Corrosion prevention methods for onshore LSAW Steel Pipes

1. External anti-corrosion coating

Epoxy powder coating

Principle and Application: Epoxy powder coating is a commonly used external anti-corrosion method. The principle is to evenly coat the surface of the steel pipe with epoxy powder through electrostatic spraying. At high temperatures, the powder melts and solidifies to form a continuous, dense protective film. This coating has excellent adhesion, chemical corrosion resistance, and good mechanical properties. For example, in some onshore oil pipelines, epoxy powder coating can effectively prevent the erosion of steel pipes by acidic and alkaline substances and moisture in the soil.

Construction points: During the construction process, the surface of the steel pipe needs to undergo strict pre-treatment, usually including sandblasting, to achieve a surface cleanliness of Sa2.5 level and appropriate roughness (generally between 40 and 100 μ m), which can ensure that the epoxy powder coating adheres better to the surface of the steel pipe. At the same time, parameters such as temperature and powder particle size during the spraying process also need to be strictly controlled to ensure the quality of the coating.

Three layer polyolefin coating (3PE)

Structure and advantages: The 3PE coating consists of a bottom layer of epoxy powder, an intermediate layer of adhesive, and an outer layer of polyolefin (such as polyethylene). The bottom layer of epoxy powder mainly plays a bonding role with the surface of the steel pipe, and the adhesive in the middle enhances the bonding force between the epoxy powder and the outer layer of polyolefin. The outer layer of polyolefin provides good weather resistance, corrosion resistance, and mechanical damage resistance. This coating system has superior comprehensive performance and can effectively resist various corrosion factors such as soil stress and microbial corrosion.

Construction process: During construction, the surface of the steel pipe is first pre treated, and then epoxy powder is sprayed. When the epoxy powder is still in a gelatinous state, the adhesive and polyolefin layer are immediately wrapped. This requires precise process control to ensure good adhesion between layers and overall coating quality.

Coal tar enamel coating

Characteristics and applicable scenarios: Coal tar enamel coating has good water resistance, chemical corrosion resistance, and electrical insulation properties. It contains coal tar components, which form a hard and smooth coating after solidification, effectively resisting the erosion of water and chemicals in the soil on steel pipes. This coating is more suitable for some highly corrosive soil environments (such as saline alkali land) or situations where electrical insulation is required.

Construction precautions: During the construction process, coal tar enamel paint needs to be heated and melted before coating, with a coating thickness generally between 3-7mm. At the same time, due to the presence of volatile organic compounds (VOCs) in coal tar porcelain paint, ventilation should be taken into account during the construction process and environmental protection requirements should be met.

2. Internal anti-corrosion coating

Cement mortar coating

Anti corrosion principle and application scope: Cement mortar coating is mainly used for internal anti-corrosion of steel pipes. The principle is to apply cement mortar on the inner wall of the steel pipe to form an alkaline protective layer. Calcium hydroxide in cement mortar can form a passive film on the surface of steel pipes, thereby preventing corrosion of the inner wall of the steel pipes. This coating is widely used in fields such as water pipelines because it not only prevents corrosion but also reduces the erosion of the inner wall of steel pipes by water flow.

Construction details: During construction, the ratio of cement mortar should be strictly controlled, and the ratio of cement to sand is generally between 1:1 and 1:3. Meanwhile, the thickness of the coating is also crucial, typically between 6-16mm. The application process should be uniform and sufficient curing time should be ensured to fully cure the cement mortar. Generally, the curing time should not be less than 7 days.

Liquid epoxy coating

Characteristics and advantages: Liquid epoxy coating is a high-performance internal anti-corrosion coating. It has good chemical stability, wear resistance, and adhesion to the inner wall of steel pipes. This coating can effectively prevent corrosive components in the conveying medium (such as oil, natural gas, etc.) from corroding the inner wall of the steel pipe. Moreover, liquid epoxy coating can be applied by spraying or brushing, and can adapt to the inner walls of steel pipes of different shapes and sizes.

Construction points: Before construction, the inner wall of the steel pipe also needs to be pre treated to ensure surface cleanliness and appropriate roughness. The curing process of liquid epoxy coatings requires appropriate temperature and humidity conditions. Generally, a curing temperature between 10-30 ℃ is more suitable, and sufficient curing time should be ensured to fully utilize the anti-corrosion performance of the coating.

3. Cathodic protection

Sacrificial Anode Protection Law

Principle and material selection: Sacrificial anode protection method is to connect a metal with a more negative potential than the protected steel pipe (such as magnesium alloy, zinc alloy, etc.) to the steel pipe. In the presence of electrolyte solutions (such as soil moisture), sacrificial anode metals will preferentially lose electrons and be oxidized, providing cathodic protection for steel pipes and preventing corrosion. For example, in some buried small oil pipelines, using magnesium alloy sacrificial anodes is a simple and effective anti-corrosion method.

Installation and maintenance points: The installation position of sacrificial anodes should be reasonable, generally evenly distributed around the steel pipe, and ensure good electrical connection between the anode and the steel pipe. During use, the potential and remaining amount of the sacrificial anode should be regularly tested. When the sacrificial anode is consumed to a certain extent (such as less than 50% of the original volume), it needs to be replaced in a timely manner.

