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2017, Volume 11, Issue 3

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Frontiers of Chemical Science and Engineering >> 2017, Volume 11, Issue 3 doi: 10.1007/s11705-017-1641-3

Towards Cr(VI)-free anodization of aluminum alloys for aerospace adhesive bonding applications: A review

. Materials innovation institute (M2i), 2628 XG, Delft, The Netherlands.. Delft University of Technology, Department of Materials Science and Engineering, 2628 CD, Delft, The Netherlands.. Fokker Aerostructures BV, 3351 LB, Papendrecht, The Netherlands.. Department of Materials and Chemistry, Research Group Electrochemical and Surface Engineering (SURF), Vrije Universiteit Brussel, 1050 Brussels, Belgium

Accepted: 2017-05-16 Available online: 2017-08-23
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Abstract

For more than six decades, chromic acid anodizing (CAA) has been the central process in the surface pre-treatment of aluminum for adhesively bonded aircraft structures. Unfortunately, this electrolyte contains hexavalent chromium (Cr(VI)), a compound known for its toxicity and carcinogenic properties. To comply with the new strict international regulations, the Cr(VI)-era will soon have to come to an end. Anodizing aluminum in acid electrolytes produces a self-ordered porous oxide layer. Although different acids can be used to create this type of structure, the excellent adhesion and corrosion resistance that is currently achieved by the complete Cr(VI)-based process is not easily matched. This paper provides a critical overview and appraisal of proposed alternatives to CAA, including combinations of multiple anodizing steps, pre- and post anodizing treatments. The work is presented in terms of the modifications to the oxide properties, such as morphological features (e.g., pore size, barrier layer thickness) and surface chemistry, in order to evaluate the link between fundamental principles of adhesion and bond performance.

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