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This work presents an experimental validation that an ultrafine grained (UFG) structure of binary Fe-Cr alloys fabricated by severe plastic deformation (SPD) can bring about dramatic improvements in the corrosion resistance. More specifically, UFG Fe-Cr alloys with subcritical chromium content for passivation (8% and 10% Cr) was found to exhibit passivity, and was resistant to corrosion in an aqueous 0.6 mol/L NaCl solution whereas coarse-grained Fe-12% Cr, which are known as a stainless steel and passive in most dilute aerated solution, was degraded by corrosion. The findings indicated that the critical threshold of Cr content required for establishing a protective layer by self-passivation in binary Fe-Cr alloy is microstructure dependent, and can be reduced by grain size reduction to a sub-micron scale by SPD.