49.1 Introduction

The characteristics of the electrochemical cell formed at an underground pipeline are discussed in this chapter. Considering the pipeline as the electrode and the soil as the electrolyte, the properties that control corrosion activity can be characterized. Technologies for detecting and monitoring the state of external corrosion of oil and gas pipelines will be described by considering the underground pipeline as electrochemical, or electrical, in nature. Modeling of external corrosion has become an important tool to implement the current preventative technologies and is used to support analyses of the information acquired from each survey or inspection method. Critical factors incorporated into modeling help to characterize not only the position of external corrosion but also the time-dependent effect of the dissolution mechanism. The factors range from macro- to microscale in nature. This chapter includes the description, quantification, and influence of critical factors for external corrosion of buried pipelines. The classification of the factors is based on the multiscale approach for onshore pipelines. Furthermore, the field and laboratory techniques used to detect, characterize, and monitor the state of the pipeline will be categorized based on the micro and macro parameters.

49.2 Background

Pipelines are exposed to various environmental elements that can interact with them and cause integrity damage. The soil, oxygen in the air, rainfall, temperature, moisture, vegetation, and electrolytes suspended in the atmosphere can interact with the pipeline and cause it to corrode. Corrosion is a time-dependent threat in pipelines exposed to underground conditions and is one of the prime causes of pipeline failure. Soil, as defined by Rim-Rukeh et al. [1], is a complex material consisting of a discontinuous and heterogeneous environment that is constituted by an organic solid phase, liquid water phase, air, and other gas phases. It is estimated that a pipeline failure occurs on an average of every 2–4 months in the United States, with the most common failure mechanism being localized corrosion [2, 3].

In soils, water and gas occupy the spaces between solid particles, and these spaces can constitute as much as half the volume of dry soil. Some of the water in the soil is bound to mineral surfaces, whereas bulk water can flow through porous soil. Fluid flow through soil is controlled by the permeability of the soil, which, in turn, depends on the size distribution of the solid particles in the soil. For example, good drainage can occur in coarse-grained sand, and atmospheric oxygen can penetrate to a depth greater than that which would be reached, for example, in fine-grained soil high in clay. Capillary action in fine-grained soil can draw water up, keeping the soil saturated with water, preventing drainage, retarding evaporation, and restricting oxygen access from the atmosphere to the pipeline.

The electrochemical corrosion processes taking place on metal surfaces in soils occur in groundwater that is in contact with the metallic structures. Damage evolution of the pipe coating, of the coating/steel interface, and of the steel beneath the coating is based on initiation conditions and time-dependent parameters that produce and influence various processes. Both the soil and the climate influence the groundwater composition and the factors that control the damage/failure process.

The damage evolution process can be defined in four stages by assuming the pipeline/coating intact conditions, as illustrated in Figure 49.1. Stage I is the initiation, which comprises the transport of species toward and within the pipe coating under cathodic protection conditions. Stage II includes the activation of the steel substrate and corrosion product formation, during which any change in volume or physical modification of the pipe coating is thermodynamically feasible. Any anomaly built due to the stresses induced by the corrosion products or third-party damage resulting in a holiday, which is a defect in the pipe coating, is an indication for Stage III, where the environment is in direct contact with the steel substrate. Stage IV, the final stage, is the metal loss leading to a leak or rupture of the pipe. The parameters of each stage that are critical to the performance of the corrosion control and protection system are explored in this chapter.

Pipelines are expected to function reliably and continuously over several decades without failure or extended damage from corrosion. Various parameters in the soil can affect pipeline damage, for example, pH, moisture content, resistivity, ion content, redox potential, physical, and chemical characteristics of the type of soil, oxygen content, and microbes present in the soil [4]. Different components that form the electrochemical cell affect the damage evolution and will indirectly or directly influence each of the damage evolution stages. The electrochemical and transport processes at the metal/coating interface are influenced by the water (or electrolyte) uptake on the metal surface as well as the chemistry of the film, in addition to the oxide layer formation on the surface and the interactions with the soil. The external interactions with the soil are environmental, electrical, and mechanical in nature and include weather, climate, stray currents, and telluric effects among others. The microenvironments due to oxygen, moisture, and other species migrating through the soil to the metal surface control these macro metal/moisture environments and influence corrosion. The macroenvironment will further influence the soil condition and corrosivity [5]. A multiscale concept becomes important for the critical factors for external corrosion in buried onshore pipelines.

