The metal/polymer interface in aluminium adhesive joints: A microanalytical study

Abstract

Aluminium adhesive joints have been prepared with metal pretreatments used in the aerospace industry, namely chromic sulphuric acid pickling (CSA, FPL-etch), phosphoric acid anodizing (PAA), and chromic acid anodizing (CAA). Chemical and physical techniques are discussed which render the interfaces accessible to electron microscopy and microanalysis. These comprise fracturing, selective removal of metal or metal plus oxide through chemical etching, angle lapping, and argon ion sputtering. The applied analytical techniques are high resolution SEM, scanning Auger, and ESCA. It is found that the porous oxide layers are completely invaded by the adhesive primer, which cures into a fibrous structure. The porous region contains more nitrogen compounds than the primer itself, possibly owing to the migration of the reactive part of the primer into the pores and to the adsorption and interphase segregation of low-molecular-weight constituents. The early stage of corrosion in one case is studied. The techniques discussed here are suitable for quality control or failure analysis. The work supports the concept of a mechanical anchoring of the adhesive within the oxide pores as the predominant adhesion mechanism.

Keywords:

• Adhesive bonding
• aluminium
• interface
• AES
• ESCA
• SEM