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STEEL CONSTRUCTION:
PROTECTION: CORROSION
Lecture 4A.2: Factors Governing Protection of Steelwork
OBJECTIVE/SCOPE
To expand upon Lecture 4A.1, giving the practical means of
protecting steelwork at a level suitable for young architects and
engineers.
PREREQUISITES
None.
RELATED LECTURES
Lecture 4A.1: General Corrosion
SUMMARY
This lecture covers the assessment of the required life design
for the successful use of protective systems and surface
preparation. The coatings commonly used to protect steel are
described and the use of stainless and weathering steels are
briefly discussed. Finally a general discussion of maintenance is
given.
1. LIFE EXPECTANCY Table 1 classifies the principal types of
environment that have a significant influence on the life
expectancy of steel.
In dry, heated buildings, e.g. offices, hospitals, warehouses,
the corrosion rates of carbon steel are usually very low. Steel can
be used without protection in such environments when it is hidden.
Elsewhere it is coated for aesthetic or hygienic reasons.
Many interiors are not dry however and steelwork requires
protection in these situations, as well as in exterior
environments.
Structures and plant usually have a "design life". If after
execution of the structure access is impossible, the initial
protective system needs to have the same life as the steel.
Economic pressures often increase the functional life of plant
significantly beyond the
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"design life". Changes in expectation usually occur after the
initial protective system is in place. It is sensible therefore to
consider this possibility at the start of every new project.
1.1 Likely Time to First Maintenance
Table 2 gives in column (a) typical lives in the general
environment quoted to prevent deterioration of the steel using
various coating systems. Column (b) gives the likely time to first
refurbishment where good appearance and the maintenance of a
readily cleaned surface are important. Neither set of figures can
allow for the influence of local conditions, e.g. heavy overnight
condensation due to the unplanned shutting down of ventilating
systems to save money.
Protective systems require regular inspection allowing
unexpected local failures to be repaired. Ideally the base steel
should never be exposed. If the first coat of the system is zinc
galvanising or metal spray then it should be considered part of the
structure, the paint coats being refurbished at intervals which
ensure it remains unexposed.
1.2 Life Between Maintenances
When there is data on the performance of a protective system on
similar structures or plant, prediction of the intervals to
maintain the top coat(s) is fairly easy. Since the initial failure
of a protective system may be sooner than anticipated, the
estimation of the interval for some breakdown to bare steel can be
complicated.
1.3 Assessment of Life Requirement
It may be necessary to assess each part of a structure
separately. For each assessment the following points should be
taken into account:
a. Required life of structure/plant.
b. Decorative and hygienic requirements. The decorative life of
a coating (and its ability to be readily cleaned) is rarely as long
as the protective life of the system, see Table 2.
c. Irreversible deterioration if scheduled maintenance is
delayed.
d. Difficulty of access for maintenance.
e. Technical and engineering problems in maintenance.
f. Minimum acceptable period between maintenance.
g. Total maintenance costs, including plant shut-down, closure
of roads, access, etc.
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2. DESIGN The design of structures and plant is based largely on
data and functional requirements which can be quantified, e.g. 'the
steelwork supports plant manufacturing a specific product and has a
life expectancy of 25 years'. The selection of a protective system
involves many factors; these factors vary widely according to the
type of structure, its complexity, its function, the general
environment, (see Table 1) the influence of microclimates and the
effects of possible environmental changes (natural and otherwise)
which may occur during the required life.
Other factors affecting selection are quantitative, e.g. time to
first maintenance, planned maintenance schedule to cover the
required life of the structure or plant, thickness of coatings,
etc. They should be viewed with caution because the degree of
variation may differ between one coating system and another.
Quotations may vary considerably for the same system
irrespective of whether it is hot dip galvanising, metal spray or
paint. Great care is necessary to ensure quotations for apparently
identical products or services do cover the same materials,
application with the same degree of control, and comparable quality
of finish in terms of both required durability and appearance.
Some of the critical conditions and circumstances that have to
be taken into account before selecting a protective system are
listed in question form in Appendix 1. Not every question is
relevant to a particular job and the importance of the relevant
questions varies. The order of relevant questions might be modified
in the light of answers to later questions. The list should be
studied as a whole before the questions are considered in
detail.
2.1 Design for Protective Systems
The design of structures and plant can influence the choice of
protective system. It may be appropriate and economic to modify the
design to suit the preferred protective system. The following
points should be noted:
a. Provide safe and easy access to and around the structure to
facilitate maintenance.
b. Design the elements:
i. to avoid pockets and recesses in which water and dirt can
collect, see Figures 1 - 5.
ii. to eliminate sharp edges and corners, see Figure 6.
iii. to provide clear access for painting e.g. to allow space to
use a paint brush or spray gun, see Figure 7.
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c. Any areas which are inaccessible after erection require a
coating system designed to last the required life of the structure.
Is this feasible or should the design be modified?
d. Certain structural sections are more suited to some coating
systems than others, e.g. hollow section are more easily wrapped
than structural shapes.
e. The method or size of fabrication may preclude or limit some
protective systems, e.g. friction grip bolts, galvanising.
f. If bimetallic corrosion is possible, additional protective
measures are necessary, see Figure 8.
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g. Where steel is likely to be in contact with other building
materials, special precautions may be necessary e.g. oak
timbers.
h. For steel structures in water, cathodic protection may be the
best solution, see Figure 9.
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2.2 Where to Apply Protection
In this case "where" means should the protective coating system
be applied on or off site.
Protective system are more durable when applied in the
fabrication shop or steel mill. Where there is a likelihood of
substantial damage occurring during transportation and erection
specifiers may prefer the final one or two coats of protection to
be applied on site. Paints specified for site use must be tolerant
of delay and a measure of intercoat contamination. The
specification should state clearly who is responsible for quality
control at each stage of fabrication and processing.
