An ORP-EIS approach to distinguish the contribution of the buried interface to the electrochemical behaviour of coated aluminium

In this work, the influence of the buried coating/metal oxide interface as a crucial parameter in hybrid systems’ durability was investigated. To evaluate these hybrid interfaces in situ and in realistic conditions/environments, an integrated approach based on the use of odd random phase electrochemical impedance spectroscopy (ORP-EIS) is employed. In this approach proposed previously by our group, in addition to the effectively shortened measurement time, supplementary information about the stochastic noise, non-linearities and non-stationarities is provided. This information allows the rapid analysis of the error structures on the experimental data as well as providing a statistical evaluation for the proposed electrochemical equivalent circuit (EEC). To elaborate on the interfacial influences, aluminium substrates received two different surface pre-treatments (tartaric sulfuric acid (TSA) anodizing and acidic pickling) and were coated with a pigmented polyurethane based and a pigmented epoxy-amine based coating. The ORP-EIS data was fitted to an appropriate EEC and the output parameters were used to distinguish between the diffusion of water and ions. It was shown that prior to the arrival of ions at the buried interface, the coating response is dominating. With further diffusion of ions and their arrival at the interface, the onset of the contribution of different surface pre-treatments was captured and quantified.