Welding of Coated Steels

Abstract:
Welding of zinc-coated steel can be done, but specific precautions
should be taken. When galvanized steel is arc welded the heat of
the welding arc vaporizes the zinc coating in the weld area. This
is because the boiling point of zinc is below the melting point of
steel. The zinc volatilizes and leaves the base metal adjacent to
the weld.
When galvanized sheet is resistance welded, the welding heat
causes less disturbance of the zinc coating than the arc
processes. Resistance welding of galvanized steel is, however,
more of a problem than arc welding.

There are many types of coated steels and some of them are welded.
For example, tern plate steel, which is tin-lead coated, and
aluminized steel. Steels coated with tin, cadmium, copper, and
lead may be soldered.
Galvanized steel is widely used and is becoming increasingly
important. Manufacturers of many items such as truck bodies,
buses, and automobiles are increasingly concerned with the effects
of corrosion particularly when chemicals are used on roads for ice
control. Galvanized metal is also used in many appliances such as
household washing machines and driers and in many industrial
products such as air conditioning housings, processing tanks, etc.
Other uses for galvanized products are for high tension electrical
transmission towers, highway sign standards, and protective
items.

There are two basic methods of galvanizing steel. One is by
coating sheet metal and the other is by hot dipping the individual
item. The coated sheet metal is produced by the continuous hot dip
process, but traffic control devices, high-tension transmission
tower parts, etc., are made by dipping each item. The continuous
hot dip or zinc coated sheet comes in eight classes based on the
thickness of the zinc coating.

Welding of zinc-coated steel can be done, but specific precautions
should be taken. When galvanized steel is arc welded the heat of
the welding arc vaporizes the zinc coating in the weld area. This
is because the boiling point of zinc (1600°F, 871°C) is below the
melting point of steel (2800°F, 1538°C). The zinc volatilizes and
leaves the base metal adjacent to the weld. The extent to which
the coating is disturbed depends on the heat input of the arc and
the heat loss from the base metal. The disturbed area is greater
with the slower welding speed processes such as oxyacetylene
welding or gas tungsten arc welding.

When galvanized sheet is resistance welded, the welding heat
causes less disturbance of the zinc coating than the arc
processes. The resistance to corrosion or rather the protection by
the zinc is not disturbed since the zinc forced from the spot weld
will solidify adjacent to the spot weld and protect the weld
nugget. Resistance welding of galvanized steel is, however, more
of a problem than arc welding.

Weld Quality
The low boiling temperature of the zinc of the coating causes it
to volatilize in the heat produced by an arc or by an oxyacetylene
flame. The zinc in the gaseous state may become entrapped in the
molten weld metal as it solidifies. If this occurs there will be
porosity in the weld metal and if sufficient zinc is available it
will cause large voids in the surface of the deposit.
The presence of the zinc in stressed welds can cause cracking and
it may also cause delayed cracking due to stress corrosion. To
eliminate this, the weld joint must be designed to allow the zinc
vapor to completely escape from the joint. Fixturing, backing
straps, etc., should be arranged to allow for the zinc to
completely escape. Other ways to avoid zinc entrapment in weld
metal is to use sufficient heat input when making the weld.

It is also important to secure complete and full penetration of
the joint. The ultimate precaution would be to remove the zinc
from the area to be welded.

When welding on galvanized steel or any coated steel, particularly
those with coatings that produce noxious fumes, positive
ventilation must be provided. Positive ventilation involves the
use of a suction hose at the weld area. When using the gas metal
arc process or the flux-cored arc process, the suction type gun
nozzles should be used. Welding on zinc or other coated steels
should never be done in confined areas.

For corrosion resistance of the weld it is sometimes advisable to
use a corrosion-resistant weld metal. This can be done by using a
bronze deposit such as a copper-zinc alloy, or a stainless steel
electrode. In any case, when arc or oxyacetylene welding is used
the area adjacent to the weld will lose the protective zinc
coating which must be repaired.

Arc Welding
When using covered electrodes, the electrode selection should be
based on the thickness of metal and the position that will be used
when welding galvanized steel. The E-XX12 or 13 will be used for
welding thinner material, the E-XX10 or 11 will be used for
welding galvanized pipe and for welding hot-dipped galvanized
parts of heavier thickness. The low-hydrogen electrodes can also
be used on heavier thicknesses.
The welding technique should utilize slow travel speed to permit
degassing of the molten metal. The electrode should point forward
to force the zinc vapor ahead of the arc. The quality of welds
will be equal to those of bare metal, assuming the weldability of
the steel is equal.

