UNIT 1 CORROSIONCorrosion is generally an Electro-chemical process, which results from an anodic reaction and at least one cathodic reaction. The corrosion of steel takes place at the anode.

The anodic reaction is expressed as follows:MM+n+ne

Where:1. M = element involved2. N = a number3. E = electronsFor iron and steel this would be expressed as:

FeFe+++2e

At least one of five basic reactions may take place at the cathode. The most common reaction applicable to the corrosion of steel is as follows:

O2+2H2O+4e4OH

Iron ore is an oxide in chemical balance with the environment; when this iron ore is converted to iron, the chemical balance is changed and the iron becomes active, i.e. it corrodes on contact with the natural environment and tries to revert back to it’s natural inert state. The natural environment usually contains moisture (which provides the electrolyte) giving the following simultaneous reactions:The products of these reactions take place in further reactions with the immediate environment leading to the formation of corrosion products, the most familiar being rust.

Corrosion reactions can be accelerated by the existence of certain criteria including:1. Variations in oxygen content on the materials surface2. The concentration of chemical salts in the electrolyte, e.g. chlorides and sulphates:3. Other metals or metal compounds of higher nobility (more Electro positive) in contact with the

steel, e.g. millscale4. Acids or alkalis5. Certain types of bacteria near the materials surface6. High temperature. The higher the temperature the greater the rate of corrosion.

The following list shows some metals/metal compounds in their order of nobility in seawater at ambient temperature. The relative positions of the metals/metal compounds in the list can change in the electrolyte or temperature; this list is known as the galvanic series.

Gold NobleSilverNickelCopperMill scaleMild steelAluminiumZincMagnesium Ignoble

If steel was in intimate contact with zinc or attached via a wire in an electrolyte, e.g. soil or water, the zinc would corrode first because the steel is more noble than the zinc. In this example the zinc becomes the anode and the steel becomes the cathode, i.e. the steel is being cathodically protected and the zinc is acting as a sacrificial anode.

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We can use the galvanic series to bring about cathodic protection to steel. There are two ways in which this can be achieved

Bi-metallic corrosionWhen two differing metals are attached closely together the less noble one will act as an anode to the more noble one cathode and will sacrifice itself

Sacrifical coatingWhen a coating whose metallic pigment particles are less noble than the steel (zinc or aluminium) are coated onto the steel they will sacrifice themselves in order to preserve the more noble steel. When coatings are used for this purpose they must contain around 90% of the metallic pigment

Notes An electrolyte is a medium, which will allow the passage of electrical current consisting of a chemical salt dissolved in water.The greater the amount of chemical salt present, the better the conductivity of the electrolyte.Millscale is an oxide of iron produced when the steel is manufactured; it is a result of the hot steel coming into contact with the air and forming an oxide layer composed of three layers;1. FeO nearest the steel2. Fe3O4

3. Fe2O3 on the outside Millscale has a total thickness between approximately 25m and 100m

It is essential for millscale to be remove from steel to be removed from the steel surface during blast cleaning for the following two reasons1) Millscale is more noble than steel and when parts of the millscale break away the exposed

areas of the steel (anode) will sacrifice themselves to preserve the more noble surrounding millscale (cathode)

2) If painted over, millscale which is loosely adhering and flaky will leave the steel, bring the paint away from the substrate thus causing early breakdown of the system due to lack of adhesion

Methods of arresting corrosion

Barrier principleThis isolates the substrate from the environment by means of a low permeability coating system. This type of system is usually made up from around four coats and contains an M.I.O (micacious iron oxide) coat, which helps give the paint system its low permeability

Sacrifical principleThis involves making a paint whose pigment particles (zinc or aluminium) are less noble than the steel onto which it is coated. The result is that the less noble coating (anode) sacrifices itself to the more noble steel (cathode)

GalvanisingInvolves the dipping of steel components into a bath of molten zinc at around 4500c this leaves a zinc coating on the steel component of between 85m - 130m and is directed under BS 429

PassivationChemical reactions which are achieved between rust and inhibitive pigments in the primer and the substrate

Sacrificial anodesBlocks of metal ignoble to steel which are attached to steel structures as a sacrificial anode

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UNIT 2 SURFACE PREPARATIONCorrect surface preparation is a vitally important stage for most coating systems, it is often the process which governs the service life of the coating system.

There are various ways to prepare a surface prior to coating Abrasive blast cleaning Wire brushing Scraping Grinding Needle gunning Chemical cleaning Waterblasting Weathering Flame cleaning Vapour degreasingThe quality of a surface preparation is governed by the amount of surface contaminate remaining on the substrate after cleaning, although it may also relate to the resultant surface texture, e.g. the surface profile on a substrate after blast cleaning.

Dry abrasive cleaningDry abrasive blasting is carried out by projecting a highly concentrated stream of small abrasive particles onto the substrate’s surface at speeds of approximately 720 km/h (450m.p.h.). The operation removes rust, scale and any other extraneous material from the substrate and also leaves an irregular profile, which provides an ideal key for the coating adhesion. Dry abrasive blasting is often the best method of surface preparation for long-term protection coating systems.AbrasivesThe degree of surface roughness and rate of cleaning is partially governed by the characteristics of the abrasive used; these beingAbrasive Size Hardness Density ShapeHuman factors Speed Angle Distance TimeBoth metallic and mineral abrasives are commonly used for blasting for example Steel or chilled iron grit Steel shot Metallic grit and shot mixed Copper slag Garnet SandThe control of substances hazardous to health regulations 1994 (coshh) do not allow the use of sand containing free silica in dry blasting operations because of the risk of silicosisThere are other abrasives, which are used, usually for specialised applications e.g. walnut shells, ceramic grits, crushed glass aluminium silicate.Effect of abrasivesGrit is angular in profile with sharp cutting edges: it shatters millscale and undercuts any surface contaminates resulting in a clean surface with a rough profile. The amplitude tends to be quite erratic with a large occurrence of rouge peaks, especially when blasting in one area to long.NotesSand is not dangerous unless it is in dust form when it can be inhaled, e.g. after fragmentation during the dry blasting operation.

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Shot is spherical, it shatters millscale but does not have sharp cutting edges to cut into a surface, however, the visual appearance of a shot blasted finish is similar to a grit blasted finish although there is less roughness to the touch. Shot blasting work hardens a steel surface to a greater degree than grit, which has the effect of reducing the chance of any stress corrosion cracking which could otherwise occur in the future. Shot also reduces the occurrence of rouge peaks but may press impurities into the surface.It is a common practice to mix metallic shot and grit to obtain a blast finish close to the ideal (a typical mix being 70-80% shot and 20-30% grit)

Surface profileThe shape of a cross-sectioned blast finish is known as the surface profile or anchor pattern.

The size of the profile as measured from the peaks to the troughs is known as the amplitude or peak to trough height, and is primarily governed by the size of abrasive used, although other factors are important, e.g. angle of impingement, hardness of surface and other characteristics of the abrasive itself. Maximum amplitudes or amplitude ranges would normally be quoted in specifications, a typical amplitude range for liquid paints would be in the region of 30 - 75m. the amplitude of a blasted surface may be measured by a number of methods including the use of surface profile needle gauge, surface replica tape, e.g. testex tape or a surface comparator.Surface profile needle gaugeThis relies on a needle reaching the bottom of the troughs on the surface profile. Because there are so many troughs of different depth, it is normal and necessary to take ten or twenty readings and calculate the average amplitude. Before taking any readings it is necessary to zero the gauge on a flat piece of glass

NotesRouge peaks are peaks that stand out above the required profile and should be avoided if applying thin coatings as they may lead to spot or flash rusting. Blast finishes produced in production should not be touched with bare hands due to contamination. All accurate measuring equipment should be issued with calibration certificates or certificates of conformance to give assurance that the readings obtained are going to be correct within a stated margin of error.

