If you require a thicker hot dip galvanized - MISCONCEPTIONS

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If you require a thicker hot dip galvanized - MISCONCEPTIONS Powered By Docstoc
					                                                                                                            Miss Conceptions

                                                                       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

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