Astrid Bjørgum

Anodising as pre-treatment for structural bonding

Degradation of adhesive bonded aluminium profiles in various environments has been investigated in terms of the tensile strength of lap shear joints and wedge testing. Prior to adhesive bonding, the extrusions were pre-treated using various anodising processes. The results showed that anodising conditions, and thus structure and thickness of the anodised films, affect adhesion and durability of the adhesive joints. Tensile lap-shear testing of samples exposed in corrosive environments showed that hot AC anodising, using either sulphuric or phosphoric acid electrolytes, performed as well as standard PAA in spite of much shorter anodising times and no etching step prior to anodising. This was confirmed by wedge testing at high relative humidity. Wedge results, however, showed adhesive fracture of samples hot AC anodised in sulphuric acid, while cohesive fracture was found for samples anodised in phosphoric acid.

The Structure and Impurities of Hard DC Anodic Layers on AA6060 Aluminium Alloy

Hard anodic layers produced under DC conditions in sulphuric acid on a commercial AA 6060 aluminium alloy have been characterised by electron microscopy and secondary ion mass spectroscopy (SIMS). Once exposed during anodising, α–Al(Fe, Mn)Si intermetallic particles start to corrode in the electrolyte. Partially corroded particles become trapped in the oxide, disturbing the local pore structure. This leads to pyramidal defects on the alloy surface and retention of isolated, unmodified fragments of alloy in the oxide. Sealing in boiling water left a low density, loosely attached surface layer of modified oxide that gave poor adhesion properties. The bulk oxide pore structure was modified by a uniform, fine network of bridges that extended down the barrier layer. Calibration of the SIMS sputtering rate allowed the concentration of alloying and impurity elements to be measured as a function of depth in the oxide and into the alloy.