Cswip 111 of painting

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(1) *CORROSION
· Corrosion can be generally defined as;
“Degradation of a metal by chemical or Electro-chemical means”.
· It is obvious that two mechanisms are involved,
Firstly an Electrical Circuit and secondly a Chemical Reaction.
Electrical Circuit ;
In corrosion circuit the current is always D.C. (Direct Current).
For corrosion circuit to exist three things are needed: Anode, Cathode and Electrolyte.
1-An Anode
Is a positively charged area? (It becomes positively charged because the atoms release two
electrons), the iron atom has 26 of each, protons and electrons, in it’s passive state
When the two electrons are released the atom still has it’s 26 protons, but now only 24
electrons.
(In this state the atom is now an ion, positively charged by two units and written as Fe++.)
(An ion is a charged particle, and can be positive or negative, a single atom or a group of
atoms, known as a molecule.)
This losing of electrons can be shown as: - Fe  Fe++ + 2e.
(The Fe++ is called a positive iron ion).
2-A Cathode is a negatively charged area (where there are more electrons than needed in its
passive state). At the cathode the electrons enter into the electrolyte to pass back to the anode.
3-An Electrolyte is a substance, which will conduct a current and be broken down by it,
(dissociate into ions). Water, Acids, alkalis and salts in solution are very efficient electrolytes.
As the electrons pass into the electrolyte it is dissociated into positive and negative ions, as
shown by the formula: -2H2O2H+ + 2OĦ.
The couple electrons back with the Hydrogen ions to form two full Hydrogen atoms, which
join together to form Hydrogen gas. The hydroxyl ions return to the anode through the
electrolyte carrying the electrons.
The Chemical Reaction;
Only the chemical reaction, (the formation of corrosion products), occurs at the Anode.
The positive iron ions, Fe++, receive the returning hydroxyl ions and ionic ally bond together to
form iron hydroxide, which is hydrous iron oxide, rust, and is shown by the formula:
Fe++ + 2OĦ Fe (OH) 2.
Corrosion only occurs at the Anode, never at the Cathode.
The corrosion triangle shows the three elements needed for corrosion to occur, Anode, Cathode
and Electrolyte.
If any one of these three is removed from the triangle, corrosion cannot occur.
Painting Inspection Grade 3/2. Rev 1 April 2004
Corrosion 1
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The one most commonly eliminated is the electrolyte. Placing a barrier between the electrolyte
and the anodic and cathodic areas, in the form of a coating or paint system does this.
If electrolyte is not in direct contact with anode and cathode, there can be no circuit, and so no
corrosion.
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Figure1.2The.corrosion.triangl
Certain factors can increase the reaction rate, listed below are some of these.
1 Temperature .
Steel, is thermodynamically unstable metal.
The hotter steel is faster in corrosion than the other cooler one.
2 Hygroscopic Salts.
A hygroscopic salt is one, which will attract water and dissolve in it.
When salts are present on a substrate and a coating is applied over them, water will be
drawn through the film and the resulting solution builds up a pressure under the film.
Eventually the film is forced up to form blisters.
These blisters are called osmotic or hygroscopic blisters, and are defined as ‘pinhead
sized water filled blisters’.
Sulphates and Chlorides are the two most common salts, chlorides predominant in
marine environments, and sulphates in industrial areas and sometimes agricultural.
3 Aerobic conditions ,
(Presence of oxygen). By introducing oxygen into the cathodic reaction the number of
Hydroxyl ions doubles.
This means that double the number of iron ions will be passivated and therefore double
the corrosion rate. Shown by: 2H2O + O2 + 4e  4OH-
4 Presence of some types of bacteria
On the metal surface, for example Sulphur Reducing Bacteria, better known as SRBs,
or MEMs, Metal Eating Microbes.
5 Acids and alkalis
6 Bi-metallic contact .
Otherwise known as Bi-Metallic Corrosion.
Corrosion 2
E
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Osmotic or hygroscopic blisters osmotic or hygroscopic blisters Metals can be listed in order
of nobility.
A noble metal is one, which will not corrode.
In descending order, the further down the list the metal is, the more reactive it is, and so, the
more anodic it is, the metal loses its electrons to become reactive ions.
The degree of activity can be expressed as potential, in volts.
The list can be called
+A Galvanic List, Electro Motive forces series or the Electro-Chemical series.
MATERIAL KNOWN POTENTIAL AV. VALUES
Graphite + 0.25 v
Silver - 0.1 v
Nickel 200 - 0.15 v
Copper - 0.35 v
Mill Scale - 0.4 v
Mild Steel - 0.7 v
Aluminium Alloys - 0.9 v
Zinc - 1.0 v
Magnesium - 1.6 v
*Millscale;
Is immediately above steel on the galvanic list.
This means that millscale is Cathodic to steel, and if left on the surface of steel will
accelerate the corrosion of the steel substrate.
Millscale is formed during the rolling operation of steel sections e.g. RSC, RSA, RSJ.
The oxides of iron form very quickly at temperatures in excess of 580c.
The first oxide formed is FeO, iron oxide, the next is Fe3O4 and last of all Fe2O3.
Common names in order are Wustite, Magnetite and Haematite.
These oxides are compressed during the rolling operation to produce blue millscale.
The thickness of millscale varies from 25 to 100 um. When it has been removed by any
surface preparation method, it can never re-cur.
Corrosion 3

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(2)*SURFACE PREPARATION METHODS
& STANDARDS
If paint applied over the corrosion reactions, and other contaminants,
1-The poor adhesion of the coating and thus the coatings life would be far from satisfactory.
2-A good surface preparation grade (degree of cleanliness) along with a suitable surface profile
can give 10 years life from a typical four-coat paint system. The same system applied over a
substrate with little or no profile and contaminant remaining might give four to six years, or even
less.
Surface Preparation
Involves removing these contaminants, and in some instances increasing the area available for
adhesion by roughening up the substrate.
Therefore two factors need to be considered when inspecting a surface preparation.
1. Degree of cleanliness
2. Surface Profile (degree of roughness)
Surfaces can be prepared for paint application in several different ways; each one varies in cost,
efficiency, ease and suitability.
a) Dry Abrasive Blast Cleaning
b) Water Blasting
c) Hand and Power Tool Cleaning
d) Flame Cleaning
e) Pickling
f) Vapour Degreasing
g) Weathering
*Dry abrasive blast cleaning;
A-Dry abrasive blast cleaning involves compressing air and forcing it along a hose and out of a
small aperture called a nozzle.
B-A pressure of 100 psi results in the air speed exiting the nozzle at approximately 450 mph.
C-If abrasive particles are mixed in with the air and travel at the same speed; they will carry a lot
of work energy. This energy is used in chipping away millscale and other detritus from the
substrate. And in shattering into small pieces and with others all the energy is used in
impinging into the steel surface, roughening the surface and increasing the surface area to
increase adhesion properties.
Because all standards refer to the amount of contamination remaining on the
surface,
(The longer the time spent on this operation,
the higher the degree of cleanliness.)
Testing of Paints
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Abrasives;
Abrasives come in many forms and can be classified in several different ways, as shown below.
None metallic (Mineral)
Expendable Metallic (Recyclable) Agricultural by-product
Copper Slag
Nickel Slag
Boiler Slag
Glass Bead
Aquamarine
Garnet
Sand
ACI (Angular Chilled Iron)
Steel Grit
Steel Shot
Grit and Shot Mix
Garnet
Walnut Shell
Coconut Shell
Eggshell
Corn Cob Husk
Peach Husk
In the context of this course we are considering the following: -
a) Sand;
It is not permitted to use sand. SI 1657 states that any mineral used as an abrasive must release
less than 1% free silica on impact. (Silica causes preumonicosis or silicosis). COSHH REGS
does not allow the use of sand containing silica for dry blasting. Sand itself is perfectly safe, but
Shattering on impact releases silica, which can be inhaled.
b) Copper Slag;
The amount of copper in the structure is extremely minute.
1-Minerals melted with the copper,
2- liquefies and forms a protective cover over the molten
Copper to prevent reaction with the atmosphere.
3-When the copper metal is run off the slag is
Rapidly cooled in cold running water
The material is supplied in grit form (random, sharp
Edges, amorphous and is very brittle), shatters into
Smaller pieces on impact, and should be used only once and then discarded and so classed as
expendable.
c) Garnet;
A natural mineral classed as being “of a diamond type
Hardness” can be either expendable or recyclable.
Cleansing units are available to extract contamination
So that the material can be reused, usually up to three
times. Doesn’t shatter on impact
but does suffer some “wear”. Supplied in
Grit form.
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d) Metallic Grit;
Steel and Iron are both metallic. Steel grit being the
Softer of the two to round off on impact and
Loses its sharp edges. Angular Chilled Iron chips
Off small slivers on impact to produce sharp cutting
Surfaces on its next cycle. Metallic abrasives are
Recyclable because the particles reduce in size slowly
. Hence it can be re-used many times and still perform
a useful function in a '‘working mix’. A working
mix is an accepted ratio of large and small particles, where the large particles cut the profile
and the smaller particles clean out the troughs.
e) Metallic Shot;
Shot is spherical and doesn’t shatter (otherwise it
would form grit). When supplied the particles are
Virtually uniform in size and shape, (not a working mix)
but like the grit they wear down slowly in size.
The particles are worn down eventually to finings,
and are drawn out of the system during cleansing.
f) Metallic Shot and Grit Mixed;
A mix of shot and grit results in a more uniform profile.
1- The grit cuts the profile
2- The shot, being unable to enter the troughs
Produced, controls the peak height and so
Greatly reduces the number of ‘rogue peaks.’
A rogue peak; Is one, which is well proud of the acceptable profile range, and if painted
over due to contraction of the paint, will leave bare metal in contact with the atmosphere,
thus allowing corrosion to occur. When rogue peaks are in concentrated area the effect is of
a rash, hence rust rashing or rust spotting.
*A typical mix ratio of Shot to Grit as used in a pipe coating mill would be
70 – 80 % shot to 20 –30 % grit.
