of structures to be hot-dip galvanized is no different from that
for good structural, surface treatment and welding practice in
general. However, certain components require special attention.
is important that larger structural components are matched to
the size of the pickling tank and zinc bath (Fig 1). Sometimes
“doubledipping”, which involves the handling of structural
components longer than the zinc bath, is employed. (Fig.2).
joints are preferred instead of welding, since welding burns off
the galvanized coating, which havs to be repaired.
weight of the construction. The galvanizing plants can only handle
constructions that the cranes can handle.
1. Schematic diagram of hot dip galvanizing.
2. Schematic diagram of double-dipping.
galvanizing involves dipping components and structures in both pretreatment
baths and molten zinc. This means that hollow structures, such as
pipes, tubing, and containers and also beam structures, which are
welded round the connection point, must be provided with holes for
drainage and venting. If a sufficient number of holes is not provided
to enable complete venting, there is a significant risk that the
structure will explode on immersion. Also, pickling acid is capable
of penetrating pores in welds. Upon dipping in the zinc bath the
residual pickling liquid is vaporized and the pressure can be so
high that the object explodes. The risk of serious injury to personnel,
or damage to materials, is great when such explosions occur.
dimensions for venting holes are given in table 1. The holes must
be located to enable total venting and to enable pickling acid and
molten zinc to run in and out easily. Illustrations show this point.
Examples of the location of the holes are given in the illustrations.
It is also advisable to contact the galvanizer to discuss alternative
hole sizes and locations. Holes can be formed by drilling, grinding
1. Suitable hole sizes for venting of tubular structures.
Galvanizing of cable trays. Tilting the goods improves the run
off of excess zinc when leaving the bath.
following guidelines are not necessarily preconditions for hot-dip
galvanizing but, if they are observed, quality is likely to be considerably
better and the galvanizing process easier.
structure should preferably be designed with easily handled components,
which are assembled after hot-dip galvanizing. Threaded unions are
a good means of achieving this. The structure or object should be
equipped with lifting holes or lifting eyes, to which lifting gear
can be connected. Avoid inwardprojecting pipe connecting stubs in
containers. Pipe connecting stubs should be located in corners and
diagonally, otherwise the container cannot be completely emptied
of zinc (fig.4).
Inwardly projecting pipe con-nections prevent containers
from being completely emptied of zinc and flux. Locate pipe
connections near comers and diagonally. It will also be
easier for the zinc to run out if the pipe con-nections
are not too small. On large containers lifting eyes should
for easier handling.
Do not weld parts of widely differing thickness together. The
object may be deformed in the zinc bath and during cooling after
dipping due to differing heating and cooling rates.
that are to move on each other should be assembled after hot-dip
galvanizing. If this is not possible, mobility can be retained by
giving the components a clearance of 1.0 mm on each side, i.e 2.0
mm larger than the shaft.
Avoid structures that can distort
with excessive variation in sectional thickness should not be combined
on the same structure or object (fig. 5). Otherwise, heating in
the zinc bath would be uneven, and the object could warp. Long,
slim structures should be avoided. Large flat sheet surfaces, where
the sheet thickness is less than 3—4 mm should be stiffened
(fig.6), or they might warp.
Examples of stiffening of sheet surfaces to prevent distortions.
7. Objects with welded-on reinforcements and lap joints must
be provided with holes if the contact area between the pieces
is larger than 70 cm2. An acid trap is created, but must be
accepted, since there is a great risk of explosion if a hole
is not made.
possible, structures should be designed so that welding can be performed
symmetrically, around an axis of flexure. The welding sequence should
be planned so that stresses are as low as possible.
combining different materials and different surface conditions
rolled steel should not be welded to cast iron, or to steel that
has been attacked by deep-seated rust, since the zinc coating will
be uneven and the appearances of the surfaces will be different.
Such materials must be galvanized in different ways, which is not
possible if they have been welded together.
types of steel, such as rimming or aluminiumkilled steel, should
not be welded to silicon-killed steel. If this is not observed,
both the appearance of the surface and the thickness of the zinc
coatings will be different.
Avoid acid traps
should be designed to avoid narrow crevices or gaps (fig. 9 and
11). As far as hot-dip galvanizing is concerned, butt welds are
better than lap welds. If lap welds must be used, they should be
welded around the entire joint. It is important to avoid the formation
of pinholes. If the gap between contact surfaces is greater than
70 cm2, venting holes must be provided to avoid the risk of explosion
acid has penetrated a crevice, it cannot be removed later. Since
molten zinc is more viscous than acid, it will seal the opening
during hot-dip galvanizing. After a while, the acid causes holes
in the coating over the opening. A rust-coloured liquid then runs
on to the surface and spoils the appearance. Corrosion in such crevices
can become so severe that the connection eventually fails.
should have an even, sealed and sand-free surface. Pores and cavities
can act as acid traps and should therefore be avoided.
should if possible be designed so that the zinc can run freely off
all surfaces when the object is withdrawn from the zinc bath (fig.8).
The coating will otherwise be uneven.
Corners on stiffening plates in e.g. beam webs should be cropped.
Drainage holes should be located as close to the web as possible.
Diagonals in lattice structures should not run up against frame
bars. Note that an acid trap is created in alternative 2. This
should be avoided.
9. Seal the ends on a double fillet weld so that acid cannot
penetrate into the gap.
In welded structures venting holes must be inserted at all junctions.
The holes must enable the zinc to run in and out freely. Closed
tubes incur the risk of explosion!
threads must be cut undersize to enable them to fit the respective
nuts after galvanizing. Suitable reduction of the thread diameter
is given in standards. Female threads are cut or cleaned to nominal
size after galvanizing. Male threads in assembled structures must
be cleaned after galvanizing. The zinc coating on male threads also
protects the steel in female threads.
marking of steel to be hot-dip galvanized should be carried out
with water-soluble paint only. Permanent marking can be done by
stamping letters or numbers on the object, or to an attached ID
plate. Stamping should be to sufficient depth to enable the marking
to be read even after galvanizing.
should be free of pores, which could act as acid traps. In case
of double-sided fillet welds, the weld must be continued over the
ends of the sheet to prevent the penetration of acid into any conceivable
methods which do not produce slag, such as MIG welding, are preferable.
If coated welding electrodes are used, all slag deposits should
be carefully removed from the weld. It is not possible to remove
welding slag through normal pickling, and any residue causes the
formation of black spots during hot-dip galvanizing.
7-10. Examples of how weldments should be designed in order to avoid
acid traps (narrow crevices).
pre-requisite for good results in hot-dip galvanizing is clean steel
surfaces. One should therefore try to keep the surfaces free of
paint, grease, oil and tar. Impurities such as these cannot be removed
by pickling and result in black, uncoated spots after galvanizing.
Surface contaminations in the form of grease, oil, tar, paint and
welding slag require removal by means of degreasing, sandblasting
or grinding. Contamination may be difficult to detect. If it becomes
apparent after galvanizing the article may need to be regalvanized,
which increases costs.
or flame cutting heats the steel and produces surfaces that react
differently when galvanized. The coating thickness may be lower
than expected and the adherence to the steel surface may be poor.
The cut surfaces should therefore be ground to remove the heat affected
zone before galvanizing.