Influence of Coastal Soil Environment on Local Corrosion of Oil and Gas Pipelines

The direct cause for the explosion accident occurred by Dong Huang pipeline is that the corrosion of pipeline was enhanced by coastal soil, which in turn resulted in thinning and then cracking of the pipe. Because localized corrosion cracking in coastal soils is a serious threat to the integrity and security for the pipeline, the effect of the coastal soil environment on localized corrosion for oil and gas pipeline was analyzed systematically. Typical corrosion environmental characteristics were acquired and then analyzed firstly for the specified coastal soil, which in general is rather different from the terrestrial soil and seabed soil distinctly. The coastal soil is characterized as a mixture of gas/liquid/solid multi-phases with high salt- and water-content and good air aeration, while it experiences dry-wet cycles periodically. The effect of the coastal soil environment on the initiation and propagation of localized corrosion for oil and gas pipelines is discussed. Finally, the trends and difficulties of the study in oil and gas pipeline localized corrosion are analyzed, and in the end, a scheme of micro-e electrochemical study is peculiarly prospected.

Key words:  coastal pipeline engineering    coastal soil environment    oil and gas pipeline    localized corrosion

Xiao TANG,Chuntao SHI,Guang CAO,Yan LI
College of Mechanical and Electrical Engineering, China University of Petroleum (East China), Qingdao 266580, China

1 Introduction
At 10:25 on November 22, 2013, the leakage of the Sinopec Donghuang oil pipeline in the Qingdao Economic and Technological Development Zone of Shandong Province caused a particularly serious accident (11.22 accident), resulting in 62 deaths, 136 injuries, and direct economic losses of 75.72 million. yuan. Through the accident investigation team on-site inspection, physical evidence testing, investigation and inquiry, and access to the data, and through comprehensive analysis, the direct cause of the accident is: the soil salinity and groundwater chloride content in the area where the oil pipeline is at the intersection with the drainage culvert High, and at the same time, the seawater is re-irrigated with tidal changes in the drainage culvert. The oil pipeline is in the dry and wet alternate seawater and salt spray corrosive environment for a long time. In addition, the pipeline is affected by road load and vibration, which leads to accelerated corrosion and rupture of the pipeline, resulting in crude oil. Leakage [1].
11.22 Accidents Corrosion thinning and rupture pipelines are located in the coastal soil environment. They have the characteristics of high soil salinity and groundwater chloride content, and are also affected by alternating dry and wet seawater and salt spray corrosion. Obviously, the coastal soil corrosive environment is the boundary area between the terrestrial soil environment and the marine soil environment. The physical and chemical properties are quite different from the terrestrial soil environment and the seabed mud environment, and it is a relatively unique corrosive environment. Oil and gas pipelines in this typical environment must have unique corrosion laws. However, at present, there are relatively few research work on coastal soil environmental corrosion in China, including the corrosion of Tianjin coastal saline concrete materials [2], the soil corrosion of Dagang Oilfield [3] and the soil corrosion of the seashore in Yeouido [4]. It is not clear. Corrosion characteristics and local corrosion mechanism of pipeline steel in coastal soil environment. Due to the lack of understanding of the local corrosion and rupture laws of pipeline steel in the marine soil environment, there is no special corrosion protection design for coastal buried pipelines in oil and gas pipeline engineering. For example, the material of Donghuang Oil Pipeline is API 5L X-60 straight seam welded steel pipe. The outer wall of the pipe is made of petroleum asphalt cloth and protected by current cathodic protection. In the actual project, due to the accelerated corrosion of the pipeline steel in the coastal soil and local corrosion, if there is a situation in which the cathodic protection is shielded or the cathodic protection is insufficient and overprotected, the pipeline is at risk of corrosion cracking. The occurrence of this accident has proved that the corrosion of oil and gas pipelines in the coastal soil environment is accelerated, local corrosion is prone to occur, and corrosion cracking is triggered under certain conditions, which seriously affects the operational safety of the pipeline. Therefore, it is necessary to refine the corrosive environmental characteristics of coastal soils and comprehensively analyze the impact of coastal soil environment on local corrosion of coastal oil and gas pipelines.