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Other properties of an abrasive have an effect on the resulting substrate also, these being.
A- Size of the particles
B- Hardness of the material
C- Density of the material
D- Shape of the particle
For example steel has a density of approximately 7.6 gm/cc and copper slag, approximately 4.2
gm/cc.
If one particle of each material, of identical size, hit a steel substrate, then it would be logical to
say that the steel would impinge further into the substrate, resulting in a deeper trough.
A spherical particle would not impinge as deeply because the large smooth surface area would
use its energy up in preening or work hardening the surface rather than cutting into it.
So a shot blasted surface is different in appearance and texture to that of grit blasted surface.
*Sizing of abrasives;
G Prefix = Grit amorphous, points and cutting edges, irregular profile.
S Prefix = Shot spherical, smoother profile.
The G or S notation is followed by a number, which denotes the particle size.
G24 or S330. BS 2451 the 24 means nominally 24 thousandths of an inch.
SAE(society automotive engineer) USING THE JJ 444444 SSIIEEVVEE SSYYSSTTEEMM..
System it represents 1/" 24
= approximately 40 thou.
New BS ref. 7079 pt EEPPAARRTTIICCLLEE SSIIZZEE DDIISSTTRRIIBBUUTTIIOONN
Uses a different method again, in metric units. G140 would mean a nominal particle size of
1.4mm
* Adhesion and Profile;
A commonly used definition of Adhesion is: - The force required to separate two
surfaces in touch.
A newly rolled plate, perfectly smooth, 1m x 1m has an apparent surface area of 1m2 and an
actual area of 1m2. Abrasive blasting roughens the surface and increases the actual area, (the
apparent area is still 1m2), thus increasing the adhesion. Two theories of adhesion are: -
1 Molecular Interference .
Because the surface is rough and uneven the paint wets, and locks into the profile, Analogy
Velcro. Physical.
2 Molecular Attraction .
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Negatively charged particles attracted to positive areas, and vice versa. Analogy Magnet
(sometimes called Ionic Bonding). Chemical.
* Profile;
Surface profile, Anchor pattern, key, Peak to trough height and Amplitude are all
expression meaning the cross section of a blasted area, as measured from the top of the peaks to
the bottom of the troughs. The surface profile requirements are given on the specification for the
job, e.g. for B. Gas 30 – 75 microns.
Shot blasted profile;
Figure 2.1 Terms relating to preparing surfaces
Grit blasted profile;
Figure 2.2 Grit blasted profile
*Hackle – A small surface lamination, which stands upright like a needle after blasting.
Approximately ≤ 13 mm. Easily removed.
*Lamination (slivers) – Appears to be a longitudinal ‘crack’, one lip curling back, any
laminations found must be referred to engineer for ultrasonic check.
Profile measurement;
If a profile requirement is specified, it is the inspector’s duty to ensure that the specification
requirements are met. This can be done in two ways.
Testing of Paints
Peak to trough
Rogue Peak Hackle
Lamination or
Sliver

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a) By measuring – using gauges with and without replica tape.
b) By assessing – using surface comparators.
The dial gauges are still very often used. The dial gauges fall into two categories, Surface
Profile Needle Gauge and Dial Micrometers with Replica Tape.
i Surface Profile Needle Gauge .
The gauge is applied to the blasted substrate and the needle can be felt to locate a trough. Then
by applying a slight pressure to allow the flat ‘foot’ of the gauge to sit firmly on the peaks of the
blasted substrate, the needle will pass into the trough as far as it can
Surface profile needle gaue.
Needle
1- We need to zero the gauge when the point of the needle is on the same plane as the flat
foot, i.e. on a smooth piece of glass.
2- Applying slight pressure to the foot to ensure that it is perfectly flat on the glass.
3- By loosening the locking screw, the bezel can now be moved. The bezel should be
moved till the zero on the gauge is immediately behind the needle.
4- Then tighten the locking screw and the gauge is ready for use.
5- Several readings are taken, usually more than ten, in random
It is normal to work to an average figure.
Positions over the substrate, and the average
Testing of Paints
Foot
Plane for zero
Distance travelled
by needle from
zero = profile
depth

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Calculated. This type of gauge
is not ideally suited for curved areas such as pipes.
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ii Dial Micrometer and Replica Tape;
“Replica tape”, or “Testex”, is also sometimes called ‘cornplaster method’. This method
provides a permanent record. The tapes are supplied in two grades: -
*Coarse Grade for measuring profiles “0.8 to 2 Thou”. 20-50um.
*Extra Coarse Grade for measuring profiles “1.5 to 4.5 Thou” 37-115um.
Mylar tough transparent
Polyester plastic
Testex Paste
Paper
Figure 2.4 Cross section of a replica tape
The procedure for using replica tape is as follows
1 Zero the dial micrometer.
2 Remove the backing paper from the replica tape, Stick the replica tape to the area to be
measured.
3 Using a pen or pencil end, rub firmly and evenly all over the area of the Mylar. This
causes the testex paste to pass into the troughs and the peaks of the blast will butt up to the
transparent Mylar.
4 Remove the replica tape and check. The Mylar area should no longer be white (now grey),
and pinpricks of light should be visible through the Mylar when held up to the light.
5 Place the testex paste area between the anvils of the micrometer and allow them too gently
close together. From the final reading on the gauge deduct two thou if using an imperial
gauge or 50um if using a metric gauge. The balance figure is the peak to trough height of
the profile.
1 mm = 1000 um
25.4 um = 0.001"
40 Thou" = 1 mm
25.4 mm = 1 inch
Testex
2 um
10 um
Micrometer is reading 93 um;
subtract 50 um for testex
plastic backing. The surface
amplitude is therefore 43 u
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Figure 2.6 Metric micrometer for testex measurement in microns
1
100 mm
10
microns
100 microns
0.10 mm
Figure 2.7 Imperial micrometer for testex measurement in 1000 of an inch
Reading the gauges.
Testing of Paints
Micrometer is reading 4.6 Thou
(0.0046"), subtract 2 thou (0.002")
for testex plastic backing, the
surface amplitude is therefore 2.6
thou (0.0026")
1
10 Thou
0.0001"
1 Thou
0.001"
Testex
(Allow 2 Thou
(0.002") for plastic
backing
Testex
(Allow 50 microns 0.05
mm for plastic backing
Micrometer is reading 80 microns
(0.080 mm) subtract 50 microns
(0.050 mm) for testex plastic
backing; the surface amplitude is
therefore 30 microns.

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There are four common scales for dial micrometers, one of which, the 2um scale is also used on
the needle gauge.
The common scales are: -
0.01 mm = 10 microns / small division
0.002 mm = 2 microns / small division
0.001” = 1 thou / small division
0.0001” = 1/10 thou / small division
Useful conversion factors are: -
1 mm = 1000 um
1 thou = 25.4 um
25.4 mm = 1 inch
2.54 cm = 1 inch
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Assessing a profile to BS 7079 Pt C ISO 8503.1
Grit and shot abrasives produce different surface profiles, therefore two comparators are
specified. One for grit blasted profiles, G. and one for shot blasted profiles, S. When a mix has
been used then the reference comparator should be G. In all instances the entire area should be
blasted to SA21/2 or SA3 grade.
Use of the comparators;
There are three methods, which can be employed to assess the roughness characteristics of blast
cleaned steel.
1 Naked Eye
2 Visual Aid, not exceeding 7x magnification
3 Tactile
(N.B. the comparators are not for assessing cleanliness.)
The comparators to BS 7079 are approximately 8 cm square with a 2 cm diameter hole in the
middle, and are divided into four segments, by smooth strips. On each strip is an arrow
Indicating the segment number. Segment one is the smoothest and the degree of roughness
progressively increases up to segment four.
Using the comparators; for messuring the “secondary profile”
With all three methods it is important to remember
that the prepared surface should not be touched
(Contamination). For the tactile method the
Fingernail or a clean wooden stylus may be used.
The principle is to compare the surface profile of
The blasted steel with the segments on the ISO/BS
Comparator, looking for two segments between
Whose profile the test surface lies.
The grading used is: -
Fine- Profiles equal to segment one and up to, but excluding segment two.
Medium- Profiles equal to segment two and up to, but excluding segment three.
Coarse- Profiles equal to segment three and up to, but excluding segment four.
Finer than fine. Any profile below the lower limit for ‘Fine’
Coarser than coarse. Any profile above the upper limit for ‘Coarse’
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Preparation of steel substrate before application of paints and related products
Rust Grades. BS 7079 Pt A, ISO 8501, SS 05 59 00
The numbers given all refer to the same book, which gives high quality pictorial standards for
condition and cleanliness before and after surface preparation, by abrasive blasting, hand and
power tool cleaning and flame cleaning.
Rust Grade A - Steel surface largely covered with adherent millscale with little if any
rust.
Rust Grade B - Steel surface, which has begun to rust and from which the millscale has
begun to flake.
Rust Grade C - Steel surface on which the millscale has rusted away or from which it
can be scraped, but with slight pitting visible under normal vision.
Rust Grade D - Steel surface on which the millscale has rusted away and on which
general pitting is visible under normal vision.
The degree of cleanliness is mainly dependent on the time spent on the area and the velocity
of the particles.
Abrasive Blasting Grades
Before surface preparation commences any oil or grease should be removed and heavy rust and
scale removed by chipping. After preparation the surface should be free from dust and debris.
Sa 1 - Light Blast Cleaning. When viewed without magnification, the surface shall
be free from visible oil grease and dirt and from poorly adhering mill scale,
rust, paint coatings and foreign matter.
Sa 2 - Thorough Blast Cleaning. When viewed without magnification, the surface
shall be free from visible oil grease, dirt, and most of the millscale, rust,
paint coatings and foreign matter. Any residual contamination shall be
firmly adhering.
Sa 21/2 - Very Thorough Blast Cleaning. When viewed without magnification, the
surface shall be free from visible oil grease and dirt and from millscale, rust,
paint coatings and foreign matter. Any remaining traces of contamination
shall show only as slight stains in the form of spots or stripes.
