Oxygen effects on passive film evolution and metastable pitting in 316L stainless steel using SECCM

The corrosion behaviour of industrially-produced 316 L stainless steel in 0.01 M NaCl solution was investigated using scanning electrochemical cell microscopy (SECCM) under potentiostatic conditions (+1.2 V vs Ag/AgCl, 60 s). Variations in micro-droplet footprint size during sequential SECCM measurements created an oxygen gradient at the metal-electrolyte interface, enabling systematic investigation of the effect of the oxygen supply rate, or aeration, on both general corrosion and metastable pitting kinetics. Chronoamperometric data from 205 individual measurements were analysed using custom quantitative descriptors and Linear Discriminant Analysis (LDA) of EBSD-derived crystallographic features. Manual classification of 91 transient events into two distinct categories – “peak” (metastable pitting) and “belly” (passive film thinning) - combined with data-driven analysis revealed previously hidden correlations. Higher aeration conditions (smaller droplets, 2–3 μm) produced 20% higher oxygen flux compared to lower aeration (larger droplets, ~8 μm), leading to increased initial dissolution currents and faster passive film formation. However, these rapidly-formed films exhibited persistently elevated passive current densities, while higher aeration conditions produced longer-duration belly-type transient events, revealing a trade-off between passive film protectiveness and repassivation capability. Strong correlations between early-time electrochemical descriptors and total corrosion charge suggest potential for rapid, non-destructive corrosion susceptibility assessment. These findings reveal distinct relationships between oxygen availability and corrosion behaviour.