2 Characteristics of corrosive environment of coastal soil
In recent years, China’s oil and natural gas imports have increased significantly. According to a report issued by China Petroleum Economics and Technology Research Institute, the dependence on foreign oil in 2014 was close to 60%, and the dependence on natural gas increased to 32.2% [5]. In order to ensure energy security and reduce costs, a series of oil and gas reserves, refineries, crude oil terminals, LNG receiving stations and other oil and gas projects have been established in coastal areas, and a large number of oil and gas pipelines have been laid. The terminals of oil and gas pipelines are often in a similar coastal soil corrosive environment, and also face serious local corrosion cracking risks. Soil salinity, dissolved chloride, infiltrated seawater and salt spray in the coastal soil environment are likely to cause high-concentration Cl-media to penetrate into the pipeline, causing coating damage or peeling [6], causing corrosion of pipeline steel. The necessary condition for corrosion perforation or cracking of pipeline steel is the local difference in surface electrochemical properties, that is, local corrosion occurs at the interface between the pipeline and the coastal soil environment. From the occurrence and development process of local corrosion, the corrosion of pipeline steel is first induced to undergo localization by some factors. The further development of local corrosion can lead to perforation or crack formation, resulting in leakage of dangerous media such as oil and gas.
Because the local corrosion of pipeline steel in the coastal soil environment seriously threatens the overall safety of the pipeline, it is necessary to systematically study the impact of the coastal soil environment on the local corrosion and development of oil and gas pipelines, and expand the local corrosion of pipeline steel in coastal soils. The understanding of the characteristics, mechanism and law of rupture, and a deep understanding of the local corrosion, rupture and the difference in the soil in the coastal soil of pipeline steel, provide scientific basis for the anti-corrosion design, maintenance management and life assessment of coastal pipeline engineering.
Oil and gas pipeline projects are generally divided into land-buried pipelines and submarine pipelines according to the use environment. They are located in terrestrial soil and seabed soil environment respectively, with different corrosion laws and adopt different corrosion control strategies. In terms of corrosion protection methods, terrestrial oil and gas pipelines generally adopt coating and impressed current cathodic protection. Commonly used epoxy resin coating and polyethylene coating [7]; submarine pipelines are coated with sacrificial anode for corrosion protection [ 8], the outer insulation layer and the weight layer can also play a role in isolating the seabed corrosive medium [9]. At present, the oil and gas pipelines in China’s coastal soil environment have not been specially designed. As part of the buried pipelines, the corrosion control methods of land pipelines are basically adopted. Coastal oil and gas engineering pipelines use corrosion protection technology for terrestrial soil environment, but in the corrosive coastal soil environment affected by marine environmental corrosion factors, there is obviously a large local corrosion and rupture hazard. Corrosion environment determines the difference of corrosion law and mechanism of action. At present, there is no research report on local corrosion of pipeline steel in coastal soil corrosion environment.
Among the two environments associated with coastal soil environments, common localized forms of pipeline steel in terrestrial soil environments include pitting, crevice corrosion, stress corrosion cracking [10], corrosion fatigue [11,12], and hydrogen induced cracking [13, 14], related factors include soil media erosion, material organization and compositional inhomogeneity [15], prestress [16], stress distribution [17], alternating load [18], hydrogen [19], plastic Shape deformation [20], wave frequency [21], applied potential [22] and cathodic protection potential fluctuation [23], etc., domestic and foreign scholars have conducted a lot of research, and will not repeat them here.
