(1) *CORROSIONxa.yimg.com/kq/groups/22734920/433820896/name/Painting... · Web viewOf 150 mm x 150...

WORLD CENTRE FOR MATERIALS JOINING TECHNOLOGY

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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: -2eH2O2H+ + 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 2004Corrosion 1 Copyright 2003, TWI Ltd


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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.

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.


E

A C


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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. VALUESGraphite + 0.25 vSilver - 0.1 vNickel 200 - 0.15 vCopper - 0.35 vMill Scale - 0.4 vMild Steel - 0.7 vAluminium Alloys - 0.9 vZinc - 1.0 vMagnesium - 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.



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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 cleanliness2. 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 Cleaningb) Water Blastingc) Hand and Power Tool Cleaningd) Flame Cleaninge) Picklingf) Vapour Degreasingg) 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.)

Painting Inspection Grade 3/2. Rev 1 April 2004Testing of PaintsCopyright 2003, TWI Ltd 4


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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 GritSteel ShotGrit and Shot MixGarnet

Walnut ShellCoconut ShellEggshellCorn Cob HuskPeach 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 particlesB- Hardness of the materialC- Density of the materialD- 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 J 444 SIEVE SYSTEMUSING THE J 444 SIEVE SYSTEM.. System it represents 1/24

" = approximately 40 thou. New BS ref. 7079 pt EPARTICLE SIZE DISTRIBUTIONEPARTICLE SIZE DISTRIBUTION 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.

a) By measuring – using gauges with and without replica tape.b) By assessing – using surface comparators.


Peak to trough

HackleRogue Peak

Lamination or Sliver


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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.

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 Calculated. This type of gauge is not ideally suited for curved areas such as pipes.


Foot

Plane for zero

Distance traveled by needle from zero = profile depth

Needle


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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.

Figure 2.4 Cross section of a replica tapeThe 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 um25.4 um = 0.001" 40 Thou" = 1 mm25.4 mm = 1 inch

Micrometer is reading 93 um; subtract 50 um for testex plastic backing. The surface amplitude is therefore 43 uPainting Inspection Grade 3/2. Rev 1 April 2004Testing of PaintsCopyright 2003, TWI Ltd 10

Mylar tough transparent Polyester plastic

Testex PastePaper

Testex

10 um

2 um


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Figure 2.6 Metric micrometer for testex measurement in microns

Figure 2.7 Imperial micrometer for testex measurement in 1000 of an inch

Reading the gauges.


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 Thou0.0001"

1 Thou

0.001"

Testex(Allow 2 Thou

(0.002") for plastic backing

Testex(Allow 50 microns 0.05 mm for plastic backing

1100 mm

10 microns

100 microns

0.10 mm

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 division0.002 mm = 2 microns / small division0.001” = 1 thou / small division0.0001” =1/10 thou / small division

Useful conversion factors are: -

1 mm =1000 um1 thou = 25.4 um

25.4 mm = 1 inch2.54 cm = 1 inch

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



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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 Eye2 Visual Aid, not exceeding 7x magnification3 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’

Preparation of steel substrate before application of paints and related productsRust 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.



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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 GradesBefore 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 NACESa 3 White Metal SP5 Grade 1

Sa 21/2 Near White Metal SP10 Grade 2Sa 2 Commercial Finish SA6 Grade 3Sa 1 Light Blast and Brush of SP7 Grade 4

Equipment;

1. Wheelabrators;



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*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.

2. Air Blasting;

Site blasting is normally carried out using expendable abrasives and open blasting systems. Open blasting systems operate using.a) A compressor.Painting Inspection Grade 3/2. Rev 1 April 2004Testing of PaintsCopyright 2003, TWI Ltd 15


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b) A pot containing the abrasives.c) Vapour Traps for oil and water (knock out pots).d) A hose, usually carbon impregnated.e) A nozzlef) 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.

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 asPressure reducers, cause turbulence and wear; so external



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Couplings should be used. Hose diameter is related to nozzle size and should have an internal diameter at least three to four times the nozzlediameter.

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.

3-Water Blasting;

Advantages;

* Using water is more environmentally friendly than open blasting.



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* 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.

High pressure water blasting up to 30 000 psi (water jetting);

Water usage is about 60 litres per minute.Painting Inspection Grade 3/2. Rev 1 April 2004Testing of PaintsCopyright 2003, TWI Ltd 18


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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.

Water usage with this method is approximately one to one and a half litres per minute, which is sufficient to control dust.



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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.

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.

Figure 2.8-pH scale


0 1 2 3 4 5 6 7 8 9 10 11 12 13 14AlkalineAcid


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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 colors 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 hullsRoad Marking - To give white or yellow lines on roadsFire Proofing - To provide resistance to fireHeat Resistant - For surfaces working at high temperaturesAnti-corrosive, and many more.

Paints can be classified by binder type.By color.By the pigment type.

The paints contain the same basic ingredients.

1. Binder2. Pigments and other additives3. 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 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)

Requirements, 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 thickness) 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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BinderThe 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. Dielectric 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 minimize 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 combinationsSolvent strength in descending order Common Names Binders

Water

Emulsions PVC/PVAVinyl’s

Acrylics – other materials e.g. epoxy

Bitumins, Polyurethane’s,Alkyds, Acrylated Rubbers

Aliphatic Hydrocarbons

White SpiritTurpentine

Turpentine substituteSolvent naphtha’sHexanes upwards

Natural oilsNatural resins

AlkydsPhenolics

Aromatic HydrocarbonsXyleneTolueneBenzene

Chlorinated Rubber

KetonesAcetone

Methyl Ethyl KetoneMethyl 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”. In the case of most paints the main constituents of the polymers are: -

H - HydrogenC - CarbonN - NitrogenO - OxygenCl - Chlorine

Although there are variations the main three polymer types are Linear, Branched and Cross-linkedPainting Inspection Grade 3/2. Rev 1 April 2004Testing of PaintsCopyright 2003, TWI Ltd 28


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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

METHANESATURATED

ETHANESATURATED

ETHYLENE OR ETHYNEUNSATURATED

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.

Figure 4.2 Ethylene molecules close together

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: -

Figure 4.3 Ethylene molecules polymerise


H H| |C...... C| |

H H

H H| |C...... C| |

H H

H H| |C...... C| |

H H

H H| |C...... C| |

H H

H H| |C C| |

H H

H H| |C C| |

H H

H H| |C C| |

H H

H H| |C C| |

H H


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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.

Figure 4.4 Branched polymers


|C H|

H C H|

H C HH|

C = C - C = C - C| | | | |

H H H H H

OH

O

OxygenAnother chain


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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 oilsSemi drying oilsNon 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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PigmentsPigments 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 / 10th 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 phosphatef) Barium metaborateg) 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.

Coloring 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 Black2. Compound of Cobalt Blue3. Compound of Chromium Greens, Yellows and Oranges4. Compound of Iron Browns, Reds and Yellows5. Compound of Calcium Reds and Yellows6. 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 clayChalk Calcium carbonateTalcum Magnesium silicateSlate flour Aluminium silicate



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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.

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.

PVCThe 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.


Theoretical Leafing layers

/ __ __ / ____ \ ___ _____\ \ / /___ / __ \ ___ ___ \ / ___

Practical De-lamination


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Figure 4.9 – The CPVC is exceeded and all particles are not wetted, the film would be porous, low in cohesive strength and adhesion.

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 upb) 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.



07-05-23 - 43 -

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 SystemsPipelines 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.



07-05-23 - 44 -

Powder Coating MaterialsAs 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.

ThermoplasticThermoplastic 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.



07-05-23 - 45 -

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.



07-05-23 - 46 -

WATER BORNE COATINGSRefers 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) Alkydsb) Epoxiesc) Polyestersd) Polyurethanee) Vinyl’sf) Acrylicsg) SiliconesEvery 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.



07-05-23 - 47 -

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.


( )

)

Feed hatch

Cascade angle

Support frame

Balls and mill base


07-05-23 - 48 -

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.



07-05-23 - 49 -

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

Typical disc


07-05-23 - 50 -

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.

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.


PasteScraper Apron

Feed hopper

Pressure adjustable bar

Refining bar

Recessed bar


07-05-23 - 51 -

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.

