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HERITAGE&CONSERVATION


SUCCESSFUL POWDER COATING ON GALVANIZED STEEL STARTS WITH GOOD DESIGN


Richard Besant is the Sales Director for Powdertech (Corby) Ltd, the architectural metal finishing company. Powdertech was established in 1988 and is a leading industry supplier. During 2012 Richard has been sharing his experience and views on the industry with Clearview’s readers. In this column he explains why it is advantageous to consider the finishing processes that a metal fabrication will be undergoing, whilst designing it.


“Our wisest customers know that a better quality product will be gained, and less money spent if the fabrication is designed with the galvanising process in mind.”


“It is probably easiest to explain why, by describing what actually happens to a piece of steel when it is galvanized. We often advise people to try and visualize how the molten zinc will flow over and into their fabrication.


Te fabricated steel section is immersed in a bath of molten zinc. Te critically important factor here is that steel and zinc have roughly the same densities (molten zinc is 93% of the specific gravity of steel) and therefore a hollow steel fabrication will float in a bath of zinc, much as an empty plastic bottle would do in a bath of water. Drilling ventilation holes into the hollow steel object allows air out and zinc in as it is immersed. Te holes cannot be drilled just anywhere – they need to be in the correct places and to be of the correct size. Ideally the piece of steel will sink rapidly, since, if it floats near the surface, the zinc coating that adheres to it at the surface will oxidize leaving a surface with a burned appearance. Te smallest amount of air trapped in the hollow steel section can cause this to happen. In the worst case the item might explode if air is left inside it.


As the coated steel section is lifted out of the molten zinc the reverse needs to happen – air needs rush in and all of the excess zinc needs to drain out. Very often this doesn’t happen completely and an excess of zinc collects inside the section making it heavier than it should be and utlising more zinc, making the galvanizing more costly.


VENTILATION HOLES Rust appearing at the sharp edge on a galvanized panel Premier Galvanising will be able to give you more advice on design


requirements, as will the Galvanisers Association. Our partnership with Premier Galvanising means that we can fully support you with all the technical information you need. We also keep ourselves up to date with best practice, techniques and industry knowledge through formal and informal training and we would encourage you to consult us in the early stages to discuss the advantages and disadvantages of various design techniques. We hope to be several pages ahead in the manual to give you the best advice to achieve the finish you want.


Another useful design tip for perfect galvanizing is to try and ensure that the zinc will flow uninterrupted along the surface. Tis can be achieved by cropping the corners of external stiffeners at the web to avoid dead pockets of air and ash.


If you would like to find out more or discuss a particular project then please call me on 01536 400890 or email Richard.b@powdertech. co.uk


For more information: visit www.powdertech.co.uk Clearview NMS « September 2012 « www.clearview-uk.com » 71


As well as drilling strategically placed ventilation holes, the fabricator needs to eliminate any sharp edges on the steel section. Sharp edges can cause fracturing of the zinc coating and localized thinning of the powder coating around the edge for up to 10mm. Rust will then start to appear from the sharp edge. Te solution is to round off the sharp edges, either by champhering or, even better, making a 5mm radius at the edge to allow the zinc and powder coating to grow and flow around it.


At Powdertech we can handle the galvanizing process for you


(Powergalv). Our aim is to provide you with a powder coated surface of the highest quality and, more than with most materials, the appearance and performance of the galvanized and powder coated duplex system is influenced by considering the design at an early stage of a project.


CLOSED HOLLOW SECTION FABRICATION


Holes should be within 10mm of each end of the section and diagonally opposed.


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