Surface replica tapeTestex tape is a trade name of a commonly used or surface replica tape. It is used in conjunction with a dial micrometer and although quite costly, has the advantage of providing a permanent record. The procedure for carrying out this test is as follows:1. Zero the micrometer ensuring the flat contact points are clean2. Remove paper backing and stick Testex tape to the surface to be measured3. Rub the Testex paste into the troughs using a blunt instrument, until the peaks can be seen

butting up to the transparent plastic.4. Remove the Testex tape from the surface and measure the overall thickness with the dial

micrometer

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Rogue peaks

Trough

Amplitude

5. Deduct 50m from the reading to obtain the amplitude. The plastic Mylar film to which the soft compound is attached is 50m thick

Surface comparatorThe roughness of the surface to be assessed is compared to the different areas on the comparator by visual examination and if necessary by scraping with a fingernail, small wooden stick or similar- never with the fleshy part of the finger as this will contaminate the blastA profile grading can be given when the area under assignment is rougher than the smoother of two adjacent areas on the comparator but not as rough as the rougher of the two areas. The profile is then graded according to the following: Fine profile - equal to or rougher than area 1 but not as rough as area 2 Medium profile - equal to or rougher than area 2 but not as rough as area 3 Course profile - equal to or rougher than area 3 but not as rough as area 4If the profile is finer than area 1 it is termed finer than fineIf the profile is courser than area 4 it is termed courser than course

Blasting gradesThe grade of a blast finish relates to the amount of surface contaminate remaining after blasting. The grade of blast finish is primarily governed by blasting time and the velocity of the abrasive particlesBS 7079: PART A1BS 7079 Preparation of steel substrates before the application of paints and related products. Part A1 of this standard is pictorial and shows rust grades prior to blasting and the degree of surface cleanliness after blasting.The surface under examinations visually compared with high quality photographs in the standard both before and after blasting. The preparation is then given a coding e.g. SA 21/2 which can be interpreted using the following extract from the standard:Rust gradesA Steel surface largely covered with adherent millscale but little if any rustB Steel surface, which has begun to rust and from which the millscale has begun to flakeC Steel surface on which the millscale has rusted away or from which it can be scraped, but

with slight pitting visible under normal visionD Steel surface on which the millscale has rusted away and on which general pitting is

visible under normal visionNotesIt is important to note that needle gauges, surface replica tape and surface comparators only give the degree of roughness and not the degree of cleanliness.BS 7079: Part A1 is the same as ISO 8501-1 and SIS 05-59-00

Preparation grades – blast cleaningPrior to blast cleaning any heavy layers of rust shall be removed by chipping. Visible oil and grease shall also be removedAfter blast cleaning the surface shall be cleaned from loose dust and debris.

Sa1 light blast cleaning when viewed without magnification the surface shall be free from visible oil, grease, dirt and from poorly adhering millscale, rust, paint coatings and foreign matter.Sa2 through blast cleaning when viewed without magnification the surface shall be free from visible oil, grease, dirt and from most of the millscale, rust, paint coatings and foreign matter. Any residual matter shall be firmly adhering.Sa21/2 very through blast cleaning when viewed without magnification the surface shall be free from visible oil, grease, dirt and from most of the millscale, rust, paint coatings and foreign matter.Any traces of contamination shall show only as slight stains in the form of spots or stripes.Sa3 blast cleaning to visually clean steel when viewed without magnification the surface shall be free from visible oil, grease, dirt and from most of the millscale, rust, paint coatings and foreign matter. It shall have a uniform metallic colour.

Comparison of blasting grades

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SSPC Bs 7079/ SIS 05-59-00 naceWhite metal sp5 Sa3 Grade1

Near White metal sp10 Sa21/2 Grade 2Commercial finish sp6 Sa2 Grade 3

Light blast and brush off sp7 Sa1 Grade 4SSPC= Steel structures painting councilNACE =National association of corrosion engineers

EquipmentCentrifugal blast unitsBlasting in factories is often carried out using rotating wheels, which throw the abrasive at the component. These units are known as Centrifugal blast wheels, are usually fixed installations and are commonly used for large production runs, e.g. on pipes in plate mills and large steel plates in shipyards.The main advantages of this system compared to air blasting systems are as follows1. Lower cleaning time2. Low abrasive consumption3. Low energy consumption4. Less labour used5. More consistent and uniform blast finish6. More environment friendly7. Safer to implement (closed system)The abrasive is fed into the centre of the wheels and to the inner edges of the attached blades by means of an impeller. The abrasive is then accelerated to the end of the blades and onto the component by centrifugal force at speeds typically between 250-350km/h.For cost reasons the abrasive would normally be reusable. The abrasive is recycled up to approximately twenty times providing it is free from oil and grease. An air-wash separator removes any dust contaminates from the recycled abrasive before it is fed back into the wheels.Air blastingPressure blasting, which is a type of air blasting system, would normally be used on site work. Vacuum blast and suction blast also come under this category of air blasting but are not as widely used due to lower efficiencyPressure blasting equipment consists of 1. A compressor providing an air supply of approx. 100p.s.i.2. A pressurised pot containing the abrasive3. Liquid separators, i.e. moisture filters4. A carbon impregnated hose5. A venturi shaped blasting nozzle6. A dead mans handle for direct operator controlThe velocity of abrasive particles leaving a blasting nozzle is primarily governed by the pressure at the nozzle, the higher the pressure the higher the velocity and therefore the higher the rate of cleaningThere is a point at which an increase in pressure does not increase the velocity substantially; this is at approximately 100psi, depending upon the abrasive used. Limiting pressures to 100psi is also advantageous for safety reasonsIt is important to keep the pressure at the nozzle as close to 100psi as possible because for every 1% loss in pressure there is approximately 11/2% loss in efficiency. The pressure at the nozzle may be measured using a hypodermic needle gauge; this is placed through the wall of the hose near the nozzle with the hole in the needle facing the nozzleBlasting nozzlesBlasting nozzles are available in a variety of materials and orifice sizes. Sometimes the nozzles are lined with relatively abrasive resistant materials, e.g. tungsten carbide for a longer working lifeTwo types of nozzle, which exist, are the straight bore nozzles and the venturi shaped nozzle. Straight bore nozzles are rarely used for blasting large surface areas because they are not as efficient as venturi nozzles. The velocity of abrasive leaving a straight bore nozzle at 100 psi is

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approx. 350km/h whereas the velocity shaped nozzle under similar conditions would be approx. 720 km/h

Venturi shaped nozzles also produce a larger blast pattern with the whole area receiving a relatively equal amount of abrasive, whereas a straight bore nozzle concentrates most of the abrasive in the central area of the blast pattern, resulting in a fringe area of lower blasting efficiencySafetyCentrifugal blast units are a closed system; i.e. human access to the blasting areas is limited. When using an open system, e.g. for site blasting applications using pressure blasting equipment, access is not usually restricted therefore warning signs are necessary and regular inspection of the equipment is required.Other safety considerations relating to pressure blasting are as follows Use of carbon impregnated hose to reduce the chance of static shock Use of a dead mans handle to stop the flow of abrasive when the operator lets go of the

nozzle Keeping hoses as straight as possible to prevent kinks which may lead to a blow out Use hoses of the correct type i.e. reinforced Use of external couplings if joining hoses together, internal couplings reduce the bore and the

eroding action of the abrasive could lead to a blow out Restricting the pressure to 100 psi The wearing of protective clothing, including an air fed helmet, boots, leather apron and

glovesWet blastingWet blasting methods are good for removing soluble salts such as chlorides from surfaces and are good for the removal of toxic coatings, e.g. red lead films because they do not create a dust.However, all wet blasting methods have similar disadvantages over dry abrasive blasting, including The availability and drainage of water The production and disposal of sludge (particularly with abrasive injection) The extra cost of supplying and mixing corrosion inhibitor (assuming the specification allows

the use of an inhibitor) The problems associated with drying large surface areas or the higher cost of water miscible

primers compared to conventional primersHigh pressure water jettingOperates at pressures up to 60,000 psi, which can be extremely dangerous. The advantages of this method are as follows: Simple to operate Highly flexible and mobile in use Suitable for removing soluble contaminates Will remove millscale at high pressures