Where the total system is applied off-site, the specification
must cover the need for care at all later stages to prevent damage
to the finished steel and set out repair procedures for the
coatings once the steelwork is erected.
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2.3 Special Areas
The protective treatment of bolts, nuts and other parts of the
structural connections require careful consideration. Ideally their
protective treatment should be of a standard at least equal to that
specified for the general surfaces.
Where high performance paint systems are to be used, it is worth
considering hot dip spun galvanised or stainless steel
fasteners.
The mating surfaces of connections made with high strength
friction grip bolts require special treatment, see Appendix 2 in
Lecture 4A.3.
3. SURFACE PREPARATION The surface preparation of the steelwork
has a major influence in determining the protective value of the
coating system.
For galvanising and metal spraying, surface preparation is an
integral part of the process and is included in national standards
for these operations. With paint systems there is usually a choice
of preparatory methods. Therefore the actual method chosen for a
specific job must be specified as part of the protective coating
treatment.
The choice between blast-cleaning and manual cleaning is partly
determined by the nature of the coatings to be applied. Coatings
applied to a degreased blast-cleaned surface always last longer
than similar coatings applied to manually cleaned surfaces.
However, some short-life coatings do not warrant the high cost of
blast-cleaning as required for long-life coatings. Details of
methods for blast cleaning surfaces are given in ISO 8504 [5].
3.1 Degreasing
Grease and dirt are best removed by proprietary emulsion
cleaners followed by a thorough rinsing with water, by
steam-cleaning, or by controlled high pressure water jets.
Where it is necessary to use white spirit or similar solvents to
remove oil or grease, the use of detergent or emulsion cleaner
should follow before completing the operation by thorough rinsing
with clean fresh water.
Degreasing by washing with solvent is not recommended because it
can lead to the spreading of a thin film of oil or grease over the
surface.
3.2 Removal of Scale and Rust
Mill-scale is made up of the surface oxides produced during the
hot-rolling of steel. It is unstable. On weathering, water
penetrates fissures in the scale and rusting of the steel surface
occurs. The mill-scale loses adhesion and begins to shed. It is an
unsatisfactory base and needs to be removed before protective
coatings are applied.
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In general, rusted steel surfaces are not a satisfactory base
for the application of protective coatings, although some primers
have a limited tolerance to residual rust left on steel surfaces
after manual cleaning. The means of removing rust and scale are
described below.
3.3 Blast Cleaning
Abrasive particles are directed at high velocity against the
metal surface. They may be carried by compressed air or
high-pressure water, or thrown by centrifugal force from an
impeller wheel. For some open blasting, high pressure water without
abrasives may be used. The various methods are listed in Table
3.
Commonly used abrasives for cleaning steelwork are listed in
Table 4 with notes on their advantages and disadvantages.
The choice of blast-cleaning method is determined by the
following factors.
a. Shape and size of steelwork
Centrifugal methods are economic for plates and simple sections;
they can also be used for large prefabricated sections, e.g. bridge
sections, but only in specially designed plants. 'Misses'
discovered by inspection can be cleaned with open-blast techniques.
For large throughput of shaped items, e.g. pipes, both open and
vacuum blasting techniques can be used in continuous and automatic
plants.
b. Effect of the stage at which cleaning is carried out
For blast-cleaning on site, open or vacuum-blasting methods have
to be used as on large fabricated sections. It is usually
impractical to use centrifugal methods.
c. Throughput
Centrifugal plants are economic for a high throughput, but even
with a low throughput the method may still be preferable to
large-scale open cleaning.
d. Environmental conditions
Despite its relatively high cost, vacuum blasting may be
necessary to avoid contamination of the immediate area with
abrasive. It should be ensured that the blast-cleaning process does
not affect adjacent materials.
e. Types of surface deposit to be removed
Wet-blasting methods, with abrasives, are particularly suitable
for removing entrapped salts in rust and for abrading old, hard
painted surfaces, e.g. two-pack epoxies, before recoating.
On new work, blast cleaning can be carried out before or after
fabrication. When it is before fabrication a "blast" or "holding"
primer is applied to prevent corrosion during
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fabrication. Areas damaged during fabrication, e.g. by welding,
require re-preparing and priming as soon as possible.
3.4 Blast Cleaning Standard
ISO 8501-1 1988 is a visual standard which shows different
degrees of blast cleaning on steel of four levels of rusting [1].
The reference prints are in colour and the standard is based on the
widely used Swedish Standard SIS055900 [2]. It is used to specify
and control the standard of abrasive blast cleaning required.
3.5 Surface Roughness
Because blasting roughens the surface, some control of the
profile produced is important. If the distance between the highest
peak and the deepest trough is too much then the peaks may not be
protected adequately, Figure 10. ISO8503-1 1988 is a standard for
surface comparators [3]. Visual comparison between the comparator,
Figure 11, and blasted surface allow the latter to be graded
"Fine", "Medium" or "Coarse" profile. The peak to valley distance
for each grade is specified in the standard; shot and grit blasted
profiles are different and there is one comparator for grit and one
for shot blasting.
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ISO8501-1 [1] is intended for use with previously unpainted
steel. ISO8501-2 [1] is being prepared and relates to the treatment
of previously painted steelwork.
In both the above standards the term Surface Cleanliness is
used. This is slightly misleading because although it refers to how
effectively mill scale and rust have been removed, it sometimes is
assumed to include chemical cleanliness. This is not so. Tests for
assessing the surface cleanliness are given in ISO 8502 [4]. ISO
8502-1 gives details of site tests for soluble iron corrosion
products and ISO 8502-3 provides a method for the assessment of
dust on the surface and these are the only standards of real use at
present. ISO 8502-2 gives a method of determining in a laboratory
the presence of chlorides and further part giving guidance on the
estimation of condensation is in course of preparation.
