The electrode potential is one of the most important parameters in the thermodynamics and kinetics of corrosion. This article discusses the fundamentals of electrode potentials and illustrates the thermodynamics of chemical equilibria by using the hydrogen potential scale and the Nernst equation. It describes galvanic cell reactions and corrosion reactions in an aqueous solution in an electrochemical cell. The article explores the most common cathodic reactions encountered in metallic corrosion in aqueous systems. The reactions included are proton reduction, water reduction, reduction of dissolved oxygen, metal ion reduction, and metal deposition. The article also presents the standard equilibrium potentials measured at 25 deg C relative to a standard hydrogen electrode for various metal-ion electrodes in a tabular form.

A potential pH diagram is a graphical representation of the relations, derived from the Nernst equation, between the pH and the equilibrium potentials (E) of the most probable electrochemical reactions occurring in a solution containing a specific element. This article describes three types of reactions for calculation and construction of E-pH diagrams: electrochemical reactions of pure charge (electron) transfer; reactions involving both electron and solvated proton transfer; and acid-base reactions of pure solvated proton transfer. It illustrates the practical use of E-pH diagrams for temperature aqueous solutions and adsorbed species and in prediction of corrosion of nickel and copper.

This article provides a general introduction to the kinetics of aqueous corrosion with an emphasis on electrochemical principles. It describes the thermodynamic basis for corrosion by determining the equilibrium potentials of electrochemical reactions from the Nernst equation. A corrosion process can be controlled by the electronic conductivity of passive films when the cathodic reaction occurs on the surface of the film and by activation control of corrosion. Passivation becomes thermodynamically possible when the corrosion potential exceeds the potential corresponding to the equilibrium between a metal and one of its oxides/hydroxides. The article schematically illustrates a current-potential or polarization curve for an anodic process.

Controlled-potential coulometry is a highly precise and accurate method primarily used for major constituent analysis of analyte substances such as alloys, compounds, nonmetallic materials and organic compounds. This article illustrates the apparatus required for controlled-potential coulometry, and provides information on its techniques and applications. It contains a table that lists the metals for which accurate methods have been developed and the basic electrochemistry of the procedures. The article explains that gold and uranium are the elements that are determined frequently in various sample types.

This article discusses the properties of aluminum surface and the applications of aluminum alloys. It explains the effects of trace elements on aluminum alloys. The article considers microstructural development of aluminum in terms of the surface and explains how it will impact corrosion resistance and surface treatment. It describes the thermodynamics of equilibrium oxidation processes and non-equilibrium corrosion processes. The article provides a discussion on aluminum oxidation under atmospheric and dynamic conditions. It presents the potential/pH (Pourbaix) diagram for aluminum under atmospheric and dynamic conditions. The article also explains the polarization effects during the formation of stable aluminum oxide under dynamic conditions. It concludes with information on the designation system for aluminum finishes.

Electrode potential is a key parameter in the thermodynamic and kinetic processes that drive aqueous corrosion. This article discusses the complexities associated with measuring electrode potential and explains where and how to use reference electrodes to improve measurement accuracy. It describes a three-electrode approach that compensates for measurement error stemming from nonequilibrium conditions. It also examines electrode materials and behaviors and offers insights on selection and operating conditions.

This article describes various methods of electrochemical analysis, namely coulometry, electrogravimetry, voltammetry, electrometric titration, and nanometer electrochemistry. The discussion covers the general uses, sample requirements, application examples, advantages, and limitations of these methods. Some of the factors pertinent to electrochemical cells are also provided. In addition, the article provides information on various potentiometric membrane electrodes used to quantify numerous ionic and nonionic species.

This article focuses on the internal corrosion of iron and copper in potable water. It tabulates the corrosion and water-quality problems caused by materials in contact with drinking water. The article provides a theoretical description of the reduction-oxidation reactions in water to analyze the causes of corrosion of metals in contact with water. It discusses the Langelier saturation index and the Larson index for determining corrosion in potable water systems. The article describes the two major ways of mitigation against corrosion in potable water systems. The first is to line the pipe surface physically such that water and dissolved oxygen cannot reach the metal surface and the second is to add chemical inhibitors to alter water chemistry.

Principles of metallic corrosion play a fundamental role in developing industrial processes that employ corrosion for constructive purposes. This article examines the changes in kinetics that occur with differentially small potential changes around the equilibrium electrode potentials of two reversible electrodes, such as copper and silver electrodes, in an electrochemical system. It provides a schematic illustration of a reversible cell with copper and silver electrodes to determine a reversible cell potential between the electrodes. An electrode becomes irreversible when the electrode reactions are displaced from equilibrium and the electrode potential is no longer at the equilibrium potential. The article describes irreversible cell potential by using galvanic cells, electrolytic cells, and corrosion cells.

