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 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 α-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 α-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.