Delft University of Technology Cr(VI)-free pre-treatments for adhesive bonding of aerospace ...

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Delft University of Technology

Cr(VI)-free pre-treatments for adhesive bonding of aerospace aluminium alloys
Abrahami, Shoshan
DOI
10.4233/uuid:d5850b96-ec5b-4639-aac0-e05f81681800
Publication date
2016
Document Version
Final published version
Citation (APA)
Abrahami, S. (2016). Cr(VI)-free pre-treatments for adhesive bonding of aerospace aluminium alloys.
https://doi.org/10.4233/uuid:d5850b96-ec5b-4639-aac0-e05f81681800
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Cr(VI)-free pre-treatments for adhesive bonding
of aerospace aluminium alloys
Ph.D. thesis
Shoshan Tamar Abrahami
This research was performed in Delft University of Technology
Department of Materials Science and Engineering
Mekelweg 2, 2628 CD Delft,
The Netherlands
This research was carried out under the project number M11.6.12473 in the framework
of the Research Program of the Materials innovation institute (M2i) in the Netherlands
(www.m2i.nl).
Cr(VI)-free pre-treatments for adhesive bonding
of aerospace aluminium alloys
Proefschrift
ter verkrijging van de graad van doctor
aan de Technische Universiteit Delft,
op gezag van de Rector Magnificus Prof. Ir. K.C.A.M Luyben,
voorzitter van het College voor Promoties,
in het openbaar te verdedigen op woensdag 7 december 2016 om 15:00 uur
door
Shoshan Tamar ABRAHAMI
Master of Science in Materials Science and Engineering,
Delft University of Technology, Delft, the Netherlands
geboren te Haifa, Israël.
This dissertation has been approved by the
Promotor: Prof. Dr. H. Terryn
Copromotor: Dr. J.M.C Mol
Composition of the doctoral committee:
Rector Magnificus chairman
Prof. dr. H. Terryn Delft University of Technology, promotor
Dr. J.M.C. Mol Delft University of Technology, copromotor
Dr. J.M.M de Kok Fokker Aerostructures, The Netherlands
Independent members:
Prof. dr. B.J. Thijsse Delft University of Technology, The Netherlands
Prof. dr. J.F. Watts University of Surrey, United Kingdom
Prof. dr. P. Marcus École Nationale Supérieure de Chimie de Paris, France
Dipl. Ing. T. Hack Airbus Group Innovations, Germany
Prof. dr. J. Sietsma Delft University of Technology, The Netherlands (reserve
member)
Keywords: Aluminium, Cr(VI)-free, Surface pretreatments, Anodizing, Adhesive
bonding, Adhesion, Durability.
ISBN 978-94-91909-43-6
Cover design: Visual Essence, Utrecht, NL
Copyright © 2016 by S.T. Abrahami
All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilized
in any form or by any means, electronic or mechanical, including photocopying, recording or by any
information storage and retrieval system, without permission from the author.
Printed by Proefschriftmaken.nl
Content
Summary VII
Samenvatting IX
Abbreviations and acronyms XI
CHAPTER 1 Introduction 1
CHAPTER 2 Literature review 7
CHAPTER 3 The role of acid-base properties in the interactions across
the oxide-primer interface in aerospace applications 47
CHAPTER 4 XPS analysis of the surface chemistry and interfacial
bonding of barrier-type Cr(VI)-free anodic oxides 67
CHAPTER 5 Effect of anodic aluminium oxide chemistry
on adhesive bonding of epoxy 87
CHAPTER 6 Interfacial bonding strength and stability of epoxy
and phenol resins on different aluminium oxide
and silane chemistries 105
CHAPTER 7 Potentiodynamic anodizing of aluminium alloys
in Cr(VI)-free electrolytes 127
CHAPTER 8 The effect of Cr(VI)-free anodizing conditions on
interfacial bonding strength and durability of adhesively
joined aluminium 141
CHAPTER 9 Conclusions & recommendations 163
Appendix 173
Acknowledgments 179
List of publications 181
Curriculum Vitae 183
V

Summary
For more than six decades, chromic acid anodizing (CAA) has been the central process
in the surface pre-treatment of aluminium for adhesively bonded aircraft structures
in Europe. Unfortunately, this electrolyte contains hexavalent chromium (Cr(VI)), a
compound known for its toxicity and carcinogenic properties. The approaching ban on
the use of hexavalent chromium (Cr(VI)) makes its elimination a high-priority R&D
topic within the aerospace industry and the Cr(VI)-era will soon have to come to an
end.
