has a suitable temperature. This is also an important reason for the rapid corrosion of the flue pipe and the upper half of the boiler shell of the horizontal external combustion boiler low-temperature hot water boiler

2.416. Machine weight: About 180kg influence of water flow rate

when there are no active ions (C1 -, so </sup), the corrosion rate increases with the increase of flow rate. When the flow rate reaches a certain value, a protective film is formed, and the corrosion rate decreases. When the flow rate increases again, the protective film formed is destroyed, which accelerates the corrosion rate. For shell type hot water boilers, because the volume of shell water is large and the flow rate of water in the boiler is low, the oxygen corrosion rate is faster in the parts where the water in the boiler flows relatively fast

3 measures to prevent oxygen corrosion

3.1 remove dissolved oxygen in boiler water

dissolved oxygen in water is a depolarizer in electrochemical corrosion, which will cause serious corrosion of boiler body and water supply pipeline, so it should be removed as far as possible. For small low-temperature hot water boilers, the relatively simple and feasible method is sodium sulfite deoxidization

3.2 control the pH value of the pot water

when the pH value is less than 7, the oxide film formed on the metal surface is soft and loose, promoting the protective effect. PH value increases. When the concentration of Oh < sup>- increases, the oxide film becomes dense and stable, which has a good effect of preventing oxygen diffusion and corrosion prevention. When the pH value is in the range of 9.5 ~ 10, the anti-corrosion effect is the best

3.3 for newly installed boilers and repaired boilers, the boiling process should be strictly followed to remove rust and oil on the metal surface. The boilers that will be out of service should also be washed to remove all kinds of attachments on the surface, and at the same time, a layer of corrosion-resistant protective film can be formed on the metal surface

3.4 control the loss of system water as much as possible, so as to reduce the amount of water replenishment of the system and control the source of dissolved oxygen

3.5 each working point of the system should be in a positive pressure state to avoid inhaling air due to negative pressure points in the system. A suitable and effective exhaust device shall be set in the circulating system so that the oxygen separated from the water can be discharged in time

3.6 do a good job in anti-corrosion during the shutdown of the boiler Generally, the hot water boiler is out of service for a long time, so it is better to choose the maintenance method

3.7 the flue pipe of horizontal external combustion shell type low-temperature hot water boiler is a component with rapid oxygen corrosion. Due to the thin wall of the flue pipe, it is easy to perforation once oxygen corrosion occurs. However, due to the high wall temperature of water pipes, oxygen corrosion should not occur in these pipes during boiler operation. The drum and downcomer that are prone to oxygen corrosion are thick, and the time to form perforation corrosion is also long. Therefore, the author believes that each user unit should incline to the water tube boiler when choosing the heating boiler type. At the same time, try to avoid "big horse pulling small car" to prevent the boiler from often "pressing fire" during operation

in a word, as long as the user units pay enough attention to the problem of oxygen corrosion and take measures to actively prevent it, the problem of oxygen corrosion of low-temperature hot water boilers can be controlled and solved from a portable and beautiful fill light cosmetic mirror on the table

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