  The durability of passivable metals and alloys is often limited by the
stability of the surface oxide film, the passive film, providing
self-protection against corrosion in aggressive environments. Improving this
stability requires to develop a deeper understanding of the surface structure
and initial surface reactivity at the nanometric or atomic scale. In this work
we applied scanning tunneling microscopy to unravel the surface structure of a
model stainless steel surface in the metallic state and its local modifications
induced by initial reaction in dioxygen gas. The results show a rich and
complex structure of the oxide-free surface with reconstructed atomic lattice
and self-organized lines of surface vacancies at equilibrium. New insight is
brought into the mechanisms of initial oxidation at steps and vacancy injection
on terraces leading to Cr-rich oxide nuclei and locally Cr-depleted terraces,
impacting the subsequent mechanism of chromium enrichment essential to the
stability of the surface oxide.