Compared with land-buried pipelines, submarine pipelines are located in marine soil environments with greater corrosive capacity. Corrosion of submarine pipelines is closely related to the marine soil environment in which it is located, and its corrosion form and degree of corrosion also depend on the anti-corrosion measures taken [24]. According to statistics, the proportion of corrosion on submarine pipelines is very large, mainly due to internal corrosion caused by pipelines, followed by external corrosion. In addition, wave-induced erosion on the cross-section of pipelines is easy in pipe joints and sections. Corrosion fatigue occurs [25]. The main factors affecting soil corrosion outside the submarine pipeline are: soil permeability, conductivity, acid and alkali, dissolved salts, bacteria, etc. [24]. Corrosion corrosion promoted by marine organisms and dissolved oxygen [26], as well as pitting corrosion in the weld zone [27], are all forms of corrosion damage to the safety of submarine pipelines. Corrosion damage to submarine pipelines is often difficult to predict accurately. Recently, Gomes et al. [28] established a polynomial chaotic corrosion growth model based on field data, which can be used for submarine pipeline optimization design. In the actual engineering design and application, in order to prevent the corrosion of the seabed soil environment, the outer layer of the submarine pipeline adopts multiple protective layers such as anti-corrosion layer, thermal insulation layer and weight layer, and assists in cathodic protection. In special cases, double-layer pipe is also used. Designed with a very high safety factor [8].
The cause of the explosion of the Donghuang oil pipeline shows that the characteristics of the coastal soil environment are significantly different from those of the terrestrial soil environment and the seabed mud environment. In this environment, there is a serious risk of local corrosion and corrosion cracking. Therefore, it is necessary to take the coastal soil environment as a A unique corrosive environment, and in-depth study of the impact of the environment on the occurrence and development of local corrosion of oil and gas pipelines.
Considering the corrosive environment itself, the coastal soil environment as a unique corrosive environment, the primary problem to be explored is the characteristic of the corrosive environment unique to the coastal soil environment. Its geographical location is located around the coastline and is the boundary between the terrestrial soil environment and the marine soil environment. It is affected by the periodic tides of seawater. Its physical composition is multi-phase and multi-phase of seawater/high-salt groundwater/sand/clay/marine atmosphere. The interface structure, there are many differences between the local position, salt content, oxygen content, water content, etc., is a very typical uneven corrosion environment. The coastal soil environment has the characteristics of high salt content, high humidity, direct contact with air, heterogeneous gas/liquid/solid three-phase structure, and alternating wet and dry changes. In contrast, the seabed soil environment is a seawater-saturated liquid/solid two-phase system that is not in direct contact with air and is a relatively stable anaerobic environment; the terrestrial soil environment generally has a relatively low salt content and no alternating wet and dry changes, but Gas/liquid/solid three-phase corrosion system in direct contact with air. Obviously, the coastal soil environment has several factors that are significantly different from the terrestrial soil environment and the seabed soil environment. It is necessary to study which elements constitute the unique corrosive environment characteristics of the coastal soil environment during the corrosion process.
3 Factors affecting local corrosion of coastal oil and gas pipelines
In the occurrence and development of local corrosion of oil and gas pipelines in coastal soil environment, relevant influencing factors include:
First of all, from the environmental point of view, in the aspects of gas permeability, electrical conductivity, acid and alkali, and dissolved salts, the coastal soil environment is different from the terrestrial soil environment and the seabed soil environment, and has its own characteristics. Relative to the terrestrial soil environment, the corrosive capacity of the coastal soil environment depends first on its chemical composition [29]. High salinity and chloride content will inevitably lead to soil corrosivity, and high moisture content, wet soil environment will easily lead to coating. Peeling, causing local corrosion of the pipeline [30]. Compared with the seabed soil environment, the coastal soil environment is in direct contact with the air, has a higher oxygen content, and is more likely to cause an oxygen concentration battery; the self-gas/liquid/solid three-phase non-uniform composition and the alternating wet and dry characteristics are also easier. Inducing local corrosion.
Secondly, considering the pipeline protective coating, the local corrosion of the oil and gas pipelines in the coastal soil environment also depends on the nature of the coating and the failure mode [31]. The coating directly affects the penetration of the soil electrolyte into the surface of the pipeline steel. The thin layer electrolyte penetrating from the coastal soil environment to the coating layer can cause the pipeline steel to have a significant difference in corrosion characteristics from the bulk phase solution due to space limitations and difficulty in material exchange. In the thin layer of soil solution under the coating, the X70 pipeline steel anode curve measured by the scanning Kelvin probe has a certain blunt characteristic, which is different from the active dissolution in the bulk solution [32]. The corrosion process under the coating is affected by factors such as aggressive ions, soaking time, oxygen concentration and dry-wet cycle [33].
