Miss Conception puts it “straight”
“Miss Conception” rectifies incorrect impressions concerning hot dip galvanizing.
If you require a thicker hot dip galvanized coating you should specify it as C4 or C5 (in terms of ISO 9223) hot dip galvanizing!
True or false?
When pre-cleaned steel is dipped into therefore potential damage, particularly on request from the South African specifier a
molten zinc at about 450°C a chemical edges. Mechanical manipulating, including heavy duty coating was required mostly for
reaction takes place whereby a coating rolling, bending and heat treatment, plus any mining or coastal conditions. This has been
structure comprising a series of iron/zinc welding of the steel, can lead to different included in the South African Bureau of
alloys, overcoated with a relatively pure zinc reactions in the zinc bath. Standards - General and Specific Permit
layer is metallurgically bonded to the steel Surface Condition Conditions issued to all Mark Scheme
substrate. Coating thickness and appearance
Varying surface roughness of the steel leads Holders, whereby a heavy duty coating of at
are therefore mainly influenced by the laws
of metallurgy, where the chemical to variations in thickness of the coating. The least 25% greater than that given in ISO
composition of the steel, steel thickness and rougher the surface of the steel, the thicker 1461, (not centrifuged) can be achieved
surface roughness, play the major roles. the coating. Depending on the type of steel without compromising specification
and the surface profile, preparation conformity.
Composition and the Metallurgy of the Steel treatment such as abrasive blasting can
Some carbon steels are more reactive to result in a 10 to 15% thicker coating. Steel Although a heavy duty coating can be
molten zinc and in spite of the modern steel that has been severely attacked by rust, or requested particularly with thick steels, a
making continuous casting process, this over pickled, also results in increased coating thick coating of in excess of 25% in most
phenomenon still occurs. With the thickness. instances is common! Should this
continuous casting process, either silicon or requirement be critical, samples should be
Although the galvanizer to a degree can
aluminium is added to the steel as a de- galvanized to achieve the required coating
influence the resultant coating thickness, his
oxidising agent. These steels are respectively thicknesses, the results monitored
influence is mostly limited. For instance he
known as aluminium-killed and silicon-killed
can: subsequent to discussions and then suitable
steels. Aluminium-killed steel generally
includes all flat steel up to 4.5mm thick, N Extend the immersion time instructions implemented.
which can end up being used to manufacture 1. For most silicon-killed steels a longer Therefore, should a specifier require a
tubes, etc. Silicon-killed steel generally immersion time will produce a thicker coating thickness in excess of that set out in
includes all hot rolled profiled steel and flat coating.
SANS 121(ISO 1461), a heavy duty coating
plate greater than 4.5mm thick. While 2. However, with some semi-reactive
may be specified. This is done in accordance
aluminium additions to steel have no effect silicon-killed steels the immersion time
with the “General and Specific Permit
on the coating structure and thickness of a required to produce the initial coating
thickness, may have to be doubled in Conditions”, by the South African Bureau of
galvanized coating, the same cannot be said
order to achieve a marginal increase in Standards issued to all Mark Scheme
for silicon, where certain amounts of silicon
coating growth. This would obviously Holders. In addition for practical purposes,
can have a major effect on increased alloy
growth during hot dip galvanizing. impact on the productivity and hence the requirement should also be discussed
Phosphorus in graded steel is usually less profitability of the galvanizer. with the galvanizer prior to finalizing the
than 0.001%, which at this level rarely has an 3. If the steel is aluminium-killed (very little order. For a number of technical and
effect on alloy growth either with silicon or silicon and no phosphorus – low practical reasons and steel types, a heavy
on its own. However in steel where reactivity) longer immersion times will duty coating may not always be achieved.
phosphorus exceeds 0,02% (generally some not necessarily produce a thicker
In addition the specifying of a C4 or C5 hot
of the commercial grade steels) reactivity to coating. See figure 25.
dip galvanized coating is incorrect because it
molten zinc will increase. N Speed up the withdrawal rate. (Slow
withdrawals result in thinner coatings, relates to ISO 9223 – Corrosion of metals
Thickness of the Steel and alloys – Corrosivity of atmospheres –
faster withdrawals thicker coatings).
The thickness of the steel influences the Classification, which by its title is a
Most galvanizers have a crawl speed
coating thickness. Generally, the thinner the classification of atmospheres and although a
installed on their crane control so that it
steel the thinner the coating. This applies
is easy for the crane operator to adhere thicker hot dip galvanized coating (or duplex
especially to aluminium-killed steels. One
to the norm of slow withdrawal, so coating) is necessary for long term durability
reason for this is that articles fabricated from
increasing the withdrawal rate may not in these atmospheres it is not the manner in
thinner steels, generally require shorter
be practical. which to specify such a coating!
immersion times. Although relatively
infrequent, it is for this reason that when N Depending on the facility, the
Note: All steel can be hot dip galvanized but should
reactive thinner steels are welded to non- components length and the crane hook
the specifier require a particular result, eg.
reactive thicker steels, inordinately thicker height, the preferred angle of exit is
architectural quality, the correct steel grade must be
coatings may result on the thinner steel. about 35 to 45° angled to the horizontal,
some galvanizers may find this specified, see table 16.
There is a far greater possibility that non-
reactive thinner steel is welded to reactive impractical or not achievable due to Due to space constraints figure 25 and table 16 have
thicker steel and the resultant thicker coating plant restraints. not been included in this article, kindly refer to the
on the thicker steel may be aesthetically less Although SANS 121 (ISO 1461) has no Association’s “Steel Protection by Hot Dip
acceptable and prone to brittleness and reference to a heavy duty coating, by popular Galvanizing and Duplex Coating Systems”.
Volume 6 Issue 3 2009 Hot Dip Galvanizing Today 37