Zinc spray, which is also referred to as metallizing, is done by melting zinc powder or zinc wire in a flame or electric arc and projecting the liquid zinc droplets by air or gas onto the surface to be coated, as seen in Figure 57. The zinc used is nominally 99.5% pure or better and the corrosion resistance of the wire or powder is approximately equal.
According to ASTM A 780, the surface to be reconditioned shall be blast cleaned to SSPC-SP5/NACE No.1 near white metal and must be free of oil, grease, weld flux residue, weld spatter and corrosion products. The blast cleaning must extend into the surrounding, undamaged, galvanized coating.
Zinc spraying of the clean, dry surface must be completed by skilled workers and should take place within four hours after preparation or prior to development of visible oxides. Spraying should also be done in horizontal overlapping lines, which yield a uniform thickness more consistent than the crosshatch technique. The zinc coating can be sealed with a thin coating of low viscosity polyurethane, epoxy-phenolic, epoxy, or vinyl resin. The details of the application sequence and procedures can be found in ANSI/AWS C2.18-93. The application of zinc spray can be done either in the galvanizer’s plant or at the job site. In addition, if high humidity conditions exist during spraying, adhesion may be degraded.
Final Repaired Product
The renovated area shall have a zinc coating thickness at least as thick as that specified in ASTM A 123/A 123M for the thickness grade required for the appropriate material category. These thickness measurements should be taken with either a magnetic or an electromagnetic gauge for best results. The plain zinc sprays or the sprays with aluminum additives both provide a good match for newly galvanized, bright surfaces. Finally, the surface of the sprayed zinc coating should be free of any lumps, coarse areas, and loose particles.