bgas-cswip -painting 3-2.pdf

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 its passive state

When the two electrons are released the atom still has its 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: -2H2O2H+ + 2O.

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++ + 2O 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. like slag on a weld. 3-When the copper metal is run off the slag is Rapidly cooled in cold running water which causes it to shatter. 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. If the situation justifies Cleansing units are available to extract contamination So that the material can be reused, usually up to three times. Doesnt 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 doesnt 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 inspectors duty to ensure that the specification requirements are met. This can be done in two ways.


Peak to trough

HackleRogue Peak

Lamination or Sliver


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a) By measuring using gauges with and without replica tape.b) By assessing using surface comparators. The dial gauges are still very often used. The dial gauges fall into two categories, Surface Profile Needle Gauge and Dial Micrometers with Replica Tape.

i Surface Profile Needle Gauge. The gauge is applied to the blasted substrate and the needle can be felt to locate a trough. Then by applying a slight pressure to allow the flat foot of the gauge to sit firmly on the peaks of the blasted substrate, the needle will pass into the trough as far as it can

Surface profile needle gaue.

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 Painting Inspection Grade 3/2. Rev 1 April 2004Testing of PaintsCopyright 2003, TWI Ltd 9

Foot

Plane for zero

Distance travelled by needle from zero = profile depth

Needle


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



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ii Dial Micrometer and Replica Tape;

Replica tape, or Testex, is also sometimes called corn plaster 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 11

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



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Assessing a profile to BS 7079 Pt C ISO 8503.1

Grit and shot abrasives produce different surface profiles, therefore two comparators are specified. One for grit blasted profiles, G. and one for shot blasted profiles, S. When a mix has been used then the reference comparator should be G. In all instances the entire area should be

blasted to SA21/2 or SA3 grade.

Use of the comparators;There are three methods, which can be employed to assess the roughness characteristics of blast cleaned steel.1 Naked 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 FineCoarser than coarse. Any profile above the upper limit for Coarse



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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.

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



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Equipment;1. Wheelabrators;

*Wheelabrators, sometimes known as (centrifugal blast units) they are ideal for long production runs on similar section components such as pipes, or bridge steelwork.

They are usually referred to the number of wheels which they operate e.g. 6 wheel. The operators of these machines prefer shot as an abrasive. The abrasive is gravity fed into the centre of the wheel. Centrifugal forces carry it to the end of the impeller where it is impelled at the component to be cleaned at a speed of 220 mph app. in a fan pattern. The fast moving metallic abrasive shatters millscale cuts a profile etc., and eventually, its energy spent, drops. The floor of the unit is open grating over a V shaped pit, in the bottom of which is a rotating screw which carries the spent abrasive plus detritus into a hopper. A conveyer system then carries the abrasives to the top of the machine, dispenses it, to start a gravity fed path back to be re-used. As an integral part of the system the abrasive passes aver a tilted plated, known as a weir plate. As the abrasive and detritus cascades over the edge of the weir plate, a current of air is drawn through it. This draws out low density materials such as rust, millscale, flakes of paint etc., and finings, abrasive worn so small that it is no longer useful.

This is known as an Air Wash Separator, The same principle is used in enclosed grit blasting pens. Meanwhile the cleansed abrasive is fed back into a common hopper with feed lined to all the wheels, to be re-used. As mentioned previously new abrasives need to be added periodically to maintain an adequate working mix.

Advantages;

1-The quality can be controlled by adjusting the feed roller speeds 2-Because the system is totally enclosed there is efficient use of abrasives.3-More operator safety because the operator is not involved.4-The systems can be far more productive (dependent on supply of components) than open blasting.

Disadvantages;

One major problem is access to bolt pockets, gussets and stiffeners etc. Because the wheels are fixed, there is no manoeuvrability, and thus shadow areas arise. One way to avoid this is manually blast difficult areas prior to machine blasting.



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2. Air Blasting;

Site blasting is normally carried out using expendable abrasives and open blasting systems. Open blasting systems operate using.a) A compressor.b) A pot containing the abrasives.c) Vapour Traps for oil and water (knock out pots).d) A hose, usually carbon impregnated.e) A 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.



