Authored by: Shao Ting, Wang Fangjing
Qingyang Vocational and Technical College
Summary:The increasing demand for oil energy driven by social and economic development has led to a greater reliance on buried pipelines for transporting oil from the Changqing Oilfield. These pipelines are primarily made of metal, which presents significant corrosion challenges. Corrosion-related leaks not only reduce oil quality but also shorten the lifespan of the pipelines, resulting in economic losses and environmental pollution. This article primarily discusses the analysis of corrosion in Changqing Oilfield's pipelines and the research on protective measures.
Keywords:Oil pipelines; metal corrosion; protection
1. The importance of pipeline anti-corrosion work
Oil pipelines are the primary conduits for transporting oil and natural gas. Due to corrosion of steel pipelines, they can cause disruptions in operations for consuming enterprises, leading to quality degradation and impacting residents' access to gas, heating steam, or hot water. Leaking underground oil pipelines not only result in significant material loss but also cause severe environmental pollution, potentially leading to catastrophic incidents such as fires, explosions, and collapses.
Given the widespread occurrence of metal corrosion, various metal anti-corrosion technologies and processes have emerged to prevent or mitigate its occurrence. To address the harmful effects of corrosion in oil pipelines, oilfields have introduced and promoted new anti-corrosion technologies, materials, and equipment, including the addition of chemicals to water and changes in oil transportation processes, to extend the lifespan of pipeline equipment, save costs, and ensure safe operations. As oil and gas development enters the later stages, more and more corrosion problems have emerged, and corrosive accidents often cause significant economic losses and serious social consequences. An analysis of the corrosion mechanisms and influencing factors in oil pipelines is of great importance for effective protection measures. Due to the complexity and variability of corrosion, no anti-corrosion technology is a universal solution. Corrosion is absolute, while anti-corrosion is relative. Therefore, we need to comprehensively and deeply understand the characteristics of the corrosion environment and implement appropriate anti-corrosion technologies and measures, along with proper material selection and design, to control corrosion in oil pipelines. This will minimize the loss of corrosion in oil pipelines.
2 Factors Affecting Pipe Corrosion
2.1 Metal Corrosion
Corrosion is the destructive degradation of metals and their surrounding environment caused by chemical or electrochemical reactions. Corrosion specifically refers to the corrosion of metals. In a broader sense, corrosion refers to the interaction between materials and the environment, leading to the degradation of material functionality. The basic mechanism of steel corrosion is shown in Figure 1. It essentially involves the oxidation of metal elements to form compounds, which results in the corrosion of the metal.
Corrosion of metals refers to damage caused by chemical, electrochemical, or physical dissolution of metals resulting from their interaction with external media. Corrosion of buried oil pipelines includes external corrosion and internal corrosion. Internal corrosion is caused by the presence of corrosive components in the medium transported through the pipeline. Internal corrosion is the main form of pipeline corrosion, primarily occurring in low-lying areas with stagnant water, at interfaces between fluids and gases, and other similar locations. Internal corrosion includes erosion and corrosion caused by multiphase flow, as well as gas corrosion. External corrosion includes corrosion due to stray currents, soil corrosion, microbial corrosion, and erosion. based on the basic characteristics of metal damage, corrosion can be classified into overall corrosion and localized corrosion.
2.2 Internal Corrosion of Oil Pipelines
Petroleum products contain a large amount of carbon dioxide, as well as dissolved salts and hydrogen sulfide. During transport, they are prone to chemical reactions with internal pipelines, especially in the later stages of oilfield development, due to water injection and extraction, which increases the water content of the transport medium, further accelerating corrosion within the pipelines. Carbon dioxide corrosion is influenced by several factors, including CO2 partial pressure, temperature, pH value, fluid flow rate, carbonate scale, and the effects of waxes. The thermal motion of CO2 in the aqueous phase is particularly important in influencing CO2 corrosion. While hydrogen sulfide (H2S) in oil and gas does not directly corrode pipelines, when it comes into contact with water in the transport medium, it will dissociate, leading to electrochemical corrosion reactions.
2.3 Corrosion of Oil Pipelines
Some pipelines are frequently exposed to the elements, including wind and rain, and contain oil products with S elements and acidic substances, which can lead to corrosion. The primary causes of corrosion on the exterior of pipelines are:1. Oxygen absorption by steel;2. Acidic corrosion caused by CO2 from atmospheric precipitation;3. Sulfide-induced corrosion on the pipeline surface due to acidic rain;4. Bacterial corrosion caused by sulfate-metabolizing bacteria, with oxygen corrosion being the most severe.