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.



07-05-23 - 52 -

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.



07-05-23 - 53 -

If this test indicates millscale presence then it should be reblasted and then retested.

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.



07-05-23 - 54 -

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.



07-05-23 - 55 -

Take a final weight of the glass, rod, and dry paint and simple calculations will give volatile/non volatile ratio by weight.

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.

Figure 10.1 Abel cup


Agitator

Spark electrode

Retort

Water bath

Thermometer


07-05-23 - 56 -

Paint density

Defined as being weight per unit of volume, density is calculated by weighing a know volume of material and using the formula: -

Density = WeightVolume

1cc (cubic centimetre (cm3)) weighs 1 gram1 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 cup2 Specific gravity cup3 Weight per litre cup4 Weight per gallon cup5 Pyknometer

Figure 10.2 Density cup with lid chamfered to centre vent on undersideProcedure 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.



07-05-23 - 57 -

Relative density or specific gravity

The density of distilled water is known to be 1gm/cc and the density of any other material can be calculated as above.

Relative Density or Specific Gravity is in effect comparing the density of another material with that of water using the formula: -

SG or RD = Density of xDensity of water

Because Relative Density is comparing, and giving a value of “times heavier than”, there are no units of value, but the digits will be exactly

the same as density.

Example

If five litres of paint weight 7.2Kg, what would be the density?

Step 1 Convert units to gms and cc’s

5L x 1000 = 5000 cc 7.2Kg = 7200gms

Step 2 Density = WT = 7200 gmsVol 5000 cc’s

Step 3 Perform calculation = 1.44 grms/cc

Therefore SG or Relative Density would be 1.44

Example for 2 pack ratio calculation

A two-pack epoxy is mixed at a ratio of five parts base to two parts activator, the given densities of which is pack A 1.25 gm/cc and pack B 0.97 gm/cc. What is the density of the mixed paint?

Five parts base at 1.25 gm/cc = 6.25 gms

Two parts activator at 0.97 gm/cc = 1.94 gms

Therefore total weight = 8.19 gm

Total volume for weight = 7 cc

Density of mix = 8.19 = 1.17 gms/cc 7



07-05-23 - 58 -

Hegman grind gauge

The Hegman grind gauge, also called a fineness of grind gauge.Is used to measure the degree of dispersion of paint. With gloss paint a perfectly smooth surface is required.The Hegman Grind Gauge is a stainless steel block approximately

17.5cm x 6.5cm x 1.4cm and is highly polished.Two grooves, or on some gauges one groove, are precision ground tapering from 100 um deep to zero along almost the total length of the gauge. A 10um increment scale is engraved along the length of the groove.Paint is added to the deepest point of the scale and drawn along to totally fill the groove using a specially profiled scraper bar.The specification BS 3900 requires that within three seconds of this operation the scale should be placed so that the eye looks almost parallel along the groove, To observe a point along the groove where, within a 3mm band, five to ten aggregates break through the surface of the paint. This actually, looking at the stated angle is the point where the surface will change from gloss, at the deep end, to matt at the shallow end.

Figure 10.3 Hegman grind gauge

Along the groove, at some point, the aggregates will rest along the bottom and protrude through the surface giving a result as below.

Figure 10.5 Aggregates protruding and resting on the bottom


3mm band

GlossMatt

Aggregates protruding

Aggregates on the bottom

0 20 40 60 80100

10 8 6 4 2 0

Cross-section below

AA

GlossMatt


07-05-23 - 59 -

Viscosity Viscosity is defined as being a fluid resistance to flow.A liquid of a high viscosity has a high resistance to flow; it will not run easily.And conversely, a low viscosity fluid runs very easily.An increase in temperature (or decrease) can have a severe effect on a fluid’s viscosity.As the temperature increases the molecules within the paint gain more molecular freedom, move more easily and thus reduce viscosity. A typical recommended temperature is standard laboratory temperature of 20oc 0.5oc.

There are several types of equipment available for measuring viscosity but they mainly fall into two categories.

1 Rotational Viscometers 2 Flow Viscometers

1 Rotational viscometers

Rotational viscometers rely on a paddle, disc or ball rotating in a liquid to measure the viscosity. The rotation can be driven by an electric motor, which gives Dynamic Viscosity measurements, or by falling weights, which gives Kinematic Viscosity measurements.

a) Dynamic viscosity

For dynamic viscosity measurements a rotothinner can be used

Figure 10.6 RotothinnerThe rotothinner, a flat circular disc with four holes drilled through it, is fixed into the chuck of the rotational viscometer (not unlike a pillar drill) and lowered into a 250-millilitre can containing the fluid under test.The can is magnetically attached to a spring-loaded conical shaped base.When the disc enters the can, a micro-switch engages the motor and starts the disc rotating. When the rotating disc enters into the paint the frictional forces between the disc and the paint molecules and the can cause the can to rotate, When the two equalise the can will stop rotating and a reading can be taken from the pointer on the scale on the conical base.


Poises


07-05-23 - 60 -

The systems international (SI) units For dynamic viscosity is, newton-second per square metre (N.s/m2)

Although on many machines the poise is still used (cgs. unit). Poise has ten subdivisions called centi-poise.

Water has a viscosity of approximately one centi-poise. One poise is equal to one dyne second per cm2.

b) Kinematic viscosity

Figure 10.7 Krebs stormer viscometer

Kinematic viscosity is measured using a Krebs Stormer Viscometer.The weight is allowed to fall, which in turn causes the paddle to rotate in the paint. More weight added results in a higher rotation speed.Weights are added until the rotation speed is 200 rpm as measured either with a digital display counter.A viscosity unit frequently used for kinematic viscosity is the stoke and centi stoke. A fluid having a viscosity of one poise and a density of 1 gm/cc has a viscosity density ratio of one stoke. (Krebs units or poise can also be used.)

c) Flow viscometers (Flow cups)

There are various types of flow cups e.g. Zahn and Frikmar, used for hot fluids, Ford, ISO and DIN used for ambient temperature materials. The flow cup is machined from Aluminium, has a capacity of 100cc, And is fitted with a stainless steel nozzle at the bottom with

Various orifice sizes, in millimetres. 4mm hole size is standard, and known as A Ford Flow Cup No4. The cup is mounted on a special stand, and has a lid with a bubble

Spirit level. The triangular base of the Stand has one fixed foot and two screws

Adjustable feet


Weight


07-05-23 - 61 -

A typical procedure for use would be: - Ensure that the equipment and paint temperatures are at 20oc 0.5oc.

1 Level off the equipment using the bubble level and adjustable screw legs.2 Put the lid to one side when levelling is complete.3 Place a suitably sized receptacle under the orifice (greater than 100cc).4 Place a finger over the nozzle orifice and fill with the paint to be tested, up to the brim,

leaving a convex meniscus.5 Using a straight edge (a ruler) quickly scrape excess material into the overflow rim on the

top of the cup.6 Simultaneously start a stopwatch (or use sweep second hand) and remove finger from the

nozzle.7 The paint will run from the orifice in a continual stream. At the first distinctive break in

the stream i.e. when it drips, stop the watch. The time in seconds is recorded as the viscosity, at the measured temperature.

Thinners added to paint over and aboveRecommended quantities could also be determined

By viscosity. To do this a sample containing

Maximum amount permitted (by manufacturers TDS) is prepared, and compared to samples taken From the operators at the point of application.

Using the flow cup, if the operators sample Runs through the cup faster than the reference Sample, then more thinners than allowed has Been added.

To find the exact percentage added, small amounts can be added to the reference sample until operator’s sample and reference sample run through in the same time.

Should the operator’s sample take longer than the reference sample, and then there is no problem. Thixotropic paints cannot be measured using a flow cup.



07-05-23 - 62 -

FILM THICKNESSES

Wet film thickness measurement; Wet film readings should be taken immediately after application, in order to obtain true readings (solvent starts to evaporate away as it exits the spray tip). WFTs can be measured by using either an eccentric wheel, or comb gauges.

1 Eccentric wheel

An eccentric wheel is a steel disc, machined to cut two grooves leaving three rims.The centre rim is machined smaller than and eccentric to the two outer rims.The inner rim is called the Eccentric Rim and the two outer, the Concentric Rims.