The gas metal arc welding process is becoming more widely used for
joining galvanized steel. For the thinner gauges the fine-wire
short-circuiting method is recommended. In this case, the
technique would be similar to that used for bare metal. The
shielding gas can be 100% CO2 or the 75% argon and 25% CO2
mixture. The selection is dependent primarily on the material
thickness and position of welding.

For certain applications, the argon-oxygen mixture is used. The
amount of spatter produced when welding galvanized steel is
slightly greater than when welding bare steel. The flux-cored arc
welding process can be used as easily as gas metal arc welding for
galvanized steel. It is recommended for the heavy gauges and on
hot-dipped galvanized parts. The highly deoxidized type of welding
electrode should be used.

The gas tungsten arc welding process is not popular for welding
galvanized steel, since it is one of the slower welding processes
and does cause a larger area of zinc adjacent to the weld to be
destroyed. In addition, the volatilized zinc is apt to contaminate
the tungsten electrode and require frequent redressing of the
electrode.

In an effort to overcome this, extra high gas flow rates are
sometimes used, which can be expensive. Other techniques may be
used as well. If a filler rod is used it may be of either the
highly deoxidized steel type or of the bronze type previously
mentioned. In this case the arc is played on the filler rod and
zinc contamination of the tungsten electrode is avoided.

The carbon arc welding process has been widely used for welding
galvanized steel. Both the single carbon torch and twin carbon
torch can be used. The twin carbon torch is used as a source of
heat much the same as the oxyacetylene flame; however, when the
single carbon is used the carbon can be played on the filler rod
and extremely high rates of speed can be accomplished. Normally in
this situation the filler rod, Type RBCuZn-A (60% Cu-40% Zn). By
directing the arc on the filler rod it melts and sufficient heat
is produced in the base metal for fusion but not sufficient to
destroy the zinc coating. This process and technique is widely
used in the sheet metal duct work industry.

Torch Brazing
The oxyacetylene torch is also widely used for brazing galvanized
steel. The technique is similar to that mentioned with the carbon
arc. The torch is directed toward the filler rod which melts and
then fills the weld joint. A generous quantity of brazing flux is
used and this helps reduce the zinc loss adjacent to the weld.

Repairing the Zinc Coating
The area adjacent to the arc or gas weld will be free of zinc
because of the high temperature of the weld. To produce a
corrosion-resistant joint, the zinc must be replaced in this area
and on the weld itself unless the nonferrous filler metal was
used.
There are several ways of replacing the zinc. One is by the use of
zinc base paste sticks sometimes called zinc sticks or galvanized
sticks sold under different proprietary names. These sticks are
wiped on the heated bare metal. With practice a very good coating
can be placed which will blend with the original zinc coating.
This coating will be thicker than the original coating, however.

Another way of replacing the depleted zinc coating is by means of
flame spraying using a zinc spray filler material. This is a
faster method and is used if there is sufficient zinc coating to
be replaced. The coating should be two to two-and-one-half times
as thick as the original coating for proper corrosion
protection.

Other Coated Metals
One other coated metal that is often welded is known as tern
plate. This is sheet steel hot dipped with a coating of a lead-tin
alloy. The tern alloy is specified in thicknesses based on the
weight of tern coating per square foot of sheet metal. Tern plate
is often used for making gasoline tanks for automobiles. It is
thus welded most often by the resistance welding process. If it is
arc welded or oxyacetylene welded the tern plating is destroyed
adjacent to the weld and it must be replaced. This can be done
similar to soldering.
Aluminized steel is also widely used in the automotive industry
particularly for exhaust mufflers. In this case, a high
silicon-aluminum alloy is coated to both sides of the sheet steel
by the hot dip method. There are two common weights of coating,
the regular is 0.40 ounces per square foot and the lightweight
coating is 0.25 ounces per square foot based on coating both sides
of the sheet steel. Here again, if an arc or gas weld is made on
aluminum-coated steel the aluminum coating is destroyed. It is
relatively difficult to replace the aluminum coating and,
therefore, painting is most often used.