Thirdly, considering the surface state of the material itself, local corrosion of oil and gas pipelines is closely related to damage and defects on the surface of pipeline steel, such as surface defects such as scratches, micropores and corrosion pits [34]. Some research efforts attempt to explain the relationship between surface damage and crack induction of pipeline steel from the mechanism [35]. The authors have carried out micro-area electrochemical tests on several types of surface damage such as surface scratches, pits and pitting pits of X70 pipeline steel. It is found that these surface defects cause differences in the electrochemical properties of pipeline steel surface corrosion. The possibility of further development of the damage is greater, which can induce the local corrosion and damage to become a crack process [36]. In addition, the mechanical parameters of the material, such as mechanics and electrochemistry, affect the occurrence and development of local corrosion in the soil environment.
In summary, there are many factors affecting the local corrosion of oil and gas pipelines in coastal soil environment, but the way these factors work, they are specifically related to the local corrosion occurrence and development process of oil and gas pipelines, among which the main factors that can induce local corrosion of oil and gas pipelines And the strengthening effect of these factors on the accelerated development of local corrosion remains to be further studied. Therefore, we must start from the core nature of the problem, systematically study the coastal soil environment to induce and strengthen the local corrosion of oil and gas pipelines, in order to obtain a deep understanding of the local corrosion mechanism of oil and gas pipelines in the coastal soil environment.
4 Influence of coastal soil environment on local corrosion of oil and gas pipelines
It is understood from the essence of electrochemistry that the occurrence and development of localized corrosion is the development of the electrochemical non-uniform state of the material/corrosive environment interface into a macroscopic anion and anode corrosion cell. These unevenness include uneven material, uneven corrosion environment and interface. The status is not uniform.
First of all, it is worthwhile to pay attention to the impact of uneven coastal soil environment on the occurrence and development of local corrosion of oil and gas pipelines. The influence of marine soil environment on local corrosion of oil and gas pipelines is mainly reflected in two aspects: on the one hand, the induction of local corrosion, and on the other hand, the accelerated strengthening of local corrosion. Many elements in the coastal soil corrosive environment with high salt, alternating wet and dry, multi-phase and multi-interface can affect local corrosion. For example, as a typical multiphase heterogeneous corrosion system, even if the oil and gas pipeline materials with relatively uniform microstructure, composition and interface state are in the coastal soil environment, it is easy to lead to the distribution of electrochemical information such as potential and current density at the pipeline corrosion interface. Non-uniformity, which induces different forms of local corrosion such as pitting and crevice corrosion; oxygen concentration battery, salt concentration battery, gap structure under coating [37], gas/liquid/solid three-phase non-uniform composition and dry Local corrosion can be induced by factors such as wet alternating characteristics; this typical uneven coastal soil environment with high Cl-concentration and strong corrosion ability can also enhance accelerated pitting corrosion, crevice corrosion, and weld galvanic corrosion. Development process of local corrosion such as stress corrosion. The problems that need to be clarified include: the main factors that can induce local corrosion of oil and gas pipelines in coastal soil environment; secondly, the coastal soil environment enhances the difference in corrosion electrochemical behavior of oil and gas pipelines, thereby accelerating the development of local corrosion. It can be summarized as the induction and strengthening mechanism of coastal soil environment to local corrosion of oil and gas pipelines. This is a key scientific issue in the mechanism of local corrosion of coastal pipeline engineering.