Sa 3 - Blast Cleaning to Visually Clean Steel. When viewed without
magnification the surface shall be free from visible oil grease and dirt, and
shall be free from millscale, rust, paint coatings and foreign matter. It shall
have a uniform metallic colour.
From the above definitions it can be seen that Sa 1 and Sa 2 are not achievable on rust grade
A and consequently there are no photographs for the grades.
The American SSPC (Steel Structures Painting Council) and NACE (National Association of
Corrosion Engineers) have their own systems and compare as below.
BS 7079 PtA SSPC NACE
Sa 3 White Metal SP5 Grade 1
Sa 21/2 Near White Metal SP10 Grade 2
Sa 2 Commercial Finish SA6 Grade 3
Sa 1 Light Blast and Brush of SP7 Grade 4
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Equipment;
1. Wheelabrators;
*Wheelabrators, sometimes known as (centrifugal blast units) they are ideal
for long production runs on similar section components such as pipes, or
bridge steelwork.
They are usually referred to the number of ‘wheels’ which they operate e.g. 6 wheel.
The operators of these machines prefer shot as an abrasive.
The abrasive is gravity fed into the centre of the wheel.
Centrifugal forces carry it to the end of the impeller where it is impelled at the component to
be cleaned at a speed of 220 mph app. in a fan pattern.
The fast moving metallic abrasive shatters millscale cuts a profile etc., and eventually, its
energy spent, drops.
The floor of the unit is open grating over a ‘V’ shaped pit, in the bottom of which is a rotating
screw which carries the spent abrasive plus detritus into a hopper.
A conveyer system then carries the abrasives to the top of the machine, dispenses it, to start a
gravity fed path back to be re-used.
As an integral part of the system the abrasive passes aver a tilted plated, known as a weir plate.
As the abrasive and detritus cascades over the edge of the weir plate, a current of air is drawn
through it. This draws out low density materials such as rust, millscale, flakes of paint etc.,
and finings, abrasive worn so small that it is no longer useful.
This is known as an Air Wash Separator,
The same principle is used in enclosed grit blasting pens.
Meanwhile the cleansed abrasive is fed back into a common hopper with feed lined to all the
wheels, to be re-used.
As mentioned previously new abrasives need to be added periodically to maintain an adequate
working mix.
Advantages;
1-The quality can be controlled by adjusting the feed roller speeds
2-Because the system is totally enclosed there is efficient use of abrasives.
3- More operator safety because the operator is not involved.
4-The systems can be far more productive (dependent on supply of components) than open
blasting.
Disadvantages;
One major problem is access to bolt pockets, gussets and stiffeners etc. Because the wheels are
fixed, there is no manoeuvrability, and thus shadow areas arise. One way to avoid this is
manually blast difficult areas prior to machine blasting.
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2. Air Blasting;
Site blasting is normally carried out using expendable abrasives and open blasting systems.
Open blasting systems operate using.
a) A compressor.
b) A pot containing the abrasives.
c) Vapour Traps for oil and water (knock out pots).
d) A hose, usually carbon impregnated.
e) A nozzle
f) A dead mans handle for operator safety.
a) Compressor;
Compressors are rated by two factors.
i Air pressure – measured in psi, pounds per square inch.
ii Capacity - the amount of air it can deliver at the pressure required, in cubic feet per min
cfm, or litres/min.
* 100 psi, which is considered to be the ultimate pressure for open blasting.
* 100 psi gives 100% efficiency.
* Using pressures over the 100 psi uses more abrasives, more fuel, more effort from the
operator, more work by the compressor, without a proportionate increase in area blasted
* Every 1-psi drop in pressure results in an efficiency drop of 11/2%. 80 psi blasting pressure
results in 70% efficiency.
b) Blast Pot;
* For site work the most common is the pressurised blasting pot.
* These are supplied in various sizes and are selected according to purpose.
* The pots are charged with abrasives and when pressurised, seal, rubber to rubber, by means of
a mushroom shaped cap.
* The abrasive is blown by air pressure into the air stream feeding the nozzle.
* The abrasive flow can be adjusted by means of a metering valve on the conical base of the
pot. This is sometimes called a ‘miser’ valve.
c) Vapour Traps;
* Air contains water vapour and when air is compressed the water vapour in the air is
compressed.
* Compression produces heat and as the air heats up its capacity to hold water increases, every
110C rise in temperature the airs capacity to hold water doubles.
* Conversely when the air-cools rapidly on expansion, exiting the nozzle, water droplets are
formed.
* Should this water contact the substrate, corrosion would result. Also atomised oil (from the
cylinder lubricants) needs to be extracted.
* Otherwise low surface energy material, oil, on the substrate will adversely affect adhesion.
The knockout pots; are on the main airline and are inverted transparent glass domes. A small
cock on the bottom allows them to be emptied, and usually are kept slightly open. In the UK
climate it is not unusual to blow downstream 20 gallons of water in an eight-hour working day.
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d) Carbon impregnated Hose;
· Because pressure drops along the length of the hose,
line lengths are better restricted to around seven to eight
metres.
· Internal couplings reduce the hose diameter and act as
pressure reducers, cause turbulence and wear; so external
couplings should be used.
· Hose diameter is related to nozzle size and should have an
internal diameter at least three to four times the nozzle
diameter.
Any specified blasting pressure could be measured using a hypodermic needle gauge.
The needle is placed through the hose near the nozzle with the needle facing towards the
nozzle.
e) Nozzles;
* The air consumption and air speed are directly related to the nozzle aperture size.
· The larger the nozzle size the more air will be needed to maintain pressure.
· Typically a ¼" nozzle will need 103 cfm to maintain 100 psi,
· Where as a ½" nozzle will need 413 cfm. Therefore big nozzle, large bore hose, needs
high capacity compressor.
* Sometimes the nozzles are lined with tungsten carbide or ceramics to reduce wear.
* The venturi shaped nozzle give a larger blast pattern with a more even spread of abrasives
and higher velocity of the particles at approximately 450 mph.
* The straight bore nozzle gives a small concentrated area of abrasive contact with a fringe area
of lower concentration and particle speed of around 200 mph.
* The stand off distance for both types varies according to hose size and nozzle aperture size,
but an average figure is around 450mm.
f) Safety to 1GE SR 21;
Safety considerations are.
i The hose should be carbon impregnated to reduce the chance of the operator getting
electric shock from static.
ii A dead mans handle should be under direct operator control for his/her own safety.
iii Hoses should be kept as straight and as short as possible to avoid kinks, and blowouts
and to maintain pressure at the nozzle.
iv Use reinforced hoses if possible.
v Use external bayonet type couplings.
vi Maintain operating pressure at 100 psi.
vii It is necessary to have warning signs advising that abrasive blasting is in progress,
viii Correct protective clothing should be worn by the operator, including direct air fed
helmet, with adequate visors, leather aprons and gloves, boots and ear protectors.
ix Warning buntings segregating the area.
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3-Water Blasting;
Advantages;
* Using water is more environmentally friendly than open blasting.
* From the safety aspect, spark free. They are ideal for removal of soluble salts, sulphates and
chlorides, (the hygroscopics).
* Complete removal needs high-pressure ranges.
* Are also ideal for removing layers of toxic materials, e.g. red lead, calcium plumbate, and zinc
chromate primers. (Passing into the air, this can then be inhaled and passed into the
bloodstream).
Disadvantages;
* Supply of large amounts of water and disposal of the resulting slurry (water and detritus as an
entity).
* And also mixing substrate inhibitors if the specification demands it. (Substrate inhibitors are
substances usually Sodium compounds, added to the water, to retard the formation of corrosion
products) Some organisations, including B G do not allow the use of inhibitors, in which case
dry blasting, to remove light oxidation, follows wet blasting.
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 High pressure water bl asting up to 30 000 psi (water jetting);
 Water usage is about 60 litres per minute.
 This system operates at about 30 000 psi.
 To work efficiently the head must be near to the surface, within 25 to 35 mm.
 At approximately 250 mm only loose and flaking material will be removed.
 Operator fatigue is a problem.
 This system will remove soluble contamination and millscale at the higher-pressure
ranges but will not cut a profile. It will only clean up the original profile on
rework areas.
 High pressure water plus abrasive injection;
 This system operates at about 20,000 psi.
 Uses abrasives, either gravity fed into the system, suction fed or mixed as
slurry.
 This system will remove Marine growths e.g. barnacles, and it us often
used in dry-docks on ship hulls. Because of the abrasives a profile is cut using this
method.
 Low pressure water plus abrasive injection;
 Uses normal blasting pressures of 100 psi. But with water as a propellant rather than air.
 The abrasive content is semi-soluble e.g. Sodium Bicarbonate crystals, talc, chalk,
 Ideal for use on non- ferrous metals and G. R. P.
 Sodium Bicarbonate is excellent for acidic or greasy situations.
 This method is very slow and controllable and can if needed, remove one coat of paint. The
abrasives have a very gentle action but leave masses of problematic slurry.
 Steam Cleaning;
 Ideal for oily and greasy situations,
 Steam production requires a heat source, (which is not conducive with the oil and gas
industry).
 Air blasting with water injection;
 Water is injected, with or without an inhibitor into the air/abrasive stream, either immediately
after it exits the nozzle or immediately before it enters the nozzle.
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 Water usage with this method is approximately one to one and a half litres per minute, which
is sufficient to control dust.
3. Hand and power tool cleaning. 7079 Pt A, ISO 8501, SS 05 59 00;
*Any hand operated or power tools, including needle guns, wire brushes, emery cloth and
grinders can be used to achieve these standards.
* Hand and power tool cleaning is often specified for short-term maintenance programmes.
**Disadvantage; of this method is the lack of surface profile. Wire brushing may produce a
burnishing, which is polishing, and a smooth shiny area does not provide good adhesion.
Burnishing needs to be treated by abrading with coarse emery.
