Alkaline etching is a common pretreatment process for aluminium alloys before anodising or application of organic coating. Removal of the mill-finish surface layer is performed by a simple dipping process, which also gives the matt finish normally required for aesthetic appearance before conventional anodising. Major disadvantage of alkaline treatment is the surface enrichment of the alloying and trace elements more noble than the aluminium substrate by dealloying. This may reduce corrosion resistance and deteriorate the optical properties of the surface. The purpose of this work is to investigate fluoride-free acid etching for aluminium alloys as an alternative to alkaline etching to prevent or reduce heavy metal enrichment, while achieving matt appearance. Alternating current is applied to increase the rate of etching as the dissolution rate of aluminium in acid solutions is slower than in alkaline solutions. The principle is based on anodic oxidation of the heavy metal components, as well as aluminium, during the anodic cycle and removal of the corrosion products and concentration gradients by hydrogen evolution during the cathodic cycle. The materials used for this study were hot-rolled aluminium alloy AA3105 (Si 0.3%, Fe 0.6%, Cu 0.1%, Mn 0.7%, Mg 0.3%) and extrusion AA6060 (Si 0.4%, Fe 0.2%, Cu 0.01%, Mn 0.02%, Mg 0.5%). Phosphoric acid was used as the etching electrolyte. Consumption of aluminium was calculated from weight loss. Appearance was analysed visually and by gloss measurements. SEM was used to study surface microstructure and morphology. Aluminium oxidation rate increased with increasing temperature, current density, electrolyte concentration and etching time. Intermetallic Al-Mn-Fe-Si particles were selectively and rapidly removed by 1 min etching at 80⁰C. In contrast, particles were exposed and remained on the surface by alkaline etching. Matt appearance without visible defects was achieved after 10 min etching at 80⁰C. Presence of phosphorus in thin oxide film formed was determined by glow discharge optical emission spectroscopy (GD-OES). Advantages foreseen of acid-based AC pretreatment over alkaline dip processes in practice are suitability for in-line coil processing, superfluity of the desmutting step, low gloss achievable with reduced metal loss, improved corrosion resistance and adhesion properties.

Acknowledgements

This work was supported by Hydro Aluminium and The Research Council of Norway.