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Straight bore

Venturi shaped

Low pressure water plus abrasive injectionOperates at 100 psi. It is claimed that this technique is very controllable and will remove one coat of paint if required. Disadvantages include high cost and low efficiency.Steam blasting, with or without abrasive injectionOperates at 100 psi. This method is ideal for surfaces contaminated with oil, grease, etc. disadvantages include high cost and low efficiency.Air blasting with water injectionWater with or without an inhibitor is injected into an air/abrasive stream.Hand and power tool cleaningHand and power tool cleaning, relates to scraping, chipping, wire brushing, sanding, grinding and needle gun cleaning.This method of cleaning, although not as effective as blast cleaning, is often used for short term protection coating systems, maintenance work or where access for blasting is restricted or damage from abrasive to the surrounding environment would occur.Wire brushing is a widely used surface preparation method but it only cleans up an existing surface, it does not re-cut a new profile. Bs 7079:part A1 defines standards of wire brushed finishes along with other hand and power tool cleaning method as follows. Prior to hand and power tool cleaning, any heavy layers of rust shall be removed by chipping. Visible oil and grease and dirt shall also be removerSt2 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 coatings and foreign matterSt3 Very thorough hand and power tool cleaning As for St 2 but the surface shall be treated much more thoroughly to give a metallic sheen arising from the metallic substrateSt3 is usually obtained by mechanical wire brushing and St2 is usually achieved by hand wire brushing. Care must be taken to avoid over brushing a particular area causing burnishing, a condition with a highly polished surface which has an adverse effect on coating adhesion.For safety reasons it may be specified that wire brushes used must be of the non sparking type i.e. phosphor bronze or beryllium bronzeNotes Bronze brushes may not be permitted because of the possibility of galvanic corrosion. Plastic bristles embedded with abrasives as an alternative.Needle gunningA needle gun or jasons hammer as it is sometimes referred to, consists of many air operated reciprocating tungsten needles. It is usually preferable for the needles to have a small cross section. Needle guns are useful for cleaning difficult surfaces such as rivet heads and welds, they also peen and stress relieve the surface. Their disadvantages are that they can leave sharp edged craters and rogue peaks and they also have a tendency to push impurities into the surface.After the same may check needle gunning the amplitude of the surface profile methods used for abrasive blast cleaning if the contour of the substrate allows.

The guns shall have needles of a small cross section, the profile created must not exceed 100m, no sharp egged craters must be left and all rouge peaks must be removed.GrindingShall only be carried out under direct supervision of the engineer.Abrasive discsMay be permitted in certain circumstances. Approval for their use must be sought and particular care must be taken on pressure containing parts not to create notches

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Compressed air

Reciprocating needles

Needle gun

Flame cleaningThe application of an oxyacetylene flame to the steel surface to be cleaned is an efficient method of removing rust, millscale and other contamination. The effectiveness of the process is due to a combination of factorsDifferential expansion- the millscale on contact with the intense heat expands at a faster rate than the steel to which it is attached and flakes offDehydration- rust is a combination of iron oxide and moisture. As the moisture is rapidly driven off the rust is dehydrated and converted to a dry powder which can be removed by wire brushingHeat penetration- the heat from the flame penetrates all the surface irregularities and removes all traces of moisture, oil, grease etc.The flame cleaning of any of fastener, rivets or bolts, should be avoided as a loss of mechanical strength may be causedFlame cleaning often requires three operatives who work in a team as follows:1. Flames cleans the surface, this gives a light grey appearance on the surface when finished2. Wire brushes the surface to remove all the dry powder3. Primes the surface, it is often necessary to apply the paint while the metal is still warm around

400c (which is about the maximum to which the hand can be comfortably applied)The warmth of the plate lowers the paint viscosity enabling it to flow more easily into irregularities and also ensures that condensation will not form on the surface.Bs 7079: part A1 shows the minimum flame cleaning standards according to rust grades, i.e. A / F1 B/ F1 C/ F1 D/ F1

Disadvantages Dealing with high temperature naked flamesDamage and warping to thin materialsHeat causing loss of strength to steel fastenings

AdvantagesMetal is perfectly dry for painting

Chemical cleaningPickling and phosphatingPickling is a chemical cleaning process, which is widely used in a factory environment for preparing items such as pipes and steel platesThe process usually involves immersing the steel in a bath of hot acid such as sulphuric acid, which has been inhibited to reduce attack by the acid on the steel. The acid dissolves a thin oxide layer at the interface with the steel causing the rust or millscale to be removed.Other acids e.g. phosphric acid and chromic acid, are used to passivate the substrate to retard corrosion reactions and also to promote adhesion. The acids react with the steel to form a thin layer on the surface, which passivates the surface and provides corrosion resistance.Procedure (HB Footners duplex process)1. Degrease- removes surface contaminates such as grease and oil by use of a suitable

solvent, e.g. xylene, usually applied by cloth2. Pickle- total immersion in a tank of acid, e.g.5-10% sulphuric acid at 65-700c, to remove

millscale, rust etc., the time taken is variable and depends on the type and degree of contamination. An inhibitor is also present in the tank

3. Wash- a clean water wash to remove acid and surface residues, usually applied by hose or spray

4. Phosphate- the technique involves a final treatment in a 1-2% phosphoric acid solution held at 800c for 1-2 minutes. This leaves a thin rust inhibitive phosphate coasting on the steel surface to which the coating should be preferably applied while it is still warm, possible after a final wash

Hydrocarbon solvent cleanersThe removal of oil or grease from a substrate using hydrocarbon solvents involves proprietary brands of degreasers which usually use solvents such as xylene, toluene and solvent naptha. Other solvents known as halogenated hydrocarbon solvents such as perchloroethane and perchloroethylene are also used

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Note halogenated hydrocarbon such as 1,1,1, trichloroethane, trichloroethylene and carbon tetrachloride were commonly used as degreasers but there use has declined, or been completely restricted, due to high toxicity. Heavy vapours of all chemical solvents are a hazard in enclosed areas.A thin film of oil invariably remains after solvent cleaning but the more solvent used the more frequent the operation, the less residual matter there is presentXylene is a commonly used degreaser but its use on painted surfaces is limited due to its solvent strength and compatibility considerations

Some non-ferrous metallic surfaces such as copper, brass and galvanised steel may be coated for anti corrosion purposes but sometimes they are coated for aesthetic reasons. Relatively reactive materials, e.g. zinc and aluminium, are often coated to prolong their effective life.Non metallic materials such as glass reinforced plastic (GRP) or concrete are often coated for appearance reasons only, although if concrete is coated its effective life or time to maintenance may be prolonged.Regardless of the reason for coating the surface must be cleaned1. Remove any oil grease by suitable solvent or proprietary degreaser2. Remove any water soluble contaminates using water, usually mixed with 2% detergent3. Rinse using clean water. Preferably use power washing equipment where the situation allows4. Sweep abrasive blasting preferred. If this is not practical , abrade using emery cloth or wire

brushIt may not be feasible to use sweep blasting as a surface preparation method with certain surfaces such as thin gauge aluminium cladding or zinc galvanising. However using a wire brush or some other abrasion method may not always be the best alternative. Etch primers or etchants are usually on these surfaces to provide a key to the substrate.For many non-ferrous substrates including aluminium and zinc a polyvinyl-butyral PVB type etch primer or a non proprietary etchant called t-wash is used. The etchant used in both cases is phosphoric acid.It is not always a requirement to apply etch primers or etchants to zinc coatings which have a dull appearance due to an oxide layer which has formed over a period of time due to reactions with the oxygen in the atmosphere. The oxide layer usually provides an adequate key for the coating system.The specification must always be consulted to determine which preparation method to use on non-ferrous substrates. For example, a specification may require t-wash etchant to be used on unweathered zinc galvanised surfaces, PVB etch primers on aluminium surfaces and abrasion using emery cloth on copper surfaces.Tests to detect surface contamination may be qualitative or quantitative. Qualitative tests will determine whether or not contamination is present but they will not show the exact quantity, although an idea of the extent of contamination will normally be determined. There are many tests for detecting contamination but some of these require a chemist or other suitable qualified person to perform; these tests tend to be mainly quantitative, i.e. a quantity is determined, e.g. in mg/m2

although this value may not be the exact amount present.

Soluble iron saltsColourless soluble iron salts may be present in pits within the substrate after blasting. If salts are present they will accelerate corrosion causing rust spots which may in turn break the bond of any applied coatings leading to the failure of the coating system.Some specifications state the maximum levels of salts permissible on a surface and express the quantity in milligram’s per square meter mg/m2

The maximum requirement may be as low as 10 mg/m2 although other specifications may state that 30 mg/m2 is the critical level. Only quantitative tests could be used to determine whether these requirements are metTest results may be misleading or totally wrong if chromate or nitrate inhibitors have been used, for example in wet blasting.

