Characterisation of oxide and hydroxide layers on technical aluminum materials using XPS

Aluminum alloys are widely used as technical lightweight materials. For industrial applications, these materials often have to be bonded, e. g. welded or brazed. To get a metallic connection by brazing, the natural oxide layer on Al-materials, which is dense and has a high melting point, has to be eliminated. This layer can be influenced by the temperature and humidity in the surrounding atmosphere. Due to the nm-thickness of the layer, the analysis of its thickness and composition is challenging. An applicable method is X-ray photoelectron spectroscopy (XPS). The present investigation shows the applicability of XPS to estimate the oxide layer thickness as well as to distinguish between Al-hydroxide and Al-oxide phases. Finally, reasons for the differences in brazeability depending on the previous storage conditions are studied. It is shown, that a storage under condensation for 9 days causes an increase of the oxide layer thickness and an aggregation of water inside the pores of the hydroxide layer. This lowers significantly the brazeability of the material.

► XPS can be used very convenient to analyse the oxidation/hydration behavior of Al-surfaces.
► The binding energy spectra cannot be used to distinguish between Al-oxide and Al-hydroxide.
► But the ratio between the oxidic bound Al and O enables the differentiation between Al-oxide and Al-hydroxide.
► Al-hydroxide is available on all technical Al-surfaces.
► The storage under condensation decreases the brazeability of Al-materials.