To use the wheel it should be placed on the surface with the zero at the six o’clock position, rolled through 180o in one direction, back to the zero and then 180o in the opposite direction, back to zero. The concentric outer rims will be wet for the full circumference, but the inner rim,

The eccentric rim will only be wet for part of the circumference,having left and re-entered the film on two occasions. The wetfilm thickness value is taken by transferring (mentally) the interface between wet and dry on both sides of the eccentric rimInto a value from the scale. The average of the two values is theWFT of the paint film.It should be noted that the eccentric wheel could only be usedon flat plate.On a pipe, for example it would be used circumferentially.

2 Comb gauges Painting Inspection Grade 3/2. Rev 1 April 2004Testing of PaintsCopyright 2003, TWI Ltd 62

Degree of eccentricity, 250 um normal

0

125

250

125


07-05-23 - 63 -

Assuming use of the SS gauges, four gauges will each have two working ends covering eight different WFT ranges.

Above each tooth is engraved a value ‘thou’ on one side and its equivalent in microns on the other side.

This represents the value of the gap from tooth end to substrate when the gauge is place firmly, perpendicularly onto the substrate.

When the gap under the tooth is full of paint it will wet the tooth. When not full it will not wet the tooth.

A procedure for this operation would be: -

a) Select the appropriate gauge with the smallest increment rise tooth to tooth.

b) Apply the gauge firmly, perpendicular to the substrate into the paint film ensuring that the two end lands are firmly on the substrate.

c) Withdraw the comb gauge and look at the teeth.

d) Two values should be recorded. The number above the last tooth wetted by the paint and the value of the next highest not wetted.

The WFT is not an absolute value but ‘in between’.NB Comb gauges should be used longitudinally on curved surfaces e.g.

pipes.

Figure 10.9 Comb gauge


Paint

Substrate

25 50 75Wet

Wet

Not wetRecorded as 50/75

Wet


07-05-23 - 64 -

WFTs can be calculated by using the following formulae, according to information given.

WFT = 100 x DFTVS

WFT = V = VolumeA Area

Figure 10.10 Contraction from evaporation

Dry film thickness;

The specification for a painting contract will state DFT criteria for each coat of paint applied. As it is the inspector’s main function to ensure that work is carried out to specification, he/she should perform as many checks as needed to ensure that the specification criteria is met. The DFT value can be determined by one of four methods.

1 Test panels2 Calculations3 Destructive test gauges4 Non destructive test gauges

1 Test panels;

Test panels are usually 150mm square plates of the same material as the component being processed. The plates undergo the same operations at the same time as the main components. Mainly used for destructive tests e.g. adhesion, they can also be used for DF. T checks.


SolventWFTSolvent

Binder

Pigment, Extenders and others

DFTVolumeSolids %

Solvent %


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2 CalculationsUsing certain formulae and information given on a materials data sheet;1 WFT = V

A3 DFT = WFT x VS

1 100

2 WFT = 100 x DFTVS 1

4 VS% = DFT x 100WFT 1

1 WFT = VA

Question:If 12 litres of paint were used to cover an area of 10m x 10m what would be the average WFT?Answer:Step 1 WFT = Volume = 12 Litres = 12 L

Area 10 x 10 100 m2

Step 2 Change to units common to volume and area = cm2 cm3

12 L x 1,000 = 12000 cm 3 = 0.012 cm100 m2 x 10,000 1,000,000 cm2

Step 3 Multiply x 10,000 (um/cm) = 120 um

2 WFT = 100 x DFTVS 1

Question:What WFT would be needed to give 50 um DFT using a paint with a VS% of 65%? Answer:WFT = 100 x DFT = 100 x 50 = 5000 = 76.92 um

VS 1 65 1 653 DFT = WFT x VS

1 100

Question:What would be the DFT if a paint with a VS content of 45% was applied at 120 um WFT?Answer:DFT = WFT x VS = 120 x 45 = 5400 = 54 um

1 100 1 100 1004 VS% = DFT x 100

WFT 1Question:What would be the VS% of a paint if it was applied at a WFT of 110 um and the DFT was 63 umAnswer:VS% = DFT x 100 = 63 x 100 = 57.27%

WFT 1 110 1



07-05-23 - 66 -

3 Destructive test gauges

These types of gauges cause damage to the film, which then needs to be repaired. (Micaceous Iron Oxide), MIO is magnetic and would cause error in the reading when using a magnetic gauge. A PIG, paint inspectors gauge is a type of destructive gauge.A reference line of a contrasting colour is drawn on the painted surface to be tested.A blade is tightened into a special slot in the PIG; pressure applied to force the blade through the paint to the substrateAnd then cut across the reference line, leaving a damage about ⅜" It is then possible to examine the damage through a focusable microscope. Measurements can be taken by means of a scale engraved on one of the lenses.

The dimensions taken from the graticule scale at this point are not in any units, as the angle of the cutter used alters, so will the representations of the graticule.A chart is supplied with each gauge, and blades of different angles.If for example the chart indicates Blade No3 will be ground to x angle, can be used on thickness less than 500 um, multiply graticule reading by 1.8. 20 unit of graticule scale would then convert to 20 x 1.8 = 36 um.

Other commonly used destructive gauges are the Ericson Test Drill and Saberg Thickness Drill. The damage caused with this is circular.


Blade

View

Damage

Reference line

View through lens with graticule scale


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4 Non destructive test gauges This category of gauges is the most widely used and can be subdivided into Electronic and Magnetic.a) Electronic

The electronic gauges work mainly on two principles. Electro Magnetic Induction and Eddy Current. The Electro Magnetic Induction is suitable for ferro-magnetic substrates Eddy Current is suitable for non ferro-magnetic substrates. Modern electronic gauges are sometimes supplied with probes suitable for both situations, and the gauges automatically change function according to the fitted probe. Both types are for measuring non-ferro magnetic coatings. Accuracy ½ %.

b) Magnetic

This classification of gauges works with permanent magnets, no batteries. Tinsley Pencil or Pull of Gauge. Sometimes called a foreman's gauge is suitable for spot checks and is not very accurate, even on modern gauges of this type 15 % accuracy is quoted.It looks very much like a pen and indeed is sometimes fitted with a pocket clip.It has a permanent magnet attached to a spring.The tension of the spring can be adjusted so that the gauge can be calibrated to work over a variety of thicknesses.

Figure 10.12 Cross section of Tinsley pencil


Screw to adjust tension

Spring

Cursor line

Permanent magnet

Scale


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The Magnetic Horseshoe gauge is a very old type of gauge still favoured for measuring hot surfaces such as metal spray.

Accuracy often quoted as better than 10% and as for all magnetic gauges, It is suitable for use in hazardous areas. This gauge works by measuring the change in magnetic flux between two magnetic poles at

the bottom of the gauge. The flux change is brought about by the thickness of the non-magnetic coating. The gauges are supplied in a wide variety of scales and are calibrated like all magnetic

gauges.

Figure 10.13 Magnetic horseshoe gauges


Lock/unlock

Knurled wheel for calibration


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The Magnetic Coating Thickness gauge, known colloquially as the 'banana gauge', measures non-ferromagnetic coatings over ferromagnetic substrates

It can, according to the manufacturer be used under water. This type of gauge relies on spring tension to break the magnetic attraction of a permanent

magnet to a ferromagnetic substrate. Because spring tension doesn't have a linear function the scales on the gauges are in

logarithmic increments. When calibrating for use it is therefore of paramount importance to calibrate using a shim as

near as possible to the paint thickness. Modern gauges of this type often quote 5 % accuracy. Procedure for calibration to BS 3900 PT C5 (now ISO 2808).

(BG Transco specify calibration on a prepared surface, therefore a plate with the same substrate surface finish as that to which the paint is applied, should be used).It is extremely important to remember that should the gauge be calibrated on a flat plate, the reading on a blasted surface would take from approximately ⅔ of the depth of the profile, giving values of up to 50 um more than the actual 'over the peak' value.

