  The iron-chromium alloy and its derivatives are widely used for their
remarkable resistance to corrosion, which only occurs in a narrow concentration
range around 9 to 13 atomic percent chromium. Although known to be due to
chromium enrichment of a few atoms thick layer at the surfaces, the
understanding of its complex atomistic origin has been a remaining challenge.
We report an investigation of the thermodynamics of such surfaces at the atomic
scale by means of Monte Carlo simulations. We use a Hamiltonian which provides
a parameterization of previous ab initio results and successfully describes the
alloy's unusual thermodynamics. We report a strong enrichment in Cr of the
surfaces for low bulk concentrations, with a narrow optimum around 12 atomic
percent chromium, beyond which the surface composition decreases drastically.
This behavior is explained by a synergy between (i) the complex phase
separation in the bulk alloy, (ii) local phase transitions that tune the layers
closest to the surface to an iron-rich state and inhibit the bulk phase
separation in this region, and (iii) its compensation by a strong and
non-linear enrichment in Cr of the next few layers. Implications with respect
to the design of prospective nanomaterials are briefly discussed.