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3.6 Flame Cut Edges
Flame cut edges have to be smooth and corners ground in order to
make a durable paint coating. A sharp corner creates a thin film
and a starting point for corrosion.
3.7 Other Methods of Surface Preparation
Manual cleaning, possibly using power assisted tools, is the
method most frequently used for practical or economic reasons,
although it is the least effective. In due course Part 3 of ISO
8504 [5] will cover hand and power tool cleaning but at present the
only relevant standard is ISO8501-1 [1] which contains two visual
preparation grades for scraping and wire-brushing [2].
4. SURFACE COATINGS As indicated in Lecture 4A.1, the common
methods of protecting steelwork are paints, galvanising, zinc or
aluminium metal spray or "duplex" systems where one of the last
three is over-coated with paint. The main characteristics of the
three groups are given in Lecture 4A.1. Appendix 1.
4.1 Paint Systems
Paints have three main components, a resinous components which
literally glues them together and is best referred to as the "film
former", pigment to give colour, weather resistance and in some
cases corrosion inhibition and, solvents to produce the correct
consistency for application, control of the drying rate, etc.
It is the film former which influences a paint's main
properties, e.g. hardness, flexibility, water resistance. For
convenience the paint types listed in Appendix 2 are divided into
three families, drying oil based paints, one pack chemical
resistant paints and 2-pack varieties. In each case the main film
formers and pigments are indicated, together with typical end uses
for each broad family.
Usually there are three components, 'primer', 'undercoat' and
'finish' in a paint system.
Primers. Their functions are to promote adhesion and protect
from corrosion. Since film thickness is a very important in
protection, two coats are frequently specified - sometimes three
when the last two are applied by brush.
Occasionally specifiers refer to the second and third coat of
primer as 'primer undercoat'. Frequently this misleads the
contractor because the branded products freely available never
feature this latter term in the product description. The specifier
is advised to label the system 'First coat', 'Second coat', etc.,
following with the appropriate generic description.
Undercoats. On steel, traditional undercoats provide the right
colour base for the finish; they adhere to the primer and little
else. The high performance undercoat is more accurately described
as an 'Intermediate coat'. It is a second barrier should the steel
be
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bared by damage or erosion. Often coats used for this function
can stand in their own right as finishes.
One important feature is to provide dry film thickness. A
traditional undercoat gives about 25m per coat; those used on steel
in other than a being environment must give a minimum of 50m, with
heavier duty types producing 100m plus.
Finishes. They supply the required colour, gloss or sheen level
and resist weathering, abrasion, and chemical attack, as
appropriate. More than one coat may be required depending on
product type, exposure, environment, colour, etc. Dry film
thicknesses per coat vary from 25m for a simple oil based product
to 100m or more for two pack epoxy coatings.
4.2 Metallic Coatings
a. Hot Dip Galvanising
The process deposits about 85m on the surface of the structural
steel. Thicker films can be obtained in some circumstances.
Galvanising must not be confused with Sheradising which achieves no
more than 30m zinc thickness or electroplating which deposits even
less thickness.
b. Strip Mill Galvanising
Strip mill galvanising utilises sophisticated plant to clean,
pickle and plate strip with non-ferrous metals under carefully
controlled conditions. The exterior surface of proprietary branded
products, e.g. building cladding is likely to be finished with a
20-25m protective layer of zinc or zinc/aluminium (the latter
varying from 5 to 55%). This layer may be overcoated on the same
production line with highly durable organic finishes of varying dry
film thicknesses.
4.3 Metal Spraying
The usual methods of applying zinc and aluminium are gas
combustion and electric arc. Very high standards of blasting and
surface cleanliness are essential. Metal spraying and sealing are
carried out by specialist contractors. Inspection must be
undertaken by qualified metal spraying inspectors.
All grades of steel can be metal sprayed and there is no size
limit. Work can be undertaken at works or on site. Aluminium is
rarely applied at thicknesses greater than 150m. In polluted or
immersed conditions zinc is applied at 200-250m.
Sprayed aluminium should be sealed. Zinc spray must be sealed if
it is to be painted or during maintenance. Sealers are applied
immediately after metal spraying and should not increase the
thickness of the metal coating. There are many sealers and it is
wise to ask the paint manufacturer for a specific recommendation
for each job.
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Both zinc and aluminium spray have good heat resistance, zinc up
to 100C and aluminium to 500C.
4.4 Metal Plus Paint Systems
Galvanising and paint. The selection of paints is more critical
than for steel. Some paints have been developed for direct
application to galvanised steel but results are variable.
Acceptable pretreatments include etch primers, proprietary
pretreatments which provide a 'key' for the paint, certain water
borne primers formulated specifically for the purpose. The paint
manufacturers advice should always be obtained.
Zinc or Aluminium Spray and Paint. Sealed spray can be
overcoated without difficulty using a wide range of coatings.
Unsealed zinc in particular is extremely difficult to paint; the
formation of zinc corrosion salts ("white rust") can cause severe
blistering.
The use of a mixed system. Non-ferrous metal plus paint systems,
can produce a layer which will outlasts either component if used
alone. However, if the environment is aggressive to zinc or
aluminium, their use is questionable as opposed to seeking to
protect them by overpainting, i.e. outside pH range of 5-12 for
zinc or 4-9 for aluminium.
4.5 Guidance on Corrosion Prevention
In order to assist the specifier of corrosion preventative
coatings in selecting the materials to use and the workmanship and
inspection requirements needed, two further standards are now in
course of preparation.
The standard dealing with paint products has been allocated the
number ISO12944 [6] and that dealing with metallic products is as
yet unnumbered [7].
These are scheduled to become available by about 1996/7.
5. MAINTENANCE OF STRUCTURES AND PLANT All protective coatings
require maintenance and there are a number of ways in which the
need becomes apparent.