1. Select a plastic shim (magnetically insulated) as near as possible in thickness to that of the paint to be measured.

2. Place the shim centrally on the calibration plate, as detailed above.3. Locate the magnet in the gauge onto the shim, apply a light pressure to ensure that the heel

doesn't wobble or rock, and wind the scale wheel on the gauge fully forward to release all tension on the spring allowing the magnet to attach to the substrate.

4. Wind the wheel slowly back, clockwise, tensioning the spring until the magnet detaches. At this point the movable cursor on the gauge is adjusted so that the red line on top of the cursor is in line with the thickness value of the shim as shown on the scale wheel.

The gauge is now ready to use.

Some 'banana' gauges do not have a movable cursor. Instead these have a fixed cursor, moulded into the case, and a movable scale, and to calibrate these gauges, the value of the shim on the scale wheel has to be moved to the cursor.



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Tests for mechanical properties on paint films

Abrasion resistance;

Ericson, Taber and Gardner are just three of many companies who manufacture specialist equipment for testing paint films.A materials resistance to abrasion can be tested using a Taber Rotary Abraser.Discs painted with the material to be tested are rotated under special abrading wheels.Sand paper or carborundum. Periodically the samples can be checked for thickness or damage inflicted.

Hardness

The hardness of a film can be tested by many methods including the Buchholz Indentor and the Sward Hardness Rocker, but one of the most frequently used for hard coatings is the Koenig Albert.

A pendulum with two spherical fulcra is free to swing on a plate painted with the material under test.

The number of swings is counted electronically. (If the fulcra penetrate the surface, more resistance will reduce the number of swings).

Flexibility BS 3900 E1

Standard panels are coated with material to be tested and bent around cylindrical mandrels of various diameters.

The flexibility of a coating is expressed as the smallest diameter mandrel over which the paint will not crack when bent.

A conical mandrel with a uniform taper from 3 mm diameter to 37 mm diameter is frequently used now.

The conical type needs only one sample to achieve a result whereas the straight mandrels need a plate for each mandrel.

Impact resistance

Each generic type of coating material used has it's own impact resistance requirements, as measured, in joules.Tubular impact testers are commonly used for this test.A weight, typically 1 Kg. is lifted up the tube to the height required and held in place by a retaining collarA painted sample is fixed under the tube. By rotating a ring within the collar the weight is released and falls onto the sample, which is then assessed for damage.Two types of test can be done, direct impact and indirect impact. Direct being onto the painted side of the sample and indirect on the non-painted side.



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Accelerated testing

Normal weathering tests are a simple process of hanging out painted panels, facing south, on an ' A' frame and periodically testing for colour retention, chalking, water absorption etc. over a period of years. However new products ready to go on the market cannot wait years for test results. The manufacturers may have spent many thousands of pounds on Research and Development of the product, and will want some return. Accelerated tests can be done which reduce testing time to months by accelerating or intensifying the conditions to which the paint will be exposed.

Some typical test cabinets used for testing specific conditions are: -

1-Humidity cabinets

For testing tropical conditions. Humidity is very high at 95% and elevated temperatures up to 55oc.

2-Salt spray cabinets

For checking paint ability to withstand salt laden environments.

3-Water soak tests

Allowing painted panels to be submerged to test for water absorption, by weighing before and after submersion.

4-Temperature cycling

Painted samples are subjected to constant temperature cycles from hot to cold. Paints in common with most materials expand and contract according to temperature. Constant expansion and contraction can result in cracking. Maximum and minimum temperature can be set and cycle time, over a running period of 1000 hours, as an example.

5-Prohesion testing

Painted sample plates are cut with a pre-damage in the form of an X, 50 mm each incision length. A 3% saltwater solution is sprayed onto the plate for 60 minutes and stopped for 60 minutes, at a constant 35oc.

The cycle continues for 1000 hours.On examination after the 1000 hours.There shall be no blistering or undercut outside of a 3 mm boundary on each side of the pre-damage.



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Drying and curing testsTwo tests to determine the drying time are: -

1-Ballotini test

Ballotini, tiny spheres of glass, or sometimes sand is trickled onto a newly painted block graduated in hour of traverse, e.g. 24 hours for the block to traverse full length under the funnel.After a specified time the block is removed, tipped onto its side, tapped lightly, and examined.The position of the last grain of sand or ballotini sticking to the surface is recorded as the drying time at that temperature e.g. 20oc 0.5oc.

Figure 10.14 Ballotini test

2-BK drying recorders

The BK gives more information than the Ballotini, which Purely indicates drying time.

The BK defines also the stages of drying. E.g. Solvent Evaporation Time, The SolGel Transition, Surface Drying Time and Final Dry Time.

Needles (stylus) are fixed to motor driven wires which Then traverse over the full length of painted glass strips 300 mm x 25 mm, in pre set times of 6, 12, 24 hours.

The needles can also be weighted if required.When the paint is wet the needle will penetrate through to

The glass.As the solvent evaporates the needle will start to cut a continuous track in the film, as drying

progresses it will cut an interrupted track, until finally dry when no scratch is visible.


2 4 6 8 10 12 14 16 18 20 22 24


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Other tests

A-Mechanical thumb test

This is a test for even through drying of paint. It simulates pressing a thumb onto a surface and applying

a twisting motion. A cam drives a weighted shaft with a semi spherical rubber end cap, allows it to drop onto the painted

plate, rotate through 270o, then lifts it off again.The plate is then visually inspected for tearing, pulling

, wrinkling etc.

B-Pencil scratch test (Wolff-Wilborn)

Pencils are graded in degree of Blackness B and degree of Hardness H. HB being the middle of the range.

Higher the number and harder or blacker the lead is. A sharpened pencil is fitted into a special steel block and pushed along the surface, starting with e.g. 3H and working up 4H, 5H etc.

The first pencil to scratch the paint lends its hardness value to the paint e.g. 5H.

C-Mechanical scratch test

A stylus with various added weights is drawn across the painted surface. The weight that causes the surface to be scratched gives its value to the hardness e.g. 500 gm.

D-Gold leaf test

A test for residual tack. A small square of gold leaf is lightly pressed onto the surface of the paint. The gold leaf is then pealed off and the area examined with a magnifying glass. No residual gold leaf should remain.

E-Thumbnail test

A quick test for hardness is to try to penetrate the paint film with the thumbnail.If the thumbnail penetrates, the film is “cheesy”.



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F-Opacity

The opposite of transparency, a test to determine the ability to hide (cover) the substrate.The following are methods to determine the wet film opacity, a combined function of pigment

concentration and refractive index, using cryptometers. Commonly used cryptometers are Pfund Cryptometers and there are two types.

1 Trough type

A wooden block with a tapered sunken trough in the middle, the bottom of which is formed by chequered black and white glass squares.Paint is added at the deep end and scraped along to fill the trough.Looking perpendicular onto the trough, find the point where the underlying square can just no longer be seen. (Look at the squares offering the biggest contrast to the paint colour).A scale running along the groove will indicate the depth of the groove at that point, and is recorded as a wet film thickness.

Figure 10.15 Pfund cryptometer, trough type2 Black and white fused plates

The second type is a black and a white glass square, fused together.On each square is an engraved scale starting at zero on the joint.Paint is applied onto the square with the most contrast and a “top plate” of clear glass placed in

position with the tapered contact edge exactly on the fusion line.Small “feet” at the end of the top plate allow a tapered film of paint to form under the glass top

plate.Look for a point along the film where the underlying black or white plate can just no longer be

seen. Note the value on the scale and multiply by the constant on the top plate.


A A

Section on A - A

70 60 50 40 30 20 10

10 20 30 40 50 60 70

Black Square White

Square Glass plate in position


07-05-23 - 75 -

Figure 10.16 Black and white fused plates H-Hiding power charts and micrometer adjustable film applicator

Various designs of black and white A4 signed cards are used for this method.Chequered, striped, zigzag, half and half and cards of ones own design can be used. The surface is coated with a solvent resistant lacquer to prevent immediate absorption.The applicator is a frame with an adjustable gate, which can be controlled by two micrometers

for vertical movement.After zeroing on a flat surface the reading on the micrometers represents the gap under the

gate.Paint is applied onto one chart and the bar applicator immediately drawn over it. If opacity is not achieved (as previous) the gate is adjusted 5 um higher and the operation

repeated on another card until the film thickness required for opacity has been attained.

