Influence of Pre-treatments on the Surface Condition of 2024-T3 Aluminium Alloy

SUMMARY

The influences of various pre-treatments of interest in adhesive bonding on the surface condition of 2024-T3 alloy have been examined. In particular, the morphologies of films formed during alkaline etching (NaOH), electropolishing (perchloric acid/ethanol), acid pickling (Na₂Cr₂O₇/H₂SO₄4 and anodzing (H₃PO₄), and the consequences of film growth to the composition of the alloy, have been determined using transmission and scanning electron microscopies and Rutherford backscattering spectroscopy. The pretreatments result generally in scalloped surfaces, with deeper cavities associated originally with second phase particles. The topography of the surface is generated primarily in the pre-treatment stages, with subsequent anodizing having a lesser influence. Enhanced attack of the alloy in the vicinity of grain boundaries occurs during acid pickling, which appears to be accentuated during subsequent anodizing. Alkaline etching and electropolishing produce compact films, respectively about 5–8 and 2–3 nm thick, on the main alloy surface. In contrast, acid pickling produces an open-textured, porous film, about 30 nm thick and of fibrous appearance in section, with a thin barrier layer at the alloy/film interface. The porous films produced by anodizing in phosphoric acid are of typical thickness in the range 200–350 nm, with a 10–18 nm thick, barrier layer. The formation of the various films results in significant enrichment of copper in the alloy to the range 4.2 × 10¹⁵ to 6.4 × ¹⁵ Cu atoms cm⁻².

Key Words:

• aluminium alloy
• 2024-T3
• surface treatment
• adhesive bonding