Rehab Part 1 PDF

# Chapter 1. Introduction Pipelines have played a key role in modern energy as they provide an economic and safe way of transportation of oil and gas products. Petroleum products are of vital importance to the preservation of industrial civilization. These products have been transported over long d...

# Chapter 1. Introduction Pipelines have played a key role in modern energy as they provide an economic and safe way of transportation of oil and gas products. Petroleum products are of vital importance to the preservation of industrial civilization. These products have been transported over long distances by buried steel pipelines. For example, in Libya there is about 3,700 km of steel main-line pipe for the transport of natural gas alone and more than 7,000 km of pipeline for transport of crude oil and refined products. Pipelines are prone to both internal and external corrosion. Internal corrosion can take different forms such as uniform corrosion, pitting corrosion, erosion corrosion, and microbiologically-influenced corrosion due to the presence of different corrosive environment such as CO2, H2S, CL, solids and bacterial activity. When buried in soil, pipelines are in contact with different corrosive environments (mainly soil which contain oxygen and water). During long-term service, the external coating may be degraded and leads to external corrosion. Stress corrosion cracking (SCC) in steel pipelines, is a major form of external corrosion results from the combination act of mechanical stresses and corrosive environmental conditions. SCC often leads to unexpected and catastrophic failures and thus requires particular attention. Simulating stress corrosion cracking is a complex but essential task for predicting failures in critical infrastructure. By integrating mechanical, electrochemical, and statistical models, engineers can gain insights into the behavior of materials under stress and corrosive environments, ultimately helping to design safer and more resilient structures. Different approaches can be used for SCC simulation which involves using computational methods and modeling techniques to understand and predict how SCC develops in materials under various conditions. Finite Element Analysis (FEA) is commonly used to analyze stress distribution in materials. By modeling the geometry of a component and applying loads, stress concentrations where SCC is likely to initiate could be identified. Fracture Mechanics Models focus on crack propagation once a crack has initiated. Techniques like Linear Elastic Fracture Mechanics (LEFM) or J-integral methods can be used. Corrosion Models predict the electrochemical processes leading to corrosion, often using software that simulates corrosion kinetics. Multi-Scale Modeling combines different scales of analysis, from atomic to macroscopic, to capture the complex interactions involved in SCC. In this study, a predictive SCC modelling was implemented to provide quantitative estimation of the effect of elastic and plastic deformation on the corrosion of steel pipeline using a small strain plasticity model and von-Mises yielding criterion. The corrosion kinetic parameters of the polarization behavior of the alloy were obtained from potentiodynamic polarization of steel in soil environment. The model was built on a Comsol Multiphysics finite element platform (COMSOL). The model assumed that active corrosion was taking place on only at the anode, while cathodic reaction of hydrogen evolution happens at the steel pipeline. Iron dissolution (anodic) and hydrogen evolution (cathodic) are considered as electrochemical reactions, using kinetic expressions that account for the effect of elasto-plastic deformations. # Chapter 2. Background ## 2.1. Fundamental Mechanisms of Metal Corrosion ### 2.1.1. Corrosion Reactions Corrosion is the process of metal or its properties deterioration due to the electrochemical reaction with the environment [1]. In 2002, the direct cost of metallic corrosion in the United States was estimates as 3.1% of the U.S. Gross Domestic Product (GDP) [2]. In 2013 the global cost of corrosion was estimated to be 3.4% of global GDP [3]. For corrosion to occur, the following components must be present: conductive solution (electrolyte), electronic path, anodic reaction, and cathodic reaction. The electrochemical deterioration of a metal (M) could be represented by the following anodic (oxidation) reaction: $M \rightarrow M^{n+} + ne$ (2.1) The general cathodic reaction is shown in equation 2.2 and depending on the electrolyte it could take one of the common forms in equations 2.3-2.6 [4]: $aOx + ne \rightarrow b Red $ (2.2) $O_2 + 2H_2O + 4e^- \rightarrow 4OH$ (oxygen reduction in neutral and basic media) (2.3) $O_2 + 4H^+ + 4e^- \rightarrow 2H_2O$ (oxygen reduction in acidic media) (2.4) $2H^+ + 2e^- \rightarrow H_2$ (hydrogen reduction) (2.5) $2H_2O + 2e^- \rightarrow H_2 + 2OH$ (water reduction) (2.6) The rate of the anodic (oxidation) reaction in equation 2.1 is equal to the rate of cathodic (reduction) reaction in equations 2.2-2.5. Here the metal is oxidized to metal ion while electrons are released and consumed during the reduction reaction. The number of electrons equals the valence of the metal ions released to the electrolyte [4]. When a metal is placed in a solution and establish a dynamic equilibrium with its own ion as represented by Equation 2.1, the electrical potential difference between the metal and the solution is given by Nernst equation [1, 4, 5]: $(E)_{M^{n+} /M} = (E^o)_{ M^{n+} /M} - \frac{RT}{nF}ln[M^{n+}] $ (2.7) The equilibrium reaction for the reduction reaction in equation 2.2 is: $(E)_{ ox/Red} = (E^o)_{ ox/Red} - \frac{RT}{nF}ln\frac{[ox]}{[Red]^b} $ (2.8) The activity of the metal, oxidizing and reducing species are represented by [M], [ox], and [Red], respectively. Eº is the standard potential of the half cell. When the standard potentials of different metals are listed