Theoretical model for evaluating macrocell corrosion of pipeline steel under elastic-plastic deformation

Abstract

The application discloses a kind of theoretical model for evaluating pipeline steel macro-battery corrosion under elastic-plastic deformation, the theoretical model includes following physical field and coupling mechanism thereof: physical field: (1) stress-strain field: the elastic-plastic mechanics constitutive model based on von Mises yield criterion and isotropic hardening criterion is used to characterize the mechanical behavior of pipeline steel;(2) electrochemical field: the electrode kinetics model based on Tafel equation is used to characterize the corrosion reaction kinetics process on the surface of pipeline steel;Coupling mechanism: the mechanical-electrochemical double-path coupling mechanism for characterizing macro-battery corrosion.The model realizes the quantitative description of the macro-battery corrosion behavior induced by elastic-plastic deformation by coupling the mechanical deformation and electrochemical corrosion process of the material.The application realizes the two-way interaction of stress-strain field and electrochemical field under the background of macro-battery corrosion, and can quantitatively reveal the regulation of deformation on the intensity and evolution process of macro-battery corrosion.

Description

Technical Field

[0001] This invention belongs to the field of civil engineering materials and relates to a theoretical model for evaluating the corrosion of pipeline steel macrocells under elastoplastic deformation. Background Technology

[0002] As a key substrate for oil and gas pipelines, the service safety of pipeline steel is crucial for ensuring the robustness and reliability of the energy supply chain and mitigating environmental risks. Under the influence of factors such as internal pressure, complex geological conditions, and third-party construction, oil and gas pipelines inevitably experience mechanical loads, leading to localized elasto-plastic deformation. This deformation not only causes degradation of the material's mechanical properties but also significantly alters its electrochemical corrosion characteristics, particularly dramatically increasing the risk of macrocell corrosion. Macrocell corrosion, due to the separation of anode and cathode regions, large corrosion driving forces, and wide affected area, often leads to rapid thinning or even perforation in localized areas, making it a significant contributing factor to sudden pipeline failures.

[0003] Currently, the industry typically relies on classical electrochemical theoretical models to predict the corrosion behavior of pipeline steel, such as corrosion rate calculations based on Tafel extrapolation and electrode process kinetics described by the Butler-Volmer equation. These traditional models are mostly based on the assumption that the material is in a stress / strain-free state. They primarily focus on uniform corrosion or micro-galvanic corrosion. While existing models can describe macro-cell corrosion caused by macro-geometric differences and environmental variations (such as oxygen concentration differences) to some extent, their fundamental limitation lies in their failure to fully consider the fundamental changes in the electrochemical properties of materials caused by elastoplastic deformation and its decisive role in the formation of macro-cells. Specifically, traditional models treat the anodic dissolution and cathodic reduction kinetic parameters (such as exchange current density and Tafel slope) as constants or only related to the environment. However, numerous studies have shown that deformation introduces high-density dislocations, vacancies, and other crystal defects into the steel, significantly increasing the surface energy and electrochemical activity of the material, leading to a negative shift in its self-corrosion potential and a decrease in polarization resistance. More importantly, this activation effect generates a significant electrode potential difference between the deformed and undeformed regions, providing a powerful driving force for the formation of macrocell corrosion, a point that is generally ignored in existing models.

[0004] In summary, developing a theoretical model that can quantitatively describe the macrocell corrosion of pipeline steel under elastoplastic deformation fills the gap in traditional corrosion theory regarding the mechanical-electrochemical coupled macrocell corrosion effect. This is of vital importance for accurately predicting corrosion damage in in-service pipelines, guiding pipeline design and safe maintenance, and preventing catastrophic accidents. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, this invention provides a theoretical model for evaluating macrocell corrosion of pipeline steel under elastoplastic deformation. This model achieves a quantitative description of macrocell corrosion behavior induced by elastoplastic deformation by systematically coupling the mechanical deformation and electrochemical corrosion processes of the material. This invention realizes the bidirectional interaction between the stress-strain field and the electrochemical field under the macrocell corrosion background, enabling quantitative revelation of the regulatory mechanisms by which deformation affects the intensity and evolution of macrocell corrosion.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] A theoretical model for evaluating macrocell corrosion of pipeline steel under elastoplastic deformation includes the following physical fields and their coupling mechanisms:

