Product criteria of suitability for hot-dip galvanizing
Description
1
Dimensions
2
3
Weight
Thin-walled articles (wall
thickness of 0,8-2 mm)
Structures without holes or
hinges
Suitability for hot-dip
galvanizing
Design
13,6 х 2,2 х 3,1 m
(LхWхD) recommended
maximum 9 t
Test galvanizing is
required
Method of loading on jig
should be considered
5
Tanks, drums, pipe coils
Unsuitable
6
Reservoirs, containers
Each structure to be
considered separately.
7
Pockets, closed cavities,
air-pockets
Unsuitable, or adaptation
is required
8
Hollow products and
Each structure to be
articles of irregular shape
considered separately
Pipes,
branch
pipes, Unsuitable
housings with blind plugs
4
9
10
Welded tube structures
Suitable, if fabrication
holes are available
11
Welding gap is less than 4
mm
Blind gaps are not
acceptable, through gaps
can be sealed off with
zinc
12
Welded structures where
different steel grades are
used
Suitable
Remarks
Galvanizing of longer articles
is possible upon prior
agreement
Limited by the crane capacity
Risk of temperature
deformation
Additional provision of holes
or hinges, or usage of special
tooling
Generally, fabrication holes are
not acceptable in such
structures, and galvanizing of
structures with enclosed
volumes is not acceptable.
Formation of air pockets or
areas with significant amount
of trapped zinc is possible.
All cavities should ensure
unimpeded inlet and outlet of
liquids, melted zinc and flux
decomposition gases.
Test galvanizing is required
There is no free outlet of zinc
from the interior volume of the
product, formation of airpockets is possible
Thorough examination of
structure for availability of
fabrication holes is required.
For requirements to fabrication
holes refer to table
Relationship between pipe
size and number of holes in
plugs at their ends
Liquid from pre-treatment area
can penetrate into blind gaps
causing explosion due to its
evaporation during immersion
into the zinc melt.
Through gaps of less than 4
mm size are subject to
intensive crevice corrosion
over time
Coating appearance on
different steel grades can differ
significantly
13
14
Internal stress
concentration zones
(unacceptable bend radius)
Different wall thicknesses
of structural components
15 Overlapping zones
16 Sheet metal products
17 Folded or flanged edges of
metal sheets
18 Edge bend radii
19
Tanks, pipes:
-for potable water
-for service water
-for process fluids transfer
25
26
27
Product surface has marks
of paint coats, highlighters,
silicon grease or sealants
Product surface has marks
of rust, scale, oils, greases,
coolants
Surface roughness
Unsuitable
Possible destruction at the
galvanizing temperature
Expansion of material
and behavior of
components with thin
and thick walls during
cooling down can differ
depending on thickness
ratio
Presence of overlapping
zones should be avoided,
where possible
Minimum and maximum
thickness ratio should not
exceed 1:5
Unsuitable
Coating contains lead
Suitable
Suitable, if fluid рН is
5,5 to 12,5 and fluid
temperature does not
exceed 35оС
Surface
Such zones will not be
galvanized
Mechanical removal of such
contaminants is required
Small overlapping zones to be
airtightly welded along the
contour. For large overlapping
zones, on the contrary, vent
holes to be provided.
Stability of sheet metal is It is preferable to use not flat
not high. Besides, if
sheets, but sheets with large
there is no possibility for bend radius.
product expansion during
hot-dip galvanizing, then
distortion can easily
occur.
Possibility of gap
Particular care is required, so
formation
that during galvanizing acid or
flux residuals did not sediment
in crevices and these
overlapping zones could be
sealed off with zinc melt.
Possibility of air-pockets It is required to ensure
formation
unimpeded inlet and outlet of
zinc; if required, special holes
should be provided.
Purpose
Suitable
These contaminants to be
removed in the pre-treatment
zone
Affects the coating
thickness
Surfaces with very deep
roughness, for example, steel
elements shot blasted using
shots with very sharp edges,
28
29
Low quality surfaces
having such defects as
voids, flakes, stripes,
wrinkles.
Thread surfaces
These are the reasons of
low quality coating or
coating unsuitable for
operation
Zinc coating thickness is
significant, thus, this
thread cannot be used
without rethreading
generally, form thicker zinc
coating, since rough surface
provides larger area for
reaction.
Mechanical removal of such
defects is required
Thread surfaces should be
isolated before galvanizing by
means of cloth tape (male
thread), heat-proof mastic
(female thread)
Welded joints
30 Welded joints:
- butt welding or doublesided welding, or singlesided welding with backing
run;
- overlapping loopback
connections continuous
along the perimeter,
without gaps;
-point or non-continuous
welding;
-pores, blowholes, cracks,
slag
inclusions,
filler
coupling of weld joins,
splatters;
-manual welding using
high silicon electrodes
31 Products requiring
preliminary degalvanizing
Suitable
Suitable
Unsuitable
Unsuitable
Deseaming is required, sand
blasting is recommended
High silicon content causes
visible difference of weld joint
from other surface
Suitable, but not
recommended
Special cases
Suitable, but test
galvanizing is
recommended
In case of thin-walled articles
thickness can be reduced to
undesirable tolerances.
Products can have worse
appearance than the ones non
subject to degalvanizing, and
this should be approved with
the customer.
Relationship between pipe size and number of holes in plugs at their ends
Pipe type
Holes quantity and size
О
□
▭
1
2
15
20
30
15
20
30
20х10
30х15
40х20
8
10
12
10
3
40
50
60
80
100
120
160
200
40
50
60
80
100
120
160
200
50х30
60х40
80х40
100х60
120х80
160х80
200х120
260х140
14
16
20
20
25
30
40
50
12
12
12
16
20
25
25
30
10
10
12
12
20
20
25
Diameters of fabrication holes should not be less than 10 mm. Actual sizes shall be selected from
experience, namely, fabrication hole area of long-profile structures should be minimum1/7 –
1/10 of sectional area of profile included in the structure. The more the size of fabrication holes
is, the smoother galvanizing proceeds and the higher is the coating quality.
Large-sized parts should have fabrication holes of 15-25 mm diameter at 50 mm distance from
the article end (parts longer than 2 m should have holes at two sides).