  Electrochemical polarization measurements were combined with surface analysis
by Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS), X-Ray
Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM) to study the
alterations of the passive film on 316L austenitic stainless steel induced by
the presence of chlorides in sulfuric acid electrolyte. The work was performed
at a stage of initiation of localized corrosion preceding metastable pitting at
the micrometer scale as verified by current transient analysis and AFM. The
results show that Cl-ions enter the bilayer structure of the surface oxide
already formed in the native oxide-covered initial surface state at
concentrations below the detection limit of XPS (< 0.5 at%), mostly in the
hydroxide outer layer where Fe(III) and Mo(IV,VI) species are concentrated but
barely in the oxide inner layer enriched in Cr(III). Their main effect is to
produce a less resistive passive state by poisoning dehydroxylation and further
Cr(III) and Mo(IV,VI) enrichments obtained in the absence of chlorides. This
detrimental effect can be suppressed by pre-passivation in a Cl-free
electrolyte, which blocks the entry of chlorides in the passive film, including
in the outer exchange layer, and enables the beneficial aging-induced
variations of the composition to take place despite the presence of chlorides
in the environment.
