Selective Dissolution of Alloys

For alloys, one has to distinguish single-phase and multiphase alloys. If an alloy is single phase, it contains the two or more metal components in a homogeneously distributed 

Log i-E plot for the dissolution of CuAu alloys with 13 and 18 at % Au in showing the current plateau and the steep current increase at the critical potentials of 0.60 and 0.80V, respectively. (From Kaesche, H., Corrosion of Metals, Springer, Berlin, Germany, 2003 Pickering, H.W. and Burne, P.J., /. Electrochem. Soc., 118,209,1971.) 

Similar studies have been performed with brasses of different composition. Cu-65Zn shows a plateau current of ca. IpA cm with a steep increase at potential E O.OV up to i 10mA cm . For Cu-30Zn (a-brass), the current plateau reaches up to E = 0.10 V, with a similar increase to i 10 mA cm. Below these potentials, both metals show a very slow Zn dissolution only, whereas the dissolution of both metal components is reported for E 0.10 V [71]. The equilibrium potential of the Cu/Cu + electrode with E = 0.337V coincides with the observed current increase for both alloys as its value decreases with the Cu content of the alloy according to the Nemst equation  

This result is different from the one for CuAu alloys. Au cannot dissolve even for potentials E E° = 1.50 V for the Au/Au + electrode because its surface is protected by a passive layer even in acidic electrolytes. In the case of CuZn alloys, both metals dissolve at E 0.1 V although with different rates. Thus, a Cu-rich phase is formed at the surface due to preferential Zn dissolution. Diffraction studies show the presence of Cu and brasses enriched in Cu. Preferential Zn dissolution requires a supply of Zn atoms by diffusion from the bulk metal via di-vacancies. Cu-86Zn starts high dissolution rates already at E -0.9V increasing to more than 1 mA cm. The critical potential of Zn dissolution is too negative for this alloy to show a plateau with small dissolution rates. Dissolution of Cu is not possible for these negative potentials similar to the situation of AuCu alloys. 

Dissociation Constants of Some Cation Complexes Kp and the Related Standard Potentials E  

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Decrease in velocity will favour all forms of localised attack in which an occluded cell is involved in the mechanism, and will also favour selective dissolution of alloys that are susceptible to this form of attack. 

The slip dissolution model assumes that plastic deformation at the crack tip is responsible for the activation. But other mechanisms can have the same effect. Tensile stress at the crack tip could, for example, break a brittle tarnish film or passive oxide film, thereby exposing the base metal to the electrolyte. Selective dissolution of alloy components at the crack tip could locally weaken the metal matrix and thus permit... 

Chen S J, Sanz F, Ogletree D F, Hallmark V M, Devine T M and Salmeron M 1993 Selective dissolution of copper from Au-rich Au-Cu alloys an electrochemical STS study Surf. Sc . 292 289... 

In principle the selective dissolution of the less noble component of a singlephase alloy would perhaps be expected and is in fact observed (dezincification of an a-brass, etc.) even though the details of the mechanism by which it occurs is not yet fully understood. In contrast, the preferential attack of the less noble phase of a two-phase alloy is not only expected and observed —the mechanism by which it occurs in practice is also quite clear. Selective dissolution of the more active phase of a two-phase alloy is best exemplified by the graphitic corrosion (or graphitisation) of grey cast iron. 

Multiphase gold or palladium-based alloys never show dissolution of Au or Pd but often exhibit progressive surface ennoblement due to selective dissolution of copper or silver from the outer 2-3 atomic layers Heat treatment often decomposes multicomponent alloys into a Pd-Cu rich compound and an Ag-rich matrix with corrosion of the latter phase in deaerated artificial saliva and S -containing media . Au-Cu-rich lamellae have similarly been observed, again with preferential attack on Ag-rich phases or matrix. These effects presumably arise from the ability of the noble alloy phases to catalyse the cathodic reduction of oxygen . 

For environments in which tin is less readily corroded than lead, corrosion resistance of the alloy decreases as the lead content increases the decrease may, in some circumstances, be sharp at a particular composition. In the more corrosive media, such as nitrite solution, a sharp increase of corrosion rate is observed as the lead content increases beyond 30waters with low contents of dissolved salts, the corrosion rate increases slowly with lead content up to about 70% and then rises more steeply, but in the general run of supply waters the ability of lead to form protective insoluble anodic products is helpful to the durability of solder. Selective dissolution of tin has been... 

Parting selective dissolution of one metal (usually the most electro-reactive) from an alloy leaving a residue of the less reactive constituents. 

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. 

Because of the high difiusivity of liquid alloys, the selective dissolution of component A results in a continuous variation of the average alloy composition, dXA/dt, which is given by... 

Frequent constituents of protective zinc coatings on steel, Zn Fe intermetaUics such as the 5-, Pi - and P-phases, represent alloys of relatively low melting point, high and considerable technical importance. A detailed electrochemical study has been performed, so far, only with the 5 phase of this alloy system. Even though the selective dissolution of Zn prevails at low overpotentials of the Zn dissolution reaction (e.g. at Ew = —0.70 V), a formation of intermediate product phases (such as the more Fe-rich Pj- and P- phases) was not observed. However, dezdncification of... 

I. K. Marshakov, A. V. Vvedenskii, V. Y. Kondrashin et al., Anodnoe Rastvorenie i Sdektivnaya Korroziya Splavov (Anodic Dissolution and Selective Corrosion of Alloys), Voronezh Gos. University, Voronezh, 1988. 

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