Tarnish reactions

The kinetics of reactions in which a new phase is formed may be complicated by the interference of that phase with the ease of access of the reactants to each other. This is the situation in corrosion and tarnishing reactions. Thus in the corrosion of a metal by oxygen the increasingly thick coating of oxide that builds up may offer more and more impedance to the reaction. Typical rate expressions are the logarithmic law,... 

We are now ready to consider some soUd state reactions that relate more directly to the real world. These include the tarnishing reaction and Pick s Laws of Diffusion. Both of these scientific areas have been rigorously studied because of their importcmce in revealing how diffusion mechanisms are related to everyday solid state reactions which occur on a daily basis. 

About 1942, Pick formulated laws which described diffusion processes in solids, similar to that already presented for the tarnishing reaction. To do this. Pick hypothesized that atoms (ions) would "hop from site to site in the manner we have already described in 4.2.2. and 4.2.4. 

Actually, we must account for all types of defects, including charged species (see 2.2.1.). To do this, let us reconsider the tarnishing reaction (Section 4.5. given above), using the genered equation ... 

Silver and silver-copper alloys are often even more sensitive to sulfur compounds—the effect is catalyzed in the presence of oxidizing species and high relative humidity [268]. The resulting surface layer—dark, thin and adherent when developed on a polished surface—usually acts as a good physical barrier that slows down the tarnishing reactions. However, since silver objects are expected to be shiny. 

Tarnishing reactions. A film of tarnish is formed on the surface of a metal by diffusion of dissolved gas A through the film from an exterior gaseous phase. The reaction rate of gas with metal is assumed to be large,... 

Primary Step. This step must take place at the interface between the two reacting phases or at the outside surface of a phase undergoing decomposition. A possible exception is that of the dissociation of a lattice point itself, as in photochemical decomposition, in which, say, ions are converted to atoms of a new phase. To illustrate this step, we may consider the simple tarnish reactions, where a metal M combines with a gaseous atom X, to give ultimately an ionic solid MX. 

OUR knowledge of reactions in the solid state has been derived mainly from the systematic study of the tarnish reactions and their inverse, dissociation processes. This is due largely to the fact that the product or reactant ionic compounds have been studied by physical methods the nature of the defect, the electronic and ionic processes and the structures are known with some degree of certainty. Moreover, tarnishing reactions are ideal for experimental study, particularly where a coherent film of the product (oxide, halide, etc.) is formed on the metal. 

Fig. 39.— Types of kinetic laws obtained in tarnish reactions (a) linear, (6) parabolic, (c) logarithmic, (d) limiting thickness.

Wagner (42) discussed tarnish reactions on a quasi-Chemical basis using anions, cations, electrons and positive holes as entities obe dng the mass action laws. As a specific example, we shall consider the formation of a silver halide layer, in which the anions are immobile and form a perfect lattice, while the cation lattice may contain vacancies and interstitial cations. There is a... 

Mott (5) has developed the theory of tarnish reactions in a more satisfactory way. One must examine the relative positions on the energy scale of the electronic levels in the M and MX layer. As a general rule we may say that the Fermi surface in a metal lies somewhere in the forbidden region between the valence and conduction bands of an insulator. When they are brought into contact the situation is as shown in Figure 40. The... 

Fig. 40.— Energy level diagram corresponding to Mott s model for tarnish reactions.

One obviously asks what happens in the earliest stages of a tarnishing reaction, when the number of corrosive atoms is very few, say a mono-layer or less. This is the concentration range where we normally speak of adsorption, yet if there is reconstruction, a distinction between adsorption and corrosion in this region is not meaningful. 

To do this, let us reconsider the tarnishing reaction (Section D-I), with the general reaction ... 

The silver tarnishing reaction involving hydrogen sulfide is a classic of solid state materials science literature, and the zinc oxidation is yet another example of a more complex protective layer corrosion problem for which wide ranges of data exist relating to purity, to kinetic conditions, etc. 

All of these processes can be described by a diffusion-controlled model originally derived to explain the tarnishing of metals and hence commonly called the tarnishing model. The derivation of this model is based on the assumptions that (a) the reaction site is immobile, (b) the concentration of reaction sites is independent of time and temperature in the absence of the tarnishing reaction, and (c) the reaction rate is very... 

In order to calculate the practical tarnishing reaction rate constant (Tammann constant) Jc, Wagner assumed that ions and electronic charge carriers are responsible for mass transport through the product layer. This reaction scheme is shown in Fig. 8-1. Local electroneutrality must always be observed, of course. 

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