be important in understanding how inadequate performance might occur, are detailed below. The metal substrate used in this application is usually galvanised steel for standard building sandwich panels, although other substrates can be used for specialised use, such as stainless steel for refrigerated buildings. These materials are developed to provide the structural and cost benefits of steel but with the increased corrosion resistance provided by the zinc coating which acts as both a protective barrier and also a sacrificial (anodic) component, corroding preferentially to the steel. There are different ways to apply a zinc coating but the type used in this application is a continuous galvanising hot-dip process, as shown in Figure 22 where the large coil is being fed into the series of baths containing the galvanising solutions. Figure Continuous galvanising plant As can be seen schematically in Figure 23 a coil of suitable cold rolled steel, of controlled standardised composition with respect to its alloying components, is initially cleaned using either an acid or caustic degreasing bath to remove dirt, oil and grease from the surface. Next is a pickling process in hydrochloric acid, which removes mill scale and other oxides. This is followed by fluxing, where any final oxidation products formed since the pickling is removed and the surface is coated with a temporarily protective layer which also facilitates a good bond in the galvanising process. The coil is then annealed at a variety of temperatures and is now ready to be immersed in a bath of molten zinc at 438-460°C (for 0.5% aluminium alloy). Generally many variants of these zinc coatings are often (erroneously) referred to as HDGS. Exact bath composition is proprietary to the steel manufacturer although these must be within guidelines established by relevant standards. Certainly it is known that the surface analysis of HDGS often shows significant variation in its surface chemistry. The types of zinc coated steel substrates, which are to be considered of particular importance commercially, are listed below in order of decreasing manufactured quantities, where the HDGS product forms the vast majority of product. The ArcelorMittal recognised designation is shown in brackets:
be important in understanding how inadequate performance might occur, are detailed below. The metal substrate used in this application is usually galvanised steel for standard building sandwich panels, although other substrates can be used for specialised use, such as stainless steel for refrigerated buildings. These materials are developed to provide the structural and cost benefits of steel but with the increased corrosion resistance provided by the zinc coating which acts as both a protective barrier and also a sacrificial (anodic) component, corroding preferentially to the steel. There are different ways to apply a zinc coating but the type used in this application is a continuous galvanising hot-dip process, as shown in Figure 22 where the large coil is being fed into the series of baths containing the galvanising solutions. Figure Continuous galvanising plant As can be seen schematically in Figure 23 a coil of suitable cold rolled steel, of controlled standardised composition with respect to its alloying components, is initially cleaned using either an acid or caustic degreasing bath to remove dirt, oil and grease from the surface. Next is a pickling process in hydrochloric acid, which removes mill scale and other oxides. This is followed by fluxing, where any final oxidation products formed since the pickling is removed and the surface is coated with a temporarily protective layer which also facilitates a good bond in the galvanising process. The coil is then annealed at a variety of temperatures and is now ready to be immersed in a bath of molten zinc at 438-460°C (for 0.5% aluminium alloy). Generally many variants of these zinc coatings are often (erroneously) referred to as HDGS. Exact bath composition is proprietary to the steel manufacturer although these must be within guidelines established by relevant standards. Certainly it is known that the surface analysis of HDGS often shows significant variation in its surface chemistry. The types of zinc coated steel substrates, which are to be considered of particular importance commercially, are listed below in order of decreasing manufactured quantities, where the HDGS product forms the vast majority of product. The ArcelorMittal recognised designation is shown in brackets:
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