St2 – Thorough hand and power tool cleaning .
When viewed without magnification the surface shall be free from visible oil, grease and dirt
and from poorly adhering millscale rust, paint coating and foreign matter.
St3 – Very thorough hand and power tool cleaning.
As for St2 but the surface shall be treated much more thoroughly to give a metallic sheen arising
from the metallic substrate.
There are no wire brushing grades for Rust Grade A as the millscale is much harder than
the bristles on the brushes, which are of non sparking alloys such as phosphor bronze and
beryllium bronze.
If needle guns, Jason’s hammers, are used they tend to leave a very coarse profile, which
invariably needs to be reduced by abrading with emery, or grinding.
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4. Flame cleaning;
The BS 7079, ISO 8501 (SS 05 5900) contains four photographs showing flame cleaning
standards from the original rust grades A, B, C, D. The designation given is AFl, BFl, CFl,
and DFl. There is only one flame-cleaning standard for each rust grade.
It is not wise to use this method of surface preparation on any fasteners relying on tension,
e.g. rivets, screws, nuts and bolts.
Three factors contribute to how flame cleaning works.
1. Expansion;
Millscale is chemically bonded to the steel and applied heat causes the materials to
expand at different rates, thus breaking the chemical bond.
2. Dehydration;
Water in the corrosion products is evaporated away, facilitating the removal of the
corrosion products.
3. Heat penetration
The heat is conducted efficiently into the substrate aiding the drying of the steel and
removal of penetrated oil or grease.
Method;
The operator slowly passes an oxygen/HC gas flame (Butane, Propane, Acetylene) over
the area to be cleaned to burn and de oxidise the corrosion products and other contaminants.
This leaves a grey coloured ash deposit.
A second operator follows on with a power brush to remove the now loose, ash deposits.
The primer can now be applied over the warm steel, reducing the need for addition of
thinners.
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Other benefits are that the heat reduces the viscosity of the paint and gives better flow
properties.
The paint can then 'wet out' better and pass into tiny cavities and irregularities on the
surface.
The heat also accelerates the drying process and keeps the steel above dew point
temperature.
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5. Pickling;
Pickling is a general term relating to the chemical removal of oxides (rust), from a metal
substrate.
The metals can be either dipped (totally immersed) in the pickling fluid or sprayed with it.
Usually aqueous solutions of acids are used for steel; they convert the oxides into soluble
salts e.g. Sulphuric Acid produces Iron Sulphate salts. Sulphuric is the most common acid
used for economic and safety reasons.
Footners Duplex System
Involves the pickling process followed by a passivation process using Phosphoric or
Chromic acid along with a small percentage of iron filings, which produces Iron Chromate or
Iron Phosphate salts, which are not soluble.
These form a rust inhibitive layer, which passivates the surface and increases the adhesion
properties. They are also extremely resistant to cathodic disbondment.
A typical process would be: -
1. Any oil or grease needs to be removed by using a suitable solvent e.g. xylene or as
specified. Oil and grease show up as fluorescent yellow/green under an ultra violet light.
2. Totally immerse in a bath of Sulphuric Acid, 5 – 10% concentration at a temperature of 65
– 70oc. Time can vary from 5 to 25 minutes depending on degree of contamination but is
invariably at the lower end.
3. Rinse using clean warm water to remove the layer of soluble salts formed. If required the
component could be coated after pickling. Likewise components can be blast cleaned and
sent on for phosphating/chromating, but the patented process is only called “Footners”
when pickled then phosphated/chromated.
4. Immerse in a bath of phosphoric/chromic acid, 2% solution at 80oc for approximately one
to two minutes with iron filing (0.5%) (And an inhibitor to prevent embrittlement). This
leaves a very thin layer of iron phosphate/chromate, which acts as a rust preventative for a
limited time.
5. Rinse in clean water, and check for pH values.
PH is a measure of acidity or alkalinity of a substance and is measured using pH
indicator strips. An indicator such as litmus will only tell if a substance is an acid or an
alkali. Indicator strips give a measure of acidity or alkalinity, based upon the scale below.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Figure 2.8-pH scale
Acid Alkaline
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This is a logarithmic scale and seven is neutral, the pH value of distilled water. From 7 to 0 the
acidity increases, and from 7 to 14 the alkalinity increases. A typical requirement after rinsing
will be in the region of pH 4.5 to 7.0, slightly less acidic than household vinegar.
6. Vapour degreasing;
Fumes from a solvent bath condense on a component suspended over the bath and
dissolve any oil or grease, which then drips back into the bath. Very rarely used because of
modern regulations regarding strong hydrocarbon solvents.
7. Weathering;
Weathering relies on co-efficient of expansion properties as mentioned in Flame
Cleaning. When left in a stockyard, open to temperature changes, day and night, the
millscale sheds. This can now leave the steel open to atmospheric corrosion, which produces
such as Sulphate salts.
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PAINT CONSTITUENTS AND BASIC
TECHNOLOGY
Paint is a material, which will change the texture colour or appearance of a surface and give
some form of protection to the underlying surface.
Paint has been classified in many ways e.g. by principle involved.
1. Barrier ;
The material forms a thick impermeable layer of a high electrical resistance e.g. urethane.
2. Passivation;
Causing a chemical reaction between the paint constituents and the substrate e.g. rust inhibitive
primers.
3. Cathodic protection;
Employs the bi-metallic principles by using a less noble metal as pigmentation e.g. zinc in zinc
rich primers.
By function.
Anti Fouling - To inhibit marine growth on ship hulls
Road Marking - To give white or yellow lines on roads
Fire Proofing - To provide resistance to fire
Heat Resistant - For surfaces working at high temperatures
Anti-corrosive, and many more.
Paints can be classified by binder type.
By colour.
By the pigment type.
The paints contain the same basic ingredients.
1. Binder
2. Pigments and other additives
3. Solvent (where applicable)
It is the chemical structure and composition of these constituents, which gives the paints their
own individual properties.
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Paints are supplied as either liquids or solids in powder form
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and can be subdivided into groups .
a) Liquid paints containing solvent
This group is still the largest in terms of sales.
It is important to realise that solvent does not relate solely to Hydrocarbon solvents, but
also includes water. Due to the modern EPA. (Environmental Protection Act)
Rquirements, manufacturers are researching into new paint technology involving vastly
reduced amounts of volatile organic compounds. Some are using water-based technology;
some are concentrating on the solvent free materials.
b) Solvent free
As the name implies these materials contain no (or in some cases a minute amount of)
solvent.
These are generally chemical curing materials, which require the mixing of two or more
components,
Usually go under the name of MCLs (Multi Component Liquids). Some MCLs are made
using solvent borne materials.
c) Powders
Virtually solvent free MCLs, which are solid at, room temperatures.
The base resin and the chemical activator, along with the other constituents required to
complete the formulation are heated up to the resins melting point, mixed into an
homogeneous liquid, cooled and ground into powder form.
In theory every particle contains all necessary ingredients to affect a cure into a protective
film. The powder can be applied onto a preheated substrate (in the case of substantial steel
thicknesses) at about 240oc, or onto thin plate electro statically and post heated.
In either case the powder melts undergoes a chemical reaction or in approximately three
minutes the reaction is complete.
The three subdivisions are all made up from the basic ingredients mentioned earlier,
Binder, Solvent, Pigment and other additives.
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Binder
The binder is the main constituent of paint and is often referred to as a film former.
Other terms are vehicle and non- volatile.
Some major considerations of a binder are: -
1. Ease of application (flow properties or viscosity).
2. Adhesion to the substrate.
3. Resistance to abrasion.
4. Resistance to chemical attack according to environment.
5. Cohesive strength, its ability to hold together as a film.
6. Dialectric strength.
7. Ability to resist the passage of water.
8. Ability to change from a liquid as applied, into a solid to provide the above properties.
Several materials satisfy the criteria above for different environmental conditions, among them
are: -
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Binder – solvent groups and compatibility
A solvent free binder, or a binder using a very weak solvent, will cause very few problems
when over coating another product.
Usually in this situation the problem would be limited to different expansion and contraction
ratios.
Providing a key by abrading can mostly rectify or at least minimise this.
A very strong chemically curing binder like epoxy, needs a strong solvent and can cause
problems over coating other materials, even when they are fully cured.
Guide to binder solvent combinations
Solvent strength in
descending order Common Names Binders
Water
Emulsions PVC/PVA
Vinyl’s
Acrylics – other materials e.g.
epoxy
Bitumins, Polyurethanes,
Alkyds, Acrylated Rubbers
Aliphatic Hydrocarbons
White Spirit
Turpentine
Turpentine substitute
Solvent naphtha’s
Hexanes upwards
Natural oils
Natural resins
Alkyds
Phenolics
Aromatic Hydrocarbons
Xylene
Toluene
Benzene
Chlorinated Rubber
Ketones
Acetone
Methyl Ethyl Ketone
Methyl ISO Butyl Ketone
Epoxy
Polyurethanes use ketones and esters with aromatic diluents.
It is not advisable to use a binder with a strong solvent over an existing coating, which uses a
weak solvent.
For example Chlorinated Rubber coated over an Alkyd would result in lifting, and
wrinkling,
Alkyd over Chlorinated Rubber would have no ill effect.
Because an Epoxy is chemically cured, there is no problem over coating with Polyurethane
two packs, chemically cured.
A hydrocarbon solvent borne Epoxy coating applied over Chlorinated Rubber would not
be advisable.
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Ethyl and Methyl Silicates do not appear on the list because they are high (or low)
temperature performance coatings, the criteria for compatibility with these materials for over
coating is working temperatures. I.e. will the over coating material withstand the operating
temperature? Usually the only material suitable is silicone. Ethyl and Methyl Silicates will
not adhere over any substrate other than bare, clean steel.
Any binder, which can be converted into a polymeric salt, can be modified to be water based
and many of the binders mentioned above fall into that category.