I-Degree of Gloss

Gloss is a measure of reflectivity. Light follows general rules and travels in a straight line.When light hits a surface it reflects off at the same angle as it strikes the surface.A modern gloss meter works on exactly this principle, a light source directs a beam of light onto the surface under test, and a photo electric cell, set at the same angle, collects the reflected light and quantifies it and converts it digitally into a percentage of the incident light.On a perfectly smooth surface it would give almost 100%. On an uneven surface some of the light is deflected and so the percentage reading would be lower.A high percentage of reflection will be gloss and a low percentage will be matt.Gloss meters for general use have two common angles, typically 60% and 20% both taken from the perpendicular, the 60% angle being the most common usage.

Figure 10.17 Degree of gloss


Incident light

Light scattered

Uneven surface

Incident light

Smooth surface

Photo electric cell

Reflected light


07-05-23 - 76 -



07-05-23 - 77 -

J-Adhesion

One of the properties required of a paint film is to ‘provide adhesion to the substrate’, Therefore an inspector is expected to test to ensure the paint is performing this function.

There are three main areas for adhesive failure within a paint system.

a) Primer to substrate failure

b) Inter-coat adhesion (between films)

c) Cohesive failure (within a paint film) SPECIALSPECIAL

CUTTER CUTTER

a) Primer to substrate failure

Primer to substrate failure is the most serious.

Failure here means no protection at all.

This is a surface contamination problem mainly.

Lack of adequate surface preparation, grease, oil, dirt, and dust are the usual causes.

b) Inter-coat adhesion

Caused by the problems above and others.

Lack of observance of recommended over-coating limits and expansion/contraction differences between materials.

c) Cohesive failure

Over thickness of a layer can entrap solvent during the drying process and thus stop polymerisation and the correct formation of the film, reducing cohesive strength.

The main reason for cohesive failure is solvent entrapment but incorrect ratio mix of a two pack can have exactly the same effect.

These failure points can be detected in several ways, some costly, requiring equipment costing several hundred pounds and some requiring an outlay of a few pounds only.



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K-‘V’ cut test

A craft knife is all that is required to perform this test.Cut through the paint, to the steel substrate, with two cuts forming an inclusive angle of approximately 30o, with leg length of approximately 13 mm.Insert the tip of the blade into the tip of the ‘V’ and try to lever off. The paint should chip across the tip of the ‘V’ clearly and cohesive without following the line of any of the faults described.It should not expose any of the substrate.

L-Cross cut (cross hatch test)

Cut through the paint using six horizontal and six vertical cuts approximately 2 mm spaces giving a 25-squared grid.Special profile cutters can be purchased for this, or a craft knife can be used.Apply an agreed tape to the area (different tapes have different degrees of stickiness and would give different results); rub smoothly onto the hatched area and then snatch off.The resulting areas of disbondment are then compared to diagrams shown in BS 3900 Pt E6 and classified according to percentage area of disbondment.

M-Dolly test

The dolly test is more expensive to use, but unlike the above gives answer in units of psi or newtons/um square, etc and so is classed as a quantitative test.

A typical procedure for the test would be: -Ensure the test area is clean and oil/grease free, lightly abrade the area and apply mixed two pack heavy duty adhesive.Firmly place the aluminium alloy dolly in position onto the adhesive ensuring that the skirted flange is to the adhesive.Leave for manufacturers recommended cure time.Place the core drill supplied around the dolly and cut through the coating to the substrate (this ensures that only the area of the dolly flange receives the pull off forces). Apply the pull off gauge and apply pull off force, (some models use a ratcheted lever, others a knurled wheel) until failure occurs.This will usually involve a loud bang and the instrument will ‘jump’ from the substrate. Examine the face of the dolly and apportion adhesive failure according to areas exposed, at the pull off force indicated on the scale.

For example with an aluminium metal spray, single coat, there could be: -

1. Adhesive to dolly failure.

2. Adhesive to aluminium failure.

3. Cohesive failure within the aluminium.

4. Aluminium to substrate failure.



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N-Hydraulic adhesion test equipment

This is a much quicker test with a higher degree of accuracy. The HATE use cyano-acrylic impact adhesives and can

usually be done approximately two hours after dolly/adhesive application, the dolly’s are mild steel and reusable because they are heated up to destroy the adhesive after use.

Big downside for this test is initial cost and usually high maintenance.

O-HOLIDAY/PINHOLE DETECTION

Holidays and pinholes in a paint film are defects which allow ingress of an electrolyteNot all defects of this nature are visible to the naked eye and we therefore need equipment to facilitate the detection.For coatings of thicknesses above 500 um it would be necessary to use a high voltage holiday detector,But for coatings of less than 500 um it is normal to use a wet sponge pinhole detector.The wet sponge pinhole detector is a very simple piece of equipment and consists of a small control box, usually pocket size, with two terminals, positive and negative.The negative terminal is connected to bare steel on the structure to be tested. The positive terminal is connected to a hand stick with a sponge on the end. Two, 1½v batteries in the control box provide the operating power.To use the detector the sponge electrode is wetted in water with a tiny amount of detergent/washing up liquid added, and squeezed out to remove excess water.After switching on and selection of operating voltage, the sponge is traversed methodically over the area.On a vertical surface it is better to work upwards.On contact with a pinhole, the wetting agent (detergent) allows immediate penetration of the water, so providing a very low resistance circuit back to the control box.A high-pitched bleep indicates the presence of a pinhole, the exact position of which is located by using a corner of the sponge. The position is then marked ready for repair.

Voltage setting

Basic models have two options for setting, 9v and 90v.More sophisticated models have an intermediate setting. For DFTs of less than or equal to 300 um the 9v setting is normal. For DFTs of 300 – 500 um 90v or 67½v intermediate sensitivity would be preferred.



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SPECIFIED COATING CONDITIONS

A manufacturers product data sheet will indicate under which ambient conditions a paint/coating can or cannot be applied.The client’s specification may sometimes be a little stricter.

A typical specification used to be: -

“It is not permissible to apply paints

1. During rain, snow, or high winds”. This clause would be sensible even in modern specifications.

2. When the air or metal temperature is down to within 3oc above the dew point temperature”. It can be overridden by giving alternate systems.

3. When the air or metal temperature is below 5oc”. Solvent evaporates very slowly at low temperatures and chemical cure rates used to be static.

4. When the relative humidity is more than 90%”. Still a very common restraint, and sometimes the benchmark for using moisture curing polyurethane’s.

Relative Humidity

“The amount of water vapour in the air expressed as a Percentage of the amount of water vapour, which could Be in the air at that same temperature”. 100% humidity, saturation, is measured as being taken Within 1" of the surface of a fast flowing river.

Dew Point

This is the temperature at which water vapour in the air will condense. Condensation cannot occur unless the relative humidity is 100%. Recalling that every 11oc drop in temperature results in the airs capacity to hold water halving, even the smallest drop in temperature results in water being released from the air, in the form of condensation.

So at 100% humidity the air temperature and dew Point temperature, and wet bulb temperature on the Whirling hygrometer is all the same value.



07-05-23 - 81 -

Q-The Whirling Hygrometer, Aspirated Hygrometer or Psychrometer

Used by coating inspectors to determine wet and dry bulb temperature readings.

Using calculators or hygrometric tables, relative humidities and dew points can be calculated.

Two thermometers are mounted in a plastic frame, fitted With A handle so that the frame can be rotated through the

Air. One of the thermometers is fitted with a wick around the bulb. The wick passes through a hole in the end of the frame and into A small container with a Screw lid, into which is put distilled Water or clean rainwater i.e. de-ionised water. The water is Drawn by capillary Action all along the wick out the area Enveloping the Thermometer bulb. This is referred to as the wet bulb and The second thermometer is the dry bulb.

The frame with the thermometers mounted should be rotated quickly about a horizontal axis. (The BS 2482 states in front of and to windward of the operator) so that the bulbs pass through the air at 4m/sec. If there is a wind the operator should face into the wind, if no wind then walk slowly into a clean air current.

