Spray galvanisation

In thermal-sprayed zinc coatings, layer thicknesses of 0.050-0.150 mm are the maximum reached. These layers consist mainly of pure zinc, with a certain amount of zinc oxide and pores depending on the layer thickness and spraying technique. The much lower protective effect of spray galvanisation at the same layer thickness is probably due to the lack of the iron-zinc alloy phase. 

Because of its resistance to corrosion, zinc may be used to coat iron. This may be done by dipping the iron into molten zinc or by spraying zinc on the iron articles, for example iron sheets. This is known as galvanising. Smaller iron articles may be coated by heating with zinc dust, a process known as sherardising, or suspensions of zinc may be used in paints. 

The faying surface of the steel should be given a metal coating of either zinc (by spraying or galvanising) or aluminium spray. 

In practice the thickest zinc coatings can be obtained by hot-dip galvanising or spraying. Table 13.7 compares the essential aspects of each coating. Plated coatings can also be produced mechanically in a wide range of thicknesses as well as electrochemically. 

Characteristics of coating Hot-dip galvanising A/etai spraying Plating Sherardising Zinc dust painting... 

Can be produced either by grii-blasiing before galvanising or by zinc spraying. Typical thickness of coaling on galvanised or zinc-sprayed structural steel. 

In the Forth, Severn and many other suspension bridges, zinc coatings have an important function. The whole main structure is of steel and has been zinc-sprayed on the external surfaces, while the main cable and hanger ropes have been coated by continuous hot-dip galvanising. Case histories of galvanised multi-truss bridges cover more than 30 years. 

Technical Notes on Zinc. Separate leaflets entitled Zinc Coalings, Galvanising, Zinc Spraying, Sherardising, Zinc Plating and Zinc Dust are available from Zinc E>evelopment Association, London... 

Electrostatic spraying PVC can be applied by the electrostatic process to continuous galvanised wire mesh. 

Certain British Standards, e.g. for galvanising or metal spraying", contain detailed sections on surface preparation. Others are concerned with acid pickling but the chief method of preparing iron and steel for a long-life protective system is by blast cleaning see Section 12.4). 

Note 8 Metal spray, paint powder and tape coatings are usually costed by the area covered which should be known reasonably accurately. Galvanising is costed by weight of steel coated i.e. by thickness of steel. Costs also vary by product. 

Corrosion behaviour of hot-dip galvanised and zinc sprayed sheet steels with and without protective coatings on the VDEh station for seawater tests on Helgoland) (in German)... 

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