High Frequency Pulse Anodising of Aluminium for Decorative Applications

This thesis focuses on generating anodised aluminium with a glossy and decorative appearance, which is attractive for use in the consumer industry. The use of aluminium ensures a light-weight and durable packaging, while the decorative appearance sends a signal of high-end product quality. The conventional technology for obtaining a cosmetic appearance of anodised aluminium, involves the use of primary aluminium (i.e. non-recycled) of very high purity in combination with a traditional anodising treatment.

High purity Al alloys are typically very costly due to the energy intensive method of producing primary aluminium, in contrast to recycled material. The present thesis investigates a special anodising method, with an expectation that recycled alloys and other low-purity Al alloys can be anodised to a cosmetic appearance, thus saving cost on the raw material.

Furthermore, it is investigated if the new anodising method can improve the aesthetic appearance of High Pressure Die Cast aluminium, which traditionally does not lend itself to decorative appearances due to heavy loading of alloying elements. While conventional anodising involves the use of a Direct Current (DC) source, the new method involves the use of a High Frequency Square Wave Pulse source.

This study investigates how the pulse anodising parameters (such as pulse frequency, voltage offset and anodic cycle potential) affect kinetics and surface appearance of commercially available Al alloys. Additionally, an effort is made to determine why high frequency pulsing yields a considerably faster process compared to a conventional DC anodising.

Characterization of surfaces was performed using Scanning Electron Microscopy, Glow Discharge Optical Emission Spectroscopy, Energy-dispersive X-ray spectroscopy, Focused Ion Beam electron microscopy and Atomic Force Microscopy. Optical characterization of high-gloss anodised surfaces was performed using a commercially available glossmeter accepted by industry.

Combining these results provided a good understanding of the mechanisms at play for the individual alloys, and the reason of High Frequency Pulse Anodising generally yielding a more decorative appearance compared to the traditional DC process. Chapter 1-3 contains introduction, literature review and a description of the materials and methods that were used throughout the project.

In addition to this, the work presented in this thesis is divided into two parts. The first part describes investigations on different Al alloys including wrought and cast materials and how the overall surface appearance can be improved by adopting high frequency pulses as an alternative to conventional DC anodising (chapter 4, 5, 6 and 7) with details of mechanisms involved.

These chapters are presented in a manuscript form suitable for journal publication. The second part of the work is directed towards the implementation of the new method, namely investigations related to upscaling (chapter 8 and 9).