Coating against corrosion

 

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The use of hot-dip coatings for steel sheet improves quality and lifetime of industrial goods. Most of the steel used for such purposes is galvanised with zinc, but for some purposes an aluminum-based coating is superiour.Flash back to the steel crisis of the 1970s: Western European capacity was far too high, many steel facilities were closed, and widescale unemployment and destruction of communities followed. In Belgium and Luxembourg, the two giants of the region’s steel industry, Arbed and Cockerill Sambre, decided to rationalise production, closing down uneconomic plants and concentrating production on modern efficient facilities. A steel-making plant in Dudelange, Luxembourg, was among the victims, but Arbed and Cockerill Sambre, which is now part of the Usinor Group, wanted to keep the jobs in the region. They launched a joint venture, Galvalange, to specialise in hot-dip aluminum-rich coatings for steel sheet. Galvalange bought the license to make its main product, Aluzinc – an aluminum-zinc coating – from the Bethlehem Steel Company in Pennsylvania, and production started in 1982. In 1982, European Community steel quotas limited production to 150,000 tons a year, but the quotas were lifted in 1986. That year saw the launch of a second product, Alugal, an aluminum-silicium coating. By 1989, Galvalange was producing 275,000 tons a year. A second production line in 1991 brought production capacity to 500,000 tons. Now the company is aiming for 600,000 tons a year.
   About two-thirds of production is devoted to Aluzinc. The other third is Alugal, a special coating used in high-temperature environments.
   The use of coated steel has made a major difference in the quality of industrial goods. You may remember the days when many cars were seen on the streets with rusting body panels, where paint would blister or the bodywork would slowly deteriorate into sharp, dangerous blades of rusty metal. Most of the steel used for such purposes is galvanized with zinc, but for some purposes an aluminum-based coating is superior.
   The Aluzinc coating consists of 55 percent aluminum and 43.4 percent zinc, with 1.6 percent silicium to help bond the coating to the steel. It is used mainly in the building industry for roofing and walls as an improved substitute for galvanized steel, since it is relatively light and is better at insulating and resisting corrosion. Aluzinc is also increasingly being used in home appliances where manufacturers want to provide a high level of corrosion-proofing, such as on the insides of washing machines.
   “There’s more demand now for better quality, and manufacturers are using corrosion-resistant products, even where you can’t normally see them,” says André Gierenz, head of operations. “They don’t want the customer to see their product all rusty behind the front panel when the repairman comes.” The market is growing by 6 to 7 percent a year, partly because the price of metal-coated steel is moving closer to that of the untreated material.
   One advantage of Aluzinc is the ease with which it can be painted once it has gone through a process called “skin-passing,” which microscopically roughens the surface so that paint will adhere better. It can also be bent and drawn more readily than other materials. A Galvalange speciality is its Special Passivation Treatment, which provides a special coating for the surface, ensuring that the panels can be bent and formed smoothly without leaving telltale “fingerprints.”
   “With our two production lines, we can cover the whole range of sheet size and thickness,” says Gierenz. “Both of the lines can manage widths of anything between 700 and 1,550 millimeters. One of them covers thicknesses from 0.2 to 1.3 millimeters, while the other covers thicknesses from 0.3 to 2 millimeters. And we’ll be moving up to 2.3 millimeters in the near future.”
   Alugal, with its 90 percent aluminum and 10 percent silicium coating, is used wherever there’s a need to withstand high ambient gas temperatures. The standard Alugal offers excellent resistance to chemical corrosion and will take temperatures of up to 700 °C. It is often used in motor vehicle exhausts, for burners in furnaces or in electrical home appliances like cookers or water heaters, where heat resistance is important. For even higher temperatures, Galvalange makes Alugal ALT, which is treated to withstand temperatures of up to 800 °C.

Michael Lawton  
a business journalist based in Köln  
photo Galvalange

 

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