The frame should be rotated for 30 – 40 seconds, or as otherwise specified, as fast as possible (to meet requirement as above) and then read the values on the thermometer, always the wet bulb first, immediately on ceasing rotation.

The water on the wet bulb uses heat energy from the air to change into water vapour, so the wet bulb will give a lower temperature reading than the dry bulb.

When rotation stops, the aspiration rate slows and so the wet bulb temperature will slowly start to rise towards that of the dry bulb.

This operation should be repeated as many times as is necessary until the following criteria is met. On two consecutive spins the readings should be within 0.2oc, wet bulb to wet bulb and dry bulb to dry bulb.

The wet bulb and dry bulb temperatures recorded can then be used to determine the RH and DP from scales or tables.

This operation should be carried out as near as possible to where the work is being done. Big difference in temperature can occur from N side to S side of a tank or down a trench and topside.



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Steel temperature measurement

The air temperature (ambient) is the temperature recorded from the dry bulb thermometer.To measure the steel substrate temperature a magnetic gauge, known commonly as a limpet gauge is used, or a digital thermometer, thermocouple, sometimes called a touch pyrometer.

MAGNETICMAGNETIC, CONTACT , CONTACT THERMOCOUPLETHERMOCOUPLE



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PAINT APPLICATION

The three main basic methods are: -

Brush application;

Roller application;

Spray application

Brush application Roller applicationRelatively slow Enable paint to be applied quicklyLabour intensive Do not give a uniform coating thickness

Uneven thicknesses Leave a distinct pattern known as roller stiple

Environmentally friendlyLess waste material

No spotting or over spray damage Extension rollers increase accessSynthetic fibres have smooth surfaces and

are of uniform thickness for the full length.

Worked paint into the profile

Available in several materials mohair, lambs wool and sponge

Curved rollers are supplied for pipe work

Doesn’t work the paint into the substrate

Not suitable on internal corners, welds, toes, bolts, rivets and plate over laps

Spray application;

Paint spray equipment can be divided into two distinctly different types. Conventional spray. Airless spray.



07-05-23 - 84 -

Conventional spray; 1 Suction feed

The paint container is underneath the gun, usually aluminium about one litre capacity, and the paint is drawn up by venturi principle to the gun.

2 Gravity feed

The paint container is above the gun and paint feeds to the gun by gravity.

3 Remote pressure pot

Figure 13.1 Remote pressure potRemote pots are supplied in several sizes and have the advantage of

Having a much greater capacity than the above Much bigger areas can be painted before refilling is required. A container (pot) is charged with paint and then sealed with a lid. Air from a compressor is fed into the top of the pot and the paint is forced out through a line to the gun.At the gun, when the trigger is operated, a tapered needle is drawn back opening the aperture, out of which the paint exits in a continual stream.Approximately 25 mm in front of the aperture, two air channels, from lugs on the cap, diametrically opposed, blow air to converge at the paint stream.At this convergeance the paint is atomised into very minute droplets, and conducted onto the workpiece.

Figure 13.2 Conventional gun


Paint

Air in Pressurised air volume

Paint out to gun

Air

AirNeedle

Trigger


07-05-23 - 85 -

Airless spray

With an airless spray the fluid (paint), is pressurised by means of a pump. Electric motor pumps and hydraulic pumps are sometimes used but the most common is

the pump operated by compressed air. These units operate by increasing the compressed air inlet pressure by a stated ratio, e.g.

35:1, by means of two pistons on a common shaft. For instance, if an air driven piston has a surface area of 35 Square inches and is exposed

to a pressure of 100 psi, a piston at the other end of the shaft with a surface area of one square inch will exert a pressure of 3500 psi. As the piston is driven down to pressurise the paint, the one-way valve at the paint inlet is

forced to close position and the paint out port is opened. When the piston reached the bottom of its stroke, the air circuit reverses and forces the

piston back upwards. As this happens the outlet port is closed and the inlet port opens to refill the cylinder with

paint. At the top of the stroke the air circuit reverses again and drives the piston down again.

The outlet pressure can be adjusted by reducing the inlet pressure from the compressor.These systems are called airless because air is not used for atomisation.Atomisation occurs by forcing the paint at extremely high pressure, usually 2000 to 2500 psi through a very small aperture, 12 to 23 thou diameter, into a volume of air offering a resistance to the paint flow.As the air and paint meet, the paint atomises.Most tips used on airless spray equipment have a facility for reversing the flow of paint through the tip.Blockages can then be cleared by turning the tip through 180o, triggering to ground or a container to clear the blockage, then reverse the tip again to its original position.A type of airless spray tip exists with an adjustable aperture size, called a Titan Tip. Pigments and extenders, especially MIO and metallic pigments can be quite abrasive and the tips are subject to wearSome are sleeved with tungsten carbide to give a loner life.


Lubricant packing

Paint pressurising piston

Paint inlet

One-way ball valves

Paint to gun

Compressed air in

Air driven piston


07-05-23 - 86 -

Data sheets for a product will recommend spraying pressure and tip sizes although each sprayer will have his/her own preference. Typical recommendations would be: -

Paint Type Tip Size Pressure PSIChlorinated Rubber 13 – 21 thou" 2400

High Build Epoxy 17 – 23 thou" 3000Zinc Rich Epoxy 17 – 23 thou" 2800

Notable differences

Conventional AirlessSlow application due to fluid delivery. Excellent application rates.

Low air pressure 40 – 75 PSI Can need 100 PSI to operate the pump.

Delivery pressure greater than 20 PSIDelivery pressures greater than 6000 PSI,

dependant on pump ratio.

Need special paint containers. Uses manufacturer’s containers.

Guns can be unwieldy, two lines to supply the gun. Single line supplies pressurised paint.

Basic equipment needs very little maintenance. Needs more maintenance due to high

pressure and moving parts.

Easier to clean after use.

Equipment needs flushing well to remove all traces of paint. Expensive

replacement.

Safety considerations Always observe manufacturers recommendations. Wear recommended safety equipment. Always depressurise the system before even minor maintenance. Regularly check a fluid line for wear and leaks. Ensure that swivels and couplings are properly tightened. Always engage the safety catch when the gun is not in use. Never point the spray gun at yourself or other people.

Electro-static spray; Both liquid and powder paints can be electro-statically applied.For liquid paints a small air driven turbine is mounted on the gun and supplies a current to the tip. The current is usually on a thumb control for adjustment and operates in the region of 85 kV.Powder paints in general are charged electro-statically by spraying the powder through an area of ionised air.



07-05-23 - 87 -

In either case the component to be coated is earthed into the same circuit and thus becomes negatively charged.The coating material is positively charged and is attracted to the component.As the coating thickness increases it has an insulation effect and the coating material is then drawn to other charged areas.The voltage can control the thickness, especially when using powder coatings. Wastage is significantly reduced and it produces a more uniform coating.Electro-static application is widely used in industry for components such as kitchen white goods, office cabinets and line pipe. (When powers are used the components are either pre heated or post heated. Line pipe and other substantial section components can be pre heated, but thin steel plate components will not maintain sufficient heat and so are electro-statically coated and then post heated).

Other paint application methods Industrial anti-corrosion systems are generally applied by the systems discussed previously, however various other accepted methods exist. Viz.

Dip coating - A component is dipped into paint and hung to dry.

Padding

- Mainly DIY Pads of mohair or foam are used to apply paint. Large pads like plaster hawks for large areas and small ones (about 25 mm square) for cutting in around door furniture and putty lines on windows.

Hot spraying- When paint is heated it reduces in viscosity (flows easier) and the cure

or drying starts quicker. It is therefore easier to apply and wets out better, and reduces the need for solvent addition.

Spin rotating

- Usually called a spinner, the equipment, consisting of a three-legged frame, each with a wheel at the end and a centrally mounted spinner is drawn along dispensing paint from the spinner. Ideal for internal coatings on line pipe sections.

Flow coating/ curtain coating

- Bitumen and Coal Tar enamel pipeline coatings, when used, are applied hot, about 200oc, to the 12 o’clock position of a pipe, the material flows down both sides to meet at 6 o’clock. The material being thermoplastic hardens as it cools and coats the pipe.

Aerosols - Pressurised cans operated by push buttons, car paint touch up kits among other used.