In the extreme, the need for maintenance is shown when a
mechanical or structural failure occurs as a complete surprise
because the building or plant has never been the subject of regular
inspections.
The need may also be manifest when visible coating failure or
corrosion is noted by accident, e.g. when casually passing through
a building.
The preferred method of determining maintenance needs is by
means of planned inspections made at regular intervals. The
comparison of the results of inspections with reliable records of
the first and subsequent inspections give the basis for defining
maintenance needs.
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The aim of maintaining coatings is to preserve a structure or
building so that it performs its required functions throughout its
designed life safely, efficiently and economically. For this
purpose a maintenance schedule for the structure or building is
used to manage properly planned inspections and to keep reliable
records.
Consideration of maintenance should start when a new project is
being planned. The specifier should take into account the effects
not only of the design upon maintenance painting, but also the
influence of the initial coating system.
Drying Oil Based Paints The paints are readily overcoated with
similar products if the surface is cleaned and if very hard,
abraded. "Upgrading" to one or two-pack chemical resistant paints
without completely removing the oil-based paint is unlikely to
prove satisfactory.
One Pack Chemical Resistant Paints They can usually be safely
overcoated with similar materials once the surface is cleaned. An
exception is a moisture curing urethane system. Such systems may
well require light blasting to obtain adhesion. Two pack products
can be applied over moisture cured urethanes, but is unusual to use
them over the more common one pack chemical resistant products,
e.g. vinyl and chlorinated rubber resin based paint. Drying oil
based paints are rarely applied over this particular class of
paints and never in wet environments.
Two Pack Chemical Resistant Paints They are usually hard and are
difficult to maintain unless lightly blasted. They are maintained
by the application of similar products or, one pack chemical
resistant materials, but never with drying oil based paints.
Galvanised Steelwork It can only be safely over-coated when all
soluble corrosion products are removed. Once removal of these
products is achieved, virtually any paints from the families noted
above can be used. Etch primers are available which assist adhesion
to the zinc surface.
Metal Sprayed Steelwork If metal sprayed steelwork has been
exposed unsealed, it is virtually unpaintable. Sealed coatings give
few problems.
The choice of a maintenance paint process depends on the
existing coating and its condition, the standard of surface
preparation possible, the working environment, time available,
safety requirements, access and, economic considerations.
The decision of whether maintenance is to be by patch painting
or a complete recoat is influenced as much by access as the state
of the existing work. For example, if much scaffolding is required
it may be more economical to repaint overall.
If there is more than 5% rusting of the substrate painting
overall will certainly be economical. The "European scale of degree
of rusting for anti-corrosive paints" presents monochrome pictures
of nine degrees of rusting from Re1 (0,05%) to Re9 (95%).
In summary, successful maintenance starts at the beginning
overall new project with the specifier projecting the consequences
of his design and choice of initial paint system into future
maintenance - can it be done and, with what? It continues with a
strict, regular
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inspection routine, the results of which are accurately recorded
and become part of a detailed maintenance schedule. It ends with
maintenance painting specifications tailored to the job in hand and
with the provision of adequate inspection to ensure the
specification is followed.
6. CONCLUDING SUMMARY When choosing a protective system, the
maintenance cycle is an important
consideration. The 'design' of the steel members and the way in
which they are jointed affects the
maintenance cycle. Poorly prepared steel surfaces prevent the
protective treatment subsequently
applied from achieving its design life. Corrosion prevention
treatments can be either organic (paint), metallic (zinc,
etc.),
duplex (metallic and organic) or cathodic. Alternatively, in
order to limit or prevent corrosion, the steel itself can be of
a
weathering or stainless grade. Regular inspection of the
structure and proper routine maintenance prevents major
remedial work being necessary to the corrosion prevention
treatment.
7. REFERENCES ISO 8500 series Preparation of steel substrate
before application of paints and related products.
[1] ISO 8501 Visual assessment of surface cleanliness
Part 1 Rust grades and preparation grades of uncoated steel
substrates and of steel substrates after overall removal of
previous coatings.
Part 2* Preparation grades of previously coated steel substrates
after localized removal of previous coatings.
[2] SIS 05 5900: 1988, Preparation of steel substrate before
application of paints and related products - Visual assessment of
surface cleanliness.
[3] ISO 8502 Tests for the assessment of surface
cleanliness.
Part 1 Field tests for soluble iron corrosion products.
Part 2 Laboratory determination of chloride clean surfaces.
Part 3 Assessment of dust on steel surfaces prepared for
painting (pressure sensitive tape method).
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Part 4* Guidance on the estimation of the probability of
condensation prior to paint application.
[4] ISO 8503 Surface roughness characteristics of blast-cleaned
substrate.
Part 1 Specifications and definitions of ISO surface profile
comparators for the assessment of abrasive blast-cleaned
surface.
Part 2 Methods of the grading of surface profile of abrasive
blast-cleaned steel. Comparator procedures.
Part 3 Method for the calibration of ISO surface profile
comparators and for the determination of surface profile - focusing
microscope procedure.
Part 4 Method for the calibration of ISO surface profile
comparators and for the determination of surface profile - Styles
instrument procedures.
[5] ISO 8504 Surface preparation methods.
Part 1 General principles.
Part 2 Abrasion blast-cleaning.
Part 3 Hand and power tool cleaning.
[6] ISO 12944* Protective paint systems for steel structures
Part 1 General Introduction.
Part 2 Classification of Environments.
Part 3 Types of Surface and Surface Preparation.
Part 4 Classification and Definitions of Paint Systems and
Related Products.
Part 5 Performance Testing.
Part 6 Workmanship.
Part 7 Design.
Part 8 Guidance for Developing Specification for New Work and
Maintenance.
[7] Metal coatings for the corrosion protection of iron and
steel in structures.
