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Coating Report
nozzle pressure be not greater than
300kPa #1 ; nozzle angle to the surface
being cleaned – 30º- 60º; sweeping
distance 450 - 600mm; abrasive–
ultrafine non-metallic grit – not less
than 0.2mm and not greater than
0.8mm; grit should not be recycled #2 ;
the blasted zinc surface must be free
from all contaminants including oil and
dust.
#1 Nozzle blast pressure of below 300kPa is of
extreme importance, especially when thick hot
dip galvanized coatings are encountered. A thick
hot dip galvanized coating is normally due to
the reactivity of the steel when immersed in
molten zinc and is caused mainly by the
element silicon in the steel. The resultant thicker
coating is normally devoid of the soft outer zinc
(eta) layer and made up largely of iron/zinc
alloys, which can be susceptible to mechanical
damage, particularly on edges of the steel,
noticeably seen after the components have been
inappropriately handled during loading and
transport. Conversely, the soft zinc or eta layer
when correctly sweep blasted is easily fluffed up
to provide a surface profile for subsequent
painting. The soft zinc layer is also more
forgiving when sweep blasting is done at
excessive blast pressures.
In order to ensure that correct sweep blasting is
carried out, a needle valve indicating blast
pressure, can be inserted through the blasting
hose within easy visual distance of the operator.
#2 Although the recycling of micro-grit is not
recommended in the Code of Practice, an article
appeared in Hot Dip Galvanizing Today No.
27, which discusses the successful recycling of
the grit when sweep blasting.
Coating thickness (µm)
Coating thickness readings taken at
the galvanizer measured far in excess
of that required by SANS 121, which
requires that for steel thickness
equal to and greater than 6mm the
local coating thickness be a
minimum of 70µm with a mean of at
least 85µm. SANS 121 does not have
a upper limit in prescribing the
coating thickness. See 4 photos top left.
Sweep blasting at the correct blast pressure will normally remove a maximum of about 10 to 20
µm of the coating, judging by the residual coating thickness measured adjacent to the coating
damage after sweep blasting, coating removal has been exceeded and in many areas, the coating
has been damaged leaving a thin iron/zinc alloy layer of 24 to 35µm.
Other comments
The photos on the opposite page show
the surface profile of the steel as a
result of acceptable roll wear as
commented by the steel producer (top)
and the consequent appearance of the
coating after hot dip galvanizing
36 Hot Dip Galvanizing Today Volume 4 Issue 3 2007