Electrochemical corrosion thermodynamics

The potential for electrochemical corrosion in a boiler results from an inherent thermodynamic instability, with the most common corrosion processes occurring at the boiler metal surface and the metal-BW interface (Helmholtz double layer). These processes may be controlled relatively easily in smaller and simpler design boilers (such as dual-temperature, LPHW heating, and LP steam boiler systems) by the use of various anodic inhibitors. 

Tower, Stephen. All About Electrochemistry. Available online. URL http //www.cheml.com/acad/webtext/elchem/. Accessed May 28, 2009. Part of a virtual chemistry textbook, this excellent resource explains the basics of electrochemistry, which is important in understanding how fuel cells work. Discussions include galvanic cells and electrodes, cell potentials and thermodynamics, the Nernst equation and its applications, batteries and fuel cells, electrochemical corrosion, and electrolytic cells and electrolysis. 

Marcel Pourbaix Thermodynamics of Dilute Aqueous Solutions Atlas of Electrochemical Equilibria in Aqueous solutions Atlas of Chemical and Electrochemical Equilibria in the presence of Gaseous Phase Lectures on Electrochemical Corrosion ... 

The thermodynamics of corrosion processes provides a tool to determine the theoretical tendency of metals to corrode. Thus, the role of corrosion thermodynamics is to determine the conditions under which the corrosion occurs and how to prevent corrosion at the metal/environment interface. Thermodynamics, however, cannot be used to predict the rate at which the corrosion reaction will proceed [1—6]. The corrosion rate must be estimated by Faraday s law and is controlled by the kinetics of the electrochemical reaction. 

Naqvi, L Saleemi, A. R. Naveed, S. (2011). Cefixime A drug as Efficient Corrosion Inhibitor for Mild Steelin Acidic Media. Electrochemical and Thermodynamic Studies. International Journal of Electrochemical Science, Vol. 6, No.l, January 2011), pp. 146 -161, ISSN 1452-3981... 

The thermodynamics of pure Mg is a foundation for understanding the electrochemical corrosion of Mg and its alloys. The stability of Mg in various environments can also provide clues for estimating the corrosion performance of Mg alloys in typical environments which is helpful towards understandiug the thermodynamic behavior of Mg alloys. This section briefly touches upou the thermodynamics of pure Mg. 

Electrochemical techniques measure electrochemical potentials and currents that are fundamentally related to the thermodynamics and kinetics of corrosion reactions. They are often used to measure the rates of uniform corrosion, to determine the tendency of localized corrosion, to study a wide range of corrosion-related phenomena such as passivation, galvanic corrosion and sensitization effects. Electrochemical corrosion testing and monitoring can be performed in a diverse range of environments in the laboratory or in the field, in a pipeline or in an autoclave. For instance, they have been successfully employed to monitor corrosion in multiphase oil/water conditions with as little as 1-2% water " ... 

The problem of corrosion is immense. One can say with certainty that all metals, except the noble ones (gold, platinum etc.), corrode to a certain extent. Some of them form oxide layers on the surface which protects them against further attack, but some, notably iron, do not form such a layer. Iron corrosion alone costs every year millions of pounds in protection and replacement. There are two main mechanisms by which corrosion occurs. One is by direct oxidation of the metal by air oxygen. Metal oxides are, as a rule, thermodynamically more stable than the metal and oxygen in their elementary states. This is the mechanism by which metals become covered with oxide even in very dry atmosphere. The second mechanism is an electrochemical one two electrode reactions take place on the surface of the corroding metal their products are the undesirable corrosion products. It is easier to understand electrochemical corrosion by considering the corrosion of copper plated iron as an illustration if the plating is broken, the iron, which is in contact with the copper, is exposed to the atmosphere, to rain and often to traffic fumes. The atmosphere contains carbon dioxide and some-... 

The main objective of this chapter is to apply the previously studied electrochemical thermodynamics and kinetics for analyzing an important electrochemical phenomenon—corrosion. Pourbaix diagrams, polarization curve, and the rate of the electrochemical corrosion are considered. Main types of corrosion protection and possible kinds of corrosion are mentioned. 

In the field of electrochemistry that deals with electrochemical corrosion among other matters, electrical work constitutes an essential computational quantity. The description of this thermodynamic quantity is formulated in chapter 6 Electrochemistry. In the following, only the final expression for electrical work is given. 

