Sector Focus: Protective CLoiastings
NSC 11
May 19
application of paint coatings are temperature
and humidity. These can be more easily
controlled under shop conditions than on
site.
• Air temperature and steel temperature
affect solvent evaporation, brushing
and spraying properties, drying and
curing times and the pot life of two-pack
materials, etc. Where heating is required,
this should only be by indirect methods.
• Paints should not be applied when there is
condensation present on the steel surface
or the relative humidity of the atmosphere
is such that it will affect the application or
drying of the coating. Normal practice is
to measure the steel temperature with a
contact thermometer and to ensure that it
is maintained at least 3°C above the dew
point. However, moisture cured paints
are available. These paints are specifically
formulated for application in damp and
humid conditions.
Metallic Coatings
There are four commonly used methods of
applying metal coating to steel surfaces. These
are hot-dip galvanizing, thermal spraying,
electroplating and sherardizing. The latter two
processes are not used for structural steelwork
but are used for fittings, fasteners and other
small items. In this article NSC looks in more
detail at the hot-dip galvanizing process.
Hot-dip galvanizing is a process that
involves immersing the steel component to
be coated in a bath of molten zinc (at about
450°C) after pickling and fluxing, and then
withdrawing it. The immersed surfaces are
uniformly coated with zinc alloy and zinc
layers that form a metallurgical bond with
the substrate. The resulting coating is durable,
tough, abrasion resistant, and provides
cathodic (sacrificial) protection to any small
damaged areas where the steel substrate is
exposed.
As the zinc solidifies, it usually assumes
a crystalline metallic lustre, often referred to
as spangling. The thickness of the galvanized
coating is influenced by various factors
including the size and thickness of the
workpiece, the steel surface preparation, and
the chemical composition of the steel. The
typical minimum average coating thickness
for structural steelwork is 85 μm. Thick steel
parts and steels which have been abrasive
blast cleaned tend to produce relatively thick
coatings up to 140 μm.
Since hot-dip galvanizing is a dipping
process, there is obviously some limitation
on the size of components that can be
galvanized. However, ‘double-dipping’ can
often be used when the length or width of
the workpiece exceeds the size of the bath.
The longest tank in the UK is currently 21
metres in length, the maximum double-dip
dimension is 28 metres, and the maximum
lift weight is 16 tonnes.
Some aspects of the design of structural
steel components need to take the galvanizing
process into account, particularly with
regards the ease of filling, venting and
draining and the likelihood of distortion. To
enable a satisfactory coating, suitable holes
must be provided in hollow sections to allow
access for the molten zinc, the venting of hot
gases, and the subsequent draining of zinc.
Further guidance on the design of articles to
be hot-dip galvanized can be found in BS EN
ISO 14713-1.
The suitability of steels for hot-dip
galvanizing should also be considered.
Structural steel that is to be hot-dip
galvanized should be clearly specified, by
invoking the appropriate options in the
material standards, e.g. Option 5 in BS EN
10025-1.
Andrew Harrison, Sales & Marketing
Director at Wedge Group Galvanizing says:
“Hot dip galvanizing is highly sustainable,
producing minimal waste, while the nonferrous
properties of the metal make it
indefinitely recyclable without loss of physical
or chemical properties.
“Galvanized products, once constructed or
installed can also be removed, re-galvanized
and re-used.”
Duplex coatings
For many applications, hot-dip galvanizing
is used without further protection. However,
to provide extra durability, or where there
is a decorative requirement, paint coatings
are applied. The combination of metal and
paint coatings is usually referred to as a
'duplex' coating. When applying paints
to galvanized coatings, special surface
preparation treatments must be used to
ensure good adhesion. These include light
blast cleaning to roughen the surface and
provide a mechanical key, and the application
of special etch primers or 'T' wash, which is
an acidified solution designed to react with
the surface and provide a visual indication of
effectiveness.
Sponsors
Protective Coatings
Gold: Wedge Group Galvanizing
Silver: Jack Tighe Ltd
Bronze: Hempel; Jotun Paints;
Joseph Ash Galvanizing;
Sherwin-Williams
Galvanized steelwork
was chosen for much
of the M&S car park
frame at Longbridge
/Metallic_coatings#Thermally_sprayed_metal_coatings
/Metallic_coatings#Hot-dip_galvanizing
/Surface_preparation
/Surface_preparation#Abrasive_blast_cleaning
/Design
/Steel_construction_products#Structural_hollow_sections