[0008] Physical field:

[0009] (1) Stress-strain field: An elastoplastic mechanical constitutive model based on the von Mises yield criterion and the isotropic hardening criterion is used to characterize the mechanical behavior of pipeline steel;

[0010] (2) Electrochemical field: An electrode kinetic model based on the Tafel equation was used to characterize the corrosion reaction kinetics of the pipeline steel surface;

[0011] Coupling mechanism:

[0012] A mechanical-electrochemical dual-path coupling mechanism specifically for characterizing macrocell corrosion was constructed, comprising the following steps:

[0013] First, an elastoplastic mechanical constitutive model based on the von Mises yield criterion and the isotropic hardening criterion is established to calculate the stress and strain field distributions of pipeline steel.

[0014] Then, the calculated equivalent plastic strain is coupled to the equilibrium potential of the anodic reaction to quantify the thermodynamic effect of plastic deformation on anodic dissolution. At the same time, the equivalent stress is coupled to the exchange current density of the cathode reaction to quantify the effect of stress state on cathode reaction kinetics, thereby accurately simulating the key characteristics of anode-cathode separation in the macro battery system.

[0015] Finally, the coupled model and the charge conservation equation are combined to form a set of nonlinear multiphysics coupled equations, and the Newton-Raphson iterative method combined with the MUMPS parallel solver is used for numerical solution to finally obtain the corrosion potential and current density distribution of the macrocell under elastoplastic deformation conditions.

[0016] A method for constructing the theoretical model for evaluating macrocell corrosion of pipeline steel under elastoplastic deformation includes the following steps:

[0017] Step 1: Establish a mechanical constitutive model that considers elastoplastic deformation:

[0018] Based on the von Mises yield criterion, the yield state of the material is determined, and the evolution of the yield surface after plastic deformation is described by the isotropic hardening criterion. Thus, a small strain plastic model that can reflect the elastic-plastic stress-strain response of pipeline steel under load is constructed. The stress field and strain field distribution of pipeline steel under macroscopic battery corrosion environment are calculated by this model.

[0019] Step 2: Construct a cross-scale coupling model of stress / strain and electrochemical reaction:

[0020] The stress field and strain field calculated in step one are used as coupling variables and introduced into the anodic and cathodic electrochemical kinetic equations of macrocell corrosion, respectively, to achieve bidirectional coupling between the mechanical field and the corrosion electrochemical field. The specific coupling relationships include: (1) Strain-anodic potential coupling: The equivalent elastic-plastic strain is used as a state variable and coupled to the equilibrium potential function of the anodic reaction to quantify the thermodynamic influence of plastic deformation on the metal dissolution trend; (2) Stress-cathode kinetic coupling: The equivalent stress is used as a state variable and coupled to the exchange current density function of the cathode reaction to quantify the influence of stress state on the kinetics of cathode hydrogen evolution or oxygen reduction reaction.

[0021] Step 3: Construct the governing equations for solving the macrocell corrosion system based on a fully coupled framework:

[0022] The coupled model established in step two is combined with the charge conservation control equation to form a highly nonlinear multiphysics coupled equation set. The Newton-Raphson iteration method is used to linearize this nonlinear problem. In each Newton iteration, the corresponding linear equation set is constructed and solved. This process is implemented by the MUMPS massively parallel direct solver to ensure the numerical stability and solution efficiency of the calculation process. Finally, the corrosion potential and current density distribution of the macrocell under elastoplastic deformation conditions are obtained.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] 1. This invention represents a theoretical leap from "pure corrosion" to "mechanical-electrochemical coupling," revealing the deep-seated mechanism of macrocell corrosion under material deformation. Compared to traditional corrosion models that only consider the chemical properties of materials, this invention uses a rigorous theoretical framework to quantitatively couple the key mechanical state of elastoplastic deformation with the macrocell corrosion process. This is not merely a model upgrade, but a paradigm shift. It reveals how stress / strain actively "regulates" the corrosion rate and distribution by altering anodic dissolution thermodynamics and cathodic reaction kinetics, providing a completely new theoretical perspective for understanding catastrophic phenomena such as stress corrosion cracking.

