What are the main causes of corrosion in boiler steel pipes of thermal power plants

1. Causes of corrosion in boiler steel pipes in heating networks.

Corrosion in heating networks can range from minor issues like perforation of boiler steel pipe walls, causing leaks, to major issues like the inability of the boiler steel pipe walls to withstand the pressure of the heating medium, leading to explosions and serious threats to user safety. Corrosion in boiler steel pipes of heating networks is divided into internal and external corrosion. External corrosion is mostly caused by environmental factors, primarily due to corrosion at the interface between the pipeline metal material and corrosive media such as soil, seawater, and the atmosphere.

There are many factors affecting internal corrosion in boiler steel pipes of heating pipelines. The main factors are as follows:

(1) Concentration of dissolved oxygen in water

(2) pH of water

(3) Temperature of water

(4) Concentration of salt

2. Causes of boiler scaling

The heating medium in urban heating network boilers is generally hot water. Water quality varies across the country, and water hardness also varies, meaning the content of calcium, magnesium, iron, and other ions in the water differs. Under the special conditions of high temperature and high pressure in a boiler, if water with hardness enters the boiler without treatment, solid deposits will firmly adhere to the boiler's water-side heating surface after a period of operation. This phenomenon is called scaling. The solid deposits adhering to the heating surface are called scale. Under certain conditions, solid precipitates can also precipitate in the boiler water, appearing as loose suspensions, called sludge. Sludge can be removed with blowdown, but if blowdown is not timely, some sludge will also deposit on the heating surface or in areas with slow water flow, transforming into scale (usually called "secondary scale"). Adding the wrong water treatment agent will accelerate scale formation. For example, simply adding sodium hydroxide to adjust the pH value will form magnesium hydroxide and calcium carbonate, which will adhere to the pipes, clogging water passages and causing significant waste of electricity and fuel. Scale formation inside the boiler drum and heating pipes is usually caused by high salt content in the incoming water. Scale is typically composed of calcium salts (with low solubility), and specific indicators include conductivity. Additionally, to prevent corrosion, attention should be paid to controlling dissolved oxygen content and pH value. During operation, pay attention to regular and continuous blowdown and chemical dosing as required.

3. Main Components of Scale in Boiler Steel Pipes

(1) Carbonate Scale: The main component is calcium and magnesium carbonates, with calcium carbonate as the primary component, sometimes exceeding 50%.

(2) Sulfate Scale: The main component is calcium sulfate, often exceeding 50%;

(3) Phosphate Scale: The main component is Ca3(PO4)2;

(4) Silicate Scale: The main component is SiO2, sometimes exceeding 20%;

(5) Mixed Scale: A mixture of various scales.

4. Hazards of Scale Formation in Heating Networks and Boilers.

Hard scale usually adheres to the surface of the boiler or boiler steel pipes. First, hard scale has very poor thermal conductivity; its thermal conductivity coefficient is tens to hundreds of times smaller than that of the boiler steel plate. Therefore, scale formation in the boiler will severely hinder heat transfer, leading to a deterioration of heat transfer on the heating surface, thus wasting fuel or electricity. Secondly, if scale hardens and adheres to the inner wall of boiler steel tubes, it can significantly increase the risk of tube rupture or even explosion due to thermal expansion and contraction, and uneven stress. Thirdly, when scale hardens, it often carries a large amount of heavy metal ions. If the boiler is used to hold drinking water, there is a risk of excessive heavy metal ions dissolving into the drinking water. Furthermore, scale affects human health, easily causing digestive and absorptive disorders and constipation, and increasing the incidence of gastritis and various types of stones. Dental tartar and periodontitis are frequently caused by plaque. Blockage in industrial boiler pipes can lead to explosions.

5. Cleaning Scale from Boiler Steel Tubes.

Mechanical and chemical descaling methods are commonly used to clean scale from boiler steel tubes.

Mechanical Descaling: When there is scale or slag inside the boiler, after shutting down the boiler and draining the boiler water, allow the boiler to cool and then flush it with water or use a spiral wire brush to remove it. If the scale is very hard, high-pressure water jet cleaning or electrically or hydraulically powered cleaning devices can be used. This method has limited applicability; mechanical descaling cannot be used during boiler operation in the heating season. Furthermore, it is labor-intensive, has a poor descaling effect, and easily damages metal surfaces. It is only suitable for small to medium-sized steam boilers with small descaling areas and simple structures that are easy for mechanical tools to scrape away scale.

Chemical descaling: This method involves acidic and alkaline chemical agents. Chemical cleaning can only clean a small portion of the heat exchanger and can damage the equipment. Adding corrosion and scale inhibitors is a commonly used descaling method that solves the water quality issue while extending the service life of the boiler and boiler steel pipes. Corrosion and scale inhibitors, as the name suggests, are chemical agents that can remove scale from inside the system and also have multiple functions such as corrosion prevention, scale inhibition, rust removal, pre-protective film formation, inhibition of bacterial and algal growth, and boiler shutdown protection. They are widely applicable to the descaling of various types of boilers and plate heat exchangers.