In-situ measurement of oxide film growth on aluminium using PM-IRRAS

The adhesion of organic coatings to aluminium is influenced significantly by the chemistry of the native surface layer on aluminium. The chemistry of the oxide layer and the changes that occur in humid environments have been of interest for many years, studied for example using XPS. Here, the chemistry of the surface layers is characterised with polarisation modulated infrared reflection absorption spectroscopy (PM-IRRAS); this technique measures the differential reflectance of a sample without the need of background spectra, as necessary in conventional IRRAS. Using simple IRRAS, only a difference spectrum can be obtained, usually with respect to an air-formed aluminium film. In contrast, PM-IRRAS allows direct acquisition of spectra from the oxide using the modulation of the infrared light from the p to the s-polarisation state to remove the contribution of species not adsorbed at the surface. This facilitates characterisation of changes to the native oxide layers during environmental exposure. According to a recently developed calibration procedure, PM-IRRAS spectra can be used to achieve quantitative analysis. This procedure is explored using anhydrous, thermally grown amorphous air-formed films on magnetron sputtered aluminium. This quantitative analysis can thus give insight into the change in surface chemistry associated with film growth under humid or immersed conditions, i.e. types of service environment for coated aluminium.