I. Corrosion Background of Thermal Power Plants

China is a country with an energy structure primarily based on coal, and this characteristic of the energy structure has led to severe corrosion situations, such as acid rain and other pollution problems. Air pollution caused by coal-fired power plants is quite serious, belonging to smoke-type pollution. Dust, sulfur dioxide (SO₂), and nitrogen oxides (NOx) are the main pollutants in the atmosphere. A thermal power plant is a complex corrosive environment. In addition to the severe atmospheric corrosion caused by coal combustion, it also involves corrosion of various tank walls, external and internal corrosion of circulating water pipes, high-temperature insulation parts, and the most corrosive environment—the flue gas desulfurization system (FGD).

Atmospheric corrosion environment of steel structures

The fuel used in thermal power plants includes heavy oil and coal, which, during operation, release large amounts of corrosive gases like sulfur compounds. These gases combine with water vapor or rainwater in the air to form acidic solutions. The deposition of coal ash on steel structures forms electrolytes, accelerating electrochemical corrosion of steel structures. Therefore, corrosion protection in thermal power plants becomes one of the key aspects of production and maintenance. In power plants, steel structures exposed to atmospheric corrosion mainly involve:

(1) Boiler steel structure

(2) Main plant building steel structure

(3) Coal conveying system: coal unloading machine, stacker reclaimer, crane, and coal conveyor bridge

(4) Outer walls of pipes

(5) Inner walls of pipes

The corrosion level of these steel structures in atmospheric environments, due to their location in the corrosive environment of thermal power plants, is classified as high-level environment C4 according to the international standard "Corrosion Protection of Steel Structures by Coating Systems" ISO 12944-2.

I. References

Current national standards, relevant regulations, and mandatory clauses;

"Construction and Acceptance Code for Anti-Corrosion Engineering" (GB50212-2002)

"Standard for Corrosion Grade and Rust Removal Level of Steel Surface before Coating" (GB8923-1988)

"Safety Regulations for Coating Operations, Safety Management Rules" (GB6514-1995)

"Quality Requirements for Anti-Corrosion Coatings" (GB6514-1991)

"Safety Regulations for Coating Operations, Paint Process Safety and Ventilation Purification" (DJ/T6931-1999)

"Construction and Acceptance Code for Industrial Equipment and Pipeline Anti-Corrosion Engineering" (HGJ229-91)

"Safety Regulations for Coating Operations, Coating Pretreatment Process Safety" (GB7692-87)

"Construction Site Boundary Noise Limits" (GB12523-90)

ISO9001 Quality Management System Documents

ISO14001 Environmental Management System Documents

GB/T28001 Occupational Health and Safety Management System Documents

I. Design Basis

Environmental Conditions: The corrosion level of the atmospheric environment on building steel structures under long-term exposure can be determined according to Table 1.

I. Main Plant Anti-corrosion

The main plant of a thermal power plant is the building that houses the primary and auxiliary equipment. The structural system of the main plant is complex, with heavy loads, large horizontal and vertical dimensions, and substantial self-weight. The fuels used in thermal power plants, such as heavy oil and coal, release a significant amount of corrosive gases like sulfur compounds during operation, which combine with moisture or rainwater in the air to form acidic solutions. The deposition of coal ash on steel structures forms electrolytes that accelerate the electrochemical corrosion of steel.

In coastal cities, the thermal power plants are located in highly saline, humid, and corrosive marine environments, with an environmental classification of C5. This requires the steel structure of the main plant to have an overall anti-corrosion lifespan of 10-15 years with high durability. The optimal steel structure protection coating system can be chosen according to GB/T30790.5, and the dry film thickness should be at least 300 microns. Graphene-based anti-corrosion coatings, such as DreamZinc, meet these requirements.

Table I: Main Plant without Fireproofing Requirements Using Graphene Coating Technology

I. Anti-corrosion Scheme for the Desulfurization and Denitrification Tower

From the brief introduction of the operating conditions of the desulfurization tower, it can be seen that the desulfurization tower undergoes drastic temperature changes, including high-temperature and low-temperature flue gas; it also involves a dual corrosion environment with moderately alkaline and strongly acidic substances; as well as the strong scouring effects of particulate matter, liquids, and gases. It is a highly complex operational environment.