* In course of preparation
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8. ADDITIONAL READING 1. Uhlig, H. H., "Corrosion and Corrosion
Control", 3rd ed, 1985, John Wiley &
Sons. 2. Durability of Steel Structures: Protection of Steel
Structures and Buildings from
Atmospheric Corrosion, ECSC Report 620.197, 1983. 3.
"Controlling Corrosion", series of booklets published by the
Department of
Industry - Committee on Corrosion. 4. Steelwork Corrosion
Protection Guide - Interior Environments (3rd Ed), 1989
(published jointly by BCSA, BS, Paint Research Association (PRA)
and Zinc Development Association (ZDA)).
5. Steelwork Corrosion Protection Guide - Perimeter Walls (2nd
Ed), 1989 (Published jointly by BCSA and BS).
6. Steelwork Corrosion Protection Guide - Exterior Environments
(2nd Ed), 1989 (published jointly by BCSA, BS, PMA (Paint Makers'
Association) and ZDA).
7. BS 5493 Code of practice for protective coating of iron and
steel structured against corrosion.
8. DIN 55928: Part 5 Corrosion protection of steel structures by
organic and metallic coatings Part 5 Coating materials and
protective systems.
9. Norsk Standard NS 5415 Anti-corrosive paint systems for steel
structures. 10. ECCS No. 48 Protection against corrosion inside
buildings 11. ECCS No. 50 Protection of steel structures against
corrosion by coatings. 12. BS 729 Specification for hot dip
galvanised coatings on iron and steel articles,
1971(1986). 13. BS 2569 Specification for sprayed metal coatings
Part 1 and 2. 14. BS 2989: 1992 Specification for continuously
hot-dip zinc coated and iron-zinc
alloy coated steel: Haz product - tolerances on dimensions and
shape. 15. BS 3083: 1988 Specification for hot-dip zinc coated and
hot-dip aluminium/zinc
coated corrugated steel sheets for general purposes.
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Table 1 Classification of Environments
INTERIOR ENVIRONMENTS
Environment category Environment Corrosion risk Examples
A Normal
(RH below 60%)
Negligible Offices
Shops
Industrial Production/Assembly
Warehousing
Hospital Wards
Schools
Hotels
B Occasional Condensation
Low Unheated Buildings
Vehicle Depots
Sports Halls
C Frequent Condensation
Significant Food Processing Plants/Kitchens
Laundries
Breweries
Dairies
Not covered - seek expert assistant
Chemical Processing Plant
Dye Works
Swimming Pools
Paper Manufacture
Boat Yards over Seawater
Foundries/Smelter
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Table 1 (continued): Classification of Environments
EXTERIOR ENVIRONMENTS
D Normal inland Low Industrial plant and supporting steelwork
Bus/train terminals
E Polluted inland Significant Tank farms, cranes, docks, power
stations
F Normal coastal High Docks, cranes, container installations,
power stations refineries
G Polluted coastal Very high Tank farms, industrial plants
supporting steelwork
Not covered - seek expert assistance
Aggressive industrial environments such as steelwork adjacent to
acid plants, salt storage depots, electroplating shops, chemical
works etc. Buried or immersed steelwork Seawater splash zones.
Table 2 Typical Protective Systems
Introduction
Whilst there are numerous protective systems available, only
twelve have been selected for this lecture.
These are eight basic paint systems (P1 to P8) on which there
can be variations of paint types (see Appendix 2); one galvanizing
system (G1); and two metal spray systems (AS1 and 2).
Whilst the systems remain unaltered between environments, the
notes vary to cover the changes that are necessary.
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762
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763
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764
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765
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766
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767
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768
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769
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770
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771
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772
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773
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774
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775
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776
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777
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778
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779
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780
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781
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782
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783
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784
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785
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786
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787
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788
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789
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Table 3 Methods of blast-cleaning (ISO 8504-1 and 2)
Methods Advantages Disadvantages
Dry methods using compressed air or centrifugal force
Automatic plants based on centrifugal throwing of the
abrasive
High production rates, lowest costs, no moisture problems. Can
be coupled to automatic application of primer, dust problems
contained.
High capital cost, high maintenance cost, lack of flexibility,
ie. not suitable for recessed areas etc.
Open blasting based on propelling the abrasive with compressed
air.
Simple to operate, very flexible and mobile in use both indoor
cabinets or special rooms or on site. Low capital and maintenance
costs.
High cost of compressed air, low efficiency, liable to moisture
entrainment from the compressed air, manually operated and a
variable profile can result, operator requires protective clothing,
serious dust problems.
Vacuum blasting based on propelling the abrasive with compressed
air and immediately recycling by suction from the blast-cleaned
surface.
No dust problems, no special protective clothing for operators,
fairly low capital costs.
Can be very slow and therefore expensive, particularly on
awkward profiles and girder sections. Where flat-plate or gun-head
automation is possible it may be considered, but liable to moisture
entrainment from the compressed air.
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Table 3 Methods of blast-cleaning - Cont'd.
Methods Advantages Disadvantages
Wet methods (hydroblasting)
Open blasting based on projecting water at very high
pressure.
Simple to operate, very flexible and mobile in use, suitable for
removing soluble containments. At very high pressure can remove
mill-scale, no dry dust hazards.
Slow if firmly held containments are to be removed, dangerous at
very high pressure if proper precautions are not taken, limitation
of drying surface before painting unless approved water-based or
moisture tolerant primers are used, requires availability of water
and drainage, operators require protective clothing.
Open blasting based on projecting water at high pressure and
entraining abrasive into the water stream.
Simple to operate, very flexible and mobile in use, suitable for
removing all firmly held contaminants as well as soluble
contaminants.
Dangerous at very high pressure if proper precautions are not
taken, limitation of drying surface before painting unless approved
water-based or moisture tolerant primers are based, required
availability of water and drainage, operators require protective
clothing.