Potassium ferricyanide test1. Spray a fine mist of distilled water onto a small area of the blast cleaned surface

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2. Wait a moment for any water droplets to evaporate then apply a Potassium ferricyanide test paper by pressing down for 2-5 seconds

3. Remove the test paper and check to see if any salts have been drawn by capillary action, they show as prussian blue spots

Note the Potassium ferricyanide test may also be referred to as the Potassium hexacyanoferrate (iii) test

Merckoquant testThis test is also known as the Eisen test and is a colourmetric quantitative test claimed to be 85% accurate down to 30 mg/m2

Bresle sample patchThis is a mercury (II) nitrate (mercuric nitrate) titration test clamed to be 95% accurate down to 10 mg/m2 Salt contamination metersThese normally give a digital readout and work by directly measuring the ionised metal salts dissolved in a quantity of water.Mill scaleMillscale is cathodic with respect to steel. This means that if any traces of millscale are present on the surface after preparation they can accelerate the corrosion of the underlying steel and disbond, leading to the eventual failure of any coating system applied. To test for the presence of millscale particles left after blasting to Sa 3 the copper sulphate test may be usedA fine mist of slightly acidic copper sulphate solution is sprayed onto a localised area of approximately 10mm in diameter. The steel turns a bright copper colour and any millscale particles show as black spotsDustThe presence of dust may be determined by applying transparent pressure sensitive adhesive tape to the test surface and then removing. The tape is then examined using a magnifying glass and an assessment is made. Standards do exist which standardise the test conditions and the way in which results are assessed. Ref.: Bs7079 Part 3Oil or greaseSimple visual assessment may reveal the presence of oil or grease, however a cotton wool swab wiped over the surface may reveal oil or grease which was not directly visible when on the surface. The use of an ultraviolet lamp may also detect oil or grease by causing it fluoresce, but a dark environment is required for this method. Another method is to drip several drops of a solvent such as xylene onto the suspect area. After a few moments remove some of the solvent and drip onto a tissue or filter paper. When the solvent has evaporated any oil or grease will show up as a brown ring on the paper.

UNIT 3 PAINT CONSTITUENTSPaints may be classified in several different ways, however one way in which paints may be sub-divided is as follows Liquid paint containing solvent Solvent free liquid paint Powder paints

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Solvent free liquid paints and powder paints are latter developments, which eliminate the need for costly and hazardous solventsLiquid paints containing solvents are the most common, although solvent free materials are being more widely used. Water based paints have been used for sometime in industrial applications, e.g. electrodeposition paints in the motor industryOpaque liquid paints consist of a liquid medium know as the vehicle (solvent and binder) plus solid pigment particles. An unpigmented paint known as varnishBinderThe binder is the film former, i.e. the component in the paint, which forms a relatively hard continuous film. It may be thought of as the adhesive that holds the pigment and other additives together. The binder contributes mainly to the durability, provides the necessary mechanical strength and physical properties and provides the adhesion, cohesion and flexibility of the coatingA paint type is normally identified by its binder Acrylic Alkyd Cellulose Chlorinated rubber PVC/PVA emulsion Epoxy Ethyl and methyl silicate Vegetable oils e.g. linseed Phenolic Polyurethane Silicone StyreneEach binder has its own characteristics, therefore a paint must be carefully selected to ensure it is able to do the work required: e.g. it would not be the right choice to coat a chemical plant with linseed oil based paint as these paints have low chemical resistance. A good choice would be an epoxy or Chlorinated rubber as these have good chemical resistanceA paint binder forms polymers when drying takes place. A polymer molecule is composed of many smaller parts, contributed by similar or dissimilar molecules which are joined together until there are hundred or thousands of atoms in the polymer molecule. This process is known as polymerisation.Polymers consist of chemical compounds made up from elements of a low molecular weight, such as carbon, hydrogen, oxygen and nitrogenMost polymers are organic and may be naturally occurring, or as is more usual in modern paints, synthetic

-A-A=A-A-A-A Linear polymer

A-A-A-A-A-A-A-A-B Branched Polymer

A-A-A-A

Cross-linked polymer

All binders are polymers and in the case of reversible or non-convertible coatings, the polymer is fully formed and therefore does not undergo further polymerisation during the drying or curing processNon- reversible or convertible binders are composed of polymers which are not fully formed and which undergo further polymerisation during the drying or curing process.ResinsNatural resins are obtained from plant secretions or plant fossils and include lac, copals and dammers. Natural resins may be hard brittle or soft semi-solids, are usually quite transparent and may have film-forming properties although they are usually used to modify the properties of oils.

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Natural resins quite often have to be modified chemically by heating etc. before they are any use to the paint industry. Some natural resins are soluble in organic solvents but not water, some natural resins classified as gums are soluble in water e.g. gum Arabic.Almost all resins used in paint formulations nowadays are synthetic e.g. epoxy, alkyd, vinyl etc., these have similar physical properties to natural resins but have different chemical composition.OilsBefore synthetic resins made their appearance, unsaturated drying oils were used as film formers. Nowadays their use is restricted, although they are commonly used in oleoresinous varnishes (oil and resin) for special useCommon drying oils are linseed and tung oil, which dry by oxidationPaints, which contain more oil than resin may be referred to as, long oil paints, these produce elastic, slow drying, paint films used for decorative purposes. Paints which contain more resin than oil may be referred to as short oil paints, these produce brittle fast drying paint films used for structural coatingsPigmentsPigments are solids in powdered form, which are derived from chemical reactions, minerals, vegetables or animals. Pigments may be organic or inorganic. Most pigments used in paints are inorganic although there are some common organic pigments. Most pigment types must be chemically inert and insoluble in vehicle in which they are dispersedPigments basically give paint film its colour and opacity (hiding power), but may also improve the paint film hardness and durability. Colour permanence when exposed to the environment i.e. light, air and moisture, is also a consideration. Opacifying pigments are typically less than 1m per particle.There are pigments, which can apply other characteristics to paint e.g. anti corrosive properties.Titanium dioxide is a white pigment and worthy of special mention because it is present in many paints over a variety of colours. Its main characteristics are high tinting strength and hiding power, low weight, good chemical inertness and resistance to heatPigments are usually classified by colour or by the primary characteristic they afford to the paint; listed below are some pigments classified by the latterOpacityDescribes how well the paint will cover over the underneath surface and is possible and is opposite to transparency. The opacity of a paint is determined mostly by the amount of pigment in the paint. The greater the pigment quantity the better covering power the paint will possessAny piece of equipment that measures opacity is called a cryptometerTwo types in common use are hiding power charts and the pfund cryptometerOpaque pigmentsThese inert pigments are used for the purpose of providing colour and opacity Carbon black Compounds of calcium red, yellow Compounds of cobalt blue Compounds of chromium green, yellow, orange Compounds of iron brown, red, yellow Titanium dioxide whiteRust inhibitive (anti corrosion) pigmentsUsed in primers for the purpose of corrosion preventionInhibitive pigments work by anodic and/or cathodic polarisation of the metal substrate. The soluble particles of some types of rust inhibitive pigment react with the moisture as it passes through to the metal making it non-corrosive. Red lead and calcium plumbate are basic and react with acidic components of the vehicle to form an inhibitor Red lead* Calcium plumbate* Zinc chromate* Zinc phosphate Boro-silicates Zinc phospho-oxide Barium metaniobate*The use of the above marked in most countries is banned or restricted due to high toxicity

