Thermodynamics corrosion reactions

Anode Corrosion Reaction. Ziac might at first appear to be an unusual choice for battery anode material, because the metal is thermodynamically unstable ia contact with water... 

Fig. 1.13 Block on inclined plane illustrating the relationship between the thermodynamic tendency — AG) of a corrosion reaction aA + bB cC + dD and the rate of conversion of metal A to corrosion products C and D...

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.

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. 

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. 

These considerations show the essentially thermodynamic nature of and it follows that only those metals that form reversible -i-ze = A/systems, and that are immersed in solutions containing their cations, take up potentials that conform to the thermodynamic Nernst equation. It is evident, therefore, that the e.m.f. series of metals has little relevance in relation to the actual potential of a metal in a practical environment, and although metals such as silver, mercury, copper, tin, cadmium, zinc, etc. when immersed in solutions of their cations do form reversible systems, they are unlikely to be in contact with environments containing unit activities of their cations. Furthermore, although silver when immersed in a solution of Ag ions will take up the reversible potential of the Ag /Ag equilibrium, similar considerations do not apply to the NaVNa equilibrium since in this case the sodium will react with the water with the evolution of hydrogen gas, i.e. two exchange processes will occur, resulting in an extreme case of a corrosion reaction. 

Thermodynamic data suggest that P -alumina is stable in sulfur and that Na20-rich //"-alumina maybe unstable to some degree. In the latter event, depletion of Na20 from / " -alumina according to Eq. (5) would result in a surface layer resistant to further corrosion. A significant corrosion reaction does occur be-... 

Passivation is defined as the state where even though a metal electrode fulfills the thermodynamic condition for dissolution (solution composition, electrode potential, etc.), a corrosive reaction scarcely proceeds. 

An important example is the corrosion of metals. Most metals are thermodynamically unstable with respect to their oxides. In the presence of water or moisture, they tend to form a more stable compound, a process known as wet corrosion (dry corrosion is not based on electrochemical reactions and will not be considered here). Moisture is never pure water, but contains at least dissolved oxygen, sometimes also other compounds like dissolved salt. So a corroding metal can be thought of as a single electrode in contact with an aqueous solution. The fundamental corrosion reaction is the dissolution of the metal according to ... 

All but one of the reactions in Figure 4 lead to the formation of the soluble TiO + ion this seems consistent with the observed changes in the visible absorption spectrum of the solid electrode. It may also be that other titanium species are formed in solution, such as peroxytitanium complexes like H Ti05 We have no direct evidence as to the identity of the solution species at this time, and have limited the candidate corrosion reactions shown in Figure 4 to those for which thermodynamic data are readily available. Nonetheless, the fact that titanium is observed in the electrolyte only upon extensive photocorrosion (and then in smaller amounts than strontium) suggests that the initial photocorrosion process involves the loss of strontium from the SrTi03, with the formation of Sr(0Ac)2 or SrSO. ... 

Thermodynamics, therefore, defines a necessary, vital precondition for corrosion it determines the direction in which an overall corrosion reaction will tend. But the determination of the rate and control of a corroding system can emerge only by a study of the electrodics of corrosion. 

What energy drives an overall corrosion reaction, e.g., Fe + 2H+ — Fe2+ + H2 It is, of course, the free energy of the overall corrosion reaction. It must be negative in value for the direction indicated, for otherwise thermodynamics forbids it to occur. This negative free energy of the overall reaction can be converted into the corresponding cell potential, AG = -nFE (n is 2 in the above reaction because two electrons are involved in the constituent electrochemical reaction, Fe - Fe2+ + 2e, and 2H+ + 2e —>... 

This book consists of nine chapters. The second chapter provides an overview of the important thermodynamic and kinetic parameters of relevance to corrosion electrochemistry. This foundation is used in the third chapter to focus on what might be viewed as an aberration from normal dissolution kinetics, passivity. This aberration, or peculiar condition as Faraday called it, is critical to the use of stainless steels, aluminum alloys, and all of the so-called corrosion resistant alloys (CRAs). The spatially discrete failure of passivity leads to localized corrosion, one of the most insidious and expensive forms of environmental attack. Chapter 4 explores the use of the electrical nature of corrosion reactions to model the interface as an electrical circuit, allowing measurement methods originating in electrical engineering to be applied to nondestructive corrosion evaluation and... 

The rate of a corrosion reaction is affected by pH (via H reduction and hydroxide formation), the partial pressnre of O, (the solubility/concentration of oxygen in solution), fluid agitation, and electrolyte condnctivity. Corrosion processes are analyzed using the thermodynamics of electrode reactions, mass transfer of the cathode reactants O2 and/or H, and the kinetics of metal dissolution reactions [157, 158]. 

We have seen in Section 26.2.1 that thermodynamics (i.e., equilibrium half-cell potentials) can be used to determine which of two half-cell reactions proceeds spontaneously in the anodic or cathodic direction when the two reactions occur on the same piece of metal or on two metal samples that are in electrical contact with one another. The half-cell reaction with the higher equilibrium potential will always be at the cathode. Thus, under standard conditions any metal dissolution (corrosion) reaction with an E° less than 0.0 V vs. SHE will be driven by proton reduction while metal dissolntion reactions with an E° less than -e1.23 V vs. SHE will be driven by dissolved... 

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