Optical corrosion probes provide several advantages, enabling remote sensing, probing in variable media (gas, liquid) and temperatures, and imaging with high spatial resolution. The optical properties (reflection, transmission) of plasmonic nanoparticles (NPs) in the proximity of a metal mirror strongly depend on the dielectric properties of the immediate environment, characteristic of localized surface plasmon resonance (LSPR) systems, and the separation between the NPs and the metal surface, forming a Fabry-Pérot interferometer with the plasmonic array.^1-3 A plasmonic corrosion probe combines a film of corroding metal of interest and a chemically and morphologically stable plasmonic nanostructure (Au NP array).⁴ Au NPs can be immobilized on the surface of the chemically active metal by adsorption from a colloid solution, or deposited by vacuum evaporation as a film of Au nano-islands.

We studied the corrosion of aluminum in hot water using the optical response of Au NPs immobilized on the chemically active metal surface. Oxidation of Al in water in the temperature range 40–100 ^oC produces a non-uniform hydroxide layer, with a dense layer adjacent to the metal and an outer porous part consisting of rough platelets of pseudoboehmite. High-resolution scanning electron microscopy (HRSEM) images (Figure 1) of a fully oxidized plasmonic probes show that the oxide layer is more dense below the Au NPs and has a rough, open structure on the solution side (above the Au NPs), closely resembling the structure with no plasmonic layer. The Au NP layer embedded in the oxide preserves its integrity, while the separation between the Au NPs and the glass after complete corrosion of the Al film remains nearly constant for Au NPs evaporated or immobilized from solution. Cross-sectional HRSEM and transmission electron microscopy (TEM) images show that the separation between the plasmonic layer and the corroding metal increases with time while preserving the morphology of the oxide layer.

Corrosion of plasmonic probes exhibits large changes in the reflection spectra. The changes were measured quantitatively in-situ using a conventional fiber-optics system. In the course of the Al corrosion the visual appearance (color) of the probe continuously varies. Changes in the reflection spectra and the probe color can be quantified, allowing evaluation of the propagation of the corrosion front. The high color contrast also allows visualization of lateral inhomogeneities in the corroding layer. The combination of plasmonic probes and image processing opens the possibility of simultaneous measurement of local corrosion rates over a macroscopic area.

Plasmonic corrosion probes are simple to implement, while the results may be useful in the design of effective optical sensors for practical applications. With certain modifications, the plasmonic corrosion probe approach may be applicable to other active metals of practical importance.

References.

1. Aussenegg, F. R.; Brunner, H.; Leitner, A.; Lobmaier, C.; Schalkhammer, T.; Pittner, F. Sensors and Actuators B-Chemical 1995, 29, 204.

2. Sannomiya, T.; Balmer, T. E.; Heuberger, M.; Voros, J. Journal of Physics D-Applied Physics 2010, 43.

3. Kedem, O.; Sannomiya, T.; Vaskevich, A.; Rubinstein, I. The Journal of Physical Chemistry C 2012, 116, 26865.

4. Tesler, A. B.; Sabatani, E.; Vaskevich, A.; Rubinstein, I. Annual Meeting of the Israel Chemical Society 2016, Tel Aviv, Israel.