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Using first-principles quantum-mechanical theory, we demonstrate that the surface chemistry of Fe-Cr alloys follows the peculiar threshold behavior characteristic of ferritic stainless steels. We find that in dilute alloys the surfaces are covered exclusively by Fe, whereas for bulk Cr concentration above ϳ10% the Cr-containing surfaces become favorable. The two distinctly dissimilar surface regimes appear as a consequence of two competing magnetic effects: the magnetically induced immiscibility in bulk Fe-Cr alloys and the stability of magnetic surfaces.