Anodizing aluminium in acid electrolytes produces a self-ordered porous oxide layer with
a thin barrier layer underneath. This special type of oxide readily adheres to the organic
resin and provides protection against corrosion. Although Cr(VI)-free candidates such
as sulphuric acid- (SAA), phosphoric acid- (PAA) and mixtures of phosphoric-sulphuric
acid anodizing (PSA) can be used to create this type of structure, the excellent adhesion
and corrosion resistance that is currently achieved by the Cr(VI)-based process is not
easily matched.
To gain a better understanding of the underlying physical and chemical mechanisms
that contribute to the adhesion and durability in these structures, this study investigates
the correlation between the oxide’s chemical and morphological characteristics, as
influenced by the anodizing electrolyte, and bond performance. The major challenge in
the mechanistic understanding of the adhesion in bonded components is to differentiate
between the different forces acting at the oxide/resin interface.
In the first part of this PhD thesis, studies focus on the role of surface chemistry. To
exclude the contribution of mechanical interlocking between the oxide and the resin,
featureless oxides were prepared by stopping the anodizing during the formation of the
barrier layer. Surface characterization of the different anodic oxides by means of Fourier
transform infrared (FTIR) and X-ray Photoelectron Spectroscopy (XPS) revealed no
significant net change in the acid-base properties of the different anodic oxides. It
was found that local chemical changes were introduced due to the incorporation of
electrolyte-driven anions. Therefore, a model was developed to quantify the relative
amounts of O2-, OH−, PO 3−, and SO 2−, showing significant changes in the type and
4 4
amount of surface species. Consequently, measurements showed that the pretreatments
and the molecule type affected oxide/molecule interfacial interactions. To evaluate
the contribution of adsorptive interaction in practice, peel tests were performed on
featureless oxides bonded with commercial aerospace adhesives. Results showed that
significant initial dry adhesion is achieved with FM 73 epoxy without mechanical
interlocking, and independent of the type of pretreatment. However, the formed
bonding was not water resistant, with the amount of applied stress needed for peeling
linearly increasing with the amount of surface hydroxyls. Moreover, the application of
a thin γ-APS silane layer before bonding with epoxy has confirmed that the stability of
VII
Summary
the interface is also determined by the nature of the bond, showing much more stable
interfaces in the presence of covalent interactions. When peel tests were performed with
a phenolic-based adhesive (Redux 775), no correlation to the surface chemistry was
found. Nevertheless, the bonded joints on the basis of the weakly acidic character of
the phenolic adhesive showed better resistance to corrosion in salt spray tests, compared
to those on the basis of the epoxy adhesive. Therefore, we conclude that both oxide
surface- and adhesive chemistries play a role in the formation and long-term stability of
the oxide/resin interface.
In the second part of this thesis industrial porous oxides were applied. Fundamental
investigations show that changing the voltage during anodizing can produce
morphological variations across the oxide thickness. The effect of the initial voltage
sweeps, however, was limited by the oxide dissolution action of phosphoric acid in PSA,
since prolonged anodizing in this electrolyte not only leads to an increase of the pore
diameter, but also completely dissolves the upper most part of the oxide. Morphological
changes were distinguished between geometrical modifications that affect the pore size
and changes in the surface roughness that was caused by extended chemical dissolution at
higher anodizing temperatures and/or phosphoric acid concentration. Measured carbon
concentration profiles within the pores using high-resolution transmission electron
microscopy (TEM) coupled with energy-dispersive X-ray spectroscopy (EDS) indicated
that resin penetration is affected by both aspects. Moreover, mechanical performance
in peel tests indicates that these parameters, rather than the oxide layer thickness are
critical for moisture-resistant adhesion. Both adhesion mechanisms: adsorption and
mechanical interlocking seem to contribute to the adhesion in these structural bonds. A
higher degree of dissolution during anodizing is beneficial for the adhesion, facilitating
a composite-like interphase. Too much dissolution, however, reduces the resistance to
bondline corrosion. Overall, the presented results illustrate the need to consider both
chemical and morphological changes in the selection of Cr(VI)-free alternatives for
structural adhesive bonding.
