Corrosion thermodynamics, chemical equilibria

When a metal (M) is immersed in a solution containing its ions (M ), several reactions may occur. The metal may lose an electron (corrosion) to form metal ions or the metal ions in solution gain electrons (reduction) and enter the solid metal state. The equilibrium across the metal-solution interface controls which reaction, if any, will occur at the metal-electrolyte interface. Because the equilibrium is determined by the equality of the partial Gibbs free-energy or chemical potentials (//) on either side of the electrode interface (i.e., Absolution=A dectrode). when any metal is immersed in the electrolyte, thermodynamics... 

The interface in question is not in thermodynamic equilibrium. For this to occur at least two different redox couples must be responsible for the value of the open-circuit potential. The overall zero current then corresponds to a chemical balance which is not null. This is the case for instance in corrosion. The open-circuit condition is not in equilibrium and the system changes with time. The open-circuit potential, which is not given by the Nernst law, depends on the properties of at least two redox couples in this instance one often refers to the term mixed potential. ... 

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. 

Electrochemical reaction kinetics is essential in determining the rate of corrosion of a metal M exposed to a corrosive medium (electrolyte). On the other hand, thermodynamics predicts the possibility of corrosion, but it does not provide information on how slow or fast corrosion occurs. The kinetics of a reaction on a electrode surface depends on the electrode potential. Thus, a reaction rate strongly depends on the rate of electron flow to or from a metal-electrolyte interface. If the electrochemical system (electrode and electrolyte) is at equilibrium, then the net rate of reaction is zero. In comparison, reaction rates are governed by chemical kinetics, while corrosion rates are primarily governed by electrochemical kinetics. 

The stability of the oxide in equilibrium with the gas phase and the layer sequence of corrosion products may be calculated on the basis of thermodynamic laws and data. The chemical corrosion, which is taking place at high temperatures and the surrounding atmosphere, is interesting at high processing temperatures of modern high-performance thermoplastics. 

Laboratory corrosion testing has been in existence for many years, and yet there is no uniformity in these attempts to evaluate the corrosion resistance of specific refractory for specific conditions. Roughly, corrosion tests may be divided into static and dynamic tests. The term static usually means that the corrosive liquid is stationary, it doesn t move against the tested refractory, and the system corrosive liquid-refractory will tend to reach equilibrium. During a dynamic test, the corrosive liquid moves against the refractory surface and the thermodynamic and chemical equilibria wmi t be reached. 

To accommodate the possible chemical reactions of the ongoing corrosion process, the calculated concentrations at c (t -i- At) (cfin Fig. 32.3) must be corrected according to the local thermodynamic equilibrium. For this purpose, the concentrations c (t + At) are transferred into a thermodynamic subroutine ThermoScript [10], which contains the commercial program ChemApp [11]. ChemApp is based on a numerical Gibbs energy minimization routine in combination with tailor-made databases [12]. In order to avoid excessive calculation times, the parallel-computing system PVM (parallel virtual machine) is used, i.e., ThermoScript distributes the individual... 

The aqueous corrosion and passivation of metal surfaces involve electrochemical processes at the electrode-electrolyte interface. Like for all chemical reactions, the two aspects of equilibrium and kinetics have to be treated. Thermodynamic data give an important first insight into layer formation. They have been used to compose potential-plT diagrams for all elements and thus for all metals [26,27]. In Chapter 1 of this book on the fundamentals of corrosion, passivity of iron has been mentioned and the calculation of some of the lines based on thermodynamic data has been described. Here a more detailed description of foe potential-pH diagrams of iron and copper are presented. 

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