Repairs to hot dip galvanized steel in conformance with SANS 121 (ISO 1461)

1. Zinc Metal Spraying

The preferred method of repairing damaged galvanized coatings is by Zinc Metal Spraying. The requirement is to lightly sweep blast the affected area without damaging the surrounding zinc coating.

The procedure for light blasting is:

• Blast pressure - 300kPa maximum.
• Media grade 0.2 to 0.8 mm.
• Media type(<5 mhos hardness) – clean silica and slags, alumina, limestone.
• Angle of blasting to surface 30-60°.
• Distance from surface 300-600mm.
• Nozzle type minimum – 10mm venturi type.
• Grit should not be recycled.

The metal sprayed coating is then applied, ideally to a thickness of 30µ greater than the minimum thickness demanded by the specification.

The repaired area should then be wire brushed to remove loose over spray and to seal surface pores which may be present in the sprayed coating. All repairs carried out by a galvanizer at his works should be undertaken by this method.

Zinc Rich Paint

Metal spraying requires specialised equipment which is not readily available at erection sites. The best alternative method for the repair of damaged areas and the coating of exposed steel where modifications have taken place, is the use of a Zinc Rich Paint with at least 82% zinc in the dry film. Ideally, this should be limited to small coating defects (typically the size of a R5 coin). It should be noted that a variety of zinc rich paints are available, with some providing better protection than others. Paint supplied in aerosol cans is normally not recommended as this material deposits an unacceptably thin coating. The recommended product is Zinc Rich Epoxy. This product provides superior corrosion protection.

For convenience of application and accurate mixing of the ingredients which make up a zinc rich epoxy, a product called Zincfix is now available in "Squish Pack" form. Zincfix is a solvent free product containing a minimum of 82% zinc in the dry film and 2% leafing aluminium for added corrosion resistance. The quantity contained in each 25ml sachet is capable of coating approximately 250cm2. Larger packs are also available. An advantage of Zincfix is that problems associated with limited pot life with two component paints are avoided.

The surface to be repaired must be thoroughly cleaned by stainless wire brushing or with the aid of an abrasive such as emery cloth (roughness 80 grit). Once applied the thickness of the dry film should be checked to ensure that it is at least 30 micron greater than the minimum thickness specified for the surrounding galvanized coating. Tests undertaken by the South African Bureau of Standards on behalf of the Hot Dip Galvanizers Association of Southern Africa show conclusively that zinc rich epoxies and especially Zincfix provide better corrosion protection than that available from other zinc rich paints.

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