Project: Cr(VI)-free pretreatments for adhesive bonding of aerospace aluminum alloys
Shoshan Abrahami is currently finishing a 4-year PhD research project at Delft University of Technology. Working in cooperation with Fokker Aerostructures (NL), her research focuses on fundamental understanding on the role of oxide properties (e.g. the surface chemistry and morphological features) on the adhesion and durability of in the adhesion and durability of structural adhesive bonding for aircrafts. She has a Master in Materials Science and Engineering from TU Delft, with MSc thesis on the recycling rare earths from NdFeB magnets in HDD’s. and a Bachelor in Molecular Science and Technology, also from TU Delft.
For more than six decades, chromic acid anodizing (CAA) has been the central process in the surface treatment of aluminium 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 European REACH regulations, the Cr(VI) era will soon have to come to an end.
Anodizing aluminium 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.
Gaining confidence in the function of new systems requires a fundamental understanding of the forces that provide this system with its excellent adhesion and durability properties. This demands knowledge on both microscopic and macroscopic levels. On a microscopic level, adhesion is achieved by atomic and molecular interactions between the oxide and the resin (primer/ adhesive). On a macroscopic level, mechanical interlocking between the substrate and the adhesive also promote adhesion. This type of interactions is introduced when the resin is able to interlock with rough surface features (e.g. cavities and oxide pores) on the substrate.
In this study, we investigate how the oxide/adhesive interface (Fig. 1.2 (a)) is affected by the nature of the electrolyte and the anodizing conditions. Since industrial oxides that are used for structural bonding are porous, two main adhesion mechanisms are generally working simultaneously. In order to differentiate between individual contributions, two types of oxides are applied in this study: featureless and porous oxides (Figure 1 (b) and (c), respectively). In order to evaluate the link between fundamental principles of adhesion and bond performance, the work in this PhD research 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.
Figure 1: Schematic illustrations of (a) the components of the adhesive bond, showing the region of interest at the oxide/adhesive interface, (b) the model system used to study the effect of oxide chemistry and (c) the industrial system used to study the effect of oxide morphology.
List of publications
S.T. Abrahami, J.M.M. de Kok, H. Terryn and J.M.C Mol, Towards Cr(VI)-free Anodization of Aluminum Alloys for Aerospace Adhesive Bonding Applications: A Review. Frontiers in Chemical Science and Engineering, 2016, in preparation.
S.T. Abrahami, J.M.M. de Kok, H. Terryn and J.M.C Mol, Adhesive Bonding Performance Investigated as a Function of Oxide and Adhesive Chemistries, in preparation.
S.T. Abrahami, J.M.M. de Kok, V.C. Gudla, R. Ambat, H. Terryn and J.M.C Mol, The effect of Cr(VI)- Free Anodizing Conditions on the Adhesion and Durability of Adhesively Bonded Aluminium, in preparation.
S.T. Abrahami, T. Hauffman, J.M.M. de Kok, J.M.C Mol, and H. Terryn, The Effect of Anodic Aluminum Oxide Chemistry on Adhesive Bonding of Epoxy., submitted.
S.T. Abrahami, T. Hauffman, J.M.M. de Kok, J.M.C Mol, and H. Terryn, The role of acid-base properties in the interactions across the oxide-primer interface in aerospace applications, Surface and Interface Analysis, 2015.
S.T. Abrahami, T. Hauffman, J.M.M. de Kok, J.M.C Mol, and H. Terryn, XPS analysis of the surface chemistry and interfacial bonding of barrier-type Cr(VI)-free anodic oxides, The Journal of Physical Chemistry C, 2015 119 (34) 19967-19975.
M. van Put, S.T. Abrahami, O. Elisseeva, J.M.M de Kok, J.M.C Mol, H. Terryn, Potentiodynamic anodizing of aluminium alloys in Cr(VI)-free electrolytes, Surface and Interface Analysis, 2015.
S.T. ABRAHAMI, Y. XIAO and Y. YANG, Rare-earth elements recovery from post-consumer hard-disc drives, Mineral Processing and Extractive Metallurgy, 2015. 124(2): p. 106-115.