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Metallic pigmentsMay be used to give metallic finishes such as those used on cars. Zinc may be used to give anti-corrosive properties by acting as a sacrificial anode by means of cathodically protecting the underlying steel, providing the metallic particles are in close contact with on another. In most situations, it is unlikely that an aluminium pigment even if closely packed will achieve cathodic protection Zinc AluminiumExtender pigmentsMore often referred to as extenders; these are not opaque and are used for increasing viscosity, reducing gloss, aiding intercoat adhesion and to improve the cohesive strength of the paint film. Some opaque pigments can also provide these characteristics, but the main advantage of using extenders is lower cost Kaolin (china clay) Chalk Talc Slate dust BarytesLaminar pigmentsLaminar pigments are small flakes, which have a leafing effect when the paint dries; this means that the flakes of pigment overlap one another like leaves on the ground. This results in an excellent coating to resist the passage of water; tensile strength is also improvedMIO (micaceous iron oxide) sometimes referred to as flaky or specular hematite, is a laminar pigment widely used in midcoats on structural steelwork and is available I many types of binder formulations. Mica, glass flakes and aluminium flakes are other laminar pigments, which have similar characteristics to MIOSolventsUsually the ability of a paint to spread over a surface is far from ideal unless the paint contains a solvent. The paint solvent must be volatile so that it evaporates from a coating of paint to leave a viscous film. The choice of solvents is important because the use of inappropriate solvents can affect the drying and gloss characteristics of a paint and in some cases can chemically react with the liquid paint which will have an adverse affect on the paint film propertiesThe important properties of a solvent are Solvent power: strong solvents e.g. acetone, are required for complicated polymers, this

enables the molecules of paint to move more easily Rate of evaporation: some solvents evaporate quicker than others, also the method of

application will effect evaporation; spraying results in faster evaporation than brush applied coatings, if the rate of evaporation is to quick problems may arise In application i.e. dry spray

Flash point: this is the minimum temperature of the solvent at which the vapours given off are flammable if a source of ignition is introduced

Toxicity: the toxicity of a solvent can be determined from its occupational exposure limit expressed in PPM

A variety of apparatus exists for measuring toxicity, a common type known as the draeger tube works on the similar principle to a breathalyser

Weak solventWater emulsion, vinyls, acrylics, PVA/PVC

Aliphatics natural oils, natural resins, isomers e.g. alkydsWhite spiritTurps+substituesHexanes

Aromatics chlorinated rubberXyleneToluene

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BenzeneStyrene

Ketones epoxiesAcetoneMEK/MIBK

Polyurethanes use ketones and esters with aromatic solventsStrong solventOther constituentsIn addition to the main ingredients of a paint, namely the binder, solvent and pigment, there are other constituents added for a variety of reasons, e.g. to aid the paint manufacturing process, to increase shelf life, to aid application, to aid film formation, to aid drying or curing, to repel bacteria or suppress plant growth, to reduce flammability and to reduce UV degradation.Other constituents include PlasticisersAdded to paint to prevent a brittle film ThrixotropicsGives a jelly like structure which gives the paint build and anti-sag properties Dryers or catalystSome binders such as oils or resins need added dryers to speed up the drying processTwo pack paint rely on a chemical reaction to bring about the drying the catalyst provides this Anti-skinning agentsHelps prevent oil-based paints from skinning over ExtendersCheap mineral powders which can be added to paint in order to make it floe more easily, increase opacity but most of all to lower the cost of the paint OthersAnti-foaming agents, preservatives, fungicides and bactericides.Solutions and dispersionsAn opaque paint is dispersion; the pigment particles are suspended in the vehicle or binder.The vehicle of a paint solution of binder dissolved in solvent; a clear varnish is also a solutionSolutions In a solution, a substance known a the solute, which can either be a solid or a liquid, is dissolved in a liquid known as the solvent to form a homogenous substance Sugar ( solute ) with water ( the solvent ) Alkyd binder ( solute ) with white spirit ( the solvent )DispersionsIn a dispersion there is no solubility, one component, which could be a solid or a liquid, is surrounded by a liquid. There are two types of dispersions: suspensions and emulsionsSuspensionsIn a suspension, solid particles are dispersed within a liquid, each particle or group of particles being surrounded and wetted by the liquid, e.g. pigment and vehicleIf a paint was in complete dispersion each pigment particle would be completely surrounded and wetted by the binder In practice, complete dispersion is rarely achieved because the pigment particle group together in small groups known as aggregates when supplied by the pigment manufacturer; these aggregates are not completely broken down by the paint mills during paint manufactureHowever the paint manufacturer must break down the aggregates to achieve the necessary degree of dispersion of fineness of grind for the particular paint; for example the final degree of dispersion must be high to obtain gloss paintsEmulsionsIn an emulsion, minute droplets of one liquid known as the dispersed phase, are dispersed in a second liquid known as the continuous phase. The dispersed phase is completely surrounded by the continuous phase but is not dissolved by itE.g. Full cream milk: cream(dispersed phase) with water (continuous phase)

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Salad dressing: vinegar(dispersed phase)with oil(continuous phase) House hold emulsion PVA/PVC co-polymer(dispersed phase)with water(continuous phase)

UNIT 4 PAINT DRYING AND CURINGDrying is defined in Bs2015: 1992 glossary of paint and related terms as the change of a coating material from the liquid state to the solid state, due to evaporation of solvent, physo-chemical reactions of the binding medium, or a combination of these processesWhen the drying process takes place during exposure to air at normal temperature, it is called air-drying; if drying is accelerated by the application of heat above ambient temperature, but below that used for stoving, it is termed forced drying. Application of heat by using an oven or infrared is termed stovingVarious terms exist in relation to the drying of a paint film

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Dust dry- when dust no longer adheres to the paint surface Surface dry or sand dry- (not a Bs 2015 term) when the paint is dry on the surface but is soft

and tacky underneath. This term primarily applies to oil based paints Touch dry- when a very slight pressure with the fingers does not leave a mark or reveal

stickiness. Freedom from residual tack is another term encountered Tack free- free from tack even under pressure Hard dry- (not defined in Bs 2015) this term is used to describe certain degrees of film

hardness when tested by specified methods. The term may also be used when the drying has reached such a stage that if desired, a further coat may be applied

Dry to handle- the state of drying when a coated item can be handled without damageDrying mechanismsThe types of drying mechanisms are Solvent evaporation Oxidation Chemical curing CoalescenceSolvent evaporationSome paint dry solely by solvent evaporation to leave a film of non-volatile solids; but a permanent chemical change does not take place. This process is known as lacquer dryingPaints, which cure solely by solvent evaporation, are known as non-convertible or reversible paints. These terms mean that the binder is a linear or branched polymer, which is fully formed in the can and does not undergo further polymerisation after application. It also means that if the paint solvent or other solvents in the case of house hold emulsion paints, is reapplied to a dried coating, the coating will resoftenNon-convertible paints such as chlorinated rubber and cellulose lacquers are examples of paint, which dry solely by solvent evaporation (lacquer dry)OxidationPaints based on drying oils, which includes most alkyd and phenolic paints dry firstly by solvent evaporation and then by oxidation. This is sometimes known as oxidative drying. On contact with the oxygen in the air a chemical reaction takes place- the paint polymerises, with the aid of dryers which are present in the paint, to form a relatively hard filmPaints which cure by oxidation are known as convertible or non-reversible paints which means that if the paint solvent is reapplied after curing, the coating will not redissolve because of the permanent change that has taken placePaints, which dry by this process, have complex polymers compared to reversible or non-convertible paints.Chemical curingChemical curing paints cure by a chemical reaction between ingredients in the paint, unlike oxidation drying which is chemical reaction between the binder and the oxygen in the atmosphere. Paints employing this drying mechanism dry initally by solvent evaporation, if a solvent is present, and then by polymerisation due to a chemical reaction with the curing agentPaint which chemically cure are convertible or non-reversible paints, and therefore have complex polymers compared to reversible or non-convertible paints.Curing agents can be added to a paint prior to application (two-pack system), or they can already be present in a paint and then activated by an external source of energy, e.g.heat, ultra-violet light, infrared light electron beam etc.Paints which melt when heat is applied as known as thermoplastic coatings. Coatings which do not melt after the application of heat are know as thermoset coatingsTwo pack paints are used taking into consideration the pot life and in some cases the induction periodThe pot life is the maximum time period after mixing in which the paint must be used; this can vary from a few minutes to a few hoursThe induction period is the minimum time period during which the mixed components are left to stand before use. This is to allow for certain chemical reactions to take place. Induction periods are typically up to 30 minutes

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Other terms for induction period are lead time and stand timeCoalescencePaints, which dry by this mechanism, dry initially by water evaporation; this allows the polymers of the paint, which are fully formed during manufacture of the polymer, to come into contact with one another and physically join togetherAcrylic emulsions and vinyl emulsions dry by this mechanism and are classed as reversible or non-convertible paints, although this is not very obvious in practice.