This article describes the Schottky defect and the Frenkel defect in oxides. It provides information on the p-type metal-deficit oxides and n-type semiconductor oxides. The article discusses diffusion mechanisms and laws of diffusion proposed by Fick. It explains the oxide texture of amorphous and epitaxy oxide layers and presents equations for various oxidation reaction rates. The article reviews different theories to describe the oxidation mechanism. These include the Cabrera-Mott, Hauffe-IIschner, Grimley-Trapnell, Uhlig, and Wagner theories.

Electrogravimetry is the oldest electroanalytical technique in which the element of interest is deposited electrolytically onto an electrode and weighed. This article discusses the principles involved in determining the electrolysis rate of the solution, and describes different methods for the separation of ion in the electrolyte and their corresponding instrumentation. Furthermore, it explores the various types of analysis, such as the separation and quantitative determination of metal ions and internal electrolysis, and provides a detailed account of the applications of electrogravimetry with examples.

Corrosion of metallic materials is governed by electrochemical kinetics, so that the general concepts developed for studying electrochemical reaction mechanisms may be applied to corrosion. This article presents the fundamental aspects of electrode kinetics. The processes of charge transfer taking place at the electrode interface within the double layer and of mass transport at the vicinity of the electrode surface are discussed. The article describes the corrosion processes, which involve anodic and cathodic reactions at specific electrode sites. Some experimental methods for devising a reliable reaction model are detailed. The article explains some reaction mechanisms for cathodic and anodic processes to illustrate the great variety of reaction mechanisms occurring at the electrode interface.

This article is a comprehensive collection of terms related to corrosion fundamentals, testing, and protection.

This article describes the Bayer process for the purification of alumina. The process includes four major stages: digestion, clarification, precipitation, and calcination. The article discusses the aluminum electrolytic process in terms of aluminum electrolysis cell design, magnetohydrodynamic forces, and cathode lining. It reviews the electrochemical reactions and thermodynamics for aluminum electrolysis standard Gibbs. The article also describes the cell operations and cell stability, as well as the key indicators of cell performance. It schematically illustrates the typical costs producing aluminum in an aluminum smelter. The article also discusses various environmental issues, such as fluoride recovery; perfluorocarbons, polycyclic aromatic hydrocarbons, and sulfur emissions; spent pot lining; and development of inert anodes and CO2 emissions.

Thermodynamic descriptions have become available for a large number of alloy systems and allow the calculation of the phase diagrams of multicomponent alloys. This article begins with a discussion on three laws of thermodynamics: the Law of Conservation of Energy, the Second Law of Thermodynamics, and the Third Law of Thermodynamics. It informs that for transformations that occur at a constant temperature and pressure, the relative stability of the system is determined by its Gibbs free energy. The article describes the Gibbs free energy of a single-component unary system and the Gibbs free energy of a binary solution. It schematically illustrates the structure of a binary solid solution with interatomic bonds and shows how the equilibrium state of an alloy can be obtained from the free-energy curves at a given temperature. The article concludes with information on the construction of eutectic and binary phase diagrams from Gibbs free-energy curves.

Zinc is one of the most used metals, ranking fourth in worldwide production and consumption behind iron, aluminum, and copper. This article commences with an overview of the applications of zinc that can be divided into six categories: coatings, casting alloys, alloying element in brass and other alloys, wrought zinc alloys, zinc oxide, and zinc chemicals. It discusses the corrosion and electrochemical behavior of zinc and its alloys in various environments, particularly in atmospheres in which they are most widely used. The article tabulates the corrosion rates of zinc and zinc coatings immersed in various types of waters, in different solutions in the neutral pH range, and in soils at different geographic locations in the United States. It concludes with information on the forms of corrosion encountered in zinc coatings, including galvanic corrosion, pitting corrosion, and intergranular corrosion.

Alloy phase diagrams are useful for the development, fabrication, design and control of heat treatment procedures that will produce the required mechanical, physical, and chemical properties of new alloys. They are also useful in solving problems that arise in their performance in commercial applications, thus improving product predictability. This article describes different equilibrium phase diagrams (unary, binary, and ternary) and microstructures, description terms, and general principles of reading alloy phase diagrams. Further, the article discusses plotting schemes; areas in a phase diagram; and the position and shapes of the points, lines, surfaces, and intersections, which are controlled by thermodynamic principles and properties of all phases that comprise the system. It also illustrates the application of the stated principles with suitable phase diagrams.

Potentiometric membrane electrodes are electrochemical devices that can be used to quantify numerous ionic and nonionic species. This class of electrochemical sensors can be divided into ion-selective and gas-sensing membrane electrodes. The first half of this article mainly focuses on the subclasses, the membrane potential, electrode selectivity limitations and the methods of analysis of the ion-selective membrane electrodes. These methods of analysis include the use of calibration curves, addition techniques, subtraction techniques, and titration. The second half outlines gas sensing membrane electrodes, and discusses important elements that must be considered in addition to the potentiometric membrane electrode to ensure proper electrode response. These elements are reference electrodes, temperature controls, recording of the potential with respect to time, electrode storages, and sample pretreatment. The article also explains the applications of the potentiometric membrane electrodes with the aid of an example.