Metal spraying

An alternative method of applying a metallic coating to steelwork is by spraying. In this case, either zinc or aluminium can be used. The metal, in powder or wire form, is fed through a special spray-gun containing a heat source which can be either an oxy-gas flame or an electric-arc. Molten globules of the metal are blown by a compressed air jet onto the previously blast-cleaned surface. No alloying occurs and the coating, which consists of overlapping platelets of metal, is porous. These pores are subsequently sealed by applying a flood coat of clear or pigmented epoxy or polyurethane coating. Further painting is then optional.

The adhesion of sprayed metal coating to steel surfaces is considered to be essentially mechanical in nature. It is therefore necessary to apply the coating to a clean roughened surface; blast-cleaning with a course grit abrasive is normally specified. This would usually be chilled-iron grit, but for steels with a hardness exceeding 360HV, alumina or silicon carbide grits may be necessary.

Coating thickness varies between 100 to 250 microns for aluminium, and 75 to 400 microns for zinc. Both metals perform similarly in most situations, but aluminium is more durable in highly industrial environments.

Metal spray coatings are usually applied in the fabrication shop. Unlike hot-dip galvanizing, there is no limitation on the size of the element and, as the steel surface remains cool, there are no distortion problems. Relative costs will vary depending on the size of the section, but metal spraying might typically be twice as expensive as galvanizing.

The protection of structural steelwork against atmospheric corrosion by metal-sprayed aluminium or zinc coatings is covered in

BS EN 22063.

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