TPT May 2013

The advantages and processes of galvanising AS a process, galvanising is incredibly flexible, long-lasting and environmentally friendly, and one which can be incorporated into a variety of projects, both large and small-scale. Hot dip galvanising is essentially the process of coating clean steel with a layer of molten zinc to protect the item from corrosion and provide a long-lasting, durable covering. Because it forms a series of zinc alloy layers with the iron in the steel it is more robust than other coatings that only bond chemically or mechanically, and also has the added advantage of fully coating the steel, inside and out. The process starts with cleaning the steel to prepare it for galvanising. Cleaning involves the complete removal of light grease, scale and dirt using a variety of techniques. Commonly, the steel product is dipped into an alkaline or acidic degreaser and then rinsed before being dipped in a hydrochloric acid rinse at room temperature. After cleaning, the product is rinsed and then usually dipped in a flux solution, which is typically made up from zinc ammonium chloride held in a temperature range between 65˚C and 80˚C. This final stage removes the last traces of oxide from the surface and coats the product with a thin film of flux. After the steel has dried, it is dipped into the molten zinc which is around 450˚C. Finally the work may be immersed in water to cool it, or allowed to cool in air. When the steel is dipped into the molten zinc, a series of zinc- iron alloy layers are formed. The main thickness of the coating is created at this time, after which the metallurgical reaction slows down so even if the steel remains in the galvanising bath for a considerable time, it will not form an overly thick protective layer. Typically items will be dipped for four to five minutes, and when they are removed a layer of pure molten zinc will be taken out on top of the alloy. It is this that cools to show the bright, shiny appearance generally associated with newly galvanised products. Zinc reacts with air to create a surface film that is insoluble to rainwater and acts as a barrier to completely prevent moisture and oxygen reaching the steel itself. The past twelve months alone have seen Wedge Group Galvanizing undertake a number of national and international projects of all shapes and sizes. One such project involved the galvanising of steel used to create a number of pipe flanges connecting pipelines carrying oil as part of petrochemical giant BP’s (Floating Production Storage and Offloading) FPSO PSVM scheme in waters off the west coast of Africa. The galvanised pipes were installed as part of a system used by the offshore industry to receive and process hydrocarbons, which then store oil until it can be offloaded onto a tanker or transported via pipelines. The FPSO PSVM project is aiming to tap into four oil fields off Angola with waters ranging from 1,500 to 2,500m deep, galvanising the steel was essential in helping reduce the risk of rust and corrosion.

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M ay 2013