Chlorinated rubber
Advantages;
1. Because of the chlorine content, high resistance to mould growth.
2. Again because of the chlorine, non-flammable after solvent release.
3. Very resistant to chemical attack e.g. Acids and Alkalis.
4. Very high resistance to water vapour transmission.
5. Material is non-toxic and provides a very durable film.
6. Very easily maintained, no abrasion needed, clean surface only.
Disadvantages were;
1. Its position on solvent compatibility list shows low resistance to solvents i.e. only resistant
to Aliphatics and Water.
2. Low temperature tolerance, 65oc maximum.
3. Spray application resulted in ‘cobwebs’.
Polymers
One of the properties expected of a binder is to change from a liquid into a solid to form a
film.
To perform this function all binders form polymers or use polymers already partially
formed.
Polymer means; literally many parts, poly = many, mer = single unit or part. Mer (meras
GK) can be a single atom, or a molecule, (a group of atoms) and can be described as being “a
string or structure of repeated units”,
Polymerisation; is the “joining together of a string or structure of repeated units”.
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In the case of most paints the main constituents of the polymers are: -
H - Hydrogen
C - Carbon
N - Nitrogen
O - Oxygen
Cl - Chlorine
Although there are variations the main three polymer types are Linear, Branched and Cross-linked
1 Linear Polymers;
The atoms or molecules which form the polymer, join on at the end of the structure, and in so
doing saturate the structure.
The process depends upon the properties of carbon, which forms the backbone of the
structure. Carbon can give away electrons, take in electrons, share electrons, or join with
itself in many ways.
H|
H – C – H
|H
H H
| |
H – C – C – H
| |
H H
H H
| |
C = C
| |
H H
METHANE
SATURATED
ETHANE
SATURATED
ETHYLENE OR ETHYNE
UNSATURATED
The Ethylene or Ethyne molecule is defined as being unsaturated, the two carbons are
sharing electrons, hence leaving potential for the spare electrons to combine with another
molecule or radicle.
H H
| |
C...... C
| |
H H
H H
| |
C...... C
| |
H H
Figure 4.2 Ethylene molecules close together
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H H
| |
C...... C
| |
H H
H H
| |
C...... C
| |
H H

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The above figure represents ethylene molecules close together. The dotted line being the
weaker bond (the secondary valency bond). This being the one that joins to the next
molecule giving: -
H H
| |
C C
| |
H H
H H
| |
C C
| |
H H
Figure 4.3 Ethylene molecules polymerise
It can be seen that linear polymers, once formed, cannot react with anything to chemically
produce another compound, and until destruction will maintain the same structure and
properties.
A linear polymer is a non-convertible or reversible material and also thermoplastic.
From the binder types the linear polymers are Acrylics, Vinyls, Chlorinated Rubber,
Asphalt and Coal Tars and Cellulosic Resins.
2 Branched polymers;
Combining oxygen with the double bonds available forms branched polymers. Oxygen, from
the atmosphere, a very reactive element, combines with a constituent of natural oils and
resins called fatty acid esters. The double bonds in these fatty acid chains are not at the end
of the structure, but in the middle.
So any combination doesn’t occur lengthways to elongate the chain, but forms a branch from
the main carbon backbone.
Because of the abundance of reactive oxygen in the atmosphere, the branching carries on
and on over several years until eventually the matrix becomes cross linked and very brittle,
and cracks and flakes off.
Binders, which fall under this category, are Natural Oils and Natural Resins, and isomers
such as Alkyds and Phenolics. By combining with another element and chemically reacting
to form another compound, these materials become non-reversible or convertible coatings,
thermosetting.
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H H
| |
C C
| |
H H
H H
| |
C C
| |
H H
|C
H
|
H C H
|
H C H H|
C = C - C = C - C
| | | | |
H H H H H
OH
O
Oxygen
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Figure 4.4 Branched polymers
3 Cross linked polymers;
Cross-linking, or chemical curing is a three-dimensional polymerisation process, which
occurs fairly rapidly using only components provided in the cans.
Because the components are in calculated amounts the cross linking stops when all the
available bonds are occupied. Some urethanes fully cure in 16 hours, some Epoxies in three
days, and others in seven days, dependant on temperature.
Figure 4.5 cross-linked polymers
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Oils
Natural oils (vegetable oils) are produced from seeds of a plant.
Well-known examples being linseed, castor, olive, coconut, soya.
In order to be usable as a paint binder the oil must be of a type that will combine with
oxygen, i.e. it must be “unsaturated”. Saturated oil cannot be used as a binder because it
will not solidify by polymerisation to form a film. Therefore, oils can be divided into three
groups.
Drying oils
Semi drying oils
Non drying oils
1 Drying oils
Drying oils are oils, which have three sets of double bonds along the carbon backbone, and
react with oxygen readily at ambient temperature.
2 Semi drying oils
Semi drying oils have one or two sets of double bonds, and may need addition heat, or some
other catalyst to promote oxidation.
3 Non drying oils
Non drying oils will not oxidise and therefore cannot be used as binders. Instead these are
used as plasticisers in paint formulation, to modify properties of a resin.
Although linseed oil and tung oil used to be referred to as rapid drying oils, the term rapid was
compared to some other oils, and in fact it could be many weeks before a reasonably resilient
film was formed. Treated natural resins have the exact opposite properties, i.e. fast drying and
very brittle. Oils and resins are mixed to give a binder with modified properties.
Long oil paint – more than 60% oil to resin, elastic, slower drying properties suitable for
domestic applications, decorative materials.
Medium oil paint – between 45 – 60% oil to resin.
Short oil paints – less than 45% oil to resin, faster drying material, suitable for steelwork. More
brittle with shorter over coating time.
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Pigments
Pigments have many properties and characteristics. They are derived from many sources,
animal, vegetable, mineral and synthetically produced, and can be in a wide variety of
particle sizes and shapes.
Pigments used in paints must remain as solid particles within the vehicle (the binder plus
the solvent if a solvent is used), and not dissolve.
If it dissolves it is known as a dye, not a pigment.
Pigment particles contribute to the paint films strength cohesively, its abrasion resistance,
durability, opacity, in some cases impermeability and resistance to ultra violet rays.
Some pigment particles are as small as 1 / 10
th micron . Pigments can be subdivided into groups
according to the main function they perform in paint.
Rust inhibitive pigments. Anticorrosive
Rust inhibitive pigments are added into primers to protect the steel substrate by passivation.
Typical materials in the category are: -
a) Red lead *
b) Calcium plumbate *
c) Coal tar *
d) Zinc chromate *
e) Zinc phosphate
f) Barium metaborate
g) Zinc phosphosilicate
The four marked with an asterisk are toxic and restricted in use.
Red lead is a basic inhibitor and works in the presence of fatty acid esters in natural oils and
resins only.
These systems provide lead soaps, which give the actual inhibition.
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Metallic Pigments
Metallic pigments are also used on a steel substrate to protect the steel by cathodic
protection.
If a metal which is less noble than steel, (more electronegative) is included in the film, and an
electrolyte e.g. water, passes through the film, contacting substrate and pigment particles, then a
circuit can be engaged whereby the pigment particles will receive the hydroxyl ions and thus
suffer corrosion in preference to the steel substrate.
In order to satisfy this requirement the metal pigment must be below the position of steel on the
galvanic list. The two most amenable metals to satisfy this are: -
1-Zinc 2-Aluminium
Zinc is the better of the two for galvanic protection but Aluminium is excellent for solar
protection, reflecting the ultra violet A and B.
Colouring pigments are used, usually know as Opaque pigments.
Opaque pigments
Opaque pigments are inert particles with excellent light scattering properties in order to give
covering power, (opacity) and colour.
1. Carbon Black
2. Compound of Cobalt Blue
3. Compound of Chromium Greens, Yellows and Oranges
4. Compound of Iron Browns, Reds and Yellows
5. Compound of Calcium Reds and Yellows
6. Titanium Dioxide White
Extender pigments
Sometimes known simply as extenders or fillers.
These materials provide some of the main properties expected of the film, such as adhesion,
cohesion, film strength and durability.
They also have a role in application and flow, levelling, and other mechanical properties of
the film, and are an aid to inter coat adhesion and can reduce gloss.
Materials used, as extenders are usually low priced readily available materials such as: -
Clays e.g. Kaolin, China clay
Chalk Calcium carbonate
Talcum Magnesium silicate
Slate flour Aluminium silicate
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Practical De-lamination
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Laminar pigments
Plate like pigments such as MIO (Micaceous Iron Oxide), Aluminium Flake, Glass Flake,
Mica and Graphite, provide excellent barriers.
These pigments have a leafing effect and in theory overlap when the coating dries.
MIO sometimes known as specular haematite is widely specified, and to be regarded as
pigment quality material quite often has to meet quite stringent requirements e.g. 85% of the
total mineral compound has to be Fe2 O3, haematite, of this 85% less than 1% should be
permeable to moisture, thus giving a paint film with high resistance to water permeation.
Theoretical Leafing layers
In theory when moisture passes into the film, on contact with the MIO platelet, it has to pass
around it, thus almost doubling the distance to reach the substrate.
Glass Flake as a laminar pigment is usually for abrasion resistance,
Aluminium Flake and MIO have good ultra violet A and ultra violet B reflectance
properties, protecting the underlying binder from attack and subsequent degradation.
PVC
The pigment to binder ratio is a very important factor in the design and manufacture of paint and
is known as the Pigment Volume Concentration. There is an ideal pigment binder ratio, which
varies from paint to paint, pigment to pigment, and this is known as CPVC, Critical Pigment
Volume Concentration. CPVC is defined in BS 2015 as “The particular value of the pigment
volume concentration at which the voids between the solid particles that are nominally touching
are just filled with binder and in the region of which certain properties are changed markedly.
Figure 4.7 Below CPVC Figure 4.8 Near CPVC Figure 4.9 Above CPVC
Figure 4.7 – Too much binder to solids ratio would give a film of good gloss properties, but poor
covering power (opacity) and with a tendency to blister (low cohesive strength).
