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:
zinc-based solders, zinc-rich paints, and zinc
spray or metallizing.
Let’s take a look at zinc based solders.
Soldering is done with a zinc-based alloy
in either stick or powder form.
The acceptable material compositions of solders
include zinc-tin-lead, zinc-cadmium,
and zinc-tin-copper.
These alloys blend well with the existing
coating appearance, and each has specified melting
and application temperatures recommended by the manufacturer.
Touching up with zinc-based solders is recommended
for horizontal surfaces only.
The surface and surrounding area should be wire brushed,
lightly ground, or mildly blast cleaned
before repair.
If the area contains weld flux or spatter
not removed by wire brushing, weld spatter
should be removed by grinding or chipping.
Wire brush the repair area during preheating,
and pre-flux if necessary.
Preheat the area to at least 600 degrees Fahrenheit ,
315 degrees Celsius.
It is important to not overheat the surface
beyond 750 degrees Fahrenheit, 400
degrees Celsius, or this will burn the surrounding
galvanized coating.
Once the area is clean and preheated, rub the
repair stick or sprinkle with the powder and spread with
a spatula to create an even layer of zinc alloy.
After application and cooling, remove
solder flux residue by rinsing with water or
wiping with a damp cloth.
The final coating thickness must meet the minimum
requirement for the material category,
The repair coating thickness can be measured with a
magnetic, electromagnetic, or eddy-current
thickness gauge.