[0025] 2. This invention breaks through the predictive bottleneck of traditional models, achieving accurate assessment across all operating conditions. Existing models are mostly limited to elastic deformation or non-destructive states, and are powerless to predict the corrosion behavior of pipeline steel in the unavoidable plastic deformation zone during laying and operation. This invention, by coupling the von Mises plasticity criterion with electrochemical equations, enables the theoretical model to accurately predict the macro-cell corrosion behavior of pipeline steel under elastic-plastic deformation, filling a technological gap in this field and extending corrosion assessment from "ideal state" to "real operating conditions."

[0026] 3. It possesses significant engineering application value, providing a core tool for pipeline safety and integrity management. The precise corrosion current and potential distribution output by the theoretical model of this invention can directly serve the safety design, risk assessment, remaining life prediction, and maintenance decision-making of oil and gas transmission pipelines. It enables the prediction of corrosion risks in high-strain areas (such as bends and recesses) during the design phase, thereby achieving proactive protection. This has immeasurable value in ensuring the safety of the nation's energy artery, preventing catastrophic accidents, and reducing operation and maintenance costs. Attached Figure Description

[0027] Figure 1 A 3D schematic diagram of the geometric model of pipeline steel in an electrolyte solution;

[0028] Figure 2 A 2D schematic diagram of the geometric model of pipeline steel in an electrolyte solution;

[0029] Figure 3 This is a schematic diagram of the mesh generation for the geometric model;

[0030] Figure 4 The uniaxial tensile stress-strain curve of pipeline steel;

[0031] Figure 5 The stress (MPa) and potential distribution (mV vs. SCE) of pipeline steel in the elastic stage;

[0032] Figure 6 The stress (MPa) and potential distribution (mV vs. SCE) of pipeline steel in the plastic stage;

[0033] Figure 7 Corrosion current density distribution (μA / cm) at the interface between pipeline steel and electrolyte solution in the elastic stage 2 );

[0034] Figure 8 Corrosion current density distribution (μA / cm) at the interface between pipeline steel and electrolyte solution during the plastic stage. 2 );

[0035] Figure 9 The total current density distribution (μA / cm) at the interface between pipeline steel and electrolyte solution during the plastic stage.2 ). Detailed Implementation

[0036] The technical solution of the present invention will be further described below with reference to the accompanying drawings, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention that do not depart from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention.

[0037] This invention provides a theoretical model for evaluating macrocell corrosion of pipeline steel under elastoplastic deformation. The model quantitatively describes the macrocell corrosion behavior of pipeline steel through stress-strain field, electrochemical field, and their coupling relationship, specifically including the following physical fields and their coupling mechanisms:

[0038] Physical field:

[0039] (1) Stress-strain field: An elastoplastic mechanical constitutive model based on the von Mises yield criterion and the isotropic hardening criterion is used to characterize the mechanical behavior of pipeline steel;

[0040] (2) Electrochemical field: An electrode kinetic model based on the Tafel equation was used to characterize the corrosion reaction kinetics of the pipeline steel surface;

[0041] Coupling mechanism:

[0042] The core of this invention lies in constructing a mechanical-electrochemical dual-path coupling mechanism specifically for characterizing macrocell corrosion: First, an elastoplastic constitutive model based on the von Mises yield criterion and the isotropic hardening criterion is established to calculate the stress and strain field distributions of pipeline steel; second, the calculated equivalent plastic strain is coupled to the equilibrium potential of the anodic reaction to quantify the thermodynamic influence of plastic deformation on anodic dissolution, and the equivalent stress is coupled to the exchange current density of the cathodic reaction to quantify the influence of stress state on cathodic reaction kinetics, thereby accurately simulating the key characteristics of anode-cathode separation in the macrocell system; finally, this coupling model is combined with the charge conservation equation to form a nonlinear multiphysics coupling equation set, and the Newton-Raphson iteration method combined with the MUMPS parallel solver is used for numerical solution, ultimately obtaining the corrosion potential and current density distribution of the macrocell under elastoplastic deformation conditions. Unlike traditional uniform corrosion models, this mechanism accurately simulates the key characteristics of the separation and distinct properties of the anode and cathode in a macrocell system by coupling the elastoplastic deformation to the anodic reaction equilibrium potential and the corresponding stress value to the cathodic reaction exchange current density. Thus, the model realizes the bidirectional interaction between the stress-strain field and the electrochemical field under the macrocell corrosion background, and can quantitatively reveal the regulatory law of elastoplastic deformation on the corrosion intensity and evolution process of the macrocell.

