Selective dissolution binary alloys

In contrast, the selective dissolution or leaching-out by corrosion of one component of a single-phase alloy is of considerable practical importance. The most common example of this phenomenon, which is also referred to as parting , is dezincification, i.e. the selective removal of zinc from brass (see Section 1.6). Similar phenomena are observed in other binary copper-base alloys, notably Cu-Al, as well as in other alloy systems. 

On the one hand, selective dissolution of transition-metal alloys in liquid aluminium might be expected in view of considerable differences in their solubilities in respective binary systems. On the other, however, in these alloys the atoms of different elements are connnected together by metallic bonds of nearly equal strength. Any of the elements can therefore scarcely be expected to leave the alloy lattice at a rate which significantly exceeds the rates of transition of other elements into liquid aluminium. 

Some basic aspects of alloy dissolution are best illustrated by the behavior of a liquid binary alloy A-B. This is due (1) to the absence of crystallization overvoltage and dissolution induced structural surface modifications [6] as well as (2) to the high diffusivity in the alloy phase that provides for the reactant supply at the alloy/electrolyte interface if one alloy component dissolves preferentially (at a higher rate than the other) (7). Provided that the standard electrode potential difference of the components, AE = E — El, is large AE > RT/F) and their charge transfer reactions are fast, one expects a schematic polarization curve as shown by Fig. 1(a). For Ea < E < Eb, only the less noble component. A, dissolves ( selective dissolution or deaUoying ), the partial anodic... 

In the case of simultaneous dissolution of a binary alloy A-B with AE RT/F and E > b. the overpotential of the dissolution of the less noble (fast dissolving) component. A, clearly exceeds that of the more noble (slow dissolving) component, B. Thus, the condition for simultaneous dissolution, Eq. (7), will only be satisfied if the dissolution rate of A is suppressed by an enrichment of B in the electrode surface. It appears, therefore, that the simultaneous (steady state) dissolution mode is always preceded by a transient period of selective dissolution. This transition from the selective to the simultaneous dissolution mode was illustrated, for example, by a y-spectroscopic analysis of... 

Theoretical Aspects Every model of selective alloy dissolution must involve a transport mechanism by virtue of which the atoms of the less noble component reach the alloy/electrolyte interface and the atoms of the more noble component aggregate. For a binary alloy, the basic transport mechanisms are as follows ... 

The percolation model of selective dissolution, an extension of the surface diffusion mechanism, based on preexisting interconnected paths of like elements in the binary alloy and effects of curvature on dissolution potential [24, 25]. 

K. Sieradzki, R. R. Corderman, and K. Shukla, Computer simulation of corrosion selective dissolution of binary alloys, Philosophical Magazine A, vol. 59, pp. 713-746, 1989. 

Figure 7.31 Selective dissolution of component A of the binary alloy AB at subcritical potentials (a) the composition of the surface before dissolution and (b) the surface enriched inB.

Actually, even the sophisticated studies combining electrochemistry and surface analysis seem unable to yield any further decisive information on the detailed mechanism of selective dissolution. Atomic arrangements in which dissolution of atoms A can proceed throughout a rough electrode structure enriched in B t) e have been recognized as amenable to percolation theory [193]. The same theory was also applied to the passivation of binary Fe-Cr alloys in which, as suggested in Ref 105, 106, passivation of the more soluble element, Fe, is enhanced by a coimected surface lattice of passive Cr atoms. The main power of percolation theory is to provide diagnostic criteria for computer simulations in terms of concentration thresholds. 

Computer simulations with a Monte Carlo algorithm of the selective dissolution of 2D square and 3D cubic lattices modeling binary alloys (A-B) are performed. 

Although less work has been done on the passivation of other binary alloys, many interesting observations have been made and a few selected examples will be considered. In the case of Ni-Fe alloys of varying composition, alloy dissolution results in surface enrichment with nickel (as a consequence of the preferential dissolution of iron) and the formation of a passive film composed of an itmer layer of... 

Obviously, the parameter y is a measure of the kinetic situation when the rate-limiting step of the selective dissolution is the solid-phase diffusion mass transfer. It is clear that the increase in y contributes to the transition to the solid-phase diffusion control of the process the similar criterion was found in [4] for chronoampero- and chronopotentiometric diffusion problems of homogeneous binary alloys SD. 

Figure 6. The Randles-Sevcik dependence for selective dissolution of a binary alloy calculated by the equation (17) (1,3,5) and equation (18) (2,4,6) at different solid phase diffusion coefficient Da = 10 ...

The analytical solution of the transient diffusion problem of selective anodic dissolution of a binary alloy in the potentiodynamic polarization mode allowed us to obtain the equations for the concentration profiles of an electronegative metal in an alloy, voltammograms as well as modified Randles-Sevcik expressions, taking into accoimt the mixed solid-liquid phase diffusion nature of the kinetic limitations of the process, equilibrium solid phase adsorption of the components, and surface roughness of an electrode. 

The use of a percolation approach clarifies substantially the selective dissolution of single-phase binary alloys. Computer simulations support the existence of a tight relationship between dealloying thresholds and site percolation thresholds. However, a careful survey of the literature over the last 10 years leads to the striking constatation... 

If a binary or multicomponent alloy is undergoing the liquid-metal attack, then its dissolution can be either selective or non-selective. In the former case, the more soluble component dissolves at a higher rate. Hence, the solid phase becomes depleted, while the liquid enriched in this... 

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