Capturing transient selective copper dissolution from 316L stainless steel by reflectometry and element-resolved measurements

Transient surface states can control the reactivity of stainless steel yet remain invisible to conventional post-mortem characterization because they may disappear as soon as electrochemical control is interrupted. Here, in situ reflective microscopy (RM) was combined with electrochemistry and matched atomic emission spectroelectrochemistry (AESEC) measurements to follow surface film evolution on 316L stainless steel. The surface progressively darkened during anodic polarization, but release from the vertex potential to open-circuit potential produced an abrupt brightening. Under the matched electrochemical sequence, AESEC detected selective Cu dissolution after potential release corresponding to a 0.4 nm CuO-equivalent layer. A multilayer Fresnel model containing a retained mixed-oxide passive film, in which CuO leaves the interface, reproduces the magnitude of the brightening under the selected optical assumptions. Residual-charge calculations admit conditional mixed-valence scenarios. Together with thermodynamic calculations, these results support selective removal of a transient Cu-containing surface component. Such short-lived states can escape detection during transfer and post-mortem analysis, which motivates coupled real-time monitoring.