Figure 49.2 illustrates the multiscale concept for the degradation mechanisms. Along the right-of-way (ROW) on a pipeline/soil system, different ecosystems will influence the local and general conditions of the pipeline. We can identify and characterize different damage evolution stages from the submillimeter to meter scales.

49.3 Critical Factors of Soil Corrosivity that Affect Pipelines

Based on a multiscale concept, various factors affect the aggressivity of the soil, which, in turn, influences the integrity and reliability of a pipeline. Most often, the corrosivity of the soil determines the mode of protection offered to a pipeline, the survey technology used, and the current estimate of reliability. Several important variables have been identified that have an influence on corrosion rates and damage evolution in soils. These include the type of soil, amount of water, degree of aeration, pH, redox potential, resistivity, soluble ionic species (salts and organics), and microbiological activity. This complexity is pictured in Figure 49.3. Some variables affecting pipeline corrosivity are discussed below.

49.3.1 Multiscale Factors Influencing External Corrosion Related to Soil Properties and Conditions

Ecosystems influence the soil properties distribution from the original state. Differences are proportional to the local properties of the electrolyte, and the accumulation of several local conditions drives the characteristics of a large area. Multiscale characteristics can be divided into macro and micro parameters. The macro, or global, parameters cover larger areas of incidence in the ROW, such as precipitation, temperature, topography, vegetation, evapotranspiration, CO₂, and humidity. The moisture conditions can be the link between macro and micro conditions, and the natural soil distribution is affected by the presence of water. The water balance depends on several factors: transport within the soil; the water input due to environmental conditions; the output influenced by drainage, soil properties, and distribution; and the accumulation of water due to the soil properties. Among the critical parameters for local micro conditions, the soil plays an important role; the soil directly interacts with the atmospheric and above-ground conditions while also covering the surface of the metallic pipeline. The soil properties can control both local and global conditions. For example, the particle size influences the moisture saturation and water content. Different parameters to consider for local conditions include the soil skeleton, pore structure, and level of compaction due to the density or porosity. The distribution of the soil properties cannot be measured under field conditions. One parameter that can be critical to soil characterization is resistivity. Historically, resistivity has often been used as an indicator of soil corrosivity. Because ionic current flow is associated with transport mechanisms leading to corrosion, high soil resistivity will arguably slow down the transport kinetics for each stage in the damage evolution process. Liu et al. [7] found that soil resistivity was the most important factor contributing to pipeline damage. The resistivity value of the soil decreases with the increase of the moisture content in the soil [8]. Obtaining a good correlation between resistivity observed in the field and in the laboratory has yet to be achieved [9]. Soil resistivity generally decreases with increasing water content and the concentration of ionic species. Soil resistivity is by no means the only parameter affecting the risk of corrosion damage; a high soil resistivity alone will not guarantee the absence of metallic and coating damage.

Soil Type	Soil Resistivity (ohm-cm)	Moisture	Corrosion Rate (mm/year)
Muskeg/sloughs/free water accumulations	<500	Always wet	Very corrosive (>1.0)
Loams/clays	500–2000	Mainly wet	Corrosive to moderately corrosive (0.5–1.0)
Gravels, sandy	2000–10000	Mainly dry	Mildly corrosive (0.2–0.5)
Arid, sandy	>10000	Always dry	Noncorrosive (<0.2)

Variations in soil resistivity along the length of a pipeline are highly undesirable because they will lead to the formation of macrocorrosion cells. For pipelines, the merit of a corrosion risk classification based on an absolute value of soil resistivity is not robust.