Drying and curing tests

Ballotini testBallotini, which are tiny glass balls, are dropped onto a wet painted panel. The time in hours is given at the side of the panel and where the Ballotini fail to stick to the painted panel drying has occurredStylus testSimilar to the Ballotini test, the stylus employs a series of trailing needles which pass over the wet painted panel, because the needles are set at different tensions it can be established when the paint is tack dry, hard dry and fully cured

UNIT 5 COATING SYSTEMSCorrosion protection methodsPaint systems protect the substrate from corrosion by a combination of one or more of the following methods The barrier principle: the substrate is isolated from the environment, which causes corrosion

by using a coating, which has a low permeability to moisture and air. This may be achieved by applying a thick coat of paint or applying a paint having low permeability, e.g. epoxy, polyurethane; or even better applying a thick, low permeability coating.

Passivation: corrosion is retarded or arrested by chemical reactions between rust inhibitive pigments in the primer, and the substrate and/or moisture passing through the paint film

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Cathodic protection: this is achieved by coating the substrate with a paint containing metallic pigments, usually zinc (aluminium in some cases) which are ignoble with respect to the substrate. Cathodic protection may also be achieved by means of metal coatings such as zinc galvanising

Layers of a paint filmPaint systems may be single layered coatings or multi layered coatings. A multi layered paint system consists of a primer, at least one midcoat and a finish coat. Each coat has its own specificationPrimerThe function of a primer is to provide maximum and lasting adhesion to the substrate, to provide a key for the next paint layer and, in most cases, to retard corrosion by means of an inhibitive pigment when primer is applied to steel substratesAlthough not always practised, it is often considered good practice to apply primers with a brush as this enables the paint to be worked into the substrates surface, thereby providing optimum wetting of the substrate and mixing in any dust particles, thus achieving optimum adhesionEtch primers, also known as wash primers, are supplied either as single pack or two pack materials which contain phosphoric acid that reacts with the substrate and which also sometimes contain an inhibitive pigment such as zinc phosphate. This type of coating is often considered as an etchant or form of surface preparation rather than a primer in the conventional senseAnother form of etch primer is the mordant solution. An example of this is t-wash which was developed by the former British rail. T-wash essentially consists of an aqueous solution of phosphoric acid and copper sulphateMordant means “ of a corrosive nature ” or “ to bite into ”Many specifications will not allow etch primers to be sprayed due to their high toxicityMidcoatMidcoats may be standard undercoats or high build coats, primarily there to serve as a barrier to prevent the passage of waterPrimers and finish coats are often thin layered coatings and are quite permeable compared to midcoats; without a midcoat the inhibitive pigment in the primer would soon be leached out, which will lead to corrosionMidcoats also build up the film thickness to provide a more even surface by filling slight surface irregularitiesFinishThe final coat in a system gives a surface its final appearance; i.e. colour and gloss. A final coat must also have solar protective properties and, on most structural work, must have a gloss finish to allow water to flow more readily from a surface and allow the surface to be cleaned easily

Types of coating systemGeneralThere are various ways to classify paint systems as shown in the following table

Classification OptionsFunction Anti corrosion

Anti foulingDecorative

Flame retardantShop primer

Moisture tolerantRoad marking

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Binder type AlkydCellulose

Chlorinated rubberPVA/PVC emulsion

EpoxyPolyurethane

VinylPigment type Alumiumn

Micaceous iron oxideZinc rich

Red oxideZinc phosphate

Colour BlackBlue

GreenRed

WhiteYellow

The following table extracted from Bs5493 shows the principle coating systems identified by their binder

Type Characteristic constituentsZinc coating(except sprayed metal): bare,

sealed or paintedZinc and /or zinc-iron alloy

Sprayed metal: bare, sealed or painted Zinc or aluminium metalOrganic zinc-rich Zinc and organic binder

Inorganic zinc-rich Zinc and silicate binderDrying oil type Drying oil, urethane oil, alkyd, modified alkyd,

phenolic varnish or epoxy ester plus pigmentSilicone alkyd Silicone-modified alkyd plus pigment

One pack chemical resistant Chlorinated rubber or vinyl copolymer resin plus pigment

One pack chemical resistant and drying oil type primer

Epoxy ester or alkyd primer with Chlorinated rubber finish

Two pack chemical resistant Epoxy or polyurethane resin(including modification with coal tar)plus pigment

Two pack chemical resistant overcoated with One pack chemical resistant travel coat and

finish

Epoxy resin overcoated with Chlorinated rubber plus pigment

Bitumens Coal tar or mineral bitumen with or without pigment, coal tar enamel

Moisture curing polyurethane’s and high molecular weight linear epoxy resins which are both one pack chemical resistant materials, are not included in the product sections because of limited experience in their use at time of Bs5493 publication Sacrificial coatingsSacrificial coatings contain pigments which cathodically protect the ion or steel substrate to which the paint is adhered, these pigment particles eventually corrode thereby sacrificing themselves by corroding in preference to the substrate. In order to have this property the sacrificial pigment must be ignoble to the material to be coated; zinc and aluminium are the most common types of pigment employedZinc rich primers are two pack paints and contain metallic zinc in high concentration. A minimum zinc content of 90% (by weight) of the D.F.T. is often specifiedIn order to work effectively the zinc particles must be held in close contact with themselves and the substrate, therefore an efficient binder is necessaryOrganic zinc rich primers usually have an epoxy binder

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Inorganic zinc rich primers often contain a methyl or ethyl silicate binder. They have excellent heat resistance properties and may be used as a single coat system or they may be overcovered with a specialised finish, e.g. silicone sealer, if requiredMetal coatings e.g. zinc galvanising and metal-sprayed coatings, are principally used for very long-term protection and do not usually come under the category of paint systemsWhen a zinc rich or a zinc metal coating is subject to minor damage, e.g. a scratch, a corrosion reaction will take place which produces zinc salts that self-seal the damaged areaPowder coatingsPowder coatings are basically solvent free paints. They may be thermosetting or thermoplastic.Epoxy powder, which gives a thermosetting coating, is commonly used nowadays for a variety of applications including underground pipelines and domestic appliances. Each thermosetting powder particle contains base and curing agent, but they do not react together until they are activated with a heat source.The component to be coated is usually preheated, the powder may be then applied by using a fluid bed or spray technique; in both cases the powder is usually applied using electrostatic methods to achieve more uniform thicknesses and to reduce powder wastage via oversprayAfter application, the coating may sometimes be post cured in a subsequent stoving operationMoisture tolerant systemsThe surface that requires coating may be below dew point temperature, for example, due to low temperature gas or liquid in a piping system. In situations like these, moisture tolerant systems may be specified for use on damp surfacesVarious definitions may be used when moisture exists on a substrate, for example1. Damp surface- surface temperature is below dew point but there is no detectable water2. Moist surface- standing water and droplets have been removed but there is a thin film of

moisture on the surface3. Wet surface- droplets and free water are present on the surfaceDefinitions differ between specifications, always consult the applicable specification for exact definitions which applyPaints for use on damp surfaces include Moisture curing- e.g. one pack polyurethane Solvent free- e.g. two pack epoxy Water displacing- e.g. some moisture tolerant chlorinated rubbers Water absorbing- e.g. some moisture tolerant chlorinated rubbersMoisture curingMoisture curing materials are mainly polyurethanes, which cure by reaction with moisture in the air; they therefore require a minimum RH % for application rather than a maximum. This value is often quite generous being as low as 35% RH, however, a maximum may still be specified- check data sheetsThe curing rate is temperature dependent (as with other materials), but being moisture curing the curing rate is also RH% dependent. Most figures quoted for overcoating/cure times are based on 65%RHA maximum of 80%RH is typically specified for moisture sensitive urethanesImmediately a can is opened and moisture is in contact with the paint, the curing reaction starts, so this single pack material has a pot life varying from 6 hours upwards. Some manufactures stipulate that the material must be used within 24 hours or within one working day

Hot enamelsThe term enamels usually applies to a hot-applied bituminous materials, i.e. coal tar and asphalt (bitumen). Enamels were, and still are for some organisations, commonly applied to the external surfaces of pipelines, both in the factory for full pipe lengths and on site for welded jointsThe term enamel was originally a marketing term applied to hard thermosetting ready mixed paints because of their superficial resemblance to vitreous enamelsThe pipe is first blast cleaned or chemically cleaned then primed with the appropriate quick drying primer. Hot enamel (~2000c) is poured (flooded) over the primed surface then a fibreglass inner wrap is applied, possibly whilst simultaneously applying a second flood coat. Another flood coat is immediately applied and then an outer wrap, consisting of fibreglass strands impregnated with the

