Topic of the Month February 2009

Figure 1: Imparting a self-rotation to the cooling gas will stabilize the flame so that the coating is applied evenly on the material: Prof. Dr. Bernhard Wielage, Gerd Paczkowski and Christian Rupprecht (from right to left) have applied for a patent for their invention. Photo: Heiko Kießling.

Corrosion is a critical factor that limits the lifetime or service life of technical products, causing billions of economic damage every year –in Germany alone, this cost is estimated at about 90 billion Euros. The publication “Lectures on corrosion and corrosion protection of materials - Part I”, issued by the Institute for Corrosion Protection in Dresden, illustrates this fact very clearly: Every minute, several tons of steel corrode somewhere in the world, e. g. in pipes of chemical plants, in drinking water systems and components of the automotive industry. Corrosive damage mechanism also concern products covered by the wire and Tube trade fairs, since these components are often used in reactive environments or are in contact with aggressive material systems.

For this reason, concepts to conserve resources in production and to increase sustainability within the product life cycle are gaining in importance. Innovative developments in corrosion protection are critical in this context and demonstrate promising perspectives.

Rust-proof without chromium

Chromate coatings have, for a long time, been protecting car bodies from uncomely rust – however, a directive of the European Parliament prohibits the use of toxic and carcinogen chromium(IV) compounds in automotive production since mid-2007 in order to eliminate the associated environmental impact and health risks. Meanwhile, several chromate-free coating systems have been put into service. However, these are often only a weak alternative – for example, they require concessions with respect to the protective effect and lend themselves only to a limited choice of substrates.

Scientists of the Fraunhofer Institutes for Silicate Research in Würzburg and for Machine Tools and Forming Technology in Chemnitz, cooperating with their colleagues of Institute for Corrosion Protection Dresden GmbH, have now developed an alternative on the basis of innovative nano composites that are applied using a sol-gel process. To form a coating, the scientists immerge pre-galvanized steel plates in the sol and apply an additional powder coating. These coatings have proved their performance in demanding and realistic load tests. For example, the plates are scored and subjected to a 360-hour salt spray test or stored in a climate chamber at 100 per cent humidity for 240 hours. According to project manager Dr. Johanna Kron, most of the so modified materials provide a corrosion protection that is equivalent to commercial yellow chromating. Compared with a chromate-free system that is on the market as well as a chromium(III) passivation process, the new coatings are often even more effective. The researchers estimate that the system, which already works well in the lab, could be introduced in the market in about five years.

Optimized high-speed flame spraying

Figure 2:Chromium-free: Test coatings on the basis of innovative nano composites that have been applied by a sol-gel process.

Various variants of the high-speed flame spraying process are used whenever components that are subjected to high mechanical stress must be protected against corrosion and wear. For this purpose, the coated surfaces are impacted at speeds of up to 2,000 meters per second with a particle jet at core temperatures of about 3,000 degrees Celsius. These extreme process parameters contribute to the formation of high-density, highly adhesive and extremely resistant coatings on a wide variety of substrates. Conventional nozzle technology, however, often results in an irregular texture of the coated material surface. This is caused by the fact that, due to the enormous speed and the temperature gradient to the ambient air, the particle flame develops a whipping proper motion at its tip, which negatively impacts the regularity of depositing process and makes it less controllable. To remedy this weak point, a team of researchers headed by Prof. Dr. Bernhard Wielage of the Technical University of Chemnitz developed a simple – but groundbreaking – solution and already applied for a patent. The engineers make use of the enveloping gas around the particle jet, which is always present to cool the spraying nozzle, to stabilize the flame.

In the current design, the cooling enveloping gas – frequently nitrogen at room temperature – now emerges from the nozzle with a self-rotation and, due to the imparted angular momentum, prevents the flame from becoming unstable and starting to “wobble”. The actual spray jet is stabilized by the rotating enveloping gas flow, focusing the contained particles on a defined trajectory. This process modification results in a dramatically improved layer quality since the particles strike the substrate more evenly and tightly focused. Another advantage is that the burner can be operated at higher powers, which will allow the processing of a significantly higher amount of material per time unit in the future. Since the nozzle add-on invented in Chemnitz is compatible with commercial HVOF burners, several renowned companies have already announced their interest in this technological innovation.

Dr.-Ing. Christoph Konetschny
Materials Consultant & Nano Exper

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