In practice, the spray pipes of the desulfurization tower are made of fiberglass, which is affected by the purity of the lime slurry and the wear caused by the contained particulate matter. The spray pipes are prone to excessive wear and perforation, and they may become clogged due to internal scaling, leading to poor atomization and affecting desulfurization efficiency. The butyl rubber anti-corrosion layer used at the bottom has a maximum temperature resistance of 110℃. However, during operation, the temperature of the original flue gas (flue gas to be desulfurized) is typically between 110℃ and 160℃, often leading to scorching at the inlet of the original flue gas. The upper glass flake anti-corrosion area, due to the issues with the spray pipes, experiences significant temperature fluctuations and has quality issues in the anti-corrosion construction, often resulting in the peeling of the glass flake layer. Some power plants use coal with a high fluorine content, which leads to the rapid corrosion and perforation of the glass flake layer.

Table 1: Use of Graphene Coating Technology on the Inner Wall of the Desulfurization and Denitrification System

Table 2: Use of Graphene Coating Technology on the Inner Wall of the Desulfurization and Denitrification System

II. Anti-corrosion Scheme for the Chimney

The chimney is an important industrial facility for venting gas

The power plant that uses coal has a steel structure for the boiler equipment, and the steel platforms for the boiler are located in harsh corrosive environments. The power plant that uses coal burns fuels such as heavy oil and coal, and during operation, a large amount of corrosive gases such as hydrogen sulfide are released, which react with steam or rain in the air to form acidic solutions. The fly ash from coal is deposited on the steel structure, forming electrolytic solutions, accelerating the electrochemical corrosion of the steel structure. Therefore, corrosion prevention for the steel structure of the boiler in coal-fired power plants becomes one of the key issues in production and maintenance.

Corrosion is the phenomenon of destruction of the steel structure of the boiler due to its interaction with the surrounding environment. The main material used in the boiler is steel made from iron, and iron is extracted by reducing iron ore in a blast furnace using coke. This reaction occurs at very high temperatures, and the resulting product and steel are unstable. When steel is exposed to a humid environment containing oxygen, it easily undergoes oxidation.

Table 1: Using Graphene Coating Technology for Boiler Steel Structure Platform

Table 2: Using Graphene Coating Technology for Pipes at Normal Temperatures (Below 120℃)

The coal handling system is one of the main auxiliary systems in thermal power plants, characterized by a wide variety of equipment and unique operation and control methods. With the development of the power industry, the installed capacity of thermal power plants has continuously increased, and the capacity of coal handling systems has grown from dozens of tons per hour to thousands of tons. This sudden increase in transportation capacity has put significant pressure on the corrosion protection of coal handling system equipment. The traditional corrosion protection methods used in coal handling systems of medium and small thermal power plants can no longer meet the needs of large thermal power plants. Therefore, corrosion issues have long affected the safe and reliable operation of coal handling system equipment. Mengneng Technology has developed an oily and water-based coating corrosion protection plan for the coal handling system.

Due to the presence of sulfur and sulfides (SO2, SO3) in coal, which chemically react with water under temperature changes to form sulfuric and sulfurous acids, direct contact with metal materials will severely corrode the metal surface. Combined with the harsh environment around the coal handling system, fluctuating temperatures and humidity in the air, and the intermittent operation characteristics of the coal handling system, the corrosion rate is accelerated. In severe cases, the metal surface may flake and become uneven, with large-scale peeling during operation, affecting the service life of the equipment.

Table 1: Graphene Coating Technology for Coal Handling Conveyor Steel Structure

Table 2: Graphene Coating Technology for Coal Storage Shed

I. Freshwater Circulating Water Pipes

The corrosion characteristics of the inner walls of circulating water pipes in freshwater and seawater are quite different. The corrosion of steel buried water pipes in freshwater is primarily electrochemical corrosion caused by the oxidation of oxygen. The presence of oxygen in freshwater is the main reason for the corrosion of steel. The corrosion process is mainly controlled by the diffusion of oxygen to the metal surface. Compared to seawater, freshwater has a much lower salt content and poor conductivity, so corrosion in freshwater is much less severe than in seawater.

