Galvanizing
There are 2 types of galvanizing Hot dip galvanizing and electro galvanizing.
Here we will deal with hot dip galvanizing and the almost identical hot dip aluminizing process where zink is replaced by aluminum. While the electro galvanizing is dealt with in the chapter about
electro plating.
As the name Hot dip implies the wire that is to be galvazined is past through a molten zink bath. During this process the zinc is metallurgically bonded to the steel forming a
progression of zinc-iron alloy layers.
The Hot Dip Galvanizing Process
Surface Preparation
This is an extremely important step in the process since zinc will
not react to improperly treated steel. The following operations are
performed:
Caustic cleaning - to remove organic matter such as dirt,
paint, oil and grease
Acid pickling - to remove scale and rust
Fluxing - to prevent oxides from forming prior to
galvanizing
Galvanizing
The material is thoroughly dried after surface prep and then totally
immersed in a bath of molten zinc at 840°F. During this process
the zinc is metallurgically bonded to the steel forming a
progression of zinc-iron alloy layers. Upon removal from the bath
the material is drained, vibrated or centrifuged to remove excess
zinc.
Quenching
The galvanized steel is then dipped in a dilute chromate solution
to give a longer lasting luster and to reduce temperature for
prompt handling and inspection.
Inspection
The process is completed by a careful inspection of the zinc coating
for thickness, adherence, uniformity and appearance.
Aluminum coated Wire
Hot dip aluminum is preferred for use in industrial environments, especially where there are high concentrations of sulfur dioxide and
other pollutants. Arc sprayed aluminum has a relatively high bond strength even over marginally prepared surfaces.
Aluminum pH range is 4 to 11.
At elevated temperatures up to about 1100° F (590° C); for example, on the fire side of fossil fuel boiler tubes.
Applications include steel pilings above and below the mud and water lines; in marine atmospheres, offshore oil platforms, ship
chain lockers, fish holding tanks; U.S. Navy applications are too numerous to mention.
Zinc coated Wire
Zinc provides greater galvanic protection than aluminum. Its greater galvanic power protects gaps in the coating better than pure
aluminum. It is marginally easier to spray pure zinc than pure aluminum by some flame or arc spray systems.
Zinc pH range is 6 to 12.
Zinc should not be used in immersion service at temperatures above 140° F (60° C).
Applications include steel bridges, the inside of potable water tanks, power transmission and communication towers, buried
iron pipe. Zinc is used in combination with galvanizing to avoid double dipping, and for general repair of galvanized steel.
A relatively new use for thermal sprayed zinc is for cathodic protection of rebar set in concrete. There are two types of systems
which use the sprayed zinc; one relies on an impressed electrical current, and the other is passive or galvanic. This is a
special application and requires a certain level of expertise. Call us with your questions.
Zinc Aluminum coated Wire
Zinc - 15% aluminum wire combines the benefits of pure zinc with the benefits of pure aluminum in the metalized coating.
It is very often used as a substitute for pure zinc because it is somewhat more chloride and sulfur dioxide resistant than pure zinc,
while retaining the greater electro-chemical activity of pure zinc.