3. Corrosion Protection Measures Research
Recent investigations and maintenance assessments of pipelines in China have revealed the following issues regarding corrosion protection:- The technology for repairing existing pipeline corrosion protection layers is relatively outdated.- Corrosion in dead zones due to cathodic protection.- Maintenance and inspection of cathodic protection devices are also relatively outdated.To ensure the safety of oil pipelines and prevent corrosion from disrupting the normal transport of oil, corrosion protection personnel have researched the following methods based on the factors and conditions affecting pipeline corrosion:- Optimizing the materials used for oil pipelines, selecting appropriate materials under different transport media and environmental conditions.- Proper surface treatment of pipeline coatings, adding protective layers to metal surfaces to prevent contact with oxygen.- Cathodic protection technology, using a metal with relatively higher chemical activity to create a cathodic environment for the pipeline metal.- Adding "corrosion inhibitors," which are special additives added to metal equipment to reduce corrosion. The principle of corrosion inhibition is opposite to that of catalysts used in chemical production. Corrosion inhibitors can reduce the activation energy, thereby slowing down the corrosion reaction. Due to its low investment and significant effects, it has become a key direction in corrosion protection technologies for pipelines and other fields.
3.1 Optimization of Materials for Oil Pipelines
In practical applications, appropriate pipe material selection should be based on the differences in conveying media and environmental conditions to control costs and meet the requirements of oil and gas engineering. The type and quality of pipe materials directly determine the corrosion resistance of the pipes, and the corrosion resistance of the pipes is related to the usage time and the probability of corrosion. Selecting highly corrosion-resistant pipe materials is an effective way to fundamentally prevent pipe corrosion. To improve the corrosion resistance of carbon steel pipes, stainless steel and alloy steel pipes can be used, but their production processes are complex and costly. Ceramic and plastic pipes, among other non-metallic pipes, are increasingly used to address this issue.
3.2 Application of Coatings on the Inner and Outer Surfaces of Oil Pipelines
Applying a coating on the inner walls of pipes not only reduces the rate of corrosion and protects the pipes, but also reduces the friction of the transported medium, thereby improving the safety of oil and gas transportation. When selecting coating materials for the inner and outer surfaces of pipes, the primary goal should be to minimize internal resistance to enhance corrosion protection. The inner coating reduces internal resistance, thereby improving the corrosion stability of the pipes. The outer coating should be selected based on the soil conditions and should consist of stable composite materials, such as epoxy resins, polyenes, and epoxy powder coatings, which have good physical and chemical properties, excellent dielectric properties, and temperature adaptation properties. Although the mechanical strength of the inner coating is lower than that of the outer coating, it still requires good adhesion and temperature performance. The inner and outer coatings are an important protective measure in pipe corrosion technology. Therefore, selecting and applying durable inner and outer coating materials according to technical specifications is the primary measure for corrosion protection.
3.3 Electrochemical Protection Measures
Electrochemical protection primarily refers to cathodic protection technology. Cathodic protection devices are installed on the surface of oil pipelines, polarizing the pipeline structure and shifting the potential to a negative cathodic state in the surrounding medium, effectively reducing the migration of electrons in metal corrosion, and thus providing corrosion protection for the pipeline. Cathodic protection measures are considered the second line of defense against pipeline corrosion. In practical applications, cathodic protection technology mainly includes two types: external current cathodic protection and sacrificial anode cathodic protection. These technologies are primarily selected and adopted based on the actual electrochemical characteristics of the soil and environmental conditions. Sacrificial anode cathodic protection does not require an external power source, is simple to operate, and has low maintenance costs. It can protect the pipeline from corrosion by consuming the protective anode. External current cathodic protection has higher detection and maintenance costs, but is suitable for the protection of long-distance oil and gas pipelines. However, it is important to note that this method requires a continuous external power source and may also lead to overprotection and hydrogen embrittlement.
3.4 Add corrosion inhibitors
Corrosion inhibitors are divided into two types: inorganic and organic. Different corrosion inhibitors should be selected based on the specific corrosion factors and application environment. Adding corrosion inhibitors can protect the inner walls of pipes and form a protective film on the inner surface of the pipes. The advantages of using corrosion inhibitors include relative simplicity and high efficiency, but the fact that corrosion inhibitors can easily flow with the transported medium is also a significant drawback. To avoid affecting the flow of oil and gas and the functionality of the corrosion inhibitor, the dosage, frequency, and method of adding corrosion inhibitors should be strictly controlled.
4. Conclusion
Factors affecting the corrosion and failure of long-distance oil and gas pipelines can be categorized into direct and indirect factors. To address different corrosion causes, the appropriate corrosion prevention method must be selected. Choosing an incorrect corrosion prevention method can lead to severe consequences. To extend the lifespan of oil and gas pipelines, it is essential to properly select corrosion-resistant coatings and methods, while also strengthening the transportation, construction, and maintenance of the pipelines. To ensure timely and accurate maintenance, advanced computer-based real-time monitoring systems should be used to monitor oil and gas pipelines during repair. No corrosion prevention method is a permanent solution. When selecting a corrosion prevention method, factors such as the surrounding environment of the pipeline, technical costs, and corrosion prevention effectiveness, which are specific to petroleum and natural gas engineering, should be comprehensively considered. Future development will focus on utilizing multiple corrosion prevention technologies, as well as developing environmentally friendly and highly effective technologies.
References:
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This article is reprinted from "Cleaning World", Volume 37, Issue 12.