Chloride-induced alterations of the passive film on 316L stainless steel and blocking effect of pre-passivation

TL;DR

This study investigates how chlorides alter the passive film on 316L stainless steel and demonstrates that pre-passivation in chloride-free conditions can prevent chloride-induced damage, maintaining the film's protective properties.

Contribution

It reveals the mechanism of chloride penetration into the passive film and shows that pre-passivation effectively blocks chloride entry, enhancing corrosion resistance.

Findings

Chlorides enter the outer hydroxide layer of the passive film at low concentrations.

Chlorides weaken the passive film by poisoning dehydroxylation and altering composition.

Pre-passivation in chloride-free electrolyte prevents chloride ingress and preserves film integrity.

Abstract

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.