in ascending or descending order, taking the standard potential of hydrogen as the zero point, the electrochemical series of metals is formed (Table 2.1) [5]. A metal will corrode (when placed in an environment) if its standard potential is more negative compared to the standard potential of the coupled cathodic reaction. The oxidation reaction can be illustrated using Pourbaix diagram where the electrochemical potential is plotted vs. the pH of water at 25° C [5]. From this diagram it is possible to identify the electrochemical state of a metal (immunity, actively corroding, or passivity) at a specific pH and potential. The Pourbaix diagrams of iron [6] is shown in Figure 2.2. The dashed line (a) and (b) in Pourbaix diagram represent the oxygen and hydrogen reduction reactions as shown in Equation 2.4 and 2.5. Between these two lines water is stable. | Type | Reduction reaction | Eº(VsHE) | | :----- | :----------------------- | :------- | | Noble | Au3+ + 3e- = Au | +1.498 | | | O2+4H++ 4e = 2H2O | +1.229 | | | Pt2+ +2e=Pt | +1.200 | | | Pd2+ + 2e =Pd | +0.987 | | | Ag2+ + 2e =Ag | +0.799 | | | Fe3+ +e = Fe2+ | +0.770 | | | Cu 2+ +2e-Cu | +0.337 | | | 2H++2e=H2 | 0.000 | | | Fe3+ + 3e Fe | - 0.036 | | | Pb2+ +2e=Pb | - 0.126 | | | Ni2+ +2e-Ni | - 0.250 | | | Co2+ +2e-Co | -0.277 | | | Cd2+ +2e-Cd | - 0.403 | | | Fe2+ + 2e =Fe | - 0.440 | | | Cr3+ + 3e Cr | -0.744 | | | Zn2+ +2e-=Zn | - 0.763 | | | Ti2+ +2eTi | - 1.630 | | | Al3+ + 3e =Al | - 1.662 | | | Mg2+ + 2e- =Mg | - 2.363 | | | Na+ + e Na | - 2.714 | | | K++e =K | - 2.925 | | Active | Li+ + e =Li | - 3.045 | ### 2.1.2. Classification of Electrochemical Corrosion Generally corrosion is classified into two main categories: uniform and localized [5], based on the amount of metal loss before components failure. Metal loss during uniform corrosion is typically much higher than that of the localized corrosion [1]. Uniform corrosion occurs when the entire surface of an exposed metal or alloy corrodes, while localized corrosion is the case when specific parts of the exposed surface corrode. Uniform corrosion can be further classified into atmospheric, galvanic, high temperature, liquid-metal, molten salt, biological, and stray-current corrosion. On the other hand, localized corrosion includes selective dissolution, stress corrosion cracking, impingement attack, pitting, crevice, fretting, and intergranular corrosion [1]. ### 2.1.3. Polarization Behavior of Electrochemical Systems In the absence of any external current, the rate of the anodic (oxidation) reaction in equation 2.1 is equal to the rate of cathodic (reduction) reaction in equations 2.2-2.5. If the two half cells are coupled, a current (Icorr = Ianodic=|Icathodic|) will flow from the anode to the cathode through the electrolyte. Icorr could be used to estimate the corrosion rate using Faraday's law as: $I_{corr} = \frac{nFW}{MT} $ (2.9) where n is the valence of the metal, F is Faraday's constant (96,500 C.mol¯¹), W is the atomic mass of the metal, M is the mass of corroded metal (g), and T is the time for which the current was applied (s). The potential difference between the corroded metal and a reference electrode is known as the open circuit potential (Eoc) or corrosion potential (Ecorr). When an external current is applied, the potential will deviate from the Eoc to another potential E, the deviation is expressed as the overpotential n=E-Eoc. Figure 2.2 shows a schematic representation of polarization diagram. For clarity, both Evans and Stern diagrams are combined and the current is expressed per unit area of the electrode in log scale (log i). In Evans diagram (condition of unpolarized metal), Eo,c and Eo,a are the Eoc of the reduced and oxidized species, respectively. io,c and io,a are the exchange current densities of the reduced and oxidized species, respectively. The intersection of the anodic and cathodic reactions represents the Eoc/Ecorr and icorr. Similarly, Eoc/Ecorr and icorr are estimated as the intersection of the extrapolation of the linear parts (Tafel line) of Stern diagram (condition of polarized metal). Under steady state and simple activation polarization conditions where the effect of mass transport is neglected, the total net current density (i=ia-ic) could be expressed as Butler-Volmer equation [5, 7]: $i = i_{corr}\frac{(1-α)ηξη]}{RT} $ (2.10) where n is the overpotential, a is the half reaction transfer coefficient, ẞa and ẞe can be calculated from Tafel slopes as, ẞa = 2.3RT/anF and ẞc = -2.3RT/(1-a)nF [1]. ## 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 flow through the pores of the soil. The porosity of soil controls the flow of fluids through the soil. For example, oxygen permeability in coarse grained soil is larger than that of fine grained sand. In other hand, water saturation is higher in fine grained sand that that of coarse grained sand. In both cases the corrosion of buried structures can be highly altered [8]. In soils, the corrosion process occurs in the ground water which is in contact with the buried structure. The pH and composition of the ground water depends on both, the soil and the climate. For example, the concentration of chloride is high in desert region making the soil very corrosive. Whereas, soils in tropical climates tend to be very acidic [9]. Therefore, the corrosion resistance of steel pipelines may vary at different locations as a result of these differences in soil composition, pH and moisture content [10]. ## 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) and a tensile stress. The mechanism of SCC of steel involves the combined action of tensile stress and a corrosive environment. This phenomenon can occur in various environments such as seawater, acidic solutions, or caustic environments, and leads to the growth of cracks in steel that may result in catastrophic failure. The mechanisms commonly proposed for SCC in steel include [11]: 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