Sustainable Aluminium Surface Applications

Increased use of recycled metal is expected to cause increased levels of trace elements in aluminium products in the near future. Also the use of less pure bauxite sources in primary aluminium production will increase the levels of trace elements. Althoug h the concentration of trace elements is small at the ppm level, their effect on the surface properties of aluminium alloys can be significant. This is already exploited in a positive sense at present in the development of sacrificial anodes for cathodic protection, active anodes for Al-air batteries and electrochemically etched aluminium capacitors. However, trace elements may also cause undesirable corrosion of aluminium alloys used in buildings, transportation industry and packaging. The objective of t he SALSA project is to understand the mechanisms by which trace elements affect the surface properties, especially corrosion and appearance of commercial aluminium alloys, providing the scientific background for developing new alloy compositions and cost effective process routes. The results obtained so far have shown the great importance of the combination of various advanced experimental techniques to understand the mechanisms behind observed phenomena. Through important developments of characteris ation instruments during the past years and having access to these, we are now able to explain mysterious corrosion failures of the past. Trace elements with low melting point are of particular significance with respect to corrosion, because some of these elements become enriched at the surface during heat treatment, causing increased corrosion susceptibility. Among several trace elements studied, lead and tin have been identified as particularly important from an industrial point of view. However, it has been shown that the influence of these elements can normally be eliminated by an appropriate surface treatment, allowing production of recycled materials with a surface quality comparable to that of products made from primary metal. The new mechanist ic understanding of corrosion processes on a nanometre scale has been acquired through extensive electrochemical and surface analytical studies of model alloys. Recent standardised corrosion testing of painted materials with commercial alloy compositions has verified these findings, and results are consistent with the expected behaviour based on model alloy studies. It has further been discovered that small quantities of tin and bismuth in pure aluminium can cause formation of a relatively thick oxide film on the surface during water quenching after heat treatment. Understanding the combined influence of trace and alloying elements on the oxidation of aluminium in superheated steam should be highly relevant for the development of new alloys and proces sing routes to allow necessary tolerance to increasing levels of trace elements in recycled aluminium.

SALSA will deal with the development of the necessary new knowledge within the area of surface science that is required in order to enable a sustainable global increase of the use of aluminium. Achievement of this principal objective will be a significa nt contribution to the realisation of the vision of the European aluminium industry in general, and the Norwegian aluminium industry in particular. It will also contribute to secure a sustainable future development of the many companies (often SMEs) that apply aluminium in its products. Realisation of the European aluminium vision involves a significant increase of recycled metal in aluminium products. The most important challenge for enabling this increase is to deal with a significant increase of trac e elements with possible determent consequences for surface related product performance with existing technology. Resolving this issue will also enable a more robust sourcing of bauxite for primary aluminium production. SALSA will address the fundamental mechanisms of how trace elements affect the surface properties of the finished product. In order to maintain product performance with an increased presence of trace elements it will be necessary to address the entire aluminium production chain including sourcing of raw materials, primary metal production, casting, homogenization, extrusion, rolling, brazing, surface treatment, coating, and product design. A true through-process approach is necessary in order to be able to propose new thermomechanical pro cess routes that compensate for the increased amount of trace elements. Corrosion resistance and coating adhesion are the surface properties of primary interest, but other properties (e.g. appearance, response to surface treatment) will also be addressed. In special cases it is expected that the SALSA approach will reveal possibilities to improve product performance by the introduction of trace elements.