TPT January 2008

C oating, J acketing & Galvanizing Technology

Steel strip galvanizing technique leads to next generation galvanized pipes

Until now only batch galvanized material was available for piping in application areas where there is a high risk of corrosion, eg ‘dry’ sprinkler systems. Weak points in the zinc coating often reduce the service life of the piping. However, welded pipes made of continuously galvanized steel, where the welds are galvanized on the inside and outside, offer a higher level of protection against corrosion. As Mr Harald Leitner (BEng), sales manager of WProfile & WPipe at Wuppermann Stahl GmbH, Austria, states, “The level of corrosion protection offered by pipes that are manufactured using our process is far superior to the protection offered by batch galvanized pipes.” Due to their increased resistance against corrosion, galvanized tubes are used in numerous application areas, including manufacture of water and sprinkler pipes, scaffolding, racking systems, fencing and playground equipment. The pipes are made of ‘black’ (ie non-galvanized steel strip), which is rolled to form a tube and then longitudinally welded. The zinc layer is subsequently deposited by way of batch galvanizing, ie by dipping the tube in a bath of molten zinc. This conventional dipping process does, however, have one major disadvantage. An intermediate layer is formed as a result

of the tube being dipped in the molten zinc for a long period of time. The metallurgical reaction of the iron with the zinc leads to the formation of intermetallic phases. The thickness of this layer increases proportionately to the temperature and the dipping period. In the case of batch galvanizing, the percentage in the overall thickness of this layer can often be as high as 70-80 per cent, while the actual zinc layer only accounts for 20-30 per cent. As comments Mr Markus Dirschlmayr BEng, marketing department: “This layer of intermetallic phases has negative properties”. The corrosion protection effect of a zinc coating is based on the fact that this metal corrodes very slowly. In areas where the underlying steel has been exposed, eg as a result of a scratch, the zinc acts as a ‘sacrificial anode’ which continues to protect the exposed steel. However, when the upper layer of metallic zinc is used up and the corrosion attacks the underlying intermetallic phases, the iron atoms react with the oxygen. This problem is further magnified by the fact that the intermetallic layers are brittle. The danger of flaking in the presence of mechanical stress, eg during the forming process, increases proportionately to the thickness of this layer. Corrosion protection is greatly reduced in such areas.

However, the disruptive intermediate layer is reduced to a minimum during continuous galvanizing. As states Mr Harald Leitner, “In contrast to the conventional procedure, our Wgalweld process uses strip stock galvanized in a continuous process prior to welding” . Contact with the zinc bath only lasts for a fraction of a second and, as a result, the intermetallic phase is extremely thin and therefore does not have a negative impact on the durability of the layer or corrosion resistance. The share of metallic zinc in the coating is over 99.9 per cent. Moreover, this process is very easy to control due to the utilisation of modern process control technology. The thickness of the layer can be set with tight tolerances. Furthermore, zinc coatings of different thicknesses can be deposited on the upper and lower surfaces. › Boundary layer – the metallurgical reaction of the iron with the zinc leads to the formation of intermetallic phases which, in the case of batch galvanizing, often makes up for 70-80 per cent (photo: Wuppermann Stahl)

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fi (Below left) protective coating – in batch galvanizing, the steel components are dipped into a huge bath full of molten zinc (photo: Klaus Vollrath ); and (below right) tube production – the steel strip, which is formed into a tube in a rolling process, is then longitudinally welded. Immediately afterwards, the inner and outer sides of the weld are continuously galvanized (photo: Klaus Vollrath)

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J anuary 2008

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