Soil resistivity can be measured by the so-called Wenner four-pin technique as illustrated in Figure 49.4, or more recently, by 3D electromagnetic survey technologies. The latter allows measurements in a convenient manner and at different soil depths. Another option for soil resistivity measurements is the so-called soil box method, whereby a sample is taken during excavation. Preferably, the sampling will be performed in the immediate vicinity of the pipeline (in a pipe trench, for example). Table 49.1 shows the effect of soil type and moisture content on the corrosive behavior of the soil.

49.3.2 Water Coverage due to Vapor Transportation and Drainage

Water in liquid form represents the essential electrolyte required for electrochemical corrosion reactions. In soils, a distinction is made between saturated and unsaturated water flow/accumulation. Unsaturated water flow refers to the movement of water from wet areas toward dry soil areas. Saturated water flow is dependent on pore size and distribution, texture, structure, and organic matter. High-moisture content facilitates faster transport of ions between the pipeline surface and the soil, causing the soil to be more corrosive. Water movement in soil can occur by the following mechanisms: gravity, capillary action, osmotic pressure (from dissolved species), and electrostatic interaction with soil particles. The water-holding capacity of soil is strongly dependent on its texture; coarse sands retain very little water, while fine clay soils store water to a high degree. Gupta et al. [13] observed that the maximum corrosivity of steel occurs when the moisture content is 65% of its water-holding capacity and termed this as the “critical moisture-holding content.” Moreover, moisture content is not the only factor that is found to control the corrosion of ferrous metals. Neale et al. [14] observed that depending on the soil type, the saturation of water in the soil vary from as low as 0.5% for sand to as high as 217% for bentonite clay. In addition, it has also been reported that low moisture content favors localized corrosion [5]. Castaneda et al. [15] created drainage maps for water accumulation at different depths. The spatial distribution of precipitation, evapotranspiration, and drainage was used to map water accumulation, as illustrated in Figure 49.5.

49.3.5 Differential Aeration Corrosion Cells

Differential cell corrosion is the most common mechanism that causes localized corrosion of pipelines. The soil corrosivity in underground pipelines can be influenced by the level of oxygen concentration at different regions of the pipeline, creating differential cell aeration, as well as by any differences in soil chemistry or pipeline surface [6]. The region of higher oxygen concentration acts as a cathode, whereas the oxygen-deficient region acts as an anode. Differential cell corrosion can be autocatalytic in nature. The chemical and electrochemical reactions and ion migration tend to generate conditions favorable to the continuation of the cell [6]. Figure 49.6 depicts a pipeline passing through two different types of soil. The potential of the pipeline in the clay soil is more anodic compared to the potential of the pipeline in the dry sandy soil, and hence, a corrosion cell is generated. The pipeline in the clay soil is corroded, whereas the pipeline in the sandy soil is protected. Differential corrosion cells can also be generated by differences in moisture content in soil or differences in oxygen diffusion through the soil.

49.4 Identifying Corrosive Environments

Ideally, any highly corrosive environments along a proposed pipeline route would be identified before pipeline installation. Various ASTM standard methods can be used to test for soil resistivity and pH. ASTM G57 is one such test for Field Measurement of Soil Resistivity. Other ASTM test methods include ASTM G51-95(2102) for measuring the pH of Soil, ASTM G200-09 for the Measurement of Oxidation–Reduction Potential (ORP) of soil, and ASTM G162-99(2010) for Conducting and Evaluating Laboratory Corrosion Tests in Soils [28–31].

49.5 Cathodic Protection and Stray Currents

After identifying the risk or likelihood of corrosion of a buried pipeline in a certain environment by evaluating the factors involved in this process, different methodologies have to be followed to either prevent or counterattack the effect of corrosion. The main methods for corrosion mitigation on underground pipelines are coatings and cathodic protection (CP) (discussed in Chapters 24 and 26, respectively).