Figure 4.8 – A film with lower gloss properties but greater cohesive strength and just enough
resin to encapsulate each particle, giving good resistance to water permeation.
Figure 4.9 – The CPVC is exceeded and all particles are not wetted, the film would be porous,
low in cohesive strength and adhesion.
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Solvents
Solvents are added to paints to 1-reduce the viscosity and 2-Ease application properties.
The solvents used in paints have to fulfil various other requirements, for example if a
solvent evaporates away too quickly the film will not dry evenly, if it evaporates too slowly
drying will be protracted and on vertical surfaces the paint is likely to sag.
The four important properties of a solvent are: -
1 Solvent Strength
Low molecular weight solvents are stronger than high molecular weight solvents and,
strong binders such as epoxies and polyurethanes, need strong solvents to ‘cut’ or separate
the molecules.
Hence Ketones and Aromatics are used for these materials.
Natural resins don’t have the same attraction between the molecules and therefore need
weaker solvents, higher molecular weight, such as Aliphatics.
2 Evaporation Rate
The evaporation rate governs at what point the polymerisation starts.
For decorative materials need a long wet edge time, so a long slow evaporation rate is
needed, otherwise dragging and ropiness would occur when joining area to area.
Industrial coatings need to dry quickly for protection and so that further coat can be applied.
3 Flash Point
The flash point of a solvent is a safety consideration. Roughly defined as “The minimum
temperature of the solvent at which the vapours given off are flammable if a source of
ignition is introduced.” The higher flash point, the safer the solvent.
4 Toxicity
Solvents, especially modern solvents, are substances hazardous to health, and therefore have
predetermined concentrations to which humans can be safely exposed.
These limits are expressed in parts per million, ppm.
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Other Additives
Other than the main constituents of paint viz, binder, solvent, pigment and extenders, there are
approximately fifty other materials, which can be added to give other, or alter existing properties.
These can be grouped into Aids to Manufacture, Aids to Storage, Aids to Application, Aids to
Film Formation, Aids to Film Curing, and others. Some are used more than others, among them
being.
Anti settling agents
An anti settling agent is an aid to shelf life. It is a thixotrope, a thickener, which also allows
a higher film thickness.
Thixotropic paints are jelly paints, non- drip, and if stirred change to normal liquid
consistency. When left they slowly revert to thixotropic consistency.
Thixotropic agents are bentones and waxes, and help keep solid particulate constituents in
dispersion within the paint. I.e. stop settlement.
Plasticisers
A plasticiser basically gives paint flexibility, reduces brittleness, and therefore needs to be
compatible with the binder and have a very low volatility in order to stay in the film for a
long time.
Alkyd resin was used extensively in Chlorinated rubber binders, but for natural resins and
their isomers Non Drying Oils are used, saturated oils, which will not polymerise.
Castor Oil, Coconut Oil and some Palm Oils fall into this category.
Driers
Also known as oxidants, used in oxidising oils and resins. These are heavy metal salts, rich
in oxygen, which are added to the paint during manufacture.
Instead of relying on atmospheric Oxygen penetrating the paint layer, the oxygen is already
there, to allow even through drying of the film.
Common salts are octoates or naphthanates of cobalt, manganese and zirconium e.g. cobalt
naphthanate. (The acids producing the salts from the heavy metals are Octoic Acid and
Naphthanic Acid)
Anti skinning
Anti skinning agents are also known as anti oxidants. These are added to oxidising paints to
retard the formation of a skin on the surface of the paint.
If a skin forms it cannot be stirred back into a solution, and must be removed. Because the
anti oxidant works against the oxidant they are added in very small controlled amounts and
are liquids usually. E.g. Methyl Ethyl Ketoxime.
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SOLUTIONS AND DISPERSIONS
Solutions
A solvent is a liquid, which will dissolve another material, liquid or solid.
A solute is the material dissolved by the solvent.
A solution is the resulting liquid. Salt and water, sugar and water are solutions, a binder and
solvent are also a solution.
Dispersions
Paint consists of solid particles suspended in the vehicle, where there is no solubility, so paint
is dispersion.
Dispersion can be either a solid or liquid dispersed within another liquid, where there is no
solubility.
A suspension
A suspension is when fine particulate solids, e.g. pigment and extenders are dispersed within
a liquid, the vehicle.
Ideally after the manufacturing process, each particle should be completely wetted by the
vehicle. However because the pigment particles are so small, they cluster together to form
agglomerates or aggregates.
In some paints, especially gloss, the size of these aggregates is a very important factor and so
it has to be checked. The aggregate size is known as Degree of Dispersion of Fineness of
Grind.
An emulsion
An emulsion is a liquid dispersed in another liquid when there is no solubility.
In vinyl or acrylic emulsion, very tiny droplets of resin are suspended within water, which
can now be seen to be a non-solvent. In an emulsion water is a carrier, not a solvent.
Water is called the continuous phase, and oil/resin is called the dispersed phase.
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DRYING AND CURING OF PAINT FILMS
Generally three terms are used to refer to drying/curing temperatures.
a) Air Drying
This refers to normal ambient temperatures.
b) Forced Drying
When heat is needed to affect a cure or accelerate the reaction it is called forced drying, but the
temperature range for forced drying is ambient to 65oc.
c) Stoving
When temperatures above 65oc are used, using ovens or infrared, the term used is stoving.
Industrial paints, with a few exceptions e.g. intumescents, are generally in the air Drying
category, and the liquid to solid transition is dependant on one of the four drying mechanisms as
follows.
1 Solvent Evaporation
Paints employing this drying mechanism are linear polymer materials.
Sometimes referred to as solution polymers.
Solution polymers dissolve in the solvent, when the paint is applied the solvent evaporates
away allowing the fully formed linear polymers, saturated, with no activity points, to come
out of solution and form a film on the substrate.
The polymers lie in a random interlocking pattern, similar to cooked spaghetti or noodles and
loosely bond together by “ secondary Hydrogen bonds”.
The solvents used by these materials are strong solvents and, when reapplied onto the paints,
easily penetrate between the polymers and split the secondary bond, allowing the polymer to
go back into solution. Materials, which can do this are, called reversible or non-convertible.
Chlorinated rubber, vinyls, acrylics, and cellulosic materials fall into this
category.
2 Oxidation
Paints using this mechanism form a film by “oxidative cross linking” (polymerisation) using
atmospheric oxygen, and in some cases, the oxygen contained in the driers.
First of all if a solvent is present, the solvent evaporates away, allowing the oxidation to
begin. Oxygen then combines with the unsaturated bonds on the fatty acid esters,
progressively linking them Together, to form the film.
Once the oxygen has reacted with the binder, it has changed the chemical structure of the
binder and cannot be removed.
These materials are therefore convertible or non-reversible. Because oxygen is in
abundance in the atmosphere the reactions continue, an infinite, until the materials crack and
peel, having formed a very complex cross-linked matrix.
Alkyds, Phenolics, natural oils and resins are materials from this category.
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3 Chemical Curing
Chemical curing paints need addition of a second material, (in some cases as in moisture
curing, water from the atmosphere) but generally the second material, the activator, is
supplied in a can, hence the term 2 pack or Multi Component Liquid.
In order to obtain the desired film the whole of the contents of both cans should be
thoroughly mixed together and instructions on the materials data sheet should be strictly
observed.
Some materials will require an induction period and most data sheets will state the 'pot life'.
Chemically curing materials are convertible or non-reversible
An induction period is
“The length of time after mixing which the paint should stand before use”. Induction time is
also called stand time or lead-time, and is recommended to allow thorough wetting of the solids.
During the induction period the chemical reaction will commence and will be either: -
a) An exothermic reaction. Giving off heat, the container will warm up
b) An endothermic reaction. Taking in heat, the container will cool forming condensation.
A typical induction period is 20 – 30 minutes.
Pot life is
The period of time after mixing in which the paint must be used, and with industrial paints,
dependant on temperature is usually 6 – 8 hours. After the recommended pot life the material
becomes very user-unfriendly and if in bulk, is quite often subject to spontaneous combustion.
2 pack materials curing agents
Amides – Epoxy curing agents, usually quote seven days to full cross-linking at 20 oC.
Amines – Epoxy curing agents, three days to full cross-linking at 20oc.
Isocyanates – Mainly used for urethanes but also for some epoxies where low temperature
application is unavoidable, -10oc being typical. Ambient temperature urethanes, especially for
pipeline use quote 16 hours to full cure.
NB. Isocyanates are very toxic and need great care during use.
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4 Coalescence
Coalescence means to physically join together.
In an emulsion the resin droplets are dispersed in the continuous phase, water.
Upon application the water evaporates away allowing the resin droplets to come close together
until they are touching.
At this stage small amounts of high boiling point solvents are concentrated in the voids
between the spheres, from where they migrate into the spheres, plasticise them and allow
them to fuse together.
In so doing they also reduce the Tg of the material (Tg = Gloss Transition and is the
temperature at which the material changes from a rubbery to a glossy solid and vice versa).
If the Tg weren’t changed, the resulting film would stay as a liquid and be easily wiped
away.
These materials e.g. acrylics and vinyl’s are reversible. It is important to remember in this
case that water is not a solvent, but if the true hydrocarbon solvent was used the material
would form a solution.
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PAINT SYSTEMS
A single layer of Fusion Bonded Epoxy or Urethane would give excellent protection
employing the Barrier Principle.
A zinc phosphate pigmented primer would be a Passivation system but would need further
protection in the form of a barrier system to protect it.
An organic zinc rich epoxy would provide galvanic protection through bimetallic
principles but would last longer with a barrier system to protect the zinc.
Primer
A primer, normally low volume solid materials, wets out the substrate
Provides excellent adhesion and also provides a key for any subsequent layer.
The binders usually have a relatively low resistance to vapour transmission, and allow water
into the film to carry tiny amounts of the rust inhibitive pigmentation onto the substrate to
form a passivating layer.
Older versions of BG specifications required that all primers should be brush applied. This
was to ensure that any dust or detritus left on a substrate was ‘worked’ into the film, and not
left lying where air could be entrapped, forming pinholes.