[0043] Electrochemical corrosion occurring at the interface between pipeline steel and electrolyte can usually be described using Tafel's empirical formula:

[0044] (1)

[0045] (2)

[0046] Among them, i a and i c These are the Faraday current densities of the anodic and cathodic reactions, i. 0,a and i 0,c This corresponds to the exchange current density, b a and b c The Tafel slopes of the anodic and cathodic reactions are η and η, respectively. a and η c This corresponds to the overpotential, and has the following expression:

[0047] (3)

[0048] in, Electrode potential, The equilibrium potential represents the reaction. For near-neutral soil electrolytes, the two electrode reactions occurring at the pipeline-steel interface are as follows:

[0049] Anode reaction: Fe → Fe 2+ +2e - ;

[0050] Cathode reaction: H + +e - →H;

[0051] For the equilibrium potential of the two reactions mentioned above It can be calculated using the following formula:

[0052] (4)

[0053] (5)

[0054] in, and These represent the equilibrium potentials of the anodic and cathode reactions, respectively. and These represent the standard equilibrium potentials for the anodic and cathodic reactions, respectively, with values ​​of 0.409 V vs. SHE and 0 V vs. SHE.

[0055] To account for the effect of load on electrochemical corrosion, an elastic-plastic deformation correction term is introduced into Tafel's empirical formula for anodic reactions:

[0056] Elastic deformation:

[0057] (6)

[0058] Plastic deformation:

[0059] (7)

[0060] in, and The change in the equilibrium potential of the anodic reaction due to elastoplastic deformation; ΔP is equal to 1 / 3 of the uniaxial tensile stress borne by the pipeline steel. When the tensile stress exceeds the yield stress, ΔP remains constant at 1 / 3 of the yield stress; V m This represents the molar volume of the pipeline steel, with a value of 7.13 × 10⁻⁶. -6 m 3 / mol; z is the charge transfer number, with a value of 2; R represents the ideal gas constant, with a value of 8.314 J / (mol K); T is the absolute temperature, with a value of 298.15 K; F is the Faraday constant, with a value of 96485 C / mol; ν and α are constants, with values ​​of 0.45 and 1.67 × 10⁻⁶ respectively. 11 cm -2 N0 is the initial density of dislocations before plastic deformation, with a value of 1 × 10⁻⁶. 8 cm -2 , ε p This is plastic strain.

[0061] For the cathode reaction:

[0062] (8)

[0063] in, It is von Mises stress.

[0064] The mechanical behavior of pipeline steel under axial tension is described using isotropic hardening functions:

[0065] (9)

[0066] (10)

[0067] Where, σ yhard σ is the hardening function. exp Let ε be the experimental stress function. eff For effective stress, σ y For yield stress, σ e Let E be the effective stress during elastic loading, and E be the Young's modulus.

[0068] Example:

[0069] Figure 1To provide a geometric model for numerical calculations of the theoretical model, the gray area represents the pipeline steel under study, with clamping areas at both ends. The lower end is fixedly clamped, while the upper end is subjected to a load. To present better experimental and simulation results, a diameter reduction was performed in the middle region of the pipeline steel, changing the diameter from 18 mm to 8 mm. The blue area represents the area containing 0.01 mol / L Cl... - It is a near-neutral electrolyte solution with a pH of approximately 6.8.

[0070] To improve the computational efficiency of the model and better present the numerical simulation results, Figure 1 The three-dimensional geometric model shown is simplified according to symmetry as follows: Figure 2 The two-dimensional geometric model shown. Figure 3 To Figure 2 The diagram showing the mesh generation of the geometric model employs a free triangular mesh. To better analyze the interface between the reinforcing steel and the electrolyte solution, the mesh was refined in the tensile region of the central steel reinforcement and at the interface. The complete mesh contains 1114 domain elements and 243 boundary elements.