On a cathodically protected pipe, the coating reduces the surface area of exposed steel on the pipeline, thereby reducing the current necessary to cathodically protect the steel. Cathodic protection is used to reduce the corrosion rate of a metal surface by making it the cathode of an electrochemical cell.

The principle for cathodic protection is illustrated in Figure 49.7 [32]. The electrons are supplied to the pipeline by using a dc source and an anode. In the case of a coated pipeline, current flows to the areas where the coating is defective. An electron current flows along the electric cables connecting the anode to the cathode, and an ionic current flows in the soil between the anode and cathode to complete the circuit.

The primary goal of cathodic protection is to lower the potential of the buried structure. As long as the cathodic current is increased, the potential of the metal decreases; for each increment that the potential of the metal is reduced, the current requirements tend to increase exponentially.

A poor CP design may be a cause of damage to the pipeline; some of the key indicators are bad current distribution, interferences, or stray currents. Stray currents are currents flowing in the electrolyte (soil) from external sources not associated with the CP system. Corrosion damage to an underground structure caused by a CP system on another structure is a form of stray current corrosion that is commonly called “interference.” Stray current damage is most commonly associated with impressed current CP systems. A foreign pipeline or other metallic structure can form a second resistance path where the current distribution can be deviated from the coverage areas that the pipeline requires to complete the protection system.

Figure 49.8 illustrates a foreign pipeline passing through a zone of positive soil potentials (area of influence) and protected by an impressed current system. In this illustration, a pipeline crosses the one selected for protection at a more remote location. The positive soil potentials will force the foreign pipeline to pick up current at points within the area of influence. This current must then complete the electrical circuit and return to the negative terminal of the DC power source. The figure illustrates this by showing the current path distribution flowing along the foreign line (nonoriginal protected) toward the point where the two lines cross and then leaving the foreign line in the vicinity of the crossing. This current then flows into the protected pipeline and returns to the rectifier. The area where the current leaves the foreign line in the vicinity of the crossing becomes a potential site for corrosion in the foreign pipeline. Usually, a small amount of current will flow along the foreign pipeline in the opposite direction from the ground bed area. This is indicated as a final current path in the figure. This current will leave the foreign pipeline at remote locations, usually in areas of relatively low soil resistivity. The severity of the effect is largely a function of the impressed voltage on the ground bed and the proximity of the foreign pipeline to the ground bed. Where the impressed voltage is high, and the foreign pipeline is close to it, the current forced onto the foreign pipeline tends to be high and can cause damage. In such instances, the foreign pipeline represents a potential threat leading to weak integrity.

Once the source is identified, the current path can be distributed to correct the problem. A better electrical balance can produce suitable current distribution and avoid the high current drainage at the potential corrosion location; connecting a resistance between the two metallic conductors (protected and foreign pipelines), the magnitude of the electrical passive element (resistance) shall be calculated in order to distribute the current drain from a specific location to higher areas. Cathodic protection design (or redesign) can be also considered as a preventive or control action. Relocation of the anodic bed, the addition of passive elements (such as resistors), and more efficient current pathways in the electrical configuration are some examples of remediation actions [11].

49.6 Monitoring and Inspection for Corrosion Characterization Under Multiscale Conditions

Corrosion in pipelines is a time-dependent threat that can be monitored under field conditions and characterized or quantified under laboratory conditions by simulating field metal/coating/soil systems. Inspection under field conditions presents some challenges due to the area extension for pipeline/soil in the right-of-way. Direct and indirect measurements can be used to monitor and inspect the state of either component of the pipeline/coating system, such as the coating, cathodic protection coating failure, and metallic activation. Table 49.4 illustrates the technologies used in the field for direct measurements on pipelines; for example, metal loss and coating failure can be detected using different technologies for in-line inspection (ILI), such as magnetic flux leakage (MFL) and ultrasound (UT).

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