Other primers exist for non-ferrous substrates such as Wash or Mordant primers, and PVB etch
primers.
Mordant means
‘Of a corrosive nature, or will bite into”,
As suggested contains an acid, Phosphoric acid.
These materials contain approximately 96% VOCs in the form of Ketones, and
approximately 4% phosphoric acid, tinted with copper phosphate (blue).
Their primary use was for etching new galvanising.
The reaction turns the surface black (zinc phosphate salts).
Some specifications allowed painting as soon as dry, but others required a water wash.
Etchants do not leave a measurable thickness.
PVB Etch primers, Polyvinyl Butyrol are principally used on Aluminium, but were used
on virtually every non-ferrous metal.
PVBs are 2 pack materials, low volume solids with a dry film thickness of 15 to 25 um.
This material also contains phosphoric acid. The acid etches the Aluminium (Aluminium
Phosphate) provided a key for the vinyl binder.
The general appearance when dry is a matt yellow translucent film, with an underlying black
or darkened substrate.
Some specifications require coating before 16 hours.
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Mid-Coats
Mid-coats are mainly barrier coats.
They are applied over the primers to prevent further water passing into the film
Mid-coats also build up the film thickness and even out any irregularities.
They also provide a key for any subsequent layer to adhere to.
Aggregates and extenders do this.
Some extender materials have particle sizes of 40 um,
If there is a high concentration of extenders in the coating then many of these large particles
will protrude through the surface, increasing the area available for adhesion.
Finishing Coats
Finishing coats of a system are mainly aesthetic.
Colour and appearance are important e.g. gloss.
To have a gloss finish the surface must be perfectly smooth, and this also helps in the
removal of dust and dirt, and natural drainage or shedding of water.
The storage facilities of volatile materials need to have solar reflective properties to reduce
boil off
Moisture Tolerant Systems
Pipelines transport many different products at different temperatures and pressures.
Gas is transported in non-insulated pipes, over huge distances subsea and subterranean.
Therefore the gas is cool. Where a pipeline comes above ground (an AGI, Above Ground
Installation) the gas in the pipes is much cooler than the ambient temperature and
condensation forms on the pipes.
The BG Transco specifications include a clause permitting the latter alternative, the use of
moisture curing polyurethane or a high sold epoxy. (Section SPA4 in paragraph 10). Three
definitions apply when referring to quantity of water present. Damp, Moist, and Wet
(Paragraph 10).
Damp and moist conditions will allow the use of the materials specified, but wet conditions
require excess water to be removed.
Single pack moisture curing polyurethane’s
Are materials, which use moisture from the atmosphere to cure, not standing water on the
substrate.
Surface preparation as per the specification, then any excess water should be swabbed off,
before brush application of the material.
Because the material cures by using air borne moisture, as soon as the lid is removed from
the can the cure reaction starts.
The more moisture there is presents in the atmosphere, the faster the cure. The criteria with
this type of material is not high RH, 100% is no problem, but low humidity. Some
manufacturers state 35% as minimum RH criteria.
Testing of Paints

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Powder Coating Materials
As mentioned earlier, powder coatings are solvent free materials, which are solid at room
temperature.
The base resin and the chemical activator, along with the other constituents required to
complete the formulation are heated up to the resins melting point, mixed into an
homogeneous liquid, cooled and ground into powder form.
In theory every particle contains all necessary ingredients to affect a cure into a protective
film. The powder can be applied onto a preheated substrate (in the case of substantial steel
thicknesses) at about 240oc, or onto thin plate electro statically and post heated.
In either case the powder melts undergoes a chemical reaction or in approximately three
minutes the reaction is complete.
Thermosetting
Thermosetting means the material will cure with the application of heat and therefore are
convertible or non-reversible materials like epoxy and urethane.
With thick steel sections like underground pipes the powders are electrostatically sprayed
onto a preheated substrate, approximately 245oc.
As soon as the powder hits the heated steel, it melts, undergoes a chemical cure and is fully
cross-linked in approximately three minutes.
This group of materials is used extensively on subsea and subterranean pipes, office furniture
and kitchen white goods.
Thinner plate sections are post heated, after electrostatic application of powder.
Thermoplastic
Thermoplastic materials soften with the application of heat,
Are linear polymer and therefore reversible or non-convertible.
Polyethylene and Polypropalene being examples of these materials.
Usually flame sprayed as repair systems on existing thermoplastic coatings.
Testing of Paints

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Sacrificial coatings
This classification of materials sacrifices itself to protect the underlying substrate.
The sacrificial component must be less noble (more electronegative) than the substrate.
Zinc and Aluminium are the most common materials used to protect ferrous substrates.
Zinc and Aluminium have relatively low melting points and so are commonly used in the
form of metal spray, applied by flame onto structural steel e.g. bridges.
Zinc is used in hot dip galvanising of steel, to totally encapsulate a section.
In this situation the zinc works as a barrier coat initially and undergoes atmospheric corrosion
itself forming corrosion products such as Zinc Sulphates and Zinc Carbonates.
To stop this natural process on the zinc it is usual to paint over the galvanising.
However, if the galvanising is damaged, exposing the steel underneath so that both metals are
in contact with electrolyte, the zinc then starts working sacrificially, corroding in preference
to the steel, producing Zinc Oxides on the damages faces until the damage is filled to exclude
electrolyte contact.
The zinc then works as a barrier again.
If the galvanising suffered damage of more than a scratch or gouge repair might be a better
option. In this instance a zinc rich epoxy might be used.
These materials contain a very high percentage content of zinc pigment. Specifications vary
but 90% by weight of the dry film is a typical requirement.
If moisture, an electrolyte, passes into a film of this nature,
Each particle of zinc needs to be in contact with at least one other,
In order to form the metallic circuit through to the steel for the electrons.
These electrons, in the form of Hydroxyl ions will then return through the electrolyte to the
zinc and the zinc will corrode, sacrificially.
In order to hold the high concentration of zinc particles together, a very strong binder is
required. This is usually an organic epoxy.
Inorganic binders such as Ethyl or Methyl Silicates are zinc pigmented but are primarily
designed for high temperature service and need sealers such as aluminium or carbon
pigmented silicones.
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WATER BORNE COATINGS
Refers to a material, which complies with COSHH Regulations and EPA requirements.
Year by year, stricter regulations are brought into force regarding solvent emissions into the
atmosphere.
For example a 60% vs paint using a hydrocarbon solvent will release 400 cc of solvent into
the atmosphere for every one litre of paint applied irrespective of thinners added and cleaning
solvents used.
Hydrocarbon compounds are known to be harmful to the environment, the ozone layer, and
human life.
Paint manufacturers have therefore taken steps to comply with these requirements by using
alternatives, in the form of Solvent Free, High Volume Solids, and Water Borne.
Many binder types can now be modified to use water among them being.
a) Alkyds
b) Epoxies
c) Polyesters
d) Polyurethane
e) Vinyl’s
f) Acrylics
g) Silicones
Every material has advantages and disadvantages. Water as a solvent, poses no problems with
compatibility over any other material but may prove problematic for adhesion. Abrasion will
almost certainly be required, but generally the following will appertain.
Advantages
1 Water is of a suitably low viscosity for any application method, brush, roller or spray.
2 Water is recyclable cheap, abundant, non-toxic and non-flammable.
3 Water is not harmful to environment, the ozone layer or to mankind.
4 Water can be applied over any existing binder type with impunity.
5 In good conditions several coats can be applied in one working day.
Disadvantages
1 Water usually needs a small amount of a co-solvent for modification.
2 In periods of high humidity drying will be retarded.
3 Needs controlled storage conditions, in low temperatures certain components may come
out of solution.
4 Not as versatile as HC’s for application windows.
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PAINT MANUFACTURE
Part of this manufacturing process is grinding aggregates to a suitable size.
For example a gloss paint with a dry film thickness of 30 um would need an aggregate size of
far less than 30 um, typically 20 um or in some instances 10 um, because an aggregate of
larger size than the nominal film thickness would protrude and deflect light.
Where as an undercoat or mid coat would require a larger degree of grind (some extender
have 40 um particle size to aid with cohesion and inter coat adhesion).
Paint manufacture basically involves three main stages, once all constituents are available.
1 Premixing
Pigment/binder/solvent are mixed in proportions suitable to give a consistency of premix or mill
base.
2 Dispersion or grinding or milling
The actual dispersion or grinding or milling of the paste from the above.
3 The letdown process
Where the remaining amounts of binder/solvent and any other additives are finally mixed prior to
quality checks and canning.
Direct charge dispersing mills
1 Ball mill
A ball mill in a horizontal steel drum, typical dimension 1m diameter x 1½m long,
Which is approximately half, filled with various types of balls.
Steel balls for darker colours and porcelain or selected flint for lighter colours.
The balls are 1" to 1½" diameter.
Mill base is added to the drum until the balls
Are covered, about 50% capacity of the drum.
The hatch is then sealed off and the drum
Started rotating so that the balls cascade down
And do not stick on the
Drum due to
Centrifugal Forces.
Shear forces are applied to the mill
Base as the balls cascade both between
The balls and balls and vessel walls.
A typical dispersion time would be
Overnight for a 50-60 gallon batch.
Testing of Paints
( )
)
Feed hatch
Cascade
angle
Support
frame
Balls and
mill base

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2 Attritor mill
The attritor mill is a vertical version of the ball mill, but more efficient and also static.
Paddles drive the balls. The mill base is continually circulated by pump from bottom to top
and gives adequate dispersion in less time.
Used to be regarded as a fixed charge M/C but largely modified now for continuous use.
3 High speed disperser
Sometimes called a high-speed dissolver.
It is analogous to a large food mixer with
a flat-toothed impeller blade at the end of
a shaft. Dispersion is achieved because of the
Extreme turbulence that occurs at very high shaft
Rotation speeds near the impeller blade.
The mill base produced then undergoes a further
Process in a Bead Mill
(Sand Mill or Pearl Mill are alternate names).
