Electrochemical mechanism of corrosion

The corrosion process of steel can be summarised with the following reaction  

These four processes are complementary, which is to say that they occur at the same rate. In fact, the anodic current (i. e. the number of electrons liberated by the anodic reaction in a unit of time), the cathodic current (i. e. the number of electrons that are consumed in the cathodic reaction), the current that flows inside the reinforcement from the cathodic region to the anodic (I and finally the current that circulates inside the concrete from the anodic region to the cathodic (Icon), must all be equal  

Luca Bertolini, Bernhard Elsener, Pietro Pedeferri, Rob P. Polder Copyright 2004 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30800-8 

The common value of aU these currents is, in electrochemical units, the rate of the overall process of corrosion. The corrosion rate will thus be determined by the slowest of the four partial processes. 

In reality, the electrical resistance of the reinforcement is always negUgible with respect to that of the concrete. Therefore, the transport of current within the reinforcement is never a slow process and thus never contributes to reducing the rate of corrosion. Instead, under particular conditions inside the concrete, each of the other three processes can take place at negligible rate and thus become the Idneti- 

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It follows that corrosion is an electrochemical reaction in which the metal itself is a reactant and is oxidised (loss of electrons) to a higher valency state, whilst another reactant, an electron acceptor, in solution is reduced (gain of electrons) to a lower valency state. This may be regarded as a concise expression of the electrochemical mechanism of corrosion . 

The essential features of the electrochemical mechanism of corrosion were outlined at the beginning of the section, and it is now necessary to consider the factors that control the rate of corrosion of a single metal in more detail. However, before doing so it is helpful to examine the charge transfer processes that occur at the two separable electrodes of a well-defined electrochemical cell in order to show that since the two half reactions constituting the overall reaction are interdependent, their rates and extents will be equal. 

During the operation of the cell (or during the direct interaction of zinc metal and cupric ions in a beaker) the zinc is oxidised to Zn and corrodes, and the Daniell cell has been widely used to illustrate the electrochemical mechanism of corrosion. This analogy between the Daniell cell and a corrosion cell is perhaps unfortunate, since it tends to create the impression that corrosion occurs only when two dissimilar metals are placed in contact and that the electrodes are always physically separable. Furthermore, although reduction of Cu (aq.) does occur in certain corrosion reactions it is of less importance than reduction of HjO ions or dissolved oxygen. 

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. 

The conductivity of the soil i important as it is evident from the electrochemical mechanism of corrosion that this can be rate-controlling a high conductivity will be conducive of a high corrosion rate. In addition the conductivity of the soil is. important for stray-currenit corrosion (see Section 10.5), and for cathodic protection (Chapter 10). 

Any fundamental classification of corrosion control must be based on the electrochemical mechanism of corrosion, and Evans diagrams may be constructed (Fig. 1.27, Section 1.4) illustrating... 

T. P. Hoar, who was co-discoverer with U. R. Evans (in 1936) of the basic facts about the electrochemical mechanism of corrosion that led to Wagner and Traud s seminal theory, told about an episode from his early days as a corrosion consultant. Approached by an automotive concern for an inhibitor to stop the distressing breakdowns of its 1930s car radiators, he busied himself in his lab over a weekend and created (stumbled upon ) a potent organic inhibitor for the system concerned. The client wanted to pay a handsome fee, but Hoar wisely tempered his enthusiasm and humbly asked for just a few cents for every time the inhibitor was used. His decision, he says, provided him with a significant income for more than a decade. 

THE BASIC ELECTROCHEMICAL concepts and ideas underlying, the phenomena of metal dissolution are reviewed. The emphasis is on the electrochemistry of metallic corrosion in aqueous solutions. Hie role of oxidation potentials as a measure of the "driving force" is discussed and the energetic factors which determine the relative electrode potential are described. It is shown that a consideration of electrochemical kinetics, in terms of current-voltage characteristics, allows an electrochemical classification of metals and leads to the modern views of the electrochemical mechanism of corrosion and passivity. 

The polarization curves shown in 4(d) are typical of systems in which anodes and cathodes are separated by corrosion products and/or solutions of some resistance. The corrosion rate (current density) depends on the average resistance between the anodes and cathodes and the potential which is measured depends on the position of the probe electrode in relation to the resistance separating the anode and cathode. In a corroding system of this type there is a current flowing through the solution which can be calculated by measuring the potential differences between two spaced electrodes in the solution and the resistance of the solution. Evans and his co-workers did this with iron in bicarbonate solutions and showed that all the corrosion could be accounted for by the currents flowing between anodes and cathodes, (2). This confirmed the electrochemical mechanism of corrosion. 

Figure 7.1 Electrochemical mechanism of corrosion of steel in concrete [5]...

Tomashov has produced a detailed scheme of control based on the electrochemical mechanism of corrosion, which has been set out in an abridged and modified form in Table 0.2. However, although more fundamental than Table 0.1, it has several limitations, since it is not always possible to define the precise controlling factor, and frequently more than one will be involved. Thus removal of dissolved oxygen (partial or complete) from an aqueous solution reduces the thermodynamics of the reaction and also increases the polarisation of the cathodic reaction, and both contribute to the decrease in the corrosion rate although the latter is usually the more significant. 

In view of the electrochemical mechanisms of corrosion, the tendency for a metal to corrode can also be expressed in terms of the electromotive force (emf) of the corrosion cells that are an integral part of the corrosion process. Since electrical energy is expressed as the product of volts by coulombs Ovules, J), the relation between AG in joules and emf in volts, E, is defined by AG = -nFE, where n is the number of electrons (or chemical equivalents) taking part in the reaction, and F is the Faraday (96,500C/eq). The term AG can be converted from calories to joules by using the factor 1 cal = 4.184 absolute joules. 

In the electrochemical mechanism of corrosion, the metal dissolution—which involves the loss of electrons vis- -vis oxidation—must be accompanied by a cathodic reaction that consumes electrons, which is typically oxygen or proton or water reduction. According to the Butler-Volmer equation, the current density for the anodic reaction varies according to... 

The overall reaction takes place at a rate determined by the more difficult of these two steps. This simple concept, independent anodic and cathodic half-reactions, is the basis of the electrochemical mechanism of corrosion. The mechanism has profound effects on the corrosion protective properties of nonmetallic coatings materials. Some tests exploit the electrochemical mechanism to determine specific corrosion susceptibilities of coatings systems such as cathodic disbond-ment and pinhole detection. Coatings that are permeable to oxygen may be vulnerable to damage due to an alternative cathodic reaction... 

In many industrial applications, MIC of different metallic constructional materials is a serious problem. It does not represent a novel form of corrosion but is based on the modification of the electrochemical processes by microorganisms. Thus, it is necessary to briefly recall the basic electrochemical mechanisms of corrosion. 

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