Alloys dissolution

H. Pickering and C. Wagner, J. Electrochem. Soc. 114 698 (1967). Paired vacancy diffusion in alloy dissolution. 

J. O M. Bockris, B. T. Rubin, A. Despic, and B. I. ovrcccch, Elect rochim.. Acta 17 97 (1972). Cu-Ni alloy dissolution the dissolution rate of each alloying component is independent of its composition in the alloys. 

Alloy dissolution — The process of anodic oxidation of an alloy electrode by application of a suitable - electrode potential. It leads to dissolved products. Under non-electrochemical conditions the dissolution of an alloy can be performed with a chemical oxidant The rate of dissolution depends on the structure and homogeneity of the processed material. When the applied potential or, in the case of chemical dissolution, the oxidation potential of the chemical oxidant lays between the formal potentials of the dissolved components, a selective... 

The composition of the alloy to be machined is important in the analysis of the problem of machinability by ECM. A difference in the tendencies of different alloy components to passivate in the electrolyte of a given composition is of prime importance. If the content of component A, which is less prone to passivate, is not too high, it is etched away from a thin alloy surface layer and the potential increases to a value at which high-rate dissolution of the more passivation-prone component B is possible. Further alloy dissolution proceeds at this potential. The surface is enriched with component B, and both components dissolve concurrently. 

Chemical analysis of the solutions after anodic dissolution have shown that the oxidation state of chromium in the dissolution products depends on the alloy composition and, correspondingly, on the potential of alloy dissolution. At potentials less positive than the potential of the onset of pure-chromium passivity breakdown, chromium dissolves from the nickel-based alloys as Cr(III). The alloys with chromium contents of not more than 15% dissolve in this manner in NaCl solution. At higher Ea, chromium from the alloy dissolves, for the most part (about 90%), in the form of Cr(VI). This is true for all alloys in Na2SC>4 (or NaNC>3) solution and for the alloys containing more than 25% chromium in NaCl solution. 

The current efficiency of alloy dissolution is calculated by the following equation ... 

Figure 6 displays plots of alloy dissolution productivity (P) versus chromium content for two different electrolytes at a current density of 8 A cm-2. The maximum in Curve 2 is associated with an effect of chromium content of the alloy on the current efficiency in Na2SC>4 and NaNC>3 solutions. The variations in the electrochemical equivalent of the alloy and in the current efficiency with an increase in the Cr content have the strongest effect on the dissolution productivity. An increase in the oxidation state of chromium at Cr 15% has a weak effect on the productivity (a small inflection in the Curve 1 for NaCl). The shape of Curve 2 in Fig. 6 for the NaNC>3 solution depends on the current density in the range of low current densities, the productivity (P) of ECM of nickel and nickel-rich alloys increases with the current density, owing to an increase in the current efficiency. 

J.B.B. Silva, I.G. Souza, A.P.G. Gervasio, Anodic electrodissolution in flow injection system a fast and efficient alternative for alloys dissolution, Quim. Nova 23 (2000) 244. 

Passivity and alloy dissolution are covered in detail in other chapters of this volume. However, Pourbaix diagrams can be used to suggest the influence of alloying on passivation or oxide film formation. It is possible for one element in an alloy to enrich in a surface oxide layer if the oxide of that metal is stable in an E/pH region where the other elements are not stable. This results in an effective extension of the passivity region for the base metal of the alloy. An example of this is stainless steel. The... 

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... 

The enrichment of the slow dissolving component, B, in an alloy surface under simultaneous dissolution conditions may be rationalized by a model of alloy dissolution that is based on the simplifying assumptions (1) that a homogeneous solid solution may be described as a heterogeneous dispersion of atomic dimensions with area fraction (surface mole fraction) X j for component j, and (2) that the alloy components dissolve independently. The partial current density ij of an alloy component j will then be given by ij = i -X j, where i is the current density of the pure metal, j, and for a binary alloy A-B, the total current density of alloy dissolution. 

Analyzing the dissolution kinetics of alloys in a more general way, Heusler attributed deviations from the ideal behavior in simultaneous alloy dissolution (that is, a composition dependence of the phenomenological rate constants of individual alloy components) to a dissolution mechanism that proceeds from kinks in the steps of low-index planes, the concentration of these... 

Moreover, it represents an important issue with respect to the theory of selective alloy dissolution (see in following text). 

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 ... 

Nucleation and growth concepts Ger-ischer s model of alloy dissolution, in addition, associated the critical potential, Ec, with nucleation constraints for (1) the formation of the product phase or (2)... 

Nanoscopic Investigations of Dealloyed Surfaces Erom the background of competitive models of selective alloy dissolution as described above, a closer microscopic examination of this process with the ultimate objective of atomic resolution and chemical information on an atomic scale appears mandatory. Ex situ transmission electron microscopy (TEM) of thin, corroded alloy films provides lateral resolution at the nanometer scale, but suffers from poor depth resolution and from structural relaxation processes that may occur after termination of the anodic polarization and transferring the samples into high vacuum. Classical TEM investigations in this field were performed under open circuit conditions in oxidizing environments (that is, at > Eq) [51,... 

The intersection of the anodic Zn-Mn alloy dissolution line and the cathodic oxygen reduction line for different electrolyte velocities gives a new value for Ecotr sFid... 

R. Vidal, A.C. West, Aluminum and aluminum alloy dissolution in concentrated phosphoric acid, J. Elec-trochem. Soc. 145 (1998) 4067. 

Fig. 7.41 The example of the ALS Vs of Cd-Ni alloy dissolution dotted line, 1), pure Cd dissolution dashed line, 2), and dissolution of a two-layer electrodeposit solid line, 3) composed of a layer of Cd-Ni alloy and a layer of pure Cd electrodeposited on top of a layer of the alloy (Reprinted from Ref. [5] with kind permission from Springer)...

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