Open blasting based on injecting low pressure water into a
compressed air stream which is carrying an abrasive.
As above. High cost of compressed air, limitation of drying
surface before painting unless approved water-based or moisture
tolerant primers are used, dust hazard reduced, operators require
protective clothing.
Open blasting using steam-cleaning.
As above. Similar to the above according to whether abrasive is
or is not entrained.
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Table 4 Classification of abrasives used for cleaning steel
Abrasive Hardness Normal usage Advantages Disadvantages
Chilled iron-grit
ISO 11124-2
60 to 80 RC Captive blasting and open blasting with recovery
systems
Relatively cheap, cleans very quickly, will chip under repeated
impact with work surface, presenting fresh cutting edges
Breaks down fairly quickly. In centrifugal wheel plants, special
protection is required to reduce wear on moving parts
Chilled iron-shot
60 to 80 RC Captive blasting only
Relatively cheap, very hard, should break down to grit in
use
As chilled iron-grit. Because of ricochet effect is not suitable
for open blasting or in open cabinets
High duty chilled iron-grit or iron-shot
55 to 64 RC Captive blasting and open blasting with recovery
Breaks down less quickly than chilled iron
More expensive than chilled iron, rendered spherical in use,
poorer and slower rate of cleaning than chilled iron
Heat-treated chilled iron-grit or iron-shot
30 to 40 RC As high-duty As high-duty As high-duty
Steel grit 60 to 67 RC
47 to 53 RC
Captive blasting mainly
Does not bread down so quickly as chilled iron, causes less wear
in centrifugal wheel plant
More expensive than chilled iron, rendered spherical in use and
is less efficient, supplied in various hardnesses but at best is
not
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so hard as chilled iron-grit and therefore cleans more
slowly
Steel shot 41 to 49 RC Captive blasting only
As for steel grit As for steel grit, produces a more rounded
surface profile than grit, ricochet effect makes it unsuitable for
open blasting
Cut steel wire
ISO 11124-5
41 to 52 RC Captive blasting only
As for steel shot and grit, wears down as fairly even sizes
High cost, rendered spherical in use and slower cleaning than
chilled iron
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Table 4 Classification of abrasives used for cleaning steel -
Cont'd.
Abrasive Hardness Normal usage Advantages Disadvantages
Aluminium oxide (corundum)
ISO 11126-7
Not common in the United Kingdom
Extremely hard Expensive, hardness of dust is a danger to
machinery unless used in sealed captive plant
Copper slag
ISO 11126-3
Open blasting only
Cheap, no silicosis hazards
Initial particles rather coarse, breaks down to dust very
quickly, angular particles tend to embed in workplace
Iron slag ISO 11126-6
Open blasting only
As for copper slag As for copper slag
Sand
(Olivine) ISO 11126-8
Open blasting Cheap In United Kingdom, Factory Inspector's
approval is required, danger of silicosis
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See Table 4
International Standards for Metallic and Non-Metallic
Blast-Cleaning Abrasives
A.1 Requirements and test methods for metallic blast-cleaning
abrasives are contained in ISO 11124 and ISO 11125.
ISO 11124 consists, at present, of the following parts, under
the general title:
Preparation of steel substrates before application of paints and
related products -Specifications for metallic blast-cleaning
abrasives:
- Part 1: Introduction
- Part 2: Chilled-iron grit
- Part 3: High-carbon cast-steel shot and grit
- Part 4: Low-carbon cast-steel shot
- Part 5: Cut steel wire
ISO 11125 consists, at present, of the following parts, under
the general title:
Preparation of steel substrates before application of paints and
related products -Test methods for metallic blast-cleaning
abrasives:
- Part 1: Sampling
- Part 2: Determination of particle size distribution
- Part 3: Determination of hardness
- Part 4: Determination of apparent density
- Part 5: Determination of percentage defective particles and of
microstructure
- Part 6: Determination of foreign matter
- Part 7: Determination of moisture
A.2 Requirements and test methods for metallic blast-cleaning
abrasives are contained in ISO 11126 and ISO 11127.
ISO 11126 consists, at present, of the following parts, under
the general title:
Preparation of steel substrates before application of paints and
related products -Specifications for metallic blast-cleaning
abrasives:
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- Part 1: Introduction
- Part 2: Silica sand
- Part 3: Copper refinery slag
- Part 4: Coal furnace slag
- Part 5: Nickel refinery slag
- Part 6: Iron furnace slag
- Part 7: Fused aluminium oxide
- Part 8: Olivine sand
ISO 11127 consists, at present, of the following parts, under
the general title:
Preparation of steel substrates before application of paints and
related products -Test methods for metallic blast-cleaning
abrasives:
- Part 1: Sampling
- Part 2: Determination of particle size distribution
- Part 3: Determination of apparent density
- Part 4: Assessment of hardness by a glass slide test
- Part 5: Determination of moisture content
- Part 6: Determination of water-soluble contaminants by
conductivity measurement
- Part 7: Determination of water-soluble chlorides
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APPENDIX 1 FACTORS AFFECTING THE CHOICE OF COATING SYSTEMS
QUESTIONS RELATED TO DESIGN, USE AND SITE REQUIREMENTS
Function
a. What is the main function of the structure?
b. What are the secondary functions of the structure?
Life
a. For how long is it required to fulfil this function?
b. What is the life to first maintenance? (It may not be
possible to decide this until further questions have been
answered).
Environment
a. What is the general (atmospheric) environment at the site of
the structure?
b. What localised effects exist or are to be expected, e.g.
fumes from stacks?
c. What other factors may affect the structure, e.g. surface
temperature and abrasion?
Appearance
a. What is the structure required to look like (colour and
finish)?
b. Is the final coat to be applied on site?
Special Properties
a. What special properties are required of the coating, e.g.
coefficient of friction?