5 Prospects for local corrosion of oil and gas pipelines in coastal soils
In view of the fact that the local corrosion of oil and gas pipelines in the coastal soil environment is related to the electrochemical process at the interface, the electrochemical induction of interface potential, current density, impedance, especially the electrochemical distribution information of the micro-regions, and the environmental induction and strengthening laws of local corrosion It is crucial. In the past, due to the limitations of testing methods, it is difficult to obtain the electrochemical distribution information of local corrosion microdomains, and the most direct experimental evidence is lacking to study its production and development mechanism. This is because classical corrosion electrochemical methods such as electrochemical polarization curves and electrochemical impedance spectroscopy can only obtain electrochemical information on the entire sample surface, giving statistical average data, and it is difficult to electrochemical signals on local regions. Make a distinction. Nowadays, the development of various micro-area electrochemical testing techniques provides a new research perspective for studying local corrosion induction and strengthening laws, and provides a powerful technical support for studying the distribution characteristics of electrochemical information in oil and gas pipelines in coastal soil environment. For example, the scanning vibration electrode (SVET) can test the surface current density distribution of the metal material in the solution, reflecting the micro-area corrosion anode-anode distribution and the relative corrosion rate; the local electrochemical impedance spectroscopy (LEIS) can not only study the full frequency of a specific point. Electrochemical impedance spectroscopy can also be used to perform scanning tests at a fixed frequency to study the impedance distribution characteristics of the electrode surface. Micro-area electrochemical testing techniques such as SVET and LEIS have been used to study the effect of stress and hydrogen combination on local anodic dissolution of X70 pipeline steel [38], and obtained ideal results, verifying the micro-area electrochemical testing technique in solution. The system characterizes the feasibility of local corrosion of pipeline steel.
However, testing the electrochemical distribution information of the oil and gas pipeline interface micro-regions in the coastal soil environment faces great technical difficulties, which are reflected in two aspects: on the one hand, because the coastal soil environment is gas/liquid/solid three-phase Uniform system, corrosive medium contains solid components such as sand, clay, etc. Scanning probe-type micro-area electrochemical testing techniques such as SVET and LEIS cannot perform interface electrochemical testing in this system; on the other hand, in practical engineering applications Oil and gas pipelines are protected by coatings [2]. Corrosion may occur only after the coating is damaged or penetrates into the soil electrolyte. The local corrosion characterization in the thin layer of electrolyte under the coating is limited by the thin layer of the electrolyte and coated. There are also major technical difficulties in the reasons such as the obstacles of the layers.
Therefore, it is necessary to comprehensively utilize a variety of micro-area scanning electrochemical techniques for the unique corrosion environment of coastal soils, and to obtain interface potential, current density, impedance spectrum and polarization resistance by designing special electrodes, test devices and experimental schemes. The electrochemical distribution information of the area can systematically study the influence of the coastal soil environment on the local corrosion occurrence and development process of oil and gas pipelines.
6 Conclusion
The cause of the explosion of the Donghuang oil pipeline shows that the characteristics of the coastal soil environment are significantly different from those of the terrestrial soil environment and the seabed mud environment. In this environment, there is a serious risk of local corrosion and corrosion cracking. Therefore, it is necessary to take the coastal soil environment as a A unique corrosive environment, systematically study the impact of coastal soil environment on local corrosion of oil and gas pipelines.
The coastal soil has the characteristics of corrosive environment which is significantly different from terrestrial soil and seabed soil, that is, it has the characteristics of high salt content, high humidity, direct contact with air, gas/liquid/solid three-phase non-uniform structure and alternating dry and wet changes. Oxygen concentration battery, salt concentration battery, gap structure under coating, uneven structure of gas/liquid/solid three-phase, and characteristics of dry and wet alternating environment factors can induce local corrosion of oil and gas pipelines, and Strengthen the development of local corrosion such as accelerated pitting corrosion, crevice corrosion, weld galvanic corrosion, stress corrosion. In-depth study of the corrosive environment characteristics of coastal soils and its impact on local corrosion of oil and gas pipelines will not only help to understand the role and mechanism of the corrosive environmental characteristics of coastal soils in the local corrosion and rupture of oil and gas pipelines, but also the coastal pipelines. Provide scientific basis for engineering design, corrosion control and long-term safe operation.
The authors have declared that no competing interests exist.

Source: China Steel Pipeline Manufacturer – Yaang Pipe Industry (www.metallicsteel.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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