HOT DIP
GALVANIZING
- Using zinc to protect
steel from corrosion (hot-dip galvanizing).
- Corrosion is caused
by the inherent tendency of metals, when subjected to air and
moisture, to revert to their original earthly forms, usually an ore state.
- Zinc seals the
underlying steel from contact with its environment. If the steel is
exposed to the elements due to mechanical damage, the surrounding zinc
corrodes
sacrificially, protecting the underlying steel from corrosive attack.
- The zinc coating on
galvanized steel is uniform: inside, outside, corners and edges.
Ash
Inclusions
Zinc ash is the oxide film
that develops on the surface of the galvanizing bath. Ash may be
burnt on the steel during dipping or picked up from the top of the bath during
withdrawal.
Ash inclusions can occur on work that is cumbersome and requires slow
withdrawal from
the bath. This ash has no adverse effect on service life. Zinc ash that is not
detrimental to
the appearance of the finished product or does not interfere with the productÕs
function,
is not cause for rejection. Gross oxide lumps due to improper skimming of the
exit surface
of the bath can reduce the effective thickness of the coating and are
unacceptable.
Matte
Gray or Mottled Coating
This
condition develops during cooling and is caused by lack of a free zinc layer on
the
coating surface. It usually appears as a localized dull patch or spider
web-like area on an
otherwise normal surface, although in extreme cases, it may extend over the
entire surface
of the steel. It is not cause for rejection unless specifically stated and
agreed to by the
galvanizer and the fabricator.
A
matte gray coating is most frequently found on heavy sections that cool slowly,
with
certain types of steel, such as those with relatively high silicon or
phosphorus content, or
severely cold-worked steel, all of which may exhibit abnormally rapid alloy
growth.
Where
the condition is caused by the nature of the base steel, the galvanizer has no
control over its occurrence. Galvanizers generally do not have prior knowledge
of a steelÕs
chemical composition. A lower galvanizing temperature and shorter immersion
time
followed (if the type of product is suitable) by rapid quenching in water to
arrest the alloy
growth may be successful in marginal cases. However, such measures are not
always
effective and matte areas in the coating may be unavoidable.
Due
to the steelÕs chemical nature, these coatings are often thicker than the
bright
galvanized coatings and provide service life in proportion to the increased
thickness. After
exposure, these coatings may take on a light yellow to brown dusty appearance
as the
alloy layers weather. The appearance of this light residue colored by the iron
content of the
corrosion-resistant layers should not be considered a sign of failure.
Rust
Stains
These
are caused by seepage from joints and seams after galvanizing or by material
being stored under or in contact with rusty steel. Rust stains of this type are
superficial and
should not be confused with failure of the underlying coating. Rust stains caused
by
seepage from an assembly can indicate a need for a modification of the design.
Surface
rust stains are not cause for rejection of the galvanized product.
Wet
Storage Stain
Wet
storage stain is the buildup of zinc oxide and zinc hydroxides on the galvanized
surface. As the name implies, wet storage stain occurs when the steel is
exposed to a
humid or moist environment without access to freely circulating air. Tightly
stacked or
nested galvanized items are particularly vulnerable to wet storage stain,
especially if they
are stored as unopened bundles for more than a few weeks.
Although
in extreme cases, the protective value of the coating may be impaired, attack
is
often superficial, despite the relative bulkiness of the zinc hydroxide.
Medium
to heavy buildup of white corrosion product must be removed; otherwise, the
essential protective film of basic zinc carbonates cannot form in affected
area. Light
deposits can be removed by brushing with a stiff bristle (not wire) brush. A
coating
thickness check should be performed on the affected areas to ensure that
sufficient zinc
coating remains after the removal of the wet storage stain.
In
advanced stages of wet storage stain, the typical white or gray corrosion
product may
blacken. When this occurs, a significant amount of coating has been lost to
corrosion and
the service life is decreased.
In
extreme cases where heavy white deposits or red rust have formed as a result of
prolonged storage under poor conditions, corrosion products must be removed and
the
damaged area repaired as detailed in ASTM A 780. Where the affected area is
extensive,
or when the wet storage stain would impair the use of the article for its
intended service,
re-galvanizing may be necessary.
Unless
present prior to shipment, the development of wet storage stains is not cause
for
rejection. You must exercise proper caution during transportation and storage
to protect
against wet storage stain.
MISCELLANEOUS TESTS
Testing
for Chromate Finishes
In
some cases, post-galvanizing chromate treatments are specified for the
prevention of
wet storage stain. The presence of chromate film on the surface of the
galvanized coating
is usually visible as a light yellow tint on the surface.
ASTM
B 201 describes a test method for determining the presence of chromate
coatings.
Embrittlement
Testing
Embrittlement
of galvanized steel is very rare and usually is the result of using high
strength steel. The design of the product and selection of the proper steel for
its suitability
to be fabricated and galvanized without embrittlement are the responsibility of
the designer
and fabricator.
For
more information: www.galvanizeit.org