4 Kady and Silverson mills
Both the Kady and the Silverson mills are suitable for rapid dispersion of aggregates in
aqueous emulsions and other water borne material.
5 Colloid mill
Also known as high-speed stone mills, usually fairly small, using stone grinding discs
containing carborundum, approximately 10" in diameter.
The top stone is stationary and the lower stone is rotating fast at speeds up to 3600 revs per
minute.
Gravity fed low viscosity slurry enters the centre of the static top stone and is passed
between the two stones by centrifugal force, where it is subjected to extreme turbulence and
shear forces to affect the dispersion.
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6 The sand mill
Also known as a bead or pearl mill, the sand mill is particularly suited to long production
runs on popular paint colours.
The mill base is pumped under pressure up through the vessel which is partially filled with
sand or other grinding mediums.
Through the centre of the vessel runs a shaft with fixed discs, which causes the abrasives to
be moving constantly.
As the mill base passes through this moving abrasive, it is subject to shear dispersion.
As the paint exits at the top it passes through a fine screen, which retains the abrasive in the
vessel.
A cold water-cooling jacket is needed because of the heat generated by friction.
Dispersion
out
Filter
Screen
Sand
slurry
Slurry in
Testing of Paints
Typical disc

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7 Triple roll mills
Three rollers made from chilled steel or granite, run parallel to each other,
And each one rotates at a different speed, and each contact face passes in the opposite
direction to the adjacent roller.
The gap between them, can be adjusted. These machines need a thick paste like mill base to
operate efficiently.
The mill base is fed into the nip between rollers one and two and the final product is taken
from roller three by means of a scraper bar.
Paste
Figure 9.4 Triple roll mills
8 Single roll mills
This system utilises a single chilled steel roller.
Mill base is gravity fed from a hopper into a small gap between a longitudinal bar and the
rotating oscillating roller.
The material is thus subjected to shear and dispersion.
The bar can be adjusted to control the gap by screws or hydraulic pressure along the length
of the bar.
There are two types of bars which can be operated, a single roll refining bar and a recessed
bar. The final product is removed by scraper bar.
Testing of Paints
Scraper
Apron
Feed
hopper
Pressure
adjustable
bar
Refining bar
Recessed bar

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SURFACE CONTAMINANTS AND TESTS FOR DETECTION
Specifications often request that certain tests are done to ensure that contamination is within set
criteria. Some tests are qualitative and some are quantitative. A qualitative test is one, which
give a result as accept/reject, pass/fail, go/no go, whereas. A quantitative test is one, which
gives a result in known units e.g. milligrams/m2.
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Test for soluble iron salts;
This is a qualitative test; it will not even differentiate between the salts. It will detect the
presence of either Sulphates or Chlorides.
This test is known as the Potassium Ferricyanide test, (Potassium Hexa-cyanoferrate).
Test papers, usually Whatman No3 laboratory filter papers are soaked in a 5 – 10% solution of
potassium ferricyanide and distilled water.
And left to dry. (The result is a lime green paper, fringed with an orange brim).
The area of blast to be tested is sprayed with a fine mist of distilled water.
Left a few seconds to allow the salts, if present, to dissolve and form a solution.
A potassium ferricyanide test paper is then applied to the area and by capillary action draws
up the solution like blotting paper.
If there are any dissolved salts they react with the potassium ferricyanide to form potassium
ferrocyanide. The ferrocyanide is Prussian blue and shows as blue spots on a lime green
background.
Test to detect soluble chlorides;
The test for detecting chloride salts is known as the Silver Nitrate Test.
As with the previous test a solution of silver nitrate, 2% with distilled water, is made and the
Whatman papers cut into strips.
The strips are then soaked in the solution and pressed onto the area under test for about
20 seconds then washed in distilled water.
The reaction between silver nitrate and any chloride salts present produces silver chloride,
which remains on the strip after washing.
If the strip is then dipped into photographic developer the chlorides show up as
black/brown.
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Other tests for salts
1 Merkoquant;
Swabbing an area makes a salts/water solution
Of 150 mm x 150 mm with distilled water, 22.5 ml.
Merkoquant strips are then dipped into the solution
And the resulting colour change is compared to
A master chart on the container.
The concentration is read off from the chart.
Bresle sample patch;
Reported as being 95% accurate. An adhesive patch with a
rubber diaphragm is stuck onto the surface and distilled
water injected and extracted several times to produce
a solution of any salts present. A process of
Mercuric Nitrate Titration can detect concentrations
of 15 mg/m2. A quantitative test.
2 Salt contamination meters;
Salt contamination meters measure the resistivity
or conductivity Of a given sample and convert this value
into a Concentration (mg/m2).
With any of the above tests, if the amount of salts present
is greater than specified, the area should be washed down
with copious amounts of clean water, reblasted and retested.
3 Test to detect the presence of millscale;
Millscale being cathodic in relation to steel can cause corrosion cells under a paint film and
subsequent early disbondment. Millscale in small quantities is permitted on a SA 2½ blast
standard, but not on an SA3. Therefore the test needs to be carried out only if the specification
requires an SA3.
By naked eye Blasted steel is dark grey in colour and millscale is dark blue, so the contrast
is difficult.
If the surface is sprayed with a fine mist of slightly acidic copper sulphate solution, the
solution ionises and tints the steel copper colour and blackens the millscale, if present, thus
providing a better contrast.
If this test indicates millscale presence then it should be reblasted and then retested.
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4 Test to detect the presence of dust on a substrate;
Any dust on a blasted substrate will adversely affect the adhesion of a paint film.
In conditions of low relative humidity, dust and finings passing down a blast hose
become electro statically charged and stick onto the substrate.
Brushing or air blowing the surface will not remove them, self adhesive tape however,
will.
If a piece of self adhesive tape is stuck onto the surface and snatched off, the
dust/finings sticks to the tape.
By then sticking the tape onto white paper the dust can easily be seen.
5 Test to detect the presence of moisture on a substrate;
Presence of moisture, even in the teeniest amount, can affect the choice of paints and if work can
be done or otherwise.
A very simple test for the presence of moisture is to sprinkle with talc or powdered chalk
and then lightly blow away. The powder will stick to areas where moisture is.
6 Test to detect the presence of oil or grease;
Other than ultra violet light,
Dropping solvent onto the suspect area, and absorbing the solution on Whatman or
blotting paper can detect oil and grease.
The solvent will evaporate and oil or grease will give a darker appearance.
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TESTING OF PAINTS FOR PROPERTIES AND
PERFORMANCE
BG Transco Specification No PA9, lists a number of tests, (and required results),
which a paint must be subjected to and comply with before acceptance as a
material suitable for use on a BG Transco site.
BS 3900, Methods of test for paints,
Is the British standard, which details these tests, for method of test and equipment. It is
subdivided into groups of tests from group A, tests on liquid paints (excluding chemical tests),
through to group H, which covers defects and rating of. The following tests are to PA9
requirements.
Tests done on paint
Determination of volatile, non volatile
This test, done to BS 3900 part B2, can only be a guide and not 100% accurate.
It relies on solvent evaporation from a test sample.
As soon as the can is opened the evaporation will start. A typical procedure would be.
· Select a clean, dry glass-stirring rod and watchglass, and weigh on a sensitive balance to the
nearest milligram.
· Place onto the watchglass approximately 2gm of paint and weigh again.
· Place the watchglass with paint into a hot air oven, no naked flame or element; repeatedly stir
to drive away the volatile content.
· Take a final weight of the glass, rod, and dry paint and simple calculations will give
volatile/non volatile ratio by weight.
Testing of Paints

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Flash point determination
As per BS 3900 part A9, using a closed Abel cup (as opposed to the open cup).
Flash point is defined as being “the lowest temperature at which solvent vapour from the
product under test in a closed cup, gives rise to an air/vapour mixture capable of being
ignited by an external source of ignition” and is a safety factor.
A high flashpoint material is safer than a low flash point material and would be determined
as follows.
· Add solvent to the Abel cup, replace lid with thermometer and agitator in place.
· Clamp the Abel cup onto a retort and lower into a water bath.
· Gently heat the water bath, which will in turn heat the solvent under test.
· Every ½oc rise in temperature activates the high frequency spark.
· The flash point temperature is reached when a blue flame flashes over the solvent.
An orange flame signifies that the flashpoint temperature has been exceeded and the test
should be redone.
Thermometer
Figure 10.1 Abel cup
Paint density
Testing of Paints
Agitator
Spark
electrode
Retort
Water bath

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TECHNOLOGY
25-08-14 - 58 -
Defined as being weight per unit of volume, density is calculated by weighing a know
volume of material and using the formula: -
Density = Weight
Volume
1cc (cubic centimetre (cm3)) weighs 1 gram
1 litre (1000 cc) weighs 1 kilogram
A density cup with a capacity of 100 cc is used for measuring density of paint. Other names
referring to the same cup are: -
1 Relative density cup
2 Specific gravity cup
3 Weight per litre cup
4 Weight per gallon cup
5 Pyknometer
Figure 10.2 Density cup with lid chamfered to centre vent on underside
Procedure for use
· Weigh the clean, empty cup and the lid on a metric scale,
Sensitivity ± 0.1gm.
· Fill the density cup with the paint, to within approximately
2mm of the brim.
· Allow any entrapped air bubbles to burst and replace the lid slowly and firmly until it seats
firmly on the shoulder of the brim.
· The chamfer in the lid allows air to be expelled as; the lid is replaced, followed by paint
over the required 100cc volume. If no paint is expelled remove the lid and add more.
· Wipe off any excess paint from the vent and weigh the filled cup.
· Deduct the weight of the empty cup from the final weight and divide by 100.
· The answer is the density in grms/cc.
From information given on the materials data sheet and calculated density of
the solvent it is possible, but difficult, to calculate the percentage of any added
solvent, although better and easier ways exist. This piece of equipment
however can be used in calculating if a 2-pack material has been mixed in the
correct proportions.
Testing of Paints

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