Alternating Current Acid Etching of Aluminium

摘要

Common pretreatment process of aluminium alloys before anodising or application of organic coatings involves alkaline etching. Removal of the mill-finish surface layer is performed by a simple dipping process. It also gives the matt finish normally required before conventional anodising for aesthetic appearance. However, the desired aesthetic quality and corrosion resistance are not always obtainable because of surface enrichment of heavy metals in solid solution and heavy metal rich intermetallic particles caused by selective etching of the electrochemically more active aluminium component. 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. Dissolution rate of aluminium in acid solutions is slower than in alkaline solutions. Therefore, electric current is applied to increase the rate of etching. The choice of alternating current (AC), instead of DC, was inspired by the sulphuric-acid based hot AC pretreatment, which has been used for several decades with success prior to organic coating. 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 material used for this study was hot-rolled aluminium alloy AA3105 (Si 0.3%, Fe 0.6%, Cu 0.1%, Mn 0.7%, Mg 0.3%). Sulphuric acid was used as the etching electrolyte. Thickness of the anodic film and the consumption of aluminium were calculated from weight loss. Appearance was analysed visually and by gloss meter. SEM was used to monitor the removal of intermetallic particles and 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 (1 min etching at 80□C) removed. Heavy metal enrichment of the matrix did not occur. Surface had matt appearance without rolling streaks or other visible defects after 10 min etching at 80 jC. In contrast, particles were exposed and remained on the surface by alkaline etching in addition to the heavy-metal enrichment mentioned above. Despite the need for external AC power for achieving comparable etching rates, the foregoing differences suggested certain important advantages of using an acid-based AC pretreatment over alkaline dip processes in practice, such as suitability for in-line coil processing, superfluity of the desmutting step and low gloss achievable with reduced metal loss.