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d) Carbon impregnated Hose;

Because pressure drops along the length of the hose, line lengths are better restricted to around seven to eight metres. Internal couplings reduce the hose diameter and act aspressure reducers, cause turbulence and wear; so external couplings should be used. Hose diameter is related to nozzle size and should have an internal diameter at least three to four times the 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 " nozzle will need 103 cfm to maintain 100 psi, Where as a " nozzle will need 413 cfm. Therefore big nozzle, large bore hose, needs

high capacity compressor. * Sometimes the nozzles are lined with tungsten carbide or ceramics to reduce wear.

* The venturi shaped nozzle give a larger blast pattern with a more even spread of abrasives and higher velocity of the particles at approximately 450 mph.

* The straight bore nozzle gives a small concentrated area of abrasive contact with a fringe area of lower concentration and particle speed of around 200 mph.

* The stand off distance for both types varies according to hose size and nozzle aperture size, but an average figure is around 450mm.

f) Safety to 1GE SR 21;

Safety considerations are.i The hose should be carbon impregnated to reduce the chance of the operator getting

electric shock from static.ii A dead mans handle should be under direct operator control for his/her own safety.iii Hoses should be kept as straight and as short as possible to avoid kinks, and blowouts

and to maintain pressure at the nozzle.iv Use reinforced hoses if possible.v Use external bayonet type couplings.vi Maintain operating pressure at 100 psi.vii It is necessary to have warning signs advising that abrasive blasting is in progress,viii Correct protective clothing should be worn by the operator, including direct air fed

helmet, with adequate visors, leather aprons and gloves, boots and ear protectors.ix Warning buntings segregating the area.



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3-Water Blasting;

Advantages;

* Using water is more environmentally friendly than open blasting.

* From the safety aspect, spark free. They are ideal for removal of soluble salts, sulphates and chlorides, (the hygroscopics).

* Complete removal needs high-pressure ranges.

* Are also ideal for removing layers of toxic materials, e.g. red lead, calcium plumbate, and zinc chromate primers. (Passing into the air, this can then be inhaled and passed into the bloodstream).

Disadvantages;

* Supply of large amounts of water and disposal of the resulting slurry (water and detritus as an entity).

* And also mixing substrate inhibitors if the specification demands it. (Substrate inhibitors are substances usually Sodium compounds, added to the water, to retard the formation of corrosion products) Some organisations, including B G do not allow the use of inhibitors, in which case dry blasting, to remove light oxidation, follows wet blasting.



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

Water usage is about 60 litres per minute. This system operates at about 30 000 psi. To work efficiently the head must be near to the surface, within 25 to 35 mm. At approximately 250 mm only loose and flaking material will be removed. Operator fatigue is a problem. This system will remove soluble contamination and millscale at the higher-

pressure ranges but will not cut a profile. It will only clean up the original profile on rework areas.

High pressure water plus abrasive injection;

This system operates at about 20,000 psi. Uses abrasives, either gravity fed into the system, suction fed or mixed as

slurry. This system will remove Marine growths e.g. barnacles, and it us often

used in dry-docks on ship hulls. Because of the abrasives a profile is cut using this method.

Low pressure water plus abrasive injection;

Uses normal blasting pressures of 100 psi. But with water as a propellant rather than air. The abrasive content is semi-soluble e.g. Sodium Bicarbonate crystals, talc, chalk, Ideal for use on non- ferrous metals and G. R. P. Sodium Bicarbonate is excellent for acidic or greasy situations. This method is very slow and controllable and can if needed, remove one coat of paint. The

abrasives have a very gentle action but leave masses of problematic slurry.

Steam Cleaning;

Ideal for oily and greasy situations, Steam production requires a heat source, (which is not conducive with the oil and gas

industry).

Air blasting with water injection;

Water is injected, with or without an inhibitor into the air/abrasive stream, either immediately after it exits the nozzle or immediately before it enters the nozzle.



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

3. Hand and power tool cleaning. 7079 Pt A, ISO 8501, SS 05 59 00;

*Any hand operated or power tools, including needle guns, wire brushes, emery cloth and grinders can be used to achieve these standards.* Hand and power tool cleaning is often specified for short-term maintenance programmes.

**Disadvantage; of this method is the lack of surface profile. Wire brushing may produce a burnishing, which is polishing, and a smooth shiny area does not provide good adhesion. Burnishing needs to be treated by abrading with coarse emery.