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METAL COATINGS Galvanising;

The coating of components with zinc.Many components both for offshore and onshore use are galvanised.Galvanising can give protection to steelwork for periods of up to 60 years dependant on exposure conditions.The components are chemically cleaned (acid), washed and fluxed, then totally immersed in a vessel containing molten zinc at approximately 450oc. When drawn out, the zinc solidifies at an average thickness of approximately 100 um.

Sheradising;

Nuts and bolts and other similar components are coated with this method.Galvanising threads would make a significant difference to the dimensions and workings of fixings and fasteners, so zinc powder, just below the melting point, is used instead. After cleaning the components are tumbled in the powdered zinc, impact fuses the zinc onto the components and in effect, “cold welds” the powder onto the metal.

Calorising;

Calorising is coating with aluminium. Aluminium has a melting point of 625oc as apposed to 425oc of zinc so it is not really practical to tumble. One way of calorising a component is to dip it into molten aluminium. The resulting exothermic reaction is so severe that is alloys the aluminium with the steel. Calorising can also be done by immersing a component in a mix of fine sand and aluminium powder and heating.

Electro-plating;

This is done by electrolytic deposition.If a current is released from an item into a metal salt solution through to a cathode, the metal salts ionise and deposit the metal ions on the cathode bar.

Hot metal spraying;

Any metal, which can be easily melted, can be sprayed. Zinc and aluminium are the most commonly used metals for spraying.They are both below steel on the galvanic list and so will provide cathodic protection to the steel, and both metals have a reasonable low melting point.Both metals have advantages and disadvantages, for instance zinc performs far better than aluminium in rural areas and alkaline environments.



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Aluminium is considered to be superior to zinc in slightly acidic environments and because of its higher melting point is more widely used on high temperature surfaces such as exhaust stacks, compressor exhausts etc. where extremely high temperatures are encountered.It is specified for use on surfaces with working temperatures of up to 540oc.Application of metal-sprayed coatings can be carried out by any of the following methods.

Powder system;

Powdered metal is fed into a heat source (usually butane or propane and pure oxygen burning) and propelled onto the substrate. Using this method a relatively low proportion of the metal powder is actually deposited on the substrate.

Electric arc system;

This method is ideal for production line type facilities such as gas bottle production and lamp standards etcWhere components are of a uniform shape and the process can be mechanised. As in a welding process the metal (to be sprayed) acts as an electrode in a circuit and the electrode melts. The molten metal is atomised and blown onto the component by means of a heated air jet. This system gives a superb fine grain finish.

Wire and pistol system;

By far the most common and widely used method for site application of metal spray.The metal wire, of a very high degree of purity, greater than 99.5%, is driven through a gun by means of two knurled wheels powered by compressed air.As the wire, 3mm – 5mm passes through to the front of the gun it passes through a ring of burners, with the flames focused about 35mm from the exit point. The fuel gases used are butane/propane and pure oxygen.The flames melt the wire and droplets of metal are propelled to the steel by the combustion gasses and compressed air.The coating is usually applied at a thickness of 100 – 125mm and is about 85% to 95% density of the original wire. This is because the resulting film is in an open cell structure due to individual particles forming a “fish scale” like structure, the interstices between the particles are not all filled.If the coating is to be subjected to high temperature services it will need sealing with a silicone sealer, aluminium or carbon pigmented.If however the metal spray is applied to give an extended major maintenance free life to an anti-corrosion system, then either an epoxy sealer or etch primer would be applied prior to the specified system.

Anodising; A treatment for aluminium, anodising is an electrolytic method of coating which results in the formation of a dense oxide.The component is immersed in a weak acid bath and oxidation is induced electrically.



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COATING FAULTSAs defined in BS 2015, Glossary of Paint and Related Terms.

Bittiness

The presence of particles of gel flocculated material or foreign matter in a coating material, or projecting from the surface of a film.

Note 1 The term seedy specifically denotes the presence of bits that have developed in a coating material during storage.Note 2 The term peppery is sometimes used when the bits are small and uniformly distributed.

Bleeding

The process of diffusion of a soluble coloured substance from, into, or through a coating material from beneath, thus providing an undesirable staining or discolouration. NB. Examples of materials, which may give rise to this defect, are certain types of the following materials: - bituminous paints, wood preservatives, oleoresins from wood knots, organic pigments and stains and coal tar. Bitumen and Coat Tar Enamels also.

Blistering

The formation of dome shaped projections or blisters in the dryfilm of a coating material by local loss of adhesion and lifting of the film from the underlying surface.

Note 1 Such blisters may contain liquid, vapour, gas or crystals.

Bloom

A deposit resembling the bloom on a grape that sometimes forms on the floss film of a coating causing loss of gloss and dulling of colour.

Chalking

The formation of a friable, powdery layer on the surface of the film of a coating material caused by disintegration of the binding medium due to disruptive factors during weathering.



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Note 1 Chalking can be considerably affected by the choice and concentration of pigment.

Cissing

The formation of small areas of the wet film of a coating material where the coating material has receded leaving holidays in the film.

Cracking

Generally the splitting of the dry film of the coating material usually as a result of ageing. Specifically a break down in which the cracks penetrate at least one coat and which may be expected to result ultimately in complete failure.

a) Hair cracking

Cracking that comprises of fine cracks, which may not penetrate the top coat, they occur erratically and at random.

b) Checking

Cracking that comprises of fine cracks, which do not penetrate the topcoat and are distributed over the surface giving the semblance of a small pattern.

c) Crazing

Cracking that resembles checking but the cracks are deeper and broader.

d) Crocodiling/alligatoring

A drastic type of crazing producing a pattern resembling the hide of a crocodile or alligator.

e) Mud cracking

A network of deep cracks that form as the film of a coating material dries, especially when it has been applied to an absorbent substrate. Mud cracking is associated primarily with highly pigmented water borne paints.



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Cratering

The formation of small bowl shaped depressions in the film of a coating material.

Curtaining/sagging

A downward movement of a coat between application and setting that results in an uneven area of coat having a thick lower edge. The resulting sag is usually restricted to a local area of a vertical surface and may have the characteristic appearance of a draped curtain.



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a) Run

A narrow downward movement of a coat that may be caused by the collection of excess quantities of paint at irregularities in the surface e.g. cracks and holes, the excess material continuing to flow after the surrounding material has set.

b) Tear

A small run resembling a teardrop.

Dry spray

The production of a rough or slightly bitty film from sprayed coating materials where the particles are insufficiently fluid to flow together to form a uniform coat.

Efflorescence

A whitish crystalline formation on bricks, mortar, plaster etc. Usually forms under paint films on the above substrates, and is not a paint defect. Caused by soluble salts crystallising on the surface.

Flaking

Lifting of the coating materials from the substrate in the form of flakes or scales.

Flocculation

The development of loosely cohesive pigment agglomerates in a coating material

Grinning through

The showing through of the substrate due to the inadequate hiding power of the coating material.



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Holidays

A defect due to faulty application techniques seen as areas where the film of a coating material is of insufficient thickness or where there is a complete absence of coating materials on random areas of the substrate.

Lifting

Softening, swelling or separation from the substrate of a dry coat as a result of the application of a subsequent coat.



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Orange peel

The uniform pock marked appearance, in particular of a sprayed film, resembling the peel of an orange due to the failure of the film to flow out to a level surface.

Pinholing

The formation of minute holes in the wet film of a coating material that forms during application and drying, due to air or gas bubbles in the wet film which burst, giving rise to small craters that fail to coalesce before the film has set.

Residual tack

The degree of stickiness remaining in the film of a coating material which, although set, does not reach the true tack free stage.

Ropiness

Pronounced brush marks that have not flowed out because of the poor levelling properties of the coating material.

Saponification

The formation of soap by the reaction of a fatty acid ester and an alkali.

Note 1 In painting practice, saponification refers to the decomposition of the medium of a film by alkali and moisture in the substrate, e.g. new concrete or rendering based on cement, sand and lime. Saponified films may become sticky and discoloured. In very severe cases the film may be completely liquefied by saponification.

Wrinkling/rivelling

The development of wrinkles in the film of a coating material during drying. Usually due to the initial formation of a surface skin.



