Nanoparticle modified anodizing-layers with increased alkali resistance

19082 BG

Bild Forschungsprojekt
Period: 01.04.2016 - 30.09.2018
Partner: Fraunhofer IKTS Dresden
Funder: AiF
Project Manager: M.Sc. Adrian Anthes
Research Group: Corrosion

Motivation and Aim

Aluminum and Al alloys are nowadays used not only for automotive and aircraft construction, but also for building claddings, windows, etc., in the construction industry. In order to protect the base metal from corrosion, the surface is anodized, whereby the native / thin oxide layer is artificially thickened. The oxide layer thus produced is several microns thick and has pores whose diameters lie in the one to three-digit nanometer range (depending on the anodizing method). The present pores are generally sealed in a downstream process step (water vapor) or cold (chemical) in order to increase the corrosion resistance. However, the thus densified alumina as a passive layer is only stable up to a pH value of approx. 8.5, whereby recurring cleaning with strongly basic cleaning chemicals (pH 13.5) leads to a corrosive attack of the surface. However, an adequate long-term stability of anodized aluminum and Al alloys against alkaline media is required by the user, which is to be achieved within the framework of the project by a corresponding surface modification.

Approach

The aim of the ongoing project is to develop an anodizing process based on sulfuric acid anodizing (SAA), which allows subsequent chemical nanotechnology impregnation to increase resistance to strongly alkaline media. The modification of the anodizing parameters (current-voltage regimes, temperature and bath composition) should directly influence the pore morphology, so that the pore diameter and shape are optimized for the subsequent impregnation. The impregnation is to be carried out using aqueous nanoparticulate zirconium dioxide dispersions since zirconium dioxide has a high resistance to strong acids and bases. In addition, the coefficient of expansion of zirconium dioxide is approximately equal to that of aluminum oxide, which means that thermal stress should not be a problem for the generated layers. The impregnated anodizing layers are then to be characterized by modern analytical methods and their corrosion behavior both electrochemically and by aging in typical basic cleaners.

Results

Aqueous zirconium dioxide dispersions (starting material: powder with a particle size of 5-25 nm) having a primary particle size of <10 nm, which are distinguished by a long-term storage stability (several weeks), can be produced. In addition, an ultrasonic method was developed with which "large" zirconium dioxide particles (5 μm and larger) could be milled to a single-digit nm range in a short time, which could drastically reduce the cost of the zirconia particles used. The pore diameter could be increased by optimizing the process parameters during sulfur acid modification, so that first immersion coatings or impregnations of the anodizing layers could be carried out.

Future work

The future work will include the further optimization of the pore design (expansion of the pores) on the other hand, the already available samples should be impregnated with aqueous zirconium dioxide dispersions and compared with industrially anodized samples (benchmark), which were also impregnated with zirconium dioxide. After successful impregnation, the samples should be characterized and tested for their corrosion resistance in basic media.

 

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Das IGF-Vorhaben Nr. 19082 BG der Forschungsvereinigung DECHEMA e.V., Theodor-Heuss-Allee 25, 60486 Frankfurt am Main wurde über die AiF im Rahmen des Programms zur Förderung der industriellen Gemeinschaftsforschung (IGF) vom Bundesministerium für Wirtschaft und Energie aufgrund eines Beschlusses des Deutschen Bundestages gefördert.

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