[0071] In the theoretical model, the corrosion-related boundary conditions were obtained by fitting measured Tafel curves. The Tafel slopes for the anodic and cathodic reactions were 0.118 V / decade and -0.207 V / decade, respectively. The corresponding equilibrium potentials could be calculated using equations (4) and (5), which were -0.859 V vs. SCE and -0.644 V vs. SCE, respectively. It is worth noting that due to the presence of Fe in the electrolyte solution... 2+ The ion concentration is extremely low; here we approximate it to be 10. -6 mol / L, and further linear extrapolation from the Tafel curve yields exchange current densities of 1.457 × 10⁻⁶ mol / L. -6 A / cm 2 and 2.353×10 -7 A / cm 2 Furthermore, the electrolyte conductivity is 0.096 S / m. On the one hand, steel bars are good conductors of electricity, with a conductivity of 106 S / m; on the other hand, the geometric model is relatively small, so the pipeline steel is treated as an equipotential body in the numerical calculation process.

[0072] The boundary conditions for the mechanical properties of pipeline steel in the theoretical model are obtained by conducting uniaxial tensile tests on the pipeline steel, such as... Figure 4 As shown, pipeline steel exhibits several stages during tensile testing, including elasticity, plastic strengthening, softening, and eventual fracture. Its yield strength σ... ys The Pa value is 806 MPa, Young's modulus E is 207 GPa, and Poisson's ratio ν is 0.33. Figure 4The stress-strain curves shown are directly incorporated into the model using interpolation functions to characterize the stress-strain relationship of the pipeline steel during the calculation process. Furthermore, to compare the corrosion behavior in the elastic and plastic stages, external loads of 2 kN and 43.2 kN were applied to the upper end of the pipeline steel, respectively, keeping the tension section of the pipeline steel in the elastic and plastic stages, respectively.

[0073] Based on the above theoretical model, geometric model and boundary conditions, the physicochemical characteristics of pipeline steel under the simultaneous action of tensile stress and corrosion conditions can be solved. This invention uses COMSOL Multiphysics 6.2 finite element software to numerically implement the above model, using solid mechanics interface, deformation geometry interface and secondary current distribution interface respectively, and setting up the MUMPS solver. The finite element simulation carried out includes three aspects: (1) elastic-plastic stress-strain analysis of pipeline steel; (2) electrochemical parameter analysis of pipeline steel-solution interface; (3) simulation and analysis of the mechanical-chemical effect of pipeline steel, that is, the coupling effect of stress / strain and electrochemical corrosion behavior in solution.

[0074] Figure 5 The figure shows the stress distribution of pipeline steel and the potential distribution in the electrolyte during the elastic stage. The maximum stress in the tensile zone of the pipeline steel is only 39.5 MPa, indicating minimal deformation and the pipeline is in the elastic stage. At this point, the effect of deformation on corrosion is also very weak, with a corresponding open-circuit potential of -728 mV vs. SCE. Although the potential in the entire interface region shows a phenomenon of being larger at the ends and smaller in the middle, the difference between the two is even less than 1 mV, which can be ignored in corrosion considerations. Figure 6 The figure shows the stress and electrolyte potential distribution of pipeline steel during the plastic stage. As can be seen, the maximum stress in the tensile zone of the pipeline steel reaches 860 MPa, far exceeding the yield strength, indicating that the steel is in the plastic stage and has undergone significant deformation. Correspondingly, the electrolyte potential is significantly lower than in the elastic stage, reaching a minimum of -743 mV vs. SCE in the middle and still reaching -741 mV vs. SCE at the ends. Overall, the potential calculated in the plastic stage is 13-15 mV lower than that in the elastic stage.

[0075] Figure 7 This presents the anodic corrosion current density at the interface between pipeline steel and electrolyte during the elastic stage, and... Figure 5 The potential change patterns are similar, with the corrosion current density of pipeline steel being approximately 3.71 μA / cm². 2 Throughout the tensile zone, the pipeline steel exhibits a uniform corrosion trend. Figure 8The figure presents the anodic corrosion current density at the interface between pipeline steel and electrolyte during the plastic stage. As shown in the figure, unlike the elastic stage, the corrosion current density of pipeline steel in the tension zone increases significantly in the middle, reaching 6.4 μA / cm². 2 This represents an increase of approximately 1.73 times. However, in the end region, the corrosion current density of the pipeline steel decreased slightly to 2.86 μA / cm². 2 This is only 0.77 of the corrosion current density corresponding to elastic deformation. To explain this phenomenon, the total current density at the pipeline steel interface is given, such as... Figure 9 As shown in the figure, in the tensile section of the pipeline steel, the anodic reaction current density dominates in the middle region, while the cathodic reaction current density dominates in the end region. Furthermore, the streamlines of the electrolyte current density in the figure reveal that in the tensile section of the pipeline steel, the current density in the electrolyte tends to flow from the middle to the ends. This current flow indicates macrocellular corrosion on the pipeline steel surface, which exacerbates corrosion in the middle region while also inhibiting corrosion in the end region to some extent. It is worth noting that in practical engineering, this may accelerate the formation of local defects, making the pipeline steel more susceptible to stress concentration.

