Surface diffusion, dealloying

The second model extends the surface diffusion model to include the importance of the atomic placement of atoms in the randomly packed alloy. The model considers that a continuous connected cluster of the less noble atoms must exist to maintain the selective dissolution process for more than just the few monolayers of the alloy. This percolating cluster of atoms provides a continuous active pathway for the corrosion process as well as a pathway for the electrolyte to penetrate the solid. This is expected to depend on a sharp critical composition of the less noble element, below which dealloying does not occur.54, (Corcoran)5... 

However, since then, several mechanisms had been advanced to explain the dealloying process. They are volume diffusion, vacancy diffusion, surface diffusion, oxide formation, percolation, and ionization and re-deposition [4]. 

In the percolation model, it is the interconnectedness of atoms of the LN component that allows the dealloying front to proceed into the alloy. However, without allowing for surface diffusion, the dissolution process stops after penetrating a short distance into the crystal as the MN component covers the surface and prevents further dissolution [11],... 

J. Erlebacher, An atomistic description of dealloying porosity evolution, the critical potential, and rate-limiting behavior, Journal of the Electrochemical Society, vol. 151, pp. C614—C626, 2004. E. G. Seebauer and C. E. Allen, Estimating surface diffusion coefficients, Progress in Surface Science, vol. 49, pp. 265-330, 1995. 

The dynamic process of dealloying was discussed using Monte Carlo simulations [30, 36, 37]. The dissolution of the less-noble atoms from the topmost surface resulted in steps and kinks, where the coordinated numbers of noble atoms increased. This induced a surface diffusion of noble atoms. The competition between the dissolution rate of less-noble metals and the surface diffusion of noble metals is considered to be a key factor that controls the morphology of the dealloyed product. In bulk alloys, surface diffusion rate of the noble atoms is slow across the extended surface, which causes a Rayleigh surface instability [37] and leads to the formation of nanoporosity. 

There are some concerns about using percolation theory itself, which limits removal to 30 % of the original composition of the more noble component [101]. In work by Halley, mention is made that at high surface potentials, percolation will not occur since the surface wiU not be fractal, and an additional feature is needed to explain dealloying beyond the limits of percolation theory [99]. Sieradzki et al. [93] overcomes this limitation by allowing a small amount of surface diffusion to occur, which wiU initiate further fractal formation at the surface. 

One recent development in the static immersion testing from Newman et al. [116] originated with Bengough and May [JJ5]. Recent experimentation [116] involved exposing samples of alpha-brass in NaCl solutions with additions to simulate the local environment in crevices or otherwise inhibited transport. Under these severe conditions, dealloying can be shown to occur without an applied potential and may be reduced or eliminated by addition of arsenic to the alloy. The authors suggest that these results support the percolation mechanism with surface diffusion for dealloying rather than mechanisms that rely on formation of metastable phases or disproportionation of the less noble element. 

Conditions for simultaneous dissolution generally involve the formation of a 2D or 3D surface layer in which the relative dissolution rates of the elements are equal to their alloy fraction. Even in cases where simultaneous dissolution is obeyed, the process must begin by an initial period of selective dissolution at least at a 2D level [198]. As briefly mentioned earlier, many mechanisms have been invoked to explain how, for a nonpassive alloy, the dissolution of the more reactive element can continue across the dealloyed structure [32,201]. Smface changes have been attributed to surface diffusion and recrystallization, redeposition, short-range atomic rearrangement, and a roughening transition by capillary effects. 

Any one of these mechanisms may apply in specific instances of dealloying. For example, twin bands in brass, visible in the completely or incompletely dezincified layer, constituted early evidence for a volume diffusion mechanism of zinc transport from the bulk alloy to the surface [26]. In the gold-copper alloy system, copper corrodes preferentially, without dissolution of gold, leaving a porous residue of gold-copper alloy or pure gold. 

Since in some cases the previously described alloy, dealloyed, and controlled-crystal-face catalysts also develop porous/hollow structures, it is of particular interest to determine to what extent the hollow structure affects the high ORR activities seen in those catalysts. Focus points for future research should include (i) developing scalable synthesis techniques and (ii) determining whether the surface and bulk diffusion rates of Pt in these hollow structures, relative to the fuel cell life, are sufficiently slow for this type of catalyst to be practical. 

One model that involves essentiaUy no diffusion in the bulk or on the surface could be characterized as dissolution and reprecipitation. Dissolution of the entire alloy surface under anodic conditions is followed by reprecipitation of the more noble component. Early in this century, many authors postulated this as the most likely mechanism for dealloying [3,37,102]. Much of the early work was done on brasses and involved chemical analysis of the solutions during dealloying. This analysis showed the presence of both copper and zinc, under most conditions. 

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