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same enamel material, is wrapped around the whole surface. This system gives a homogenous, continually bonded layer with spiral wound reinforcing. The thickness range is typically 3-4mmNote: bitumen and coal tar enamels are not compatible. They are also reversible materials and so will soften and bleed with the application of hydrocarbon solvents.Heating a blade and moulding the surrounding material into the void can easily repair pinholesTape wrapping systemsHot applied tapesIn general, these are only used in conjunction with hot applied enamels. They have very poor adhesion properties when applied onto a smooth surface, e.g. plastics. The tape material is melted by blowtorch or similar and fused to the primed material. The tape is then spirally wrapped around pipes and similar usually with an overlap of up to 55%Cold applied laminate tapeUsually consists of an outer polyvinyl chloride (PVC) or polyethylene (PE) film to which is bonded a mastic layer of synthetic rubber or rubber modified bitumen. Interleaving wax paper prevents adhesion between adjacent layersThe tape is applied to a primed substrate and spirally wrapped around the pipes and similar usually with an overlap of up to 55%. Being thermoplastic, temperature affects the ease of application and behaviour during serviceSelf adhesive overwrap tapesNot normally used as a wrapping system on their own but used as an overwrap system to hold other materials in place, e.g. fillers and grease based tapesThese tapes are PVC or PE with a thin layer of pressure sensitive adhesive. Used only on smooth substrates because they will not adhere properly to rough substratesGrease based tapesAre composed of a synthetic fibre bandage impregnated with petrolatum grease. Highly mouldable, flexible and can be used on any substrate providing it is free of loosely bonded contaminationFillersNormally used to modify contours of valves and flanges etc. to facilitate the use of tape systems. Typically based on petrolatum grease, bitumen and rubber, consideration need to be given to substrate and overwrapping material compatibilityPlastic coatingsThere are many types of plastic used in the coating industry, e.g. polypropylene, polyethylene and polystyrene. Common systems include heat shrinkable materials and materials, which are applied, hot then shrunk onto the component by the application of cold waterElastomeric coatingsElastomeric coatings have elastic properties and may be considered as synthetic rubber coatings. Elastomeric materials include neoprene, syntactic polyurethanes and EPDM (ethylene propylene diene monomer). After application some systems are vulcanised whilst others come in two-pack form and chemically cure

Unit 6 PAINT MANUFACTUREManufacturing paint is a relatively simple process. The skill in making paint is how the paint maker utilises the vast array of various ingredients at his disposalPaint is manufactured in paint mills whose main job is to grind down the pigment to the finest possible degree whilst at the same time mixing the ingredients to smooth high quality paint

Types of paint millDirect charge mills, which take and mix all the raw ingredients together. Types are- Ball mill Attritor mill High speed dispenser

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Pre mix mills, take the ingredients which have been pre-mixed to a rough consistency and refine this mixture to a high quality paint this system is used fir large production runs. Types are- Sand pearl Bead mill Colloid mill Single roll Triple roll

Typical manufacturing processSelect and assemble raw materialsMill the paintCarry out any quality testing requiredCan the paintWarehouse and distribute the paint

Fineness of pigment grindDuring the paint milling process the pigment content of the paint is ground down to a very fine degree. This is required in order to give the finished paint film smoothness, but more importantly enables the paint to carry a high degree of gloss where this is required. If a pigment has only been coarsely ground the surface finish will be relatively rough and when subjected to a light box test will appear somewhat dull due to the reflected light being scattered rather than reflected

The fineness of grind gauge, which is also called a hegman gauge, is a block of stainless steel around 175mm long by 65mm wide. It has a channel running along its length going from 0m to 100m; the depth is identified along its length. Paint is applied to the channel and then a flat edge scraper is pulled from the 100m deep end towards the shallow end. At some point along the way as the paint in the channel becomes shallower pigment particles will break the surface. Where between 5-10 particles break the surface this is said to be the fineness of grind

UNIT 7 PAINT COLOURSBs 5252 Framework for colour co-ordination for building purposes establishes a framework within which 237 colours have been selected as a source for all building colour standards and the means of co-ordinating them. It is not itself a range of colours for any particular product and is not used to specify British Standard Colours. A colour is only standardised when it is included in another British Standard, such as those that have been derived for paints BS 4800, vitreous enamel BS4900, Plastics BS4901 and sheet and tile flooring BS4092. Such standards contain only a portion of the total colours contained in BS5252 and are selected to meet design requirements within relevant technical and economic constraints

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0

50

100

BS 4800BS4800 –paint colours for building purposes specifies 100 colours, which have been selected from the framework of the 237 colours contained in BS5252Each colour in BS4800 is identified by three parts as followsHue:The first part signifies hue or colour and consists of an even number of two numerals (e.g. 04) twelve main hues are used and numbered02 red purples04 reds06 yellow reds08 yellow reds10 yellows12 yellow greens14 greens16 green blues18 blues20 purple blues22 violets24 purplesGreyness:The second part signifies greyness, i.e. the apparent amount of difference in greyness between one colour with another. Five grades are used; each defines by a single letter. There are four steps of diminishing greyness from A (maximum) to D (minimum). Beyond this colours are pure of free from greyness and graded as E, e.g. pure yellows are prefixed 10-eWeight:When coded only two on the above grades it was found that the yellow hues (yellow-red and yellow green) at minimum greyness looked heavy in comparison with other colours. This was overcome by raising the value of the yellowish colours. The result was more uniform in weight. The weight is given in pairs of numbers from 01-56. Groups of colours within each of the five greyness ranges are graduated from high to low valueEach of these graduations is numberedA greyness 01 to 13B greyness 15 to 29C greyness 31 to 40 D greyness 43 to 45E greyness 49 to 56

E.G. 18 E 53

Hue (colour) greyness weight

02-24 in A-E Colour saturationEven numbers A is neutral; E is pure colour 01-56

The above example signifies a deep bright blueBlack, white or any grey maybe up from only black and white are not colours but are called neutrals and are always pre-fixed by 00-Black is 00-E-53White is 00-E-55BS4800 can be used to identify existing colours when maintenance painting and is a method by which all manufactures can make exactly the same colour

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UNIT 8 PAINT TESTING GENERALThere are a large number of paint/paint film tests, which are carried out, some of which may be unique to one paint manufacturer or client’s specification. Bs 3900- Methods of Test for Paints – Is a specification widely referred to, which covers procedures, apparatus and related information on widely used test methods for paints, varnishes and similar products. The general introduction setting out the scope of the series is intended to be read in conjunction with each of the parts which are issued in loose leaf form and can be obtained separatelyA. Tests on liquid paints (excluding chemical tests)B. Tests involving chemical examination of liquid paint and dried paint films

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C. Tests associated with paint film formationD. Optical tests on paintsE. Mechanical tests on paint filmsF. Durability tests on paint filmsG. Environmental tests on paint films (including test for resistance to corrosion and chemicals)H. Designation of intensity, quality and size of common types of defect: general principles and

rating schemes

UNIT 9 ARTIFICIAL WEATHERINGFormerly natural weathering of painted samples provided information on how well paints performed when subjected to various environmental conditions. This however was a slow process, which sometimes took years to bring back results. In order to speed up this process a number of accelerated weathering devices were devised which brought back test results in a much shorter time

Common tests in use1) Salt spray box: simulates how paints behave under marine conditions

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2) Water soak test: tests for paint film permeability3) Tropic box: simulates how paints behave in a high humidity environment4) Temperature cycling: tests for paint film flexibility by subjecting samples to alternate high and

low temperatures5) Cold check testing: tests for low temperature contraction cracking

UINT 10 FLASHPOINTFlashpoints give an indication of fire risk and are defined as “ 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 “Flashpoint determination of paints or solvents may be carried out in accordance with Bs3900 part A9 using a closed cup of the ABEL type1. Fix the Abel cup containing the substance for assessment into a bath of water

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2. Apply heat source to the water bath and monitor the temperature of the substance in the Abel cup