VIII
Samenvatting
Al meer dan zes decennia lang is chroomzuur anodiseren (CAA) de belangrijkste
stap in de oppervlakte-voorbehandeling van aluminium panelen in de Europese
vliegtuigbouw. Helaas bevat chroomzuur zeswaardig chroom (Cr(VI)), een zeer giftige
en kankerverwekkende stof. Het aanstaande verbod op het gebruik van deze stof maakt
de eliminatie ervan een hoge prioriteit binnen de luchtvaartindustrie en het Cr(VI)-
tijdperk zal op afzienbare termijn tot een einde moeten komen.
Het anodiseren van aluminium in zure elektrolyten resulteert in een poreuze oxidelaag
met daaronder een dunne barrièrelaag. Dit type oxide is uitermate geschikt voor
lijmhechting en beschermt bovendien het substraat tegen corrosie. Hoewel anodiseren
in Cr(VI)-vrije alternatieven zoals zwavelzuur (SAA), fosforzuur (PAA) en mengsels van
fosfor- en zwavelzuur (PSA) kunnen worden gebruikt om een dergelijke structuur te
creëren, is de uitzonderlijk goede hechting en corrosiebestendigheid van CAA moeilijk
te evenaren.
Om een beter begrip van de onderliggende fysische en chemische mechanismen die
bijdragen aan de hechting en de duurzaamheid van deze structuren te verkrijgen,
is in deze studie het verband tussen de kwaliteit van adhesie en de chemische en
morfologische kenmerken van het oxide onderzocht. De nadruk is gelegd op de invloed
van het elektrolyt op de hechting en duurzaamheid van lijmverbindingen. De grootste
uitdaging bestaat erin de verschillende bijdragen aan hechting in deze gelijmde panelen
te onderscheiden.
Het eerste deel van dit proefschrift richt zich erop de rol van de oppervlaktechemie te
bestuderen. Om de bijdrage van de mechanische hechting tussen het oxide en de lijm uit
te sluiten, is gebruik gemaakt van modeloxiden. Deze zijn bereid door het anodisatie-
proces te stoppen tijdens de vorming van de barrièrelaag. Oppervlaktekarakterisering
door middel van Fourier transform infrared (FTIR) spectroscopie en X-ray
photoelectron spectroscopie (XPS) vertoonde geen significante veranderingen in de
zuur-base eigenschappen van de verschillende anodische oxiden. Lokale chemische
veranderingen komen echter voort uit de opname van elektrolyt-afhankelijke anionen.
Het ontwikkelde model om de relatieve hoeveelheden van O2-, OH-, PO 3- en SO 2-
4 4
te kwantificeren, bracht significante verschillen in type en hoeveelheid anionen aan
het licht. Aanvullende metingen hebben aangetoond dat de voorbehandeling en het
type organisch molecuul de interacties aan het grensvlak tussen molecuul en oxide sterk
beïnvloeden. Om de bijdrage van adsorptie in de praktijk te evalueren, zijn pelproeven
met commerciële lijmen uitgevoerd. Resultaten toonden aan dat een goede initiële (droge)
hechting met FM 73 epoxy lijm kan worden bereikt zonder mechanische verankering
en onafhankelijk van het type elektrolyt. Desalniettemin, de gevormde binding is niet
waterbestendig en de mechanische belasting nodig voor het doen bezwijken van de
lijmverbinding neemt lineair toe met de hydroxylfractie aan het oppervlak. Wanner
IX