Claims

1. A theoretical model system for evaluating macrocell corrosion of pipeline steel under elastoplastic deformation, characterized in that... The theoretical model includes the following physical fields and their coupling mechanisms: Physical field: (1) Stress-strain field: An elastoplastic mechanical constitutive model based on the von Mises yield criterion and the isotropic hardening criterion is used to characterize the mechanical behavior of pipeline steel; (2) Electrochemical field: An electrode kinetic model based on the Tafel equation was used to characterize the corrosion reaction kinetics of the pipeline steel surface; Coupling mechanism: A mechanical-electrochemical dual-path coupling mechanism used to characterize macrocell corrosion; The construction of the coupling mechanism includes the following steps: First, an elastoplastic mechanical constitutive model based on the von Mises yield criterion and the isotropic hardening criterion is established to calculate the stress and strain field distributions of pipeline steel. Then, the calculated equivalent plastic strain is coupled to the equilibrium potential of the anodic reaction to quantify the thermodynamic effect of plastic deformation on anodic dissolution. At the same time, the equivalent stress is coupled to the exchange current density of the cathode reaction to quantify the effect of stress state on cathode reaction kinetics, thereby accurately simulating the key characteristics of anode-cathode separation in the macro battery system. Finally, the coupled model and the charge conservation equation are combined to form a set of nonlinear multiphysics coupled equations, and the Newton-Raphson iterative method combined with the MUMPS parallel solver is used for numerical solution to finally obtain the corrosion potential and current density distribution of the macrocell under elastoplastic deformation conditions.

2. A method for constructing a theoretical model for evaluating macrocell corrosion of pipeline steel under elastoplastic deformation, characterized in that... The method includes the following steps: Step 1: Establish a mechanical constitutive model that considers elastoplastic deformation: Based on the von Mises yield criterion, the yield state of the material is determined, and the evolution of the yield surface after plastic deformation is described by the isotropic hardening criterion. Thus, a small strain plastic model that can reflect the elastic-plastic stress-strain response of pipeline steel under load is constructed. The stress field and strain field distribution of pipeline steel under macroscopic battery corrosion environment are calculated by this model. Step 2: Construct a cross-scale coupling model of stress / strain and electrochemical reaction: The stress field and strain field calculated in step one are used as coupling variables and introduced into the anodic and cathodic electrochemical kinetic equations of macrocell corrosion, respectively, to achieve bidirectional coupling between the mechanical field and the corrosion electrochemical field. Step 3: Construct the governing equations for solving the macrocell corrosion system based on a fully coupled framework: The coupled model established in step two is combined with the charge conservation control equations to form a highly nonlinear multiphysics coupled equation system. The Newton-Raphson iterative method is used to linearize this nonlinear problem. In each Newton iteration, the corresponding linear equation system is constructed and solved. This process is implemented using the MUMPS massively parallel direct solver to ensure numerical stability and solution efficiency. Finally, the corrosion potential and current density distribution of the macrocell under elastoplastic deformation conditions are obtained. In step two, the specific coupling relationships include: (1) strain-anodic potential coupling: the equivalent elastic-plastic strain is used as a state variable and coupled to the equilibrium potential function of the anodic reaction to quantify the thermodynamic influence of plastic deformation on the metal dissolution trend; (2) stress-cathode kinetic coupling: the equivalent stress is used as a state variable and coupled to the exchange current density function of the cathode reaction to quantify the influence of stress state on the kinetics of cathode hydrogen evolution or oxygen reduction reaction.

Images