In-situ nanoscale thickness monitoring of passive films based on electrochemical-surface plasmon resonance sensing technology

Gold (Au) is frequently employed as a critical connector component in precision electronic instruments due to its exceptional stability. However, it remains susceptible to corrosion deterioration in complex environments such as in harsh marine settings. The passive film that forms on the Au surface is vital for corrosion protection, making the monitoring of the structure and thickness of this nanoscale film essential for ensuring the safe operation of the Au. A terminal-reflective optical-fiber structure was utilized both as the working electrode for an electrochemical (EC) sensing and as the surface plasmon resonance (SPR) sensing element, thus creating an integrated dual-function optical fiber EC-SPR sensor. This sensor enabled detection of the formation of Au passive films at various constant voltages in a high-salinity environment, alongside monitoring changes in nanoscale thickness. Based on the polarization curves, the passivation of Au was conducted at different constant voltages. During this process, the EC signal and the SPR resonance wavelength were monitored in real time to track the passivation progression. The in-situ EC-SPR spectra revealed the dynamic changes occurring on the Au surface during the formation. Notably, as the voltage increased, the thickness of the passive film correspondingly increased, yielding equivalent passive film thicknesses of 10 (±0.3) nm, 14 (±0.4) nm, and 18 (±1.1) nm at constant voltages of 0.4 V, 0.6 V, and 0.8 V, respectively. This integrated optical fiber dual-function sensor facilitates online measurement of nanoscale Au passive film thickness, providing a novel monitoring approach for corrosion detection and the safe operation of deep-sea equipment.