Corrosion Resistant Nanocoatings - News Item

NANOMAG is developing corrosion-resistant nanocomposite coatings to replace hazardous chromium-based coating treatments for magnesium alloys applicable specifically to the needs of the automotive, aeronautics and aerospace industries.

Reducing the weight of motor vehicles is a primary means of decreasing their fuel consumption. Eliminating 100 kg of weight allows a fuel saving of 0.5 l/100 km – thereby cutting the output of environment-damaging exhaust emissions.

Magnesium (Mg) – which is one third lighter than aluminium and 80% lighter than steel – has been used increasingly for this purpose since its first appearance in racing cars during the 1920s. Applications of Mg-based alloys now range from transmission casings, intake manifolds and cylinder head covers to wheels, body sections, steering wheel cores and column components.

Properties such as a high strength-to-weight ratio and good damping of noise and vibration, coupled with the ease of component manufacture using die-casting techniques, have also led to widespread use in sectors such as aerospace and portable electronics.

However, demand from the automotive industry is the main driver of a 7% annual growth in world magnesium consumption, which is currently quoted by the International Magnesium Association as 415,000 tonnes/year. In fact, the use of Mg alloys in the automotive and electronic industries together has reportedly increased by 350% over the past 10 years, and is expected to grow at 25% per year until 2008.

Widely regarded as the Achilles’ heel of these otherwise versatile materials is their proneness to corrosion and staining in wet and salt-laden atmospheres. The application of durable anodic or conversion coatings typically provide protection against such effects. Anodic coatings are tougher, harder and have better wear properties than conversion coatings, but their cost is too high for mass production.

Chromate-based conversion coatings are cheaper, but the hexavalent chromium involved is both carcinogenic and a hazardous air pollutant. Directive 2000/53/EC on end-of-life vehicles makes explicit reference to the reduction or elimination of its use in cars. And, while European regulations are also in place to control the disposal of process waste and to protect staff in the workplace, a viable alternative is urgently needed.

The GROWTH NANOMAG project aims to provide an alternative by developing clean and environment-friendly nanocomposite coatings that will be more economical while also offering superior resistance to corrosion and abrasion. Co-ordinated by Centro Ricerche Fiat (CRF), it brings together a team deploying the expertise and complementary skills of research institutes, industrial research centres and end users from seven countries, including Switzerland and Israel.

One of the most important goals for the automotive industry of the future is to extend the use of Mg alloys to the external components of the car. The doorframe, for example, represents a major challenge, both from the point of view of casting such a large part and for the high corrosion-resistance requirements in a situation where the magnesium structure is joined to dissimilar materials, giving rise to the risk of galvanic corrosion.

For parts such as wheels, which have an aesthetic function, hard, scratch-resistant undercoats for paint or lacquer finishes are required, while wear- and erosion-resistance are essential to allow the use of magnesium alloys in the construction of pistons and turbine components.

NANOMAG is addressing these needs by exploring a number of potential ‘clean’ processes: plasma-enhanced chemical vapour deposition (PECVD), plasma-assisted physical vapour deposition (PAPVD), sol-gel technology and the Keronite electrolytic ceramic coating process.

PECVD initially appears to be the most promising for the production of nano-structured coatings. It entails the application of an electrical field to a low-pressure gas mixture, creating a glow discharge. If an organic molecule is introduced into the plasma, it breaks into reactive fragments (radicals) that can bombard metal surfaces and interact to form coatings.

This process gives rise to minimal atmospheric emissions and waste production, and is extremely versatile. The concentration of the reactive species, and the energy and density of positive ion bombardment, can be varied over a wide range. In addition, the plasma approach allows pre-treatment of the substrate before the deposition stage, removing surface contamination and modifying a 1 to 3 nm layer to improve coating adhesion.

Starting from organosilicon precursors such as hexhamethyldisiloxane, hexamethyl-disilazane and tetraethoxysilane, it is possible to deposit thin (10 to 10,000 nm) SiOx films that are characterised by high chemical and thermal stability, and by low gas permeability. Nano-structured coatings can also be applied as multiple or graded layers, with variable chemical composition and properties as a function of thickness.

Integral to the three-year NANOMAG project will be an evaluation of the environmental burdens associated with the various products and processes, through identification and quantification of energy and materials used and wastes released.

Assessment will cover the entire life cycle, from the extraction and processing of raw materials; through manufacturing, transportation and distribution to use, re-use, maintenance, recycling and final disposal. It will determine the environmental impact in terms of global warming and greenhouse effect, acidification, nutrient enrichment, photochemical smog and stratospheric ozone depletion.

Achievement of the scientific targets of the project will have a major impact in improving the sustainability of transport – not only by reducing fuel consumption and emissions, but also because Mg parts require low energy for recycling.

The quality of life for workers and European citizens in general will further be enhanced by the elimination of polluting substances and dangerous chemicals during the coating process. A positive effect on employment is also expected, as lower coating costs will lead to wider use of magnesium alloys, increased competitiveness for Europe’s transport industry and a demand for the construction of new coating.

Posted 30th June 2003

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