St2 Thorough hand and power tool cleaning . When viewed without magnification the surface shall be free from visible oil, grease and dirt and from poorly adhering millscale rust, paint coating and foreign matter.

St3 Very thorough hand and power tool cleaning. As for St2 but the surface shall be treated much more thoroughly to give a metallic sheen arising from the metallic substrate.

There are no wire brushing grades for Rust Grade A as the millscale is much harder than the bristles on the brushes, which are of non sparking alloys such as phosphor bronze and beryllium bronze.

If needle guns, Jasons hammers, are used they tend to leave a very coarse profile, which invariably needs to be reduced by abrading with emery, or grinding.



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4. Flame cleaning;

The BS 7079, ISO 8501 (SS 05 5900) contains four photographs showing flame cleaning standards from the original rust grades A, B, C, D. The designation given is AFl, BFl, CFl, and DFl. There is only one flame-cleaning standard for each rust grade. It is not wise to use this method of surface preparation on any fasteners relying on tension, e.g. rivets, screws, nuts and bolts.

Three factors contribute to how flame cleaning works.

1. Expansion;

Millscale is chemically bonded to the steel and applied heat causes the materials to expand at different rates, thus breaking the chemical bond.

2. Dehydration;

Water in the corrosion products is evaporated away, facilitating the removal of the corrosion products.

3. Heat penetration

The heat is conducted efficiently into the substrate aiding the drying of the steel and removal of penetrated oil or grease.

Method;

The operator slowly passes an oxygen/HC gas flame (Butane, Propane, Acetylene) over the area to be cleaned to burn and de oxidise the corrosion products and other contaminants. This leaves a grey coloured ash deposit.

A second operator follows on with a power brush to remove the now loose, ash deposits.

The primer can now be applied over the warm steel, reducing the need for addition of thinners.



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Other benefits are that the heat reduces the viscosity of the paint and gives better flow properties.

The paint can then 'wet out' better and pass into tiny cavities and irregularities on the surface.

The heat also accelerates the drying process and keeps the steel above dew point temperature.



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5. Pickling;

Pickling is a general term relating to the chemical removal of oxides (rust), from a metal substrate.

The metals can be either dipped (totally immersed) in the pickling fluid or sprayed with it. Usually aqueous solutions of acids are used for steel; they convert the oxides into soluble

salts e.g. Sulphuric Acid produces Iron Sulphate salts. Sulphuric is the most common acid used for economic and safety reasons.

Footners Duplex System

Involves the pickling process followed by a passivation process using Phosphoric or Chromic acid along with a small percentage of iron filings, which produces Iron Chromate or

Iron Phosphate salts, which are not soluble.

These form a rust inhibitive layer, which passivates the surface and increases the adhesion properties. They are also extremely resistant to cathodic disbondment.

A typical process would be: -

1. Any oil or grease needs to be removed by using a suitable solvent e.g. xylene or as specified. Oil and grease show up as fluorescent yellow/green under an ultra violet light.

2. Totally immerse in a bath of Sulphuric Acid, 5 10% concentration at a temperature of 65 70oc. Time can vary from 5 to 25 minutes depending on degree of contamination but is invariably at the lower end.

3. Rinse using clean warm water to remove the layer of soluble salts formed. If required the component could be coated after pickling. Likewise components can be blast cleaned and sent on for phosphating/chromating, but the patented process is only called Footners when pickled then phosphated/chromated.

4. Immerse in a bath of phosphoric/chromic acid, 2% solution at 80oc for approximately one to two minutes with iron filing (0.5%) (And an inhibitor to prevent embrittlement). This leaves a very thin layer of iron phosphate/chromate, which acts as a rust preventative for a limited time.

5. Rinse in clean water, and check for pH values.

PH is a measure of acidity or alkalinity of a substance and is measured using pH indicator strips. An indicator such as litmus will only tell if a substance is an acid or an alkali. Indicator strips give a measure of acidity or alkalinity, based upon the scale below.

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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4- PAINT CONSTITUENTS AND BASIC TECHNOLOGY

Paint is a material, which will change the texture colour or appearance of a surface and give some form of protection to the underlying surface.

Paint has been classified in many ways e.g. by principle involved.

1. Barrier;

The material forms a thick impermeable layer of a high electrical resistance e.g. urethane.

2. Passivation;

Causing a chemical reaction between the paint constituents and the substrate e.g. rust inhibitive primers.