The industrial economy depends heavily on electrochemical processes. Electrochemical systems have inherent advantages such as ambient temperature operation, easily controlled reaction rates, and minimal environmental impact (qv). Electrosynthesis is used in a number of commercial processes. Batteries and fuel cells, used for the interconversion and storage of energy, are not limited by the Carnot efficiency of thermal devices. Corrosion, another electrochemical process, is estimated to cost hundreds of millions of dollars aimuaUy in the United States alone (see Corrosion and CORROSION control). Electrochemical systems can be described using the fundamental principles of thermodynamics, kinetics, and transport phenomena. 

Cathodic protection (CP) is an electrochemical technique of corrosion control in which the potential of a metal surface is moved in a cathodic direction to reduce the thermodynamic tendency for corrosion. CP requires that the item to be protected be in contact with an electrolyte. Only those parts of the item that are electrically coupled to the anode and to which the CP current can flow are protected. Thus, the inside of a buried pipe is not capable of cathodic protection unless a suitable anode is placed inside the pipe. The electrolyte through which the CP current flows is usually seawater or soil. Fresh waters generally have inadequate conductivity (but the interiors of galvanized hot water tanks are sometimes protected by a sacrificial magnesium anode) and the conductivity... 

Evans considers that corrosion may be regarded as a branch of chemical thermodynamics or kinetics, as the outcome of electron affinities of metals and non-metals, as short-circuited electrochemical cells, or as the demolition of the crystal structure of a metal. 

It follows from the electrochemical mechanism of corrosion that the rates of the anodic and cathodic reactions are interdependent, and that either or both may control the rate of the corrosion reaction. It is also evident from thermodynamic considerations (Tables 1.9 and 1.10) that for a species in solution to act as an electron acceptor its redox potential must be more positive than that of the M /M equilibrium or of any other equilibrium involving an oxidised form of the metal. 

Uusitalo, E. and Heinanen, J., Corrosion of Steel in Soft Waters , Corros. Sci., 2, 281 (1962) Littlewood, R., Diagrammatic Representation of the Thermodynamics of Metal-fused Chloride Systems , J. Electrochem. Soc., 109, 525 (1962)... 

Pourbaix, M., Recent Applications of Electrode Potential Measurements in the Thermodynamics and Kinetics of Corrosion of Metals , Corros., 25, 267 (1969) de Nora, O., Gallone, P., Traini, C. and Meneghini, G., On the Mechanism of Anodic Chlorate Oxidation , J. Electrochem. Soc., 116, 147 (1969)... 

Before considering the principles of this method, it is useful to distinguish between anodic protection and cathodic protection (when the latter is produced by an external e.m.f.). Both these techniques, which may be used to reduce the corrosion of metals in contact with electrolytes, depend upon the electrochemical mechanisms that result from changing the potential of a metal. The appropriate potential-pH diagram for the Fe-H20 system (Section 1.4) indicates the magnitude and direction of the changes in the potential of iron immersed in water (pH about 7) necessary to make it either passive or immune in the former case the stability of the metal depends on the formation of a protective film of metal oxide (passivation), whereas in the latter the metal itself is thermodynamically stable and egress of metal ions from the lattice into the solution is thus prevented. 

Although important contributions in the use of electrical measurements in testing have been made by numerous workers it is appropriate here to refer to the work of Stern and his co-workerswho have developed the important concept of linear polarisation, which led to a rapid electrochemical method for determining corrosion rates, both in the laboratory and in plant. Pourbaix and his co-workers on the basis of a purely thermodynamic approach to corrosion constructed potential-pH diagrams for the majority of metal-HjO systems, and by means of a combined thermodynamic and kinetic approach developed a method of predicting the conditions under which a metal will (a) corrode uniformly, (b) pit, (c) passivate or (d) remain immune. Laboratory tests for crevice corrosion and pitting, in which electrochemical measurements are used, are discussed later. 

Immunity the state of a metal whose corrosion rate is low or negligible because its potential is below (less positive than) that of equilibrium with a very small concentration (or activity of its dissolved ions. The metal is thus regarded as thermodynamically stable. Pourbaix has suggested that the small metal ion concentration be 10 mol dm (Atlas of Electrochemical Equilibria in Aqueous Solutions, p. 71, Pergamon/ CEBELCOR, Oxford (1966)). 

A. Anderko. Simulation of FeC03/FeS scale formation using thermodynamic and electrochemical models. Nace Int Corrosion Conf (Corrosion 2000) (Orlando, FL, 3/26-3/31), 2000. 

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