Zinc Spray Metallizing for Galvanized Steel Touch-Up or Repair
Whether in the plant or in the field,
ASTM A780 details how to repair damaged areas of a hot-dip galvanized coating.
The specification includes three acceptable materials for touch-up and
repair of a hot-dip galvanized coating: zinc based solders, zinc rich
paints, and the subject of this video, zinc thermal spray or metallizing. Metallizing deposits molten zinc droplets onto the surface of a galvanized coating. Zinc
wire, ribbon, or powder are melted in a flame or electric arc and projected by
air or gas to repair the uncoated area. The zinc used must be pure zinc.
Metallizing repair closely matches the appearance of the galvanized coating.
Metallizing requires skilled workers and special equipment and can be applied in
the plant or at the job site, however at either site, follow the
manufacturer’s instructions and be aware of the recommended temperature and
humidity conditions, curing time, and thickness per application. To prepare the
surface, ensure the area is clean, dry, free of oil grease, weld flux residue,
weld spatter, and zinc or iron corrosion products. Blast clean the surface to SSPC/SP5 NACE number 1 white metal. The blast cleaning should extend into the
surrounding galvanized coating to ensure a smooth repair coating. The zinc spray
coating should be applied as soon as possible after surface preparation and
before oxides develop. Spraying should be done in horizontal, overlapping lines to
develop a uniform thickness. The repair area should be free of any lumps, course
areas, or loose particles. The final coating thickness for the repair area
must meet the minimum requirement for a galvanized coating under the specific
steel material category. The metallized coating thickness can be verified by a
magnetic, electromagnetic, or eddy current gauge.