The corrosion of the outer wall of buried circulating water pipes is mainly due to soil corrosion. Soil is a complex mixture of various substances, filled with air, water, and various salts, which gives the soil the characteristics of an electrolyte. The surface condition of the circulating water pipe, defects in the pipe, and the uneven inclusion of materials, when in contact with the soil, will form electrode potential differences, creating electrogalvanic corrosion. The differences in the properties of the soil can create large potential differences, leading to macro galvanic corrosion.

Table 1: Inner Wall of Freshwater Circulating Water Pipes

Table 2: Outer Wall and Top Wall of Freshwater Circulating Water Pipes

II. Seawater Circulating Water Pipes

Seawater circulating water pipes, due to long-term exposure to seawater, face a complex corrosive environment. The corrosion characteristics are mainly reflected in the following aspects. First, seawater is composed of complex components, including high concentrations of sodium chloride and other salts, which accelerate the corrosion of metals. Additionally, the dissolved oxygen content in seawater is relatively high, especially under flowing conditions, where the ample oxygen supply promotes the oxidation reaction on the metal surface, accelerating the corrosion rate. Seawater also contains various microorganisms and algae that form biofilms on the pipe walls, altering the local environment and causing localized corrosion.

Secondly, the flow state significantly affects corrosion. High-speed water flow can scour the pipe wall, leading to mechanical wear and erosion corrosion; low-speed water flow tends to cause sediment accumulation, forming under-deposit corrosion. Temperature variations are also an important factor. An increase in temperature accelerates the rate of chemical reactions, including corrosion reactions, while high temperatures also reduce the durability of anti-corrosion coatings. Localized corrosion phenomena, such as pitting, crevice corrosion, and under-deposit corrosion, are common and typically occur in specific areas of the pipe wall, resulting in severe localized damage.

Moreover, the contact of dissimilar metals leads to galvanic effects, accelerating the corrosion of the metal with a lower potential. Residual and applied stresses can also increase the material’s susceptibility to stress corrosion. To address these corrosion characteristics, various anti-corrosion measures can be taken, such as selecting corrosion-resistant materials (e.g., stainless steel, titanium alloys), using corrosion-resistant coatings or linings, employing cathodic protection techniques, controlling flow rates, regularly cleaning pipes, and performing regular monitoring and maintenance. By integrating these measures, the service life of seawater circulating water pipes can be effectively extended, ensuring the safe and stable operation of the system.

Table 4: Inner Surface of Seawater Circulating Water Pipes

Table 5: Outer Surface of Seawater Circulating Water Pipes

I. Corrosion Protection Requirements for Cooling Towers

The corrosion of cooling towers includes the corrosion of reinforced concrete and steel components.

The concrete components of hyperbolic cooling towers, including cooling tower walls, air ducts, beams, columns, water collection pools, oil separation pools, etc., show signs of corrosion such as loosening, spalling, or exposure of rebar, or increased voids in the concrete walls due to erosion, freeze-thaw damage in winter, which exacerbates corrosion, as well as microbial corrosion, ultimately leading to the failure of the cooling tower.

The requirements for corrosion-resistant coatings for cooling towers:

(1) Good adhesion to the concrete surface.

(2) Excellent water resistance, resistance to water erosion, spraying, and splashing.

(3) Excellent anti-corrosion performance.

The coating scope for seawater cooling towers is divided as shown in the table below:

After operation, the concrete surface of the wet zone in the cooling tower is exposed to a dynamic seawater environment, including flowing, splashing, and overflowing. The coating surface must be resistant to seawater, salt mist, alkali, impact, and abrasion.

The coating in the dry zone of the concrete should have strong corrosion resistance and weather resistance, with good gloss retention, color stability, and crack resistance.

II. Use of Graphene Coating Technology for Cooling Tower Interior Walls

Mengneng Technology, based on the long-term corrosion protection design requirements for power plant cooling towers, combined with decades of experience in power plant corrosion protection across various regions, has developed corresponding corrosion protection solutions, construction processes, and quality control plans to comprehensively achieve the long-term corrosion resistance and aesthetic durability of the power plant cooling towers.