Corrosion in Soils PDF

## Corrosion in Soils ### 2.2. Corrosion in Soils - The spaces between solid soil particles are filled with water and gas. - Water is either bound to soil minerals or flows through the pores of the soil. - The porosity of soil controls the flow of fluids through the soil. - Oxygen permeability i...

## Corrosion in Soils ### 2.2. Corrosion in Soils - The spaces between solid soil particles are filled with water and gas. - Water is either bound to soil minerals or flows through the pores of the soil. - The porosity of soil controls the flow of fluids through the soil. - Oxygen permeability in coarse grained soil is larger than that of fine grained sand. - Water saturation is higher in fine grained sand than that of coarse grained sand. - The corrosion of buried structures can be highly altered. ### 2.3. Stress Corrosion Cracking (SCC) #### 2.3.1. Mechanisms of SCC in Steel - SCC results from the conjoint action of three components: - A susceptible material. - A specific chemical species (environment). - A tensile stress. - This phenomenon involves the combined action of tensile stress and a corrosive environment. - It can occur in various environments such as seawater, acidic solutions, or caustic environments. - The mechanisms commonly proposed for SCC in steel include: 1. **Hydrogen Embrittlement:** - Hydrogen diffuses into the steel under stress, leading to brittle fracture. - Hydrogen reduces the strength of steel by interacting with dislocations or voids in the metal structure. 2. **Anodic Dissolution:** - In environments with aggressive species, the metal undergoes anodic dissolution at the crack tip, which accelerates crack propagation. - This can happen in carbon steels and stainless steels, particularly in chloride-containing environments. 3. **Film Rupture and Repassivation:** - For some steels, a passive oxide film forms on the surface, protecting it from corrosion. - However, under stress, this film can rupture, allowing corrosive elements to attack the metal underneath. - The film may reform (repassivation), but if this process is repeated, it can lead to crack initiation and propagation. #### 2.3.2. Internal Stresses in Pipelines - Internal stresses in pipelines arise from various factors, including pressure changes, temperature fluctuations, and external loads. - These stresses can significantly impact structural integrity of the pipeline and must be carefully managed to avoid failures. 1. **Pressure-Induced Stresses:** - These include hoop stress, which acts circumferentially, and longitudinal stress, both of which are caused by the internal pressure of the fluid within the pipeline. - Hoop stress, being twice as large as longitudinal stress, is of particular concern for pipe integrity, as it can lead to cracking under high pressure. - Hoop stress (σ₁) for a thin wall pipe can be determined using the equation: $σ_h = \frac{[P_i - P_o]D}{2t}$ where: - $P_i$ = internal pressure. - $P_o$ = external pressure. - $D$ = outside diameter of the pipeline. - $t$ = minimum wall thickness of the pipeline. 2. **Thermal Stresses:** - Temperature variations, such as those encountered during the transportation of heated or cooled fluids, induce expansion or contraction in the pipe material. - When the pipe's natural expansion is restricted (e.g., by supports), thermal stress builds up. - This type of stress can lead to deformation, fatigue, and eventual failure over time. 3. **External Loads and Shear Stresses:** - Pipelines are also subjected to external forces like wind, seismic activity, or even soil movement. - These external loads generate shear stresses, which can lead to torsional forces acting on the pipe, particularly at bends or junctions. #### 2.3.3. Longitudinal Strain Due to Soil Movement - Strain is called longitudinal strain when the length of a body changes as a result of the applied force. - It is the proportion of a body's change in length to its original length. - It is calculated by measuring the change in length per unit length. - Torsion strain is another name for this type of strain. - Soil movement is frequently very slow, sometimes taking up to 50 years before the onset of pipeline failures. - Longitudinal strain in pipelines due to soil movement, such as landslides or ground subsidence, is a critical concern for pipeline integrity. - Soil movement generates axial forces along the pipeline, leading to significant longitudinal strain. - This occurs due to friction between the soil and the pipeline, which depends on various factors such as soil type, pipeline material, burial depth, and the rate of ground displacement. - In scenarios like landslides or thaw slumping, the longitudinal strain increases with the resistance of the surrounding soil, as pipelines may experience both tensile and compressive strains. - Higher operating pressure within the pipeline also amplifies these strains, making the structure more susceptible to damage at lower soil displacement thresholds. - The angle of the slope can further impact strain levels, with steeper slopes generally causing higher axial forces and greater strain. #### 2.3.4. Hydrogen Damage - Hydrogen damage refers to various forms of degradation that metals, particularly steels, experience when exposed to hydrogen. - This phenomenon can exacerbate SCC in stainless steel by promoting crack initiation and growth. - The primary mechanism behind this damage includes hydrogen embrittlement, hydrogen-induced cracking, and high-temperature hydrogen attack. 1. **Hydrogen Embrittlement:** - Occurs when hydrogen atoms penetrate the metal, reducing its ductility and causing cracks to propagate. - HE can drastically reduce the tensile strength and fracture toughness of materials, which is particularly problematic in industries like petrochemical and power generation where high-pressure hydrogen environments are common. 2. **Hydrogen-Induced Cracking:** - Typically happens in pipelines and storage vessels when hydrogen atoms create internal pressure pockets, leading to blistering and cracking. - This form of hydrogen damage is accelerated in environments with high levels of hydrogen sulfide often seen in the oil and gas industry. 