When
the clean iron or steel component is dipped into the molten zinc (which
is commonly at around 450°C) a series of zinc-iron alloy layers are
formed by a metallurgical reaction between the iron and zinc. The rate
of reaction between the steel and the zinc is normally parabolic with
time and so the initial rate of reaction is very rapid and considerable
agitation can be seen in the zinc bath. The main thickness of coating
is formed during this period. Subsequently the reaction slows down and
the coating thickness is not increased significantly even if the
article is in the bath for a longer period of time. A typical time of
immersion is about four or five minutes but it can be longer for heavy
articles that have high thermal inertia or where the zinc is required
to penetrate internal spaces. Upon withdrawal from the galvanizing bath
a layer of molten zinc will be taken out on top of the alloy layer.
Often this cools to exhibit the bright shiny appearance associated with
galvanized products.
Post galvanizing treatment can include quenching into water or air cooling.
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Conditions
in the galvanizing plant such as temperature, humidity and air quality
do not affect the quality of the galvanized coating. By contrast, these
are critically important for good quality painting.
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