LSiescttinogr sFocus: Protective Coatings
Protective coatings for
structural steel
10 NSC
May 19
Paint systems for steel structures have
developed over the years to comply
with industrial environmental
legislation and in response to
demands from bridge and building owners
for improved durability performance.
Previous five and six coat systems have been
replaced by typically three coat alternatives,
and the latest formulations have focussed on
application in even fewer numbers of coats,
but with increasing individual film thickness.
Composition of paints and film
formation
Paints are made by mixing and blending three
main components:
• Pigments are finely ground inorganic or
organic powders which provide colour,
opacity, film cohesion and sometimes
corrosion inhibition.
• Binders are usually resins or oils but can
be inorganic compounds such as soluble
silicates. The binder is the film forming
component in the paint.
• Solvents are used to dissolve the binder
and to facilitate application of the paint.
Solvents are usually organic liquids or
water.
Paints are applied to steel surfaces by many
methods but in most cases this produces a
'wet film'. The thickness of the 'wet film' can
be measured, before the solvent evaporates,
using a comb-gauge. As the solvent
evaporates, film formation occurs, leaving the
binder and pigments on the surface as a 'dry
film'. The thickness of the 'dry film' can be
measured, usually with an electro-magnetic
induction gauge. In general the corrosion
protection afforded by a paint film is directly
proportional to its dry film thickness.
A Paint System
Protective paint systems usually consist of
primer, intermediate/build coats and finish
coats. Each coating ‘layer’ in any protective
system has a specific function, and the
different types are applied in a particular
sequence of primer followed by intermediate/
build coats in the shop, and finally the finish
coat (or top coat) either in the shop or on site.
• Primers are applied directly onto the
cleaned steel surface, or in the case of
duplex systems, the sealed metal coating,
to wet the surface, to provide good
adhesion for subsequently applied coats
and to provide corrosion inhibition.
• Intermediate coats are applied to ‘build’
the total film thickness of the system.
Generally, the thicker the coating the
longer the life. Intermediate coats are
specially designed to enhance the overall
protection and, when highly pigmented,
decrease permeability to oxygen and water.
The incorporation of laminar pigments,
such as micaceous iron oxide (MIO),
reduces or delays moisture penetration in
humid atmospheres and improves tensile
strength. Modern specifications now
include inert pigments such as glass flakes
to act as laminar pigments. Undercoats
must remain compatible with finishing
coats when there are unavoidable delays in
applying them.
• The finish coat provides the required
appearance, the surface resistance of the
system and the first line of defence against
weather and sunlight, open exposure, and
condensation.
• Stripe coats are additional coats of paint
that are applied locally to welds, fasteners
and external corners to build a satisfactory
coating thickness at edges and corners
where paint has a tendency to contract and
thin upon drying.
The various superimposed coats within a
painting system have to be compatible with
one another so generally all paints within a
system should be obtained from the same
manufacturer and used in accordance with
the manufacturer’s recommendations.
Application of paints
The method of application of paint systems
and the conditions of application have a
significant effect on the quality and durability
of the coating. Standard methods used to
apply paints to structural steelwork include
application by brush, roller, conventional air
spray and airless spray/electrostatic airless
spray.
Airless spray has become the most
commonly used method of applying paint
coatings to structural steelwork under
controlled shop conditions. Brush and roller
application are more commonly used for site
application, though spraying methods are also
used. ‘Stripe’ coatings applied to edges and
sharp corners are usually applied by brush.
The principal conditions that affect the
Safeguarding structural steelwork with the correct corrosion
protection system will give a steel frame a long life and reduce
ongoing maintenance costs. NSC provides some key facts
regarding the use and application of paint coatings and hotdip
galvanizing.
Spraying is the most
commonly used method
of applying paint to
structural steelwork
/Bridges
/Paint_coatings
/Paint_coatings#Primers
/Paint_coatings#Intermediate_coats
/Appropriate_specifications
/Paint_coatings#Finish_coat
/Paint_coatings#Stripe_coats
/Paint_coatings#Application_of_paint_coatings