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C O L O U R

Colour can affect mood and perception, and can create illusions.

White light, light emitted from the noonday sun is a combination of electromagnetic wavelengths from 400 nanometers to 700 nanometers, blue through to red. When white light strikes an object, certain frequencies are absorbed and others reflected. It is the reflected frequencies that the human eye translates into colour. Colour has three attributes, which are: -

1 Hue

Refers to the basic colour e.g. red, yellow, green and blue. Can be represented in circle form, clockwise, red yellow green blue red.

2 Brightness

Sometime called lightness, it refers to the amount of lightness or darkness of the colour. The degree of reflectivity of the surface receiving the light governs this property and is sometimes called value or reflectance value.

3 Saturation

How vivid colour appears. It is measured in terms of the difference of a colour from the neutral grey with the same degree of brightness. Lower saturation, greyer the colour. The terms chroma and intensity, and sometimes weight, are also used.

Black and white and the greys in between are called “achromatic” colours, they lack hue and saturation. Anything perceived, as having colour is “chromatic”.



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The three attributes can be related to a three-dimensional model of a helix.

Figure 16.1 Three-dimensional helix

The Munsell colour system

An American system, which identifies colour by its three attributes, Hue Chroma and value (Reflectance value).

In the Munsell system, Hue is divided into five basic colours Red, Yellow, Green, Blue and Purple each identified by its initial letter, with a second dimension between each, giving ten basic colours. Value is defined in eleven steps from white to black and chroma has fifteen steps.


Red

Red purplePurplePurple blue

Blue

Blue green

Green

Green yellow Yellow

Yellow red

White

Black

Saturation

Brightness

Hue


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The BS 4800 colour system

This BS specifies 100 colours selected from 237 used in the BS 5252.

The BS 4800 uses the same basic colours but expands to thirteen, including a neutral. The colours are numbered from 02 to 24, 00 being neutral, achromatic, using even numbers only.

Lightness is identified by capital letters A to E, where A is maximum lightness and E is minimum lightness. The chroma is given by number, the third part of the coding, from 01, in single digit rises to 56. The higher the number, the stronger the colour.

The BS 5252, framework for colour co-ordination for building purposes

The BS 5252, framework for colour co-ordination for building purposes, selects a framework of 237 colours as a source for all building colour standards and a means of co-ordinating them. It is not itself a rang of colours.



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HEALTH AND SAFETY

Control of substance hazardous to health regulations 1988 generally abbreviated to COSHH regulations.

These regulations provide a framework to help to protect personnel at the workplace against health risks from substances, which are hazardous.

For the purpose of COSHH regulations, substances hazardous to health include.

a) Substances or preparations listed as being toxic, very toxic, harmful, corrosive or irritant in part 1A of Chemicals (Hazard Information for Packaging) Supply.

b) Substances with MEL or OES as detailed in schedule one of COSHH or if Health and Safety Commission has approved an OEL.

c) Harmful microorganisms.d) Dust of any kind in substantial concentrations.e) Any other substance creating comparable hazards to peoples health such as pesticides and other

chemicals used on farms.

Hazard warning symbols

- Black symbol of skull and crossbones on an orange square with the words Toxic or Very Toxic printed below

- Black diagonal cross on an orange square with the words Harmful or Irritant printed below.

- Black symbol showing a tilted test-tube dripping onto a hand with a chunk out, adjacent to a test tube dripping onto a stone flag. Orange background with the word corrosive printed below.


NToxic or Very Toxic

Corrosive

Harmful or Irritant


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Responsibilities

It is the employer’s duty to assess the risk to employees on his/her premises and any other premises, which might be visited during the execution of duties. Training establishments are responsible for trainees.

It is an employer’s duty to prevent, wherever possible, exposure to hazardous substances, but if it is not reasonably practical to totally prevent exposure then protective clothing, masks etc. should be issued to minimise exposure.

COSHH regulations require that regular monitoring should be carried out and records kept, particularly in situations where there could be serious risk to health if control measures were to fail or deteriorate.

Guidance note EH 40 (occupational exposure limits), is a document published by the HSE, which lists all substances known to be hazardous to mankind. It gives details in table form of common names, chemical formulae and chemical names of hazardous substances.

The Hydrocarbon solvents used in modern paint formulations are hazardous to health and are listed in EH 40.

Xylene is one such solvent and has an Occupational Exposure Limit (OEL) of 100 ppm (parts per million). This means that air containing more than 100 ppm would be considered to be a hazard to the health of personnel exposed to it. There are two categories of OEL.

1 Maximum Exposure Limit (MEL).

“The maximum concentration of an airborne substance, averaged over a reference period, to which employees may be exposed by inhalation under any circumstances and is specified, together with the appropriate reference period, in Schedule one of COSHH.”

2 Occupations Exposure Standard (OES).

“The concentration of an airborne substance, averaged over a reference period, at which, according to current knowledge, there is no evidence that it is likely to be injurious to employees if they are exposed to inhalation, day after day, to that concentration, and which is specified in a list approved by HSE.”

When referring to reference periods above, long term exposure limits are averaged over an eight hour reference period and short term exposures over ten minute reference periods.

If the EH 40 specifies that a substance have an MEL then the quoted figure must not be exceeded at any time, but kept as low as is reasonably practical.



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With an OES it is permissible to exceed the stated figure provided that the average over a reference period is below the stated figure. Exposures

OEL examples of some solventsSolvent Name OEL in ppmAlcohol’s Methanol

Ethanol200

1000Ethers Ethyl Ether

Isopropyl Ether400250

Esters Methyl AcetateEthyl Acetate

200400

Ketones AcetoneMethyl Ethyl Ketone

750200

Aromatics XyleneToluene

10050

Aliphatics White SpiritHexane

100500

Chlorinated Hydrocarbons 1.1.1 TrichloroethaneTrichloroethylene

350 ab100 ac

Dräger tube and Dräger bellows

Figure 17.1 Dräger tube

One way of monitoring the toxicity of the air is by dräger tube and dräger bellows.

The dräger tube is a glass tube about 110 mm long with moulded nipples at each end. One half of the tube is filled with chemical crystals (sensitive to the material testing for) and are held in position by fine wire mesh plugs. A cellophane sleeve, incorporating a scale in ppm is wrapped around the tube. There is also an arrow on the sleeve indicating the way in which the tube is to be inserted into the bellows.


a = MELb = Maximum short term exposure 450.c = Maximum short term exposure 150.

N =

5

Dräger


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The bellows are hand operated and are a one way air system, as the bellows are depressed, air is expelled from a slot at the back, when released, air is drawn in through a small rubber grommet like aperture at the front. The bellows incorporate two compression springs and stops, and two retaining chains, so that every depression and release exchanges an air volume of 100 cc exactly.

Figure 17.2 Cross-section of dräger bellows

Using the tubes and bellows

Using a special fitting situated on the bellows, the nipples are snapped off both ends of the tube, which is then inserted into the aperture on the bellows in the direction indicated by the arrow. The crystals should be adjacent to the bellows. The bellows are then depressed and released according to the number expressed as n =, as written circumferentially around the centre of the tube. Each depression and release slowly draws 100 cc of air through the open end of the tube, through the crystals and into the bellows. As the air containing the hazardous material passes into the crystals, a chemical reaction takes place, resulting in a colour change in the crystals. The extent of the colour change along the scale is recorded in ppm.

NB. Many variation of crystal combinations exist for monitoring a variety of different toxicants, all have a different requirement for number of depressions and different colour changes. The tube for monitoring the concentrations of Xylene needs five depressions and the colour change is from white to reddish brown.


Limiting chain

Discharge valve

Front plate

Break-off husk

Pump head

Sieve


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Some materials in common use in the coatings industry do not evaporate into gas or fumes, they remain instead as tiny particles of solids suspended in the atmosphere. Dräger Tube cannot therefore detect materials of this nature. They are quantified by the unit’s milligrams per cubic metre rather than ppm.

Three materials, which fall into this category, are Asphalt, Coal Tar and Isocyanates. Asphalt is considered to be fairly safe with an OEL of 5 m/gm per m3. Isocyanates are very toxic with an MEL of 0.02 m gm/m3.


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