Contents| Early consultation between galvanizer, fabricator and designer is the key to obtaining the best results from the galvanizing process. Design features which aid the access and drainage of molten zinc will improve the quality of the coating. Good design requires: - Means for the access and drainage of molten zinc - Means for escape of gases from internal compartments (venting) It is important to bear in mind that the steelwork is immersed into and withdrawn from a bath of molten zinc at about 450C. Thus any features, which aid the access and drainage of molten zinc, will improve the quality of the coating and reduce costs. General principles 1. Holes for venting and draining should be as large as possible. Further information is given in table 4. 2. Holes for venting and draining should be diagonally opposite to one another at the high point and low point of the fabrication as it is suspended for galvanizing. 3. With hollow sections sealed at the ends, holes should be provided, again diagonally opposite one another, as near as possible to the ends. 4. Where holes are provided in end plates or capping pieces, they should be placed diagonally opposite one another, off centre and as near as possible to the wall of the member to which the end plate is connected. 5. Internal and external stiffeners, baffles, diaphragms, gussets etc, should have the corners cropped to aid the flow of molten zinc. Base metal and combinations Plain carbon steel, some low-alloy steels and iron and steel castings can all be galvanized. A fabrication consisting of a variety of materials with different surface conditions should be avoided as this could affect the uniformity and appearance of the coating. Where differing materials are used, grit blasting the entire assembly can minimise any differences which may arise due to differing effects of pre-treatment. Preferably, the fabrication should be of similar steel type throughout. | Size and shape When the length or depth of the item exceeds the size of the bath, special techniques may be employed to facilitate dipping. The galvanizer should be consulted. Moveable parts Adequate clearance on mating surfaces, such as hinges, should allowed if they are to move freely after galvanizing. An extra clearance of at least 1mm is usually sufficient. Overlapping surfaces Overlapping surfaces should be avoided as far as possible. Care must be taken not to specify sealed articles for galvanizing. If overlaps are completely sealed by welding there is a risk of explosion during dipping due to increased pressure of any entrapped air. If overlaps are not completely sealed there is a danger of cleaning fluid entering the cavity and then weeping out and causing localized staining. Castings Castigns must be grit blasted before galvanizing as embedded sand from the casting process cannot be removed by conventional chemical cleaning. When designing castings to be galvanized. Distortion Efforts can be made at the design stage and elsewhere to minimi residual stresses, for example: 1. Controlling welding procedures during fabricatio. 2. Arranging weld seams symmetrically. The size of weld seams should be kept to a-minimum. 3. Avoiding large changes in structural cross-section which may increase distortion and thermal stress in the galvanizing bath. Surface contamination Clean steel surfaces are an essential requirement for good hot dip galvanizing. Contamination in the form of grease, tar paint and weld slag cannot be removed by chemical cleaning and may result in black bare spots after hot dip galvanizing. Fabricator should takes responsibility to ensure that articles being delivered free from contamination. (otherwise, an additional charge for shot-blasting may be added) |
Illustration of Draining and Venting Holes for Fabricators
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