Similarities and Differences Between Dealloying and Alloying-induced Passivity, and Current Modeling Challenges

Atomistic simulation of dealloying has made great strides since Erlebacher introduced his MESOSIM code in 2004 [1]. Most recently, the model has been adapted to understand the parting limit for dealloying [2]. Similar progress in the simulation of alloy passivity, based on a percolation model [3], might appear equally promising, but there is a fundamental difference. In the percolation approach to alloy passivity, selective dissolution of one element (e.g. Fe in Fe-Cr) exposes atoms of the other element (e.g. Cr in Fe-Cr) that form a 3D polymeric oxide network without initially moving from their lattice positions. Such a model is appealing, but does not provide passivity at 15 or 20% Cr if Fe can dissolve from terrace sites. In simulation, passivation only occurs at these realistic Cr contents if Fe dissolution is restricted to less-highly coordinated sites, or if the Cr oxide network is considered to provide a steric hindrance to Fe dissolution from terrace sites that lie close to oxidized Cr. The latter proposition, while physically reasonable (the Fe ions must become solvated to dissolve, and thus must be able to occupy a certain volume) creates severe problems for a realistic simulation. We have approached the problem step by step, and will report progress at this conference.

A different but related approach to the problem of alloy passivity involves the consideration of connectivity within a pre-formed binary oxide. This idea is mainly associated with E. McCafferty. A difficulty with such a model, when applied to the Fe-Cr system, is that an arbitrary enrichment multiplier for Cr within the oxide has to be assumed for the geometric criterion to work. Progress on atomistic simulations along these lines will also be reported.

1 J. D. Erlebacher, J. Electrochem. Soc., 151, C614-C626 (2004).

2 D.M. Artymowicz, R.C. Newman and J.D. Erlebacher, Philos. Mag., 89, 1663-1693 (2009).

3 K. Sieradzki and R.C. Newman, J. Electrochem. Soc., 133, 1979‑1980 (1986).