Maintenance
a. What access is there going to be for effective
maintenance?
b. What is the possibility of effective maintenance?
Health and Safety
a. Are any problems to be taken into account during initial
treatment?
b. Are any problems to be taken into account during maintenance
treatment?
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Tolerance
Does the coating need to be tolerant of:
a. indifferent surface preparation
b. indifferent application techniques
c. departures from specification?
QUESTIONS RELATING TO COATING SYSTEMS
Coating systems
a. What coating systems are suitable?
b. Are these systems readily available?
c. Are the system elements mutually compatible?
d. If paints, can the coats be applied by:
brush roller airless spray other?
e. Can the system, or parts, be applied on site?
Coating facilities
a. Are the coating facilities readily available:
i. for factory application ii. for site application?
b. Do they cover all sizes and shapes of fabrication?
c. Do they permit speedy application?
d. Do the facilities permit work to adequate standards?
Compatibility with engineering and metallurgical features
a. Is the design and jointing of the structure compatible with
the preferred coating technique?
b. Does surface preparation (blasting, pickling) or application
of coating affect the mechanical properties of the steel in any way
that matters?
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c. Is the system compatible with cathodic protection?
Delays
What delays should be allowed between:
a. fabrication and first protective coating;
b. application of primer and undercoat;
c. application of undercoat and finishing coat;
d. final shop coat and erection;
e. erection and final treatment?
Transport, storage and handling
How well does the coating withstand:
a. excessive or careless handling;
b. abrasion and impact;
c. early stacking;
d. exposure to seawater during transit?
Experience
a. What is known of the consistent performance of the
coating?
Export
a. What special precautions should be taken when the steelwork
is exported?
Maintenance
a. Is the deterioration of the coating rapid and serious if
maintenance is delayed?
b. What is the likely maintenance system? (Including surface
preparation).
Costs
a. What are the approximate costs of:
i. the basic system; ii. any additional items;
iii. transport; iv. access?
b. What are the approximate costs of maintenance?
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800
APPENDIX 2 PAINT TYPES BLAST PRIMERS
These primers have been the cause of some confusion; they are
therefore dealt separately here.
They are used pre- or post-fabrication, normally in-shop and
under controlled conditions.
a. Pre-fabrication primers are designed for use with automated
blasting and painting plant. However, increasingly fabricators
apply them by hand-held airless or high pressure conventional spray
very successfully.
The most important types are:
Type I
One or two-pack polyvinyl butryal/phenolic: zinc
tetroxychromate: DFT 15-20m.
Type II
Two-pack epoxy: zinc phosphate or zinc tetroxychromate: DFT
25m.
Type III
Two-pack epoxy: zinc metal DFT 10-20m. Note: Metallic zinc
coatings (including zinc spray and galvanising) can give rise to
health hazards even in open shop conditions when welded or flame
cut.
b. Post-fabrication can be Types I to III; some have higher
volume solids, give extended durability but are slower drying. The
specifier should state the type and indicate whether use pre- or
post-fabrication is required. The manufacturer's application rates
must be followed carefully, particularly when overcoating with
chemically resistant paints, e.g. over generous application of a
Type I blast primer can lead to intercoat failure (splitting).
One pack zinc metal and two-pack zinc ethyl silicate coatings
are available for specific uses.
Very often the anti-corrosive primer which is the first coat of
a chosen system is specified as the post-fabrication primer.
DRYING OIL BASED PAINTS
These paints dry by reaction with atmosphere oxygen. Widely
used, they are based on vegetable or fish oils suitably treated,
e.g. by heat, and reinforced with synthetic or naturally occurring
resins. They do not withstand direct chemical attack nor immersion
conditions.
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PRIMERS
There are two basic types, relatively slow drying products whose
use is limited to site application and faster-drying versions which
can be used in-shop and on site. In general the latter type have
lower volume solids. All are for use beneath oil-based systems;
some can be used beneath one pack chemical resistant systems.
Typical binders are:
Drying oil Drying oil modified alkyds Epoxy ester Urethane oil
Oil modified phenolic resin.
Typical anti-corrosive pigments include:
Zinc phosphate or zinc chromate with red lead and calcium
plumbate still used in primers designed for site use. All but zinc
phosphate impose limitations in use.
Dry film thicknesses vary between 25-75m depending upon volume
solids, application method and service use.
Undercoats (Intermediate coats)
With the exception of unreinforced drying oils, all the binders
noted under 'Primers' may be used.
Pigmentation is typically titanium dioxide for whites and tints,
organic and inorganic chemically resistant pigments for colours.
Micaceous iron oxide pigments are used to give increased film
thickness, improved edge cover and good weather resistance.
Dry films are between 25-50m thick depending upon volume solids,
application method and service use.
These products are for use beneath oil based gloss and micaceous
iron oxide finishes.
Finishes
High gloss finishes in BS 4800 and RAL colours and low-sheen
subdued colours in micaceous iron oxide paints have excellent
weather resistance but do not resist direct chemical attack or
complete immersion in water.
Typical binders are oil or urethane modified alkyds, epoxy
esters and oil modified phenolics.
Pigments are various grades of rutile titanium dioxide,
light-fast coloured pigments and micaceous iron oxide or
aluminium.
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Dry film thicknesses vary between 25-50m. In this respect, the
same criteria apply as for undercoats.
ONE-PACK CHEMICAL RESISTANT PAINTS
All but one of the products in this range dry by solvent
evaporation. The exception, moisture-curing polyurethanes, will be
dealt with last. A wide range of film formers is available,
typically plasticised chlorinated rubber, solution vinyl copolymers
and acrylic resins, acrylated polymers. The differences between
products based on these resins and others are subtle with
individual manufacturers having built up experience over many years
with one or two resin systems.
