Research project on new bonding technology concluded

A research project carried out recently by Krupp VDM GmbH in conjunction with Zeppelin Silo- und Apparatebau GmbH and Sika Chemie GmbH delivered highly positive results. The aim of the project, co-funded by the steel applications foundation Stiftung Studiengesellschaft Stahlanwendung, was to demonstrate the production maturity of a highly corrosion-resistant metallic composite developed by Krupp VDM GmbH and to show that the material could be manufactured on a large scale. The study concluded that this new, low-cost technology can now be used for the construction and repair of chemical plants and environmental equipment.

VDM`s development bonds a max. one millimeter thick layer of a nickel-base material to a substrate, such as structural steel, using a polyurethane adhesive. The highly corrosion-resistant nickel-base alloy is suitable for bonding with a variety of substrate materials and can therefore be applied in areas of plant construction currently using mainly glass fiber-reinforced plastic or rubber coatings for reasons of cost. The composite of nickel-base material and structural steel displays significantly higher corrosion and abrasion resistance and is thus more durable. Corrosion damage in existing plants can also be repaired quickly and inexpensively by affixing a thin layer of nickel-base metal to the damaged area. Depending on the design, it can even be possible to dispense with welding.

As well as examining suitable adhesives, carrying out large-scale bonding trials and developing suitable welding and testing procedures, research work also focused on field trials under working conditions. The main aim of the bonding trials was to develop large-size composite plates for use as prefabricated stock material in plant construction. The only limit the research team came up against was the fact that Krupp VDM`s Altena facility can only roll sheet to a maximum of 8,000 x 2,500 millimeters. It was shown that the bonding process presented no problems even on plates of 20 square meters.

Long-term field trials in major plant construction projects showed the bonded metal composite to be suitable for a number of applications, mainly in the construction and remediation of clean gas ducts. One reason for this is that the gas temperature in such applications averages around 60°C. Twelve-month exposure trials performed on three composite plates placed at various points of the pilot plant run by Würzburg, Germany-based Noell-KRC Energie- und Umwelttechnik GmbH showed that the adhesive bond starts to fail at higher temperatures. But in saturated clean gas at around 60°C the bond remains stable, and the exposed specimens remained firmly bonded despite the strong corrosion damage to the unprotected boiler steel.

At the Wilhelmshaven power station of E.ON Energie AG, the development team was able test the metallic composite both for new construction and repairs. To simulate new construction, two bonded composite plates were welded into an existing clean gas duct at right angles. The rectangular duct is made of structural steel with an organic coating on the inside to protect against corrosion. For test purposes, part of the steel duct wall was removed and replaced by 2,500 x 1,000 millimeter composite plates.

The high standard of the welding technique developed for the composite material was demonstrated when fitting the plates, comprising an eight millimeter layer of structural steel, a one millimeter thick adhesive layer and a one millimeter sheet of Nicrofer 5923 hMo. First the structural steel of the duct wall and the structural steel layers of the composite plates were joined by TIG welding. The joints between the duct wall and the thin Nicrofer layer were sealed with one millimeter thick cover strips, likewise made of Nicrofer 5923 hMo, which were fillet welded using the TIG process. This was a particularly demanding process, as the cover strips had to be welded to the one millimeter thick Nicrofer layer of the composite plate without compromising its corrosion resistance by allowing it to mix with iron from the structural steel or thermal decomposition products from the adhesive. A subsequent die penetration test showed the welds to be first class.

To prove its suitability as a repair material, the composite was used in an area of clean gas duct E where the organic coating had been damaged by abrasion. In this case, 2 one millimeter sheets of Nicrofer 5923 hMo were affixed directly to the coating. To this end, the organic coating was further roughened; at the points where fillet welds were to be provided to seal the adhesive joints, the coating was ground away completely.

The field trial at E.ON Energie is still continuing. Three inspections, after 1,841 hours, then again after 6,179 hours and a third four months later revealed no damage to the sheets or welds, and even beneath deposits there were so indications of crevice corrosion.

The versatility of the new bonding technology is demonstrated by an application in one of the flue gas scrubbers at the Boxberg lignite-fired power station of German utility VEAG. Lime spray lances made of glass fiber-reinforced plastic were in need of repair as their resin sealing had worn away completely in some areas due to abrasion. The lances were protected from further damage by affixing curved Nicrofer sheets. As it was not possible to seal the adhesive against the scrubber emulsion, retainer belts of Nicrofer 5923 hMo were used to ensure the half shells remained permanently affixed. The repairs were carried out in early August 1998; since then the scrubber has been running more or less permanently, with only a few short downtimes. To date the adhesive-bonded Nicrofer half shells show no signs of damage.

Krupp VDM, Sika and Zeppelin have jointly developed a method of completely refurbishing a metal clean gas duct at the Niederaussem power station of RWE, another German utility. The inside of the clean gas duct is lined with stainless steel, beneath which there is a layer of rubber. Pitting corrosion in the welds has made refurbishment necessary. The solution put forward by the project partners of repairing the duct by affixing Nicrofer 5923 hMo would save the operator a great deal of money, as there would be no need to tear down the existing structure, located at a height of 40 meters. The proposed solution is based on the experience gathered in closing inspection openings which the operator had cut into the clean gas duct to examine corrosion damage to the welds. For this, one millimeter thick Nicrofer foils were affixed to the stainless steel and the edges sealed with a diffusion-proof GRP compound. It was not possible to weld the edges due to the layer of rubber below the stainless steel lining.

The results of the research project have now been filed for patent with the German and European patent offices. A leading German plant construction company has added the metal composite to its range, thus widening the spectrum of construction materials it offers for chemical processing equipment and reactors.

For further reference: ThyssenKrupp AG, Trade Press, Bernd Overmaat
Tel: +49-211-824-36012, Fax: +49-211-824-36035
e-mail: overmaat@tk.thyssenkrupp.com