External current cathodic protection method

Working principle and system composition: External current cathodic protection method applies cathodic current to the protected steel pipe through an external power source, causing the surface of the steel pipe to polarize to a relatively negative potential, thereby suppressing the corrosion of the steel pipe. The system mainly consists of a power supply, an auxiliary anode, a reference electrode, and connecting cables. The power supply provides direct current, and the auxiliary anode introduces the current into the electrolyte solution. The reference electrode is used to monitor the potential of the steel pipe.

Operation and control points: During operation, the magnitude of the applied current should be adjusted reasonably according to the environmental conditions of the steel pipe (such as soil resistivity, pipeline length, etc.). At the same time, the potential of the steel pipe should be monitored in real time through a reference electrode to maintain it within an appropriate protective potential range (generally for carbon steel pipelines, the protective potential is between 0.85V and 1.2V, relative to the saturated copper sulfate reference electrode). If the potential exceeds the range, it may lead to overprotection or underprotection, affecting the anti-corrosion effect.

Selection of anti-corrosion methods for onshore LSAW steel pipes

1. Consider the environment in which the pipeline is located

soil environment

Acidity and corrosive substances: If the pipeline is located in soil with strong acidity or alkalinity, such as near chemical plants or coastal saline alkali land, anti-corrosion coatings with good chemical corrosion resistance should be selected. For example, 3PE coating (three-layer polyolefin coating) or epoxy powder coating has good resistance to acidic and alkaline environments. The epoxy powder bottom layer in the 3PE coating can adhere well to the surface of the steel pipe, the outer polyolefin layer can resist the corrosion of chemical substances, and the adhesive in the middle ensures the integrity of the coating system.

Soil moisture and resistivity: In environments with high humidity and low soil resistivity, steel pipes are more prone to electrochemical corrosion. For this situation, cathodic protection measures are particularly important. In some rainy areas in the south, the combination of sacrificial anode protection method or external current cathodic protection method with anti-corrosion coating can effectively prevent steel pipe corrosion. Sacrificial anodes (such as magnesium alloy anodes) can continuously provide cathodic protection current to steel pipes in humid soil electrolyte environments.

climatic conditions

Temperature changes and ultraviolet radiation: In areas with large temperature differences between day and night or strong ultraviolet radiation, the weather resistance of anti-corrosion coatings needs to be considered. For example, in desert or plateau areas where temperature changes are severe, 3PE coatings or coal tar enamel coatings with polyolefin outer layer can effectively resist temperature changes and ultraviolet radiation. After curing, the coal tar enamel coating forms a hard surface, which can effectively prevent coating cracking caused by temperature changes, and also has a certain resistance to ultraviolet radiation.

2. Select based on the characteristics of the conveying medium

Corrosivity of the medium

Oil and natural gas: When transporting oil and natural gas, due to the possible presence of corrosive components such as sulfides and carbon dioxide, the inner walls of steel pipes need to have good anti-corrosion measures. Liquid epoxy coating is a suitable choice as it can effectively prevent the corrosion of these corrosive media on the inner wall of steel pipes. Liquid epoxy coatings have good chemical stability and wear resistance, and can maintain their anti-corrosion performance for a long time in environments in contact with oil and natural gas.

Water: For water pipelines, cement mortar coating is a commonly used internal anti-corrosion method. The alkaline environment of cement mortar can form a passive film on the inner wall of steel pipes, preventing them from being corroded by dissolved oxygen and trace acidic substances in water. Moreover, the cement mortar coating can reduce the erosion of water flow on the inner wall of the steel pipe, extending the service life of the steel pipe.

3. Consider the service life and maintenance requirements of the pipeline

Long term usage requirements: If the service life of the pipeline is required to be longer (such as more than 30 years), 3PE coating with superior comprehensive performance is a good choice. The three-layer structure can provide long-term stable anti-corrosion protection, and under normal use, the aging rate of the coating is relatively slow. Meanwhile, in conjunction with cathodic protection measures, the service life of the pipeline can be further extended.

Maintenance convenience: Sacrificial anode protection method is relatively easier to maintain compared to external current cathodic protection method. If the pipeline is located in an area with inconvenient transportation or poor maintenance conditions, sacrificial anode protection may be more appropriate. Because sacrificial anodes only need to be replaced with new ones regularly after being consumed to a certain extent, without the need for complex electrical equipment and professional maintenance personnel to adjust current and other operations.

4. Consider cost factors

Initial investment cost: The initial investment cost of different anti-corrosion methods varies greatly. For example, the initial investment cost of epoxy powder coating is relatively low, mainly including the cost of steel pipe surface pretreatment and electrostatic spraying equipment and materials. Due to its complex structure and construction process, the initial investment cost of 3PE coating is relatively high, including the cost of three-layer materials, winding equipment, and precise construction. In the case of limited budget, it is necessary to comprehensively consider the balance between anti-corrosion effect and cost.

Operation and maintenance costs: Cathodic protection methods incur operation and maintenance costs. The cathodic protection method with external current requires the consumption of electrical energy, and electrical equipment needs to be regularly inspected and maintained, resulting in relatively high costs. The sacrificial anode protection method does not require an external power source, but it requires regular replacement of sacrificial anodes, which also incurs certain costs. When choosing anti-corrosion methods, it is necessary to consider these long-term operating and maintenance costs, and conduct a comprehensive evaluation based on the economic life of the pipeline.