3. Cathodic protection;

Employs the bi-metallic principles by using a less noble metal as pigmentation e.g. zinc in zinc rich primers.

By function.

Anti Fouling - To inhibit marine growth on ship 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 colour.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)

Rquirements, manufacturers are researching into new paint technology involving vastly reduced amounts of volatile organic compounds. Some are using water-based technology; some are concentrating on the solvent free materials.

b) Solvent free

As the name implies these materials contain no (or in some cases a minute amount of) solvent.

These are generally chemical curing materials, which require the mixing of two or more components,

Usually go under the name of MCLs (Multi Component Liquids). Some MCLs are made using solvent borne materials.

c) Powders

Virtually solvent free MCLs, which are solid at, room temperatures.

The base resin and the chemical activator, along with the other constituents required to complete the formulation are heated up to the resins melting point, mixed into an homogeneous liquid, cooled and ground into powder form.

In theory every particle contains all necessary ingredients to affect a cure into a protective film. The powder can be applied onto a preheated substrate (in the case of substantial steel thicknesses) at about 240oc, or onto thin plate electro statically and post heated.

In either case the powder melts undergoes a chemical reaction or in approximately three minutes the reaction is complete.

The three subdivisions are all made up from the basic ingredients mentioned earlier, Binder, Solvent, Pigment and other additives.



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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. Dialectric strength.7. Ability to resist the passage of water.8. Ability to change from a liquid as applied, into a solid to provide the above properties.

Several materials satisfy the criteria above for different environmental conditions, among them are: -



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Binder solvent groups and compatibility

A solvent free binder, or a binder using a very weak solvent, will cause very few problems when over coating another product. Usually in this situation the problem would be limited to different expansion and contraction ratios. Providing a key by abrading can mostly rectify or at least minimise this. A very strong chemically curing binder like epoxy, needs a strong solvent and can cause problems over coating other materials, even when they are fully cured.

Guide to binder solvent combinationsSolvent strength in descending order Common Names Binders

Water

Emulsions PVC/PVAVinyls

Acrylics other materials e.g. epoxy

Bitumins, Polyurethanes,Alkyds, Acrylated Rubbers

Aliphatic Hydrocarbons

White SpiritTurpentine

Turpentine substituteSolvent naphthasHexanes 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.

PolymersOne of the properties expected of a binder is to change from a liquid into a solid to form a film. To perform this function all binders form polymers or use polymers already partially formed. Polymer means; literally many parts, poly = many, mer = single unit or part. Mer (meras GK) can be a single atom, or a molecule, (a group of atoms) and can be described as being a string or structure of repeated units, Polymerisation; is the joining together of a string or structure of repeated units.



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In the case of most paints the main constituents of the polymers are: -

H - HydrogenC - CarbonN - NitrogenO - OxygenCl - Chlorine

Although there are variations the main three polymer types are Linear, Branched and Cross-linked

1 Linear Polymers; The atoms or molecules which form the polymer, join on at the end of the structure, and in so doing saturate the structure.

The process depends upon the properties of carbon, which forms the backbone of the structure. Carbon can give away electrons, take in electrons, share electrons, or join with itself in many ways.

H|

H C H|

H

H H| |

H C C H| |

H H

H H| |

C = C| |

H H

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


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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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

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 doesnt 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.


H H| |C C| |

H H

H H| |C C| |

H H

H H| |C C| |

H H

H H| |C C| |

H H

|C H|

H C H|

H C HH|

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

H H H H H

OH

O

OxygenAnother chain


07-12-13 - 33 -

Figure 4.4 Branched polymers

3 Cross linked polymers; Cross-linking, or chemical curing is a three-dimensional polymerisation process, which

occurs fairly rapidly using only components provided in the cans. Because the components are in calculated amounts the cross linking stops when all the

available bonds are occupied. Some urethanes fully cure in 16 hours, some Epoxies in three days, and others in seven days, dependant on temperature.

Figure 4.5 cross-linked polymers



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OilsNatural 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.

Colouring pigments are used, usually know as Opaque pigments.

Opaque pigments

Opaque pigments are inert particles with excellent light scattering properties in order to give covering power, (opacity) and colour.