3. **High-Temperature Hydrogen Attack:** - Involves the reaction of hydrogen with carbon present in steels at elevated temperatures, leading to the formation of methane within the material, which causes internal decarburization and cracking. - This is especially critical in refinery equipment where high-temperature hydrogen is prevalent. ### 2.4. Modeling of Stress Corrosion Cracking #### 2.4.1. Elastoplastic Stresses - Elastoplastic is related to the state of stress between the elastic limit of a material and its breaking strength in which the material exhibits both elastic and plastic properties. - Elastoplasticity is a rate independent phenomenon which, when activated, causes the development of permanent deformation. - It is usually considered as the limit case of viscoplasticity, in which the notion of critical stress exists and causes yielding in the material. - In metals the development of plastic strains is due to the creation of dislocations in the crystalline structure. - The evolution of the permanent deformation may or may not be linked to the criterion under which plasticity occurs, allowing us to categorize the elastoplastic mechanisms into associated and non-associated. #### 2.4.2. Strain Plasticity Model - Elasto-plastic models are needed to describe the behavior of materials where stresses exceed the yield point. - The basic characteristics of elasto-plastic behavior capture the loading and unloading stress-strain paths and the state of multi-axial stress corresponding to the onset of the plastic flow (yield criterion). - Elasto-plastic modeling includes: - Plasticity Tresca Model. - Plasticity von Mises Model. - Large Strain Analysis. - Plasticity Drucker - Prager Model. #### 2.4.3. Von-Mises Yielding Criterion - Von Mises stress is a critical concept in the field of material mechanics and is essential for understanding the behavior of materials under complex loading conditions. - Named after Richard von Mises, who developed the theory in the early 20th century, this stress measure is particularly useful in predicting yielding and failure in ductile materials. - Von Mises stress, often denoted as $σ_v$, is a scalar stress value derived from the stress state at a point within a material. - It is particularly important in the context of yielding, as it provides a criterion for determining whether a material will yield under a given set of loading conditions. - According to the von Mises yield criterion, yielding occurs when the von Mises stress reaches a critical value, which is typically equivalent to the yield strength of the material in uniaxial tension. - This criterion is widely used in engineering applications, particularly in the design and analysis of structures and components subjected to complex loading scenarios. - The relevance of von Mises stress extends beyond theoretical considerations; it is integral to practical engineering applications such as finite element analysis (FEA). - Engineers utilize von Mises stress to assess whether components can withstand operating conditions without failing. - By analyzing the stress distribution within a structure, engineers can optimize designs for safety and performance. - The mathematical formulation of von Mises stress is derived from the stress tensor, which encompasses normal and shear stresses acting on a material. - For a three-dimensional stress state, the von Mises stress is expressed as: $σ_y = [(σ_1 – σ_2)² + (σ_2 − σ_3)² + (σ_3 – σ_1)2] 1/2$ - This formula captures the influence of all three principal stresses and emphasizes the importance of shear stress in the yielding process. #### 2.4.4. Comsol Multiphysics Platform - COMSOL Multiphysics is modeling software that can simulate various physical phenomena, including electrochemistry and corrosion. - It can be used to model SCC systems for buried pipes in different environments and conditions. - This platform allows calculation of the conductivity of the electrolyte and current flow in the soil field surrounding the pipes and anode surfaces. The interface flow between the soil and any buried surface. - It is possible to display 3D or 2D design to identify which parts of the structure are susceptible to corrosion. - By using COMSOL Multiphysics for simulate of SCC of buried pipes, one can design, optimize, and evaluate protection systems in a cost-effective and way. - COMSOL Multiphysics can help improve the reliability and durability of buried pipes that are prone to corrosion. - To use COMSOL Multiphysics for simulate SCC of buried pipes, the following steps should be included: 1. Define the geometry of the model, which consists of the pipe network, the soil domain, and the anodes. - The pipe network can be represented by edges or surfaces, depending on the level of detail required. - The soil domain can be a 2D or 3D region that surrounds the pipe network. 2. Define the physics of the model, which consists of the current distribution in the pipe network and the soil domain. - The current distribution can be solved by using the Current Distribution Interface for pipes or domains. - The current distribution interface allows specifying the electrolyte conductivity, the electrode potential, the electrode kinetics, and the external current sources. - The electrode kinetics can be defined by using experimental polarization data or empirical equations. 3. Define the mesh of the model, which consists of discretizing the geometry into a finite number of elements. - The mesh can be generated automatically or manually by using different meshing algorithms and parameters. - The mesh quality and size can affect the accuracy and efficiency of the solution. 4. Define the study of the model, which consists of setting up the solver and performing the simulation. - The study can be a stationary or a time-dependent analysis, depending on the type of problem. - The study can also include parametric sweeps, optimization, or sensitivity analysis, depending on the objective of the simulation.