Effect of pre-treatment on the durability of epoxy-bonded AA6060 aluminium joints

Extruded AA6060 aluminium has been subjected to various surface pre-treatments before application of a one component 14 structural epoxy adhesive, XD4600. The durability of joints has been assessed by wedge adhesion and filiform corrosion (FFC) testing of panels coated with adhesive. A chromate-free Ti-Zr-based pre-treatment provided improved durability relative to an alkaline etch and deoxidation pre-treatment, but was clearly inferior to chromating in terms of adhesion and FFC resistance. Excessive Ti-Zr oxide deposition occurred in the vicinity of intermetallic alpha-Al(Fe,Mn)Si particles during pre-treatment, which significantly reduced the performance of bonded joints during wedge test. The intermetallics furthermore played a crucial role in promoting FFC, as demonstrated by complete FFC immunity of an AA6060 model analogue alloy (AlMg0.5Si0.4) free from alpha-Al(Fe,Mn)Si particles. The Ti-Zr-based pre-treatments provided limited protection against FFC on AA6060, apparently due to a limited effect of the Ti-Zr oxide conversion coating in reducing the cathodic activity of the intermetallic particles. In search for chromate-free pre-treatments for painted or adhesive bonded aluminium, inhibition of the cathodic activity on the Fe containing particles is suggested as an important factor to prevent FFC. (C) 2003 Elsevier Ltd. All rights reserved.