The main characteristics which they have in common are excellent
water resistance (including immersion), good resistance to
inorganic acids and adequate alkali resistance. In this latter
respect, two-pack chemical resistant systems withstand severe
attack better. Theoretically, no paint based on the resins quoted
in the previous paragraph are proof against attack by organic
acids, animal fats, etc., but in practice there are many examples
where they have proved more than adequate. Paint manufacturers will
advise on specific cases.
Because these paints dry by solvent evaporation they form films
at low temperatures and will dry satisfactorily in polluted
atmospheres. Intercoat adhesion both initially and for maintenance
is good because the resins remain soluble in the solvents used in
the paints. Conversely, solvent resistance is relatively poor.
Maximum heat resistance is circa 65C.
In this group must be included waterborne resin systems, e.g.
vinyl acrylic copolymers. Although relatively new (they were
introduced within the last decade) they show great promise,
particularly as metal primers. Since they coalesce rather than
forming a film by simple solvent loss, their mechanical properties
are better than might be expected from a one-pack paint.
Also in the group are one-pack moisture-curing polyurethane
resin-based paints. These must not be confused with oil or alkyd
containing products which are 'reinforced' by the addition of a
urethane component. Moisture-curing varieties dry like two-pack
paints, undergoing a complex chemical reaction in which moisture
acts as the 'curing' agent. Once cured, these paints possess most
of the attributes associated with two-pack polyurethane paints. A
significant advantage is their ability to form films at low
temperatures. Obviously this feature must be exploited with
caution; water or ice formed at the paint/surface interface must
degrade its performance.
Primers are available for shop and site application based on all
these resin systems. Since their corrosion inhibiting properties
are inferior to primers irrespective of which inhibitive pigment is
chosen, some manufacturers produce an oil-modified primer
specifically formulated for use in a one-pack chemical resistant
paint process (excluding moisture curing polyurethanes). Usually
these are not recommended for severe exposure or immersed
conditions. They are particularly useful for site application.
Zinc phosphate pigments are widely used as the inhibitive
pigment.
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803
Dry film thicknesses vary between 25-65m depending upon volume
solids, application method and service use.
Undercoats (Intermediate coats)
Any of the resins noted above may be used. These coats are both
weather and chemically resistant; indeed many proprietary products
are designated 'Thick Coatings' and suitable both as intermediate
and finishing coats.
Rutile titanium dioxide pigments are widely used in whites and
tints. Light fast and chemically resistant pigments are used for
colours, with micaceous iron oxide used both for its weather
resistance and ability to improve the mechanical properties of the
paint film.
Dry film thicknesses between 50-100m per coat depend upon volume
solids, dimensions and complexity of the steelwork, application
method, surface and ambient temperatures. Additionally, solvent
release is relatively slow and inhibits the thickness which can be
safely applied to avoid solvent entrapment producing bubbles or
pinholes.
Finishes
Finishes are based on the same resin types as used in
undercoats/intermediate coats. The same pigment types are also
used. Where finishes are sold specifically for this purpose they
have better resistance to severe exposure conditions and chemical
attack than dual purpose products. High gloss finishes are
available. Many BS 4800 colours can be produced although the need
for chemical resistance rules out some.
Dry film thicknesses vary between 25-100m per coat. Their
achievement is governed by the considerations noted under
'Undercoats'.
TWO-PACK CHEMICAL RESISTANT PAINTS
These two-part coatings form films by a complex chemical
reaction. The reaction is temperature dependent. Most products
cannot be used at surface and ambient temperatures below 10C,
although a few are capable of 'curing' at 5C. It is important to
differentiate between the film drying and attaining full chemical
resistance - the process referred to as 'curing'. Once this is
complete, the coatings are tough, abrasion resistant and resistant
to a very wide range of acids, alkalies, oils and solvents even
when fully immersed. The time interval between coats can be
critical, particularly with two-pack urethanes. The principal
difficulty being to ensure good intercoat adhesion.
Primers
A wide variety is available for both shop and site use. Most are
suitable as post-fabrication primers only. They are used beneath
both one and two-pack chemical resistant paints.
The most widely used anti-corrosive pigment is zinc
phosphate.
Typical binders are:
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804
two-pack epoxy two-pack urethane.
Dry film thicknesses between 25-75m are achieved, depending upon
volume solids, application method and service use.
Undercoats (Intermediate Coats)
These products are used beneath one and 2-pack high performance
finishes.
Typical binders are:
2-pack epoxy 2-pack urethane or urethane acrylic
Isocyanate-cured epoxy Epoxy: Tar Epoxy: Pitch Urethane tar or
pitch.
Pigmentation is typically titanium dioxide in whites and tints,
with light-fast chemically resistant pigments in colours. Micaceous
iron oxide is used to improve film build, weathering and mechanical
properties. It also facilitates overcoating.
Dry film thicknesses are influenced by the same criteria as the
primers. They vary between 75-200m.
Lecture 4A.2: Factors GoverningProtection of
SteelworkOBJECTIVE/SCOPEPREREQUISITESRELATED LECTURESSUMMARY1. LIFE
EXPECTANCY1.1 Likely Time to First Maintenance1.2 Life Between
Maintenances1.3 Assessment of Life Requirement
2. DESIGN2.1 Design for Protective Systems2.2 Where to Apply
Protection2.3 Special Areas
3. SURFACE PREPARATION3.1 Degreasing3.2 Removal of Scale and
Rust3.3 Blast Cleaning3.4 Blast Cleaning Standard3.5 Surface
Roughness3.6 Flame Cut Edges3.7 Other Methods of Surface
Preparation
4. SURFACE COATINGS4.1 Paint Systems4.2 Metallic Coatings4.3
Metal Spraying4.4 Metal Plus Paint Systems4.5 Guidance on Corrosion
Prevention
5. MAINTENANCE OF STRUCTURES AND PLANT6. CONCLUDING SUMMARY7.
REFERENCES8. ADDITIONAL READING