1. Carbon 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.

Figure 4.9 The CPVC is exceeded and all particles are not wetted, the film would be porous, low in cohesive strength and adhesion.Painting Inspection Grade 3/2. Rev 1 April 2004Testing of PaintsCopyright 2003, TWI Ltd 37

Theoretical Leafing layers

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

Practical De-lamination


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SolventsSolvents 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 dont 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 AdditivesOther 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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5-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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6-DRYING AND CURING OF PAINT FILMSGenerally 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 werent changed, the resulting film would stay as a liquid and be easily wiped away.

These materials e.g. acrylics and vinyls 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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7-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.

PrimerA primer, normally low volume solid materials, wets out the substrate Provides excellent adhesion and also provides a key for any subsequent layer.The binders usually have a relatively low resistance to vapour transmission, and allow water into the film to carry tiny amounts of the rust inhibitive pigmentation onto the substrate to form a passivating layer. Older versions of BG specifications required that all primers should be brush applied. This was to ensure that any dust or detritus left on a substrate was worked into the film, and not left lying where air could be entrapped, forming pinholes.

Other primers exist for non-ferrous substrates such as Wash or Mordant primers, and PVB etch primers.

Mordant means

Of a corrosive nature, or will bite into, As suggested contains an acid, Phosphoric acid. These materials contain approximately 96% VOCs in the form of Ketones, and approximately 4% phosphoric acid, tinted with copper phosphate (blue). Their primary use was for etching new galvanising. The reaction turns the surface black (zinc phosphate salts). Some specifications allowed painting as soon as dry, but others required a water wash. Etchants do not leave a measurable thickness. PVB Etch primers, Polyvinyl Butyrol are principally used on Aluminium, but were used on virtually every non-ferrous metal.PVBs are 2 pack materials, low volume solids with a dry film thickness of 15 to 25 um. This material also contains phosphoric acid. The acid etches the Aluminium (Aluminium Phosphate) provided a key for the vinyl binder. The general appearance when dry is a matt yellow translucent film, with an underlying black or darkened substrate. Some specifications require coating before 16 hours.



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Mid-CoatsMid-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 CoatsFinishing 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 polyurethanes 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.



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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.

ThermosettingThermosetting 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.



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Sacrificial coatings This classification of materials sacrifices itself to protect the underlying substrate.

The sacrificial component must be less noble (more electronegative) than the substrate.

Zinc and Aluminium are the most common materials used to protect ferrous substrates.

Zinc and Aluminium have relatively low melting points and so are commonly used in the form of metal spray, applied by flame onto structural steel e.g. bridges.

Zinc is used in hot dip galvanising of steel, to totally encapsulate a section.

In this situation the zinc works as a barrier coat initially and undergoes atmospheric corrosion itself forming corrosion products such as Zinc Sulphates and Zinc Carbonates.

To stop this natural process on the zinc it is usual to paint over the galvanising.

However, if the galvanising is damaged, exposing the steel underneath so that both metals are in contact with electrolyte, the zinc then starts working sacrificially, corroding in preference to the steel, producing Zinc Oxides on the damages faces until the damage is filled to exclude electrolyte contact.

The zinc then works as a barrier again.

If the galvanising suffered damage of more than a scratch or gouge repair might be a better option. In this instance a zinc rich epoxy might be used.

These materials contain a very high percentage content of zinc pigment. Specifications vary but 90% by weight of the dry film is a typical requirement.

If moisture, an electrolyte, passes into a film of this nature,

Each particle of zinc needs to be in contact with at least one other,

In order to form the metallic circuit through to the steel for the electrons.

These electrons, in the form of Hydroxyl ions will then return through the electrolyte to the zinc and the zinc will corrode, sacrificially.

In order to hold the high concentration of zinc particles together, a very strong binder is required. This is usually an organic epoxy.

Inorganic binders such as Ethyl or Methyl Silicates are zinc pigmented but are primarily designed for high temperature service and need sealers such as aluminium or carbon pigmented silicones.



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8-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) Vinylsf) 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 HCs for application windows.



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9-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 1m 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 1" 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


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2 Attritor mill

The attritor mill is a vertical version of the ball mill, but more efficient and also static.Paddles drive the balls. The mill base is continually circulated by pum

bgas-cswip -painting 3-2.pdf

Documents

corrosion corrosion

corrosion circuit

corrosion triangle

couple electrons

positive iron ions

iron atom

hydrogen ions

negative ions