3. Activate the source of ignition every 1/2

0c of rise in temperature4. The flashpoint temperature is identified when a blue flame flashes over the substance being

assessed

Note: if an orange flame is observed the temperature is too high and overheating has occurred. The material under test should replaced and the test restarted

UNIT 11 VICOSITYProcedure for measuring viscosity using a ford flow cup no4:1. Bring temperature of paint to200c+/- 0.50c2. Level the apparatus, then with the end of one finger over the orifice of the cup, rapidly fill it

with paint3. Allow a moment for air bubbles to rise, then draw a flat edge across the top of the cup to wipe

off the paint level with the edges

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Thermometers

Ignition

Water bath

Support

4. Remove finger from the orifice and start stopwatch simultaneously with the commencement of the paint stream. The watch is stopped when the first distinctive break in the paint stream occurs

5. The time taken in seconds is taken as the viscosityThis procedure can be used to determine the quantity of any added thinners. There is no direct relationship between the time value obtained and the percentage of added thinners. A comparison has to be obtained by preparing a number of control samples using different percentage of thinners added to pain taken from a freshly opened canA thrixotropic paint needs to be worked to reach the free flowing stage, therefore the viscosity cannot be assessed with a flow cup: a rotational viscometer or another type of viscometer, which works the paint, must be used

UNIT 12 DENSITYDensity is the weight per unit volume and is therefore found by the following formula

WeightDensity ---------

VolumeThe unit used for measuring the density of paint is usually grams per cubic centimetre (g/cm3)1cm3 of water =1 millimetre =1 gram

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1000 cm3 of water =1 litre =1 kilogramThe density of a paint will be higher than that of water, the density of a solvent will be lower than that of water, and the density of a curing agent may be higher or lower than that of water

Density cupProcedure for measuring density using a 100cm3 density cup1. Weigh the cup to the nearest decigram using laboratory balance with a 1000g capacity and a

sensitivity of +/- 0.1g2. Remove cover and fill with paint to 2.5mm of the brim3. Carefully replace the cover so that air and any excess paint is expelled through the vent4. Wipe off any surplus paint from the cover and re-weigh5. Determine the weight of paint by subtraction6. Divide weight by 100 if the density in g/cm3 is requiredThis procedure can be applied to determine the quantity of any added thinners. The weight of a sample of paint taken from a paint kettle could be compared with control samples which have been prepared by adding differing percentages of thinners to the paint taken from a freshly opened can. There is a relationship between the obtained weight and the percentage of added thinners if the pre-mix density of thinners and density of paint is known. It is also possible using this procedure to determine whether two-pack paints have been mixed in the correct proportionsRelative densityRelative density or specific gravity is the density of any substance compared to the density of water

Density of xSpecific gravity (sg)=----------------------

Density of water

Because the density of water is 1g/cm3 the figure obtained from the sg formula will be the same as that obtained from the density formula, the difference is that the answer for the sg formula will have no units, i.e. it is a dimensionless ratio

Example1. What is the density of a paint if 5 litres weighs 7,35kg?

weightA. Density ---------

Volume

30

100 cc

Vent

7.35kgB. Density ---------

5 litres

7.35 x 1000 gramsC. Density ---------

5 x 1000 cm3

D. Density = 1.47g/cm3

2. A two-pack paint is mixed at a ratio of seven parts base to two parts curing agent; the densities are 1.59g/cm3 and 0.78g/cm3 respectively. What is the density of paint after mixing?

a) 7 parts base 1.59 x 7 = 11.13b) 2 parts curing agent 0.78 x 2 = 1.56 c) 9 parts combined 11.13 + 1.56 = 12.69d) density 12.69 / 2 =1.41 g/cm3

Note the sg would be 1.41

UNIT 13 WET FILM THICKNESSThe wet film thickness is taken immediately after coating has been applied so that any deviation from the specified thickness range can be immediately rectified while the paint is still wet, thereby reducing the amount of dried coatings which are outside the specified thickness tolerances. Also any calculations based on volume solids will be meaningless if a lot of solvent has evaporatedThe wet film thickness may be found by using a comb gauge or an eccentric wheel

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W.F.T.

Procedure for measuring w.f.t. using a comb gauge1. immediately after application of the paint, the comb gauge should be placed firmly onto the

substrate in such a way that the teeth are normal to the plane of the surface2. the gauge should then be removed and the teeth examined in order to determine the shortest

one to touch the wet paintfilm. The film thickness should be recorded as lying between the last tooth touching and the first non tooth as shown on he tooth calibrations marked on the gauge

3. at least two further readings should be taken in different places in order to obtain representive results over the full coated area

NOTE: The w.f.t. is sometimes recorded as the average between the last touching tooth and the first non-touching toothThe wet film thickness may be found by calculationCalculation

VOLUMEW.F.T. = -------------- AREA

100W.F.T. = -------- X D.F.T.

V.S.(%)

UNIT 14 DRY FILM THICKNESSThere are four methods of determining the dry film thickness of paint non destructive test gauges destructive test gauges test panels calculationNon destructive test gaugesMeasuring the d.f.t. directly with a non-destructive test gauge is the most widely used method; there are a variety of gauges available with various scale ranges magnetic film gauge (banana gauge)

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PAINT Substrate

0

250

Scale

Comb gauge

Eccentric wheel

pull-off gauge or tinsely pencil magnetic horseshoe gauge eddy current/electromagnetic gaugeMagnetic film gauge (banana gauge)The banana gauge, as it is most widely referred to, may only be used for measuring the thickness of non-ferromagnetic coatings applied over ferromagnetic substrates. Prior to use, the gauge must be calibrated

Magnetic film thickness gauge

Calibration procedure1. choose a magnetically insulated shim of known thickness, close to the thickness of the paint

you expect to find, e.g. don’t choose a 25m shim to calibrate if you expect the coating to be in excess of 300m; this will reduce the accuracy

2. place the shim on the same substrate surface as the finish on which the paint to be measured is attached, e.g. if the paint is on a blasted surface, calibrate the gauge on an uncoated blasted surface

3. place the magnet onto the shim and press firmly on the instrument, wind the scale wheel forwards ( away from yourself ) until the magnet is definitely attached to the shim/substrate

4. Gradually wind the wheel backwards slowly until the magnet detaches itself. At this point, move the cursor on the instrument to the thickness of the shim as shown on the scale wheel. With some instruments the scale itself must be moved to line up with the fixed cursor. When using the latter type of instrument, rotate the wheel to zero to locate the position of the scale adjuster

The instrument is now calibrated and may be used to measure the d.f.t. of a non-magnetic paint films to within a claimed accuracy of +/- 5% in some cases

Pull off gaugeThis type of gauge may only be used for measuring the thickness of non-ferromagnetic coatings applied over ferromagnetic substrates. They are not very accurate compared to other non-destructive test gauges

The pull off gauge, or tinsley pencil as it most widely referred to, consists of a magnet at the tip of the instrument, which attached itself to the coated substrate. The gauge is then slowly pulled away from the coated substrate at normal

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Ferromagnetic substrate

Paint

Scale wheel

Magnet

Scale

Magnet

incidence until the magnet detaches itself, at this point the indicator on the body of the gauge is read (you have to be quick because the magnet and indicator are spring loaded). Calibration is required before use

Tinsley pencil

Magnetic horseshoe gaugeThe magnetic horseshoe type gauge works by measuring the change in magnetic flux between two poles of a magnet, the change of magnetic flux depends on the coating thickness. The accuracy of these instruments is claimed to be +/- 10% and as with the other magnetic gauges, may only be used for measuring the thickness of non-ferromagnetic coating applied over ferromagnetic substratesEddy current and electromagnetic gaugesThe most accurate of the non-destructive gauges for measuring the d.f.t. are eddy current and electromagnetic gauges of which there are many types. If calibrated correctly, accuracy is likely to be within +/-5%Eddy current gauges are used on non-ferromagnetic conductive substrates; electromagnetic gauges are used on ferromagnetic substrates such as ferritic steelMany Eddy current and electromagnetic gauges also have statistical capabilities and some will download and upload information from computersDestructive test gaugesDestructive test gauges cut into the paint film and should therefore only be used where necessary due to the cost of repairing the damaged coatingThey are sometimes used on paint films containing M.I.O. pigment; M.I.O. is ferromagnetic and therefore non-destructive test gauges, which rely on a non-magnetic coating, cannