Corrosion: kinetics thermodynamics

There are a number of mechanisms that pose potential problems to predicting dissolution rate kinetics as the system approaches saturation. Part of this conundrum originates from current models of glass corrosion kinetics that cannot yet incorporate these unanticipated phenomena into a mathematical equation that is consistent with the constraints of thermodynamics or kinetics. These phenomena include (1) alkali-hydrogen exchange (2) dissimilar reactivity of... 

The primary limitation for Pourbaix diagrams is that they are constructed purely from thermodynamic data. They are simply visual representations of the thermodynamic data. As such, they provide no information about corrosion rates. It is possible for substances that are thermodynamically unstable to be metastable, and exert a strong influence on the corrosion kinetics. For instance, Ni is quite resistant to acids, even though thermodynamics predicts that Ni + should be the stable phase at potentials above the Ni/Ni + reversible potential because of the metastabihty of NiO. On the other hand, some thermodynamically stable phases provide little protection. 

The tendency of metals to change with loss of energy into the disordered, more stable thermodynamic state represented by cathode local corrosion processes is particularly critical. Compounds are the reason for corrosion. However, the kinetic of metals to change with loss of energy into the disordered, more stable thermodynamic state represented by compounds is the reason for corrosion. Kinetic barriers, mainly provided by the formation of protective layers, often make metals resistant to corrosion and allow them to be used in technological applications. 

Exact corrosion kinetics must be modeled by solving the second law of Pick for the geometry of the case at hand. However, in some cases a net effect may be calculated from simple thermodynamics, as for closed system conditions in active corrosion [8], For the case of diffusion through scales it has been demonstrated that quasi-steady-state modeling is often a good approximation for an exact solution, at least for conditions tD/x > 2 [9] (where t = time, D = diffusivity, X = layer thickness). Some basic solutions for situations with instant singular corrosion can also be found in the literature [10]. 

We know that corrosion is a kinetic phenomenon. Aluminum is a fine material in water and in oxygen thanks to its corrosion kinetics and notwithstanding its instability and lead, a material that is known to be inert in oxygen, is pyrophoric in air when finely divided. Thus when one is asked to develop a material that is corrosion-resistant, it is useless to consult the Ellingham diagrams, to look for low solubilities, to find the redox potentials, or to consult other thermodynamic data. Kinetics is the key to processes, and this is true not only for corrosion, but also for other chemical reactions (except those near equilibrium). Salient examples in chemical engineering are crystal growth and catalysis. 

The electrode potential exerts a powerful control over corrosion kinetics, just as the chemical potential or the electrochemical potential does in thermodynamics. The deviation of the electrode potential E from its equilibrium value E given by the Nemst equation. 

Spence, J. W., and Haynie, F. H. (1990). Derivation of a damage function for galvanized steel structures Corrosion kinetics and thermodynamic considerations. Corrosion Testing and Evaluation Silver Anniversary volume. ASTM STP 1000. ASTM, Philadelphia, pp. 208-224. 

Thermodynamics gives an indication of the tendency of electrode reactions to occur, whereas electrode kinetics addresses the rates of such reactions. The reactions of concern are mainly corrosion reactions, hence, it is more appropriate to call the kinetics of such reactions as corrosion kinetics. In order to understand the theory of aqueous corrosion, it is important to develop a complete understanding of the kinetics of reaction proceeding on an electrode surface in contact with an aqueous electrolyte. Methods which are used to study the rate of a reaction involve the determination of the amount of reactants remaining in products after a given time. In aqueous corrosion, it is very important to appreciate the nature of irreversible reactions which take place on the electrode surface during corrosion. 

The reason for the success of Evans diagrams in corrosion is that they combine thermodynamic Victors ( values) with kinetics factors (i values). The usefulness of corrosion kinetics in the study of corrosion rates is, therefore, obvious. The exchange current densities have been included in the polarization diagram by Stem, and such diagrams are called Stem diagrams. Evans diagrams do not include exchange current densities. 

Key words nanoscale materials, corrosion prevention, galvanic corrosion, corrosion kinetics, electrochemical thermodynamics. 

One can distinguish between a tliennodynamic and kinetic stability to corrosion. C2.8.2.1 THERMODYNAMIC CONSIDERATIONS... 

The three elements necessary for corrosion are an aggressive environment, an anodic and a cathodic reaction, and an electron conducting path between the anode and the cathode. Other factors such as a mechanical stress also play a role. The thermodynamic and kinetic aspects of corrosion deterrnine, respectively, if corrosion can occur, and the rate at which it does occur. 

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. 

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. 

Environments are considered in detail in Chapter 2, but some examples of the behaviour of normally reactive and non-reactive metals in simple chemical solutions will be considered here to illustrate the fact that corrosion is dependent on the nature of the environment the thermodynamics of the systems and the kinetic factors involved are considered in Sections 1.4 and 1.9. 

In this section the interaction of a metal with its aqueous environment will be considered from the viewpoint Of thermodynamics and electrode kinetics, and in order to simplify the discussion it will be assumed that the metal is a homogeneous continuum, and no account will be taken of submicroscopic, microscopic and macroscopic heterogeneities, which are dealt with elsewhere see Sections 1.3 and 20.4). Furthermore, emphasis will be placed on uniform corrosion since localised attack is considered in Section 1.6. 

It should be noted that Fig. 1.15 (top) is based entirely on thermodynamic data and is therefore correctly described as an equilibrium diagram, since it shows the phases (nature and activity) that exist at equilibrium. However, the concepts implicit in the terms corrosion, immunity and passivity lie outside the realm of thermodynamics, and, for example, passivity involves both thermodynamic and kinetic concepts it follows that Fig. 1.15 (bottom) cannot be regarded as a true equilibrium diagram, although it is based on one that has been constructed entirely from thermodynamic data. 

The rate (or kinetics) and form of a corrosion reaction will be affected by a variety of factors associated with the metal and the metal surface (which can range from a planar outer surface to the surface within pits or fine cracks), and the environment. Thus heterogeneities in a metal (see Section 1.3) may have a marked effect on the kinetics of a reaction without affecting the thermodynamics of the system there is no reason to believe that a perfect single crystal of pure zinc completely free from lattic defects (a hypothetical concept) would not corrode when immersed in hydrochloric acid, but it would probably corrode at a significantly slower rate than polycrystalline pure zinc, although there is no thermodynamic difference between these two forms of zinc. Furthermore, although heavy metal impurities in zinc will affect the rate of reaction they cannot alter the final position of equilibrium. 

The above considerations show that the rate of a corrosion reaction is dependent on both the thermodynamic parameter and the kinetic parameters rjj and rjj. It is also apparent that (q) the potential actually measured when corrosion reaction occurs on a metal surface is mixed, compromise or corrosion potential whose magnitude depends on E, and on the Ej, -I and Ej, -I relationships, and (b) direct measurement of 7 is not possible when the electrodes are inseparable. 

Figures 1.27a to d show how the Evans diagram can be used to illustrate how the rate may be controlled by either the polarisation of one or both of the partial reactions (cathodic, anodic or mixed control) constituting corrosion reaction, or by the resistivity of the solution or films on the metal surface (resistance control). Figures 1. lie and/illustrate how kinetic factors may be more significant than the thermodynamic tendency ( , u) and how provides no information on the corrosion rate.

Equations 1.83 and 1.84, or the equations derived from them (1.85 to 1.89), may be used to calculate and E an., providing the various parameters involved are known. The equations also serve to illustrate how and corr, depend upon a thermodynamic factor ( r,ceii. °r r.c and E, ) and the kinetic factors a and / o for each of the half reactions that constitute the corrosion reaction. 

It must be emphasised that although, the rate of anodic dissolution of iron increases with,increase in. pH this will not necessarily apply to the corrosion rate which will be dependent On a number of other. factors, e.g. the thermodynamics and kinetics of the cathodic reaction, film formation, etc. 

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)... 

Pourbaix, M., Recent Applications of Electrode Potential Measurements in the Thermodynamics and Kinetics of Corrosion of Metals , Corrosion, 25, 267 (1%9)... 

The corrosion rate of many important metals and alloys is controlled by the formation of a passive film, and the thermodynamics and kinetics of their formation and breakdown are dealt with in Section 1.2. 

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