Chromate-free Pre-treatment of Aluminium for Adhesive Bonding

Abstract

The increasing use of aluminium in automotive and transport applications is primarily driven by its high strength to weight ratio, enabling substantially improved fuel economy and reduced CO2 emissions when substituted for heavier materials. However, the change of material presents new challenges with respect to design and methods of joining. Structural adhesive bonding offers several advantages compared with welding, but a major limitation is concern about the durability of joints in wet and corrosive environments. The pre-treatment of the aluminium surface prior to bonding is the key to long service life. Pre-treatments successfully employed by the aerospace industry cannot be used in automotive production, where cheaper and more environmentally friendly pre-treatments are required. Specifically, the use of chromates is unacceptable. Hence, there is a need to develop chromate-free pre-treatments that will consistently provide the required level of performance, while being acceptable both in terms of general engineering practice and economy. To accomplish this task, basic knowledge of the processes occurring on the aluminium surface during pre-treatment, properties of the modified surface, and mechanisms of joint degradation are necessary.

The purpose of the present work has therefore been to contribute to a better understanding of how the aluminium substrate affects the formation and properties of conversion coatings for adhesive bonding. In particular, a commercial chromate-free fluorotitanate/zirconate based process has been investigated and compared with conventional chromate treatment. The materials chosen for this work were commercially extruded AA6060-T6 aluminium and a structural single-part epoxy adhesive. To complement the studies of the commercial alloy, model analogues of the AA6060 aluminium matrix and α-Al(Fe,Mn)Si phase particles present in the commercial alloy were also investigated.

It was observed that the α-Al(Fe,Mn)Si particles played an essential role in the formation and properties of Ti-Zr oxide conversion coatings on AA6060 aluminium. The particles were significantly nobler than the aluminium matrix in the pre-treatment solution. An alkaline diffusion layer therefore developed around the particles during pre-treatment due to oxygen reduction and hydrogen evolution reactions. As Ti-Zr oxide precipitation was favoured at high pH, the conversion layers normally deposited at and in the vicinity of the cathodic particles. The conversion layers formed consequently exhibited considerable lateral variations in thickness. In addition to substrate microstructure, bulk pH and agitation of the conversion bath were important factors controlling the extent of Ti-Zr oxide deposition and its distribution on the surface. On areas well away from the cathodic particles coverage was generally very poor, although a high density of small (<50 nm) oxide particles was deposited, presumably with a composition similar to the continuous conversion layer close to the α-Al(Fe,Mn)Si particles. The cathodic activity of the particles was only slightly reduced by formation of the Ti-Zr oxide conversion coating. In combination with poor coverage of the aluminium matrix, these conversion coatings are therefore not expected to improve the corrosion resistance of aluminium significantly.

In contrast to the above mechanism, the chromate conversion coating (CCC) formed by a redox reaction between chromate ions and aluminium. A relatively thick, porous chromium oxide layer developed over the aluminium matrix of AA6060, while a significantly thinner film was formed on the α-Al(Fe,Mn)Si particles. The morphology of the CCC covering the matrix was influenced by the hardening Mg2Si phase, primarily by promoting nucleation of the CCC. Despite the thin film (<50 nm) formed on the α-Al(Fe,Mn)Si particles by chromating, the cathodic activity was significantly reduced. Inhibition of the cathodic reactivity at these particles is suggested as an important factor contributing to the high performance of chromate pretreatments on aluminium.

Testing of epoxy-bonded AA6060 aluminium joints in humid environment showed that Ti-Zr based pre-treatment provided improved adhesion relative to alkaline etching and deoxidation only. However, Ti-Zr based pre-treatment was inferior to chromating. Rapid, interfacial crack growth during wedge testing was particularly observed for adherends with a relatively thick Ti-Zr oxide deposit, suggesting that excessive Ti-Zr oxide deposition should be avoided. Furthermore, as the substrate microstructure (i.e. type, area fraction and distribution of cathodic sites) strongly affected the Ti-Zr oxide deposition, the pre-treatment conditions should be adapted to the specific alloy in order to achieve optimum performance.

In the presence of chlorides, degradation of adhesive-bonded joints may be accelerated by a filiform corrosion (FFC) type of mechanism. The α-Al(Fe,Mn)Si particles in AA6060 played a crucial role in promoting FFC, as demonstrated by complete FFC immunity of the iron-free AA6060 model analogue alloy. Ti-Zr based pre-treatment provided less protection against FFC relative to chromate pre-treatment. The good FFC resistance of CCCs was partly attributed to a better inhibition of the cathodic activity at the α-Al(Fe,Mn)Si particles.

The cathodic α-Al(Fe,Mn)Si particles present on the surface of AA6060 aluminium could be effectively removed by different etch treatments. However, selective removal of surface intermetallics did not prevent FFC because filament growth was supported by cathodic activity on particles that become exposed in the filament tail as a result of the corrosion process.

Based on lap shear testing, hot AC anodising in sulphuric acid to a film thickness of about 0.2 µm showed promise as another chromate-free pretreatment for durable adhesive bonding. The performance was better than a conventional chromic-sulphuric acid based etch treatment. While hot AC anodising did not significantly inhibit the cathodic activity on the α-Al(Fe,Mn)Si particles, good resistance against FFC was still obtained due to the oxide film covering the whole aluminium matrix. Based also on separate durability data recently available, hot AC anodising is considered as a robust alternative to chromating for adhesive bonding of aluminium in certain industrial applications.