Formation of Blisters on Painted Aeronautic Al 7050 Pieces: Fine Characterisation and Exploration of the Underlying Mechanism

Abstract

Protection of metal substrates against corrosion is a major challenge for a wealth of industrial applications as in aeronautics, especially when investigating CrVI-free, environmentally friendly options. In this work, we addressed the case of the widely used Al 7050 alloy, undergoing a 2-step anticorrosion treatment based on surface anodising and silica sealing, and for which blistering is sometimes uncontrollably detected. Although not systematically associated to non-usability of the industrial pieces, the aim of this work was to identify the weak element of such multi-layered surfaces, and to investigate in detail the underlying mechanism of blister formation. Cross-section observations using ionic polishing and X-ray tomography unveiled that the silica sealing layer was at the origin of the formation of the blisters. Using an original setup to record pH change as a function of time and electrode-substrate distance, we succeeded to precisely quantify the pH drop occurring at the surface of the substrate upon silica gel formation. Analysis by Raman, XPS and NMR complementary techniques of synthesised, self-supported silica samples then allowed characterising different types of silica structures, associated with a variable number of non-bridging oxygens as structural defects. This characterisation served as reference for the analysis of actual aluminium-based treated samples. Results revealed that the blistering effect observed was directly related to experimental parameters such as the silica concentration in the sealing bath and temperature, and that blistering was due to local swelling effects occurring for defective silica structures containing non-bridging oxygens. Provided that such parameters are mastered by industrial manufacturers, the silica sealing layer should not undergo swelling, thus preventing the formation of blister formation on the painted metal pieces, and could on the other hand be considered as a smart strategy to fill the porous structure of anodised layers for anticorrosive purposes.

Keywords: Protection, blistering, silicates, Surface characterisation, XPS, NMR