I. MAIN CAUSES OF CORROSION IN CHEMICAL PLANTS

During the chemical production process, there are many corrosive media present and produced. The types, chemical compositions, concentrations, pH values, impurities, moisture, and oxygen content of these media are all external causes of corrosion. The faster the flow rate of the media, the more prone it is to corrosion because during the flow process, the media will erode the protective film, creating eddies, turbulence, and bubbles, leading to severe impact wear and cavitation corrosion. In addition, improper material selection is another cause. If the equipment surface comes into contact with corrosive media, and the equipment itself is not corrosion-resistant, surface corrosion will occur. The rougher the surface, the more likely it is to corrode. The manifestations of this include leakage, early wear, damage, and abnormal sounds.

Uniform surface corrosion can occur in two forms: with or without a protective film. Corrosion without a film is very dangerous, as the corrosion process proceeds at a constant speed, mainly due to incorrect material selection. A lack of corrosion protection measures or poor construction quality also creates an environment for corrosion damage. Different environments and material selections lead to different corrosion conditions. In the manufacturing process of equipment, it is often difficult to achieve consistency between material selection and environmental corrosion conditions. Temperature, concentration, and pressure vary, leading to different material selections and varying corrosion situations. Furthermore, poor supervision during construction or low construction quality makes corrosion problems inevitable. Other causes of corrosion include overheating, overpressure in operation, poor equipment management, and a lack of attention, all contributing to corrosion damage. Typically, the higher the temperature and pressure of the medium, the faster the corrosion, as corrosion is a chemical reaction. For every 10°C increase in temperature, the corrosion rate increases by 1 to 3 times.

I. Types and Composition of the Chemical Industry

The chemical industry refers to the collective term for enterprises and organizations engaged in the production and development of chemical products. It includes sectors such as chemicals, petrochemicals, metallurgy, energy, light industry, petroleum, environmental protection, pharmaceuticals, and military industries. These sectors are involved in engineering design, fine and daily chemicals, energy and power, technological development, production management, and scientific research.

The industry is diverse, and the scale of enterprises varies greatly. Based on the variety of products, chemical enterprises can be classified into chemical conglomerates and enterprises producing single products (such as synthetic material plants, soda plants, sulfuric acid plants, fertilizer plants, etc.). A chemical plant generally consists of three parts: main production workshops (from raw material treatment to finished product processing), auxiliary production workshops (power plants or distribution stations, power workshops, water treatment workshops, etc.), and production management facilities (office buildings, material and finished product warehouses, garages, etc.).

I. Reference Standards

Current national standards, relevant regulations, and mandatory standard provisions;

"Code for Construction and Acceptance of Building Anti-corrosion Engineering" (GB50212-2002)

"Standard for Corrosion and Rust Removal Levels of Steel Surface Before Coating" (GB8923-1988)

"Safety Regulations for Coating Operations and 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)

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

"Safety Regulations for Coating Operations and Safety of Surface Preparation Process" (GB7692-87)

"Noise Limits at Construction Sites" (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 atmospheric environments on steel structures can be determined according to Table 1.

Pipelines are currently the most common means of transportation in chemical plants, characterized by high transportation efficiency, strong safety, and high economic benefits. The raw materials in chemical plants have characteristics such as flammability, explosiveness, and diffusion. If a pipeline leaks, it can have adverse effects on the surrounding environment, threaten the safety of personnel, and cause severe economic losses and environmental pollution. Therefore, enhancing the protection against pipeline corrosion is very important. The reasons for corrosion in chemical plants are complex, involving many influencing factors, making pipeline corrosion protection an important issue in the chemical industry.

1. Anti-corrosion Plan for Pipelines at Normal Temperature

Pipeline corrosion refers to the chemical and electrochemical reactions between the surface of metal materials and environmental media, leading to the degradation and damage of the materials. The common corrosion in chemical plant pipelines is mainly caused by corrosive substances such as CO2, H2S, C1-, dissolved oxygen, and bacteria in crude oil. These substances directly interact with the metal, causing chemical corrosion.

Chemical corrosion is not highly harmful, but the main reason for the formation of pits and perforations on the surface of steel pipes comes from electrochemical corrosion. When metal undergoes an electrochemical reaction, the areas with lower electrode potential tend to lose electrons and become anode, while areas with higher electrode potential gain electrons and become cathode. In the presence of O2 and H2O, Fe(OH)2 forms hydrated iron oxide, leading to corrosion.

In order to improve the corrosion resistance of the coating and maximize its anti-corrosion effect, the following requirements for the anti-corrosion coating performance are proposed based on the actual pipeline corrosion situation: 1) Good stability against corrosive media; 2) Good impermeability to ensure low water absorption; 3) Good mechanical strength; 4) Excellent electrical insulation.

Table 1: Anti-corrosion Technical Design for the External Surface of Galvanized Pipelines

II. Insulated/Cold Insulation Pipeline Anti-corrosion Plan

Corrosion Under Insulation (CUI) refers to a type of corrosion that occurs on the surface of pipelines wrapped with insulating materials in chemical plants. Data shows that the annual financial losses caused by corrosion under insulation in the chemical industry can reach billions of dollars, leading to equipment failures, shutdowns, product leaks, a sharp increase in maintenance costs, and, more seriously, posing significant safety risks. In recent years, with the increasing awareness of safety in chemical plants and the growing focus on CUI corrosion behaviors.

Typically, CUI occurs due to the infiltration of condensate or rainwater into the insulation system, leading to localized corrosion on the external surface of pipelines or equipment in chemical plants. When moisture is present beneath the insulation system, pipelines made of materials such as carbon steel, low-alloy steel, and austenitic stainless steel are highly susceptible to electrochemical reactions, pitting, stress cracking, and crevice corrosion. Since the corroded surface is covered by the insulation layer, CUI is generally difficult to detect. Depending on the material of the equipment, CUI can be divided into two types: corrosion occurring on carbon steel (primarily manifesting as pitting or material thinning) and corrosion occurring on stainless steel (primarily manifesting as pitting or stress corrosion cracking).

DreamHeat 300, a water-based nano thermal insulation coating, is applied to the outer surface of pipelines transporting high-temperature fluids and storage containers for thermal media. With a coating thickness of 2.0-5mm, it reduces the surface temperature from 300℃ to 78℃, decreasing heat loss and achieving energy efficiency of about 15%, thus improving production efficiency while also preventing worker burns. DreamHeat 300 is made from hollow ceramic suspended microparticles and water-based nano materials, offering integrated thermal insulation and anti-corrosion protection.

The micro-nano porous fillers and the coating formulation provide: extremely low thermal conductivity; the use of thin coatings significantly reduces heat conduction and thermal radiation, achieving excellent thermal insulation performance.

Table 2: High-Temperature Pipeline Coating Anti-corrosion Design

III. Buried Pipeline Anti-corrosion Plan

Some chemical pipelines are buried underground, and the surrounding soil environment can have a significant impact on the pipelines. The composition of the soil is very complex, containing large amounts of water, carbon dioxide, acidic and basic salts, and various metal ions, which can easily interact with the pipelines and cause ion conduction, electrochemical decomposition, and other chemical reactions, leading to pipeline corrosion. Factors such as moisture content, pH, resistivity, and temperature of the soil also affect the rate and degree of pipeline corrosion. In general, higher soil temperatures, higher humidity, stronger acidity, or rapid alternation of soil moisture can accelerate various electrochemical and chemical reactions in the pipeline material, leading to faster corrosion. Furthermore, the deeper the pipeline is buried, the higher the surrounding soil temperature tends to be. Pipeline corrosion can also be influenced by microorganisms in the soil and surrounding plant roots, as well as stray currents that may interfere, causing electrode reactions and exacerbating electrochemical corrosion of the pipeline.

I. CORROSION OF TANKS

1. Corrosion of the Outer Wall of the Tank

Chemical plants usually have large storage tanks. The corrosion of the outer wall of the tank is mainly atmospheric corrosion. Acidic gases in the chemical area air, when exposed to rainwater or cooling spray water in summer, cause oxygen depolarization reactions on the steel surface under the liquid film. When the temperature periodically decreases, the moisture containing electrolytes will condense on the tank's surface, forming a continuous electrolyte solution film, leading to corrosion. With the development of the chemical industry, the atmosphere has become highly corrosive. In coastal areas, there are large amounts of chloride ions and other salts in the air. When these ions settle on the tank's outer wall, they form an electrolyte film containing dissolved salts. Chloride ions have strong permeability and can cause severe local corrosion. Tanks located in areas with heavy sandstorms will experience mechanical wear due to sand or dust in the wind, which will damage the surface coating of the tank.

2. Corrosion of the Bottom Plate and Edge Plate of the Tank

Due to capillary action, underground water rises, and the tank's bottom is often in a humid environment. Also, since the bottom plate requires welding, the corrosion-resistant coating near the weld is damaged, resulting in severe corrosion at these points. The compaction of the sand cushion layer around the oil tank's bottom plate is significantly lower than that in the tank's center, causing uneven oxygen content in the sand pores, leading to the formation of galvanic cells. The center of the tank bottom plate becomes the anode, and the surrounding area becomes the cathode, causing the central area to corrode more severely than the outer areas.

When the tank is used for heating or storing intermediate products, if the oil tank temperature is high, the underground water around the bottom plate evaporates, increasing the salt concentration and worsening corrosion. After using the tank for a period, the change in oil volume load causes tank deformation. Additionally, the expansion and contraction of the bottom plate due to temperature changes lead to cracks between the bottom plate and the foundation. These cracks expand and contract with the tank's operation, providing a pathway for external corrosive agents, such as rainwater, to enter, causing crevice corrosion. Thus, the tank bottom plate and edge area are highly susceptible to corrosion and require special attention.

The underground area of the tank field is a complex electrical current zone. If the tank has no cathodic protection but the pipeline network is protected, stray currents may interfere with the tank, especially when electric welding, electrified railways, or DC power equipment are nearby. The presence of stray currents accelerates the corrosion of the tank's bottom plate and edge plate, complicating corrosion control.

3. Corrosion Under Insulation

Corrosion under insulation refers to the corrosion of steel structures, pipelines, storage tanks, or equipment covered by insulation layers for high-temperature insulation or low-temperature preservation. Under normal operating conditions, especially with thermal cycling, condensation of moisture beneath the insulation layer causes the accumulation of localized electrolyte solutions, leading to corrosion of the steel. This corrosion is particularly severe because it is often difficult to detect. For aesthetic reasons, a layer of stainless steel or aluminum foil is often applied to the exterior of the insulation. As a result, petrochemical companies often discover corrosion under insulation too late, leading to various failure accidents. Especially when the operating temperature is below 150°C, moisture condensation tends to occur beneath the insulation, resulting in a long-term high-temperature and humid environment for the base material. This accelerates corrosion at a rate several times higher than conventional corrosion. Studies show that the probability of corrosion under insulation significantly increases after 5 years of operation, and after 10 years, 60% of insulation layers contain corrosive condensate. For the petrochemical industry, the unique aspect of corrosion under insulation is that it often involves thermal cycling, which, coupled with traditional high-temperature coatings, increases the internal stress in the coating due to the difference in thermal expansion coefficients between the coating and steel, causing early coating failure and further exacerbating the corrosion problem. Corrosion under insulation is a critical issue that requires special attention in petrochemical refining sectors. Some pipelines and storage tanks now use integrated heat-insulating and anti-corrosion coatings, which eliminate the gap between insulation and coating layers, thus solving the problem of corrosion under insulation.

II. ANTI-CORROSION COATINGS FOR THE EXTERNAL WALLS OF TANKS

The external walls of tanks can be categorized according to the specific environmental conditions, such as atmospheric corrosion environments for steel structures, insulated tank walls, cave storage tank walls, the underside of tank bottoms, and the connection areas of bottom edges to foundations. Different parts of the tank face different corrosion environments, so distinct anti-corrosion strategies should be employed.

1. Conventional Tank External Walls

This area is typical of atmospheric corrosion environments. Due to the varying locations, climatic conditions, and corrosion environments of the tanks, reference can be made to ISO 12944-5-2018 "Protective Coating Systems," GB/T 50393-2017 "Corrosion Protection Engineering Technical Standards for Steel Oil Storage Tanks," and SY/T 0320-2010 "Technical Standards for External Anti-Corrosion Layers of Steel Tanks," to design coating systems according to corrosion environments and durability requirements. With the development of coating technology, and in response to owners’ concerns about expensive repair costs, the introduction of life cycle cost analysis (LCCA) concepts has led to the use of high-performance coatings to reduce maintenance frequency. The most commonly used anti-corrosion coating system consists of an epoxy zinc-rich primer or inorganic zinc silicate primer, an epoxy micaceous iron intermediate coat, and an acrylic polyurethane finish coat. Table 3-3-55 is a typical coating system designed for a 15-25 year service life in C5 corrosion environments according to ISO 12944-5-2018.

2. Insulated Tank External Walls

This area involves anti-corrosion beneath insulation. The temperatures here are generally not as high as in chemical pipelines, where temperatures exceeding 200°C are rare. Reference can be made to SY/T 0320-2010, HG/T 5178-2017, and NACE SP 0198-2010 standards for designing anti-corrosion systems based on different temperature ranges.

3. Tank Bottom Plates

Tank bottoms are usually laid with asphalt sand cushion layers, and the underside does not directly contact the soil or rock layers, though it is still subject to various corrosion factors. Moreover, the corrosion-resistant coating is applied first to the bottom plate before welding, but the coating around the weld is burned off during welding, resulting in uneven material near the weld, which lacks coating protection and thus is highly prone to corrosion. GB/T 50393-2017 specifies that the coating should be used in combination with external cathodic protection for the tank bottom surface; depending on the temperature and soil corrosion levels, different coating thicknesses of epoxy and phenolic epoxy coatings are recommended. Another method is to design weldable inorganic zinc silicate primer within a 100mm range of the weld, while using epoxy or phenolic epoxy coatings for other areas. This helps avoid corrosion at the welds before cathodic protection is applied.

III. ANTI-CORROSION COATINGS FOR THE INTERNAL WALLS OF TANKS

1. Crude Oil Tank Interior Anti-Corrosion Coatings

Crude oil often contains a significant amount of impurities, leading to the retention of water and impurities at the tank bottom. The resulting solution is acidic and highly corrosive. Moreover, the properties and corrosiveness of crude oil vary by origin. Therefore, when designing anti-corrosion systems for crude oil storage tanks, the first step is to analyze the corrosiveness of the crude oil before selecting the appropriate coating system. This section discusses the typical coatings used for crude oil storage tanks, while special cases should be addressed based on relevant experiences and regulations.

2. Intermediate Product Tank Interior Anti-Corrosion Coatings

The corrosiveness of intermediate products is less than crude oil, but they may be stored at higher temperatures. For intermediate product tanks, epoxy anti-static coatings or phenolic epoxy anti-static coatings are selected according to the temperature range. For temperatures ≤80°C, epoxy anti-static coatings are used, and for temperatures between 80-120°C, phenolic epoxy anti-static coatings are used. Additionally, because the bottom and top are more prone to corrosion, the coating thickness in these areas should be greater than in other parts of the tank. Some manufacturers use a “inorganic zinc silicate primer + epoxy or phenolic epoxy anti-static coatings” combination, which utilizes the excellent corrosion resistance, heat resistance, and anti-static properties of inorganic zinc silicate while also providing superior anti-corrosion performance.

3. Anti-corrosion Coatings for Finished Oil Tank Interiors

Due to the high purity and low impurity content of finished oil, its corrosiveness is minimal. The design of finished oil storage tanks can directly refer to the design of intermediate product storage tanks, typically using epoxy or phenolic epoxy anti-static coatings. The thickness of the coating on the bottom and top will be thicker than on other parts of the tank wall. Unlike intermediate product storage tanks, finished oil storage tanks, due to their lower temperature, lower impurity content, and a pH value that is generally neutral, can use an inorganic zinc silicate primer to meet both corrosion protection and anti-static requirements. Alternatively, a system combining "epoxy zinc-rich primer + anti-static coating" can be used to meet both the corrosion protection and anti-static requirements.

4. Anti-corrosion Coatings for Sewage Tank Interiors

The chemical composition of sewage is complex, containing various inorganic and organic impurities as well as corrosive media. Based on the corrosive nature of the chemicals in the sewage, epoxy coatings, phenolic epoxy coatings, or vinyl ester coatings can be chosen. Vinyl ester coatings offer superior chemical resistance compared to epoxy or phenolic epoxy coatings. Common vinyl ester coatings used for linings include standard bisphenol A type and phenolic epoxy type. The phenolic epoxy type offers better temperature and chemical resistance. Vinyl ester coatings typically use glass flake as a functional filler, which enhances the shielding effect by creating a labyrinth effect in the paint film. Additionally, glass flake helps reduce internal stress caused by volumetric shrinkage during the curing process of vinyl ester, improving crack resistance.

5. Anti-corrosion Coatings for Fire Water Tank Interiors

The medium inside fire water tanks is primarily fresh water. For this area, high solid epoxy coatings, solvent-free epoxy coatings, or phenolic epoxy coatings are generally used.

1. Outdoor Chemical Plant Steel Structure Corrosion Protection Plan

Steel structures vary in types and environments, and even for the same steel structure facility, the corrosion protection measures differ due to the different environments of each part. These structures can be classified as indoor or outdoor steel components, which are exposed to various corrosive environments such as chemical gases, industrial atmosphere, marine atmosphere, acidic water, and soil. To ensure long-term use without extensive repairs, reasonable corrosion protection coatings must be applied. The specific plan should be determined according to the corrosive environment and medium. Here, we recommend corrosion protection coatings based on the usage environment of large steel structures, the medium environment of each part, surface treatment requirements, and operational conditions.

Outdoor steel components: Steel structures in outdoor environments are affected by sunlight, sandstorms, rain, snow, frost, and seasonal temperature changes. Oxygen and moisture in the atmosphere are the main factors causing corrosion of outdoor steel structures, leading to electrochemical corrosion. The air in chemical parks contains SO2, CO2, NO2, CI2, H2S, and NH3, which are the most harmful to steel. These components can destroy the passivation film on the metal surface, and their acidic nature forms acid rain, thus accelerating corrosion.

Table 1: Use of Graphene Paint Technology for Outdoor Steel Structures

2. Indoor Chemical Factory Steel Structure Anti-corrosion Plan

Indoor steel components: The corrosion of steel at room temperature is primarily electrochemical corrosion. When steel structures are used in an indoor atmospheric environment at room temperature, the steel is corroded by the action of moisture, oxygen, and other pollutants (such as untreated welding slag, rust, and surface dirt) in the air. When the relative humidity of the air is below 60%, the corrosion of steel is minimal; however, when the relative humidity increases to a certain value, the corrosion rate of steel increases suddenly, and this value is called the critical humidity. Under normal temperature conditions, the critical humidity for general steel is between 60% and 70%. In chemical factory workshops where the air is polluted by acid mist, or in coastal areas where the air contains salt, the critical humidity is much lower, and a water film is easily formed on the steel surface. At this time, welding slag and untreated rust (iron oxide scale) act as the cathode, and the steel structure components (substrate) act as the anode, causing electrochemical corrosion in the water film. The water film formed by the absorption of moisture from the air on the steel surface is the determining factor for corrosion. The relative humidity of the air and the content of pollutants are important factors affecting atmospheric corrosion.

Table 2: Application of Graphene Coating Technology for Steel Structures in Indoor Factories