Colorimetric Detection of the Early Stages of Aluminum Oxidation Using Plasmonic Gold Nano–Island Films

Corrosion is a natural process of degradation of metallic parts reacting with their environment, and is the major cause of failure of metal structures in–service. Hence, detection of the early stages of environmental oxidation of metal parts is of great importance. Here we present a colorimetric method for monitoring the surface conditions of aluminum using the localized surface plasmon resonance (LSPR) response of gold nanoparticles (NPs) pre–deposited onto the aluminum surface. Upon oxidation, metallic aluminum transforms to transparent dielectric aluminum oxide (or hydroxide). In water the formed oxidation product adopts a two–layer structure: the bottom layer is thinner and compact, while the top layer is thicker and highly porous. The gold nano–island layer, initially deposited (by evaporation/annealing) on the aluminum surface, maintains its integrity after corrosion in water and is located between the two aluminum oxide layers. Water oxidation of aluminum substrates with an overlayer of plasmonic gold nano-islands leads to a pronounced color change, which can be monitored in situ using reflection spectroscopy or colorimetrically by the naked eye. Theoretical calculations show that the major spectral change during corrosion is attributed to the interplay between plasmonic scattering of the Au NP layer reacting to the changing environment, and interference within the bottom aluminum oxide layer, acting as an etalon sandwiched between the Au islands and the underlying aluminum. The top, porous oxide layer has only a marginal contribution. The results indicate that plasmonic gold NPs are well suited for real–time monitoring of the corrosion kinetics of aluminum and its alloys in aqueous environment and in air. The technique is cost–effective and can be carried out in the transmission or reflection configuration.