Galvanizing is the practice of coating clean, oxide-free iron or
steel with a thin layer of zinc to protect the surface against
corrosion. The zinc coating provides protection to the iron or
steel in two ways: (1) it shields the base metal from the
atmosphere and (2) because it is more electronegative than iron
or steel, the zinc gives cathodic or sacrificial protection.
Even if the surface becomes scratched and the base metal is
exposed, the zinc is slowly consumed while the iron or steel
remains protected from corrosion.
The hot dip process is adaptable to coating nearly all types of
fabricated and non-fabricated products such as wire, tanks,
sheets, strip, pipes and tubes, fittings, hardware, wire cloth,
hollow ware, and structural assemblies.
Galvanizing consists of three fundamental steps: (1) surface
preparation, (2) galvanizing, and (3) finishing.
The preparation step consists of cleaning and pickling
operations that free the surface of dirt, grease, rust and
scale. Clean, oxide-free work is galvanized by immersion into
molten zinc. Finishing operations include removing excess zinc,
quenching and inspection.
Good surface preparation of metal parts is essential for the
production of high quality galvanized coating. Surface
preparation usually involves a combination of methods for
producing a steel surface which is clean and oxide-free.
A variety of methods is available and more than one sequence of
surface preparation steps may be used successfully in a
- Mechanical surface preparation
- Abrasive blasting
- Brush and hand finishing
- Alkaline cleaning
Galvanizing is accomplished by immersion of the work in molten
zinc at a temperature of 820-860 Fº (438-460 Cº) for a period of
about one to five minutes, depending upon the thickness of the
pieces to be coated. A layer of molten top flux should be kept
on the zinc at the end of the kettle where the articles enter.
The pieces should be withdrawn at the opposite end of the
galvanizing kettle, through a clean zinc surface. For carrying
the articles to be galvanized into, through and out of the
molten zinc, steel tongs, baskets or various types of hooks are
used. These may be handled with an air or electric hoist.
The period of time the work is left in the zinc bath varies with
the thickness of the steel, the amount of preheat and the
thickness of coating desired. The reaction between the clean
steel and the molten zinc to give an alloy layer proceeds
rapidly for the first one or two minutes and then continues at a
decreased rate. The longer the immersion time, the heavier and
more brittle the alloy layer becomes.
After the work comes from the molten zinc, excess zinc should be
removed by shaking or by centrifuging in the case of small
items, such as bolts, nuts, etc.
The hot dip galvanized product should meet recognized and
accepted ASTM standards for coating weight and distribution,
finish and adherence.
The heavier (or the thicker) a zinc coating, the longer the
protection to the base steel. However, due to higher costs,
brittleness of the alloy layer, poor adhesion, and appearance, a
compromise must always be reached.