Mobile anodising of damaged aluminium surfaces for monuments conservation

Martina Raedel, Bundesanstalt für Materialforschung und -prüfung, Berlin

Aluminum was increasingly used in architecture from the 1950s and 1960s. Since this epoch is now moving into the focus of heritage protection, aluminium is appearing more frequently in metal conservation. Until now, aged aluminum surfaces, which are usually both anodized and dyed, are often not considered as cultural heritage in building renovation. The parts were completely stripped and newly anodized or the components were replaced. On the other hand, it must be stated that there is no satisfying procedure to “repair” damaged coloured anodized aluminium surfaces. Available repair sets are basically coloured coatings combined with an epoxy putty to repair smaller damages and do not provide a sufficient corrosion protection.

The presented research project wants to eliminate this desideratum and is conceived particularly for application in monuments care. It aims to develop a mobile anodizing process, with which damaged anodized aluminium surfaces can be repaired in such a way that the surfaces can be coloured afterwards. The three-year project is carried out by the Federal Institute for Materials Research and Testing in cooperation with Haber & Brandner Metal Conservation GmbH. In addition to the background of the project idea, the paper presents the results achieved so far as well as further steps planned in the project.
In the first work phase, the anodising process was transferred from workshop application to the laboratory scale. Processing parameters such as electrolyte composition, voltage and current strength, temperature and anodizing time were determined in dependence of the desired layer thickness. The oxide layers produced were characterized by methods such as layer thickness measurement, ESEM/EDX and hardness tests. The critical points required to produce a stable anodizing layer suitable for colouring and sufficiently thick for corrosion protection were determined.
Subsequently, the work steps required for anodising the aluminium and compacting of the surface, were transferred to a mobile form of application. A process was developed with which the growth of an oxide layer of more than 20 µm can be realized in a time span of 30 – 40 minutes. Since the electrolytic solution is applied in form of gel pads, it can also be applied to sloping surfaces. The surface to be treated can be limited by covering the surrounding areas with a reversible coating. Several important innovations have already been achieved so far, such as an increase in layer thickness, the application to sloping surfaces and the limitation of the areas to be treated. The durability of the anodised aluminium samples produced to date will shortly be tested by exposing them to outdoor weathering as well as in a climate chamber.
Further steps planned include investigations into mobile dyeing of the surface and a comparison of mobile compaction methods. The repair of artificially damaged surfaces will demonstrate the practicability of the developed method. In the final phase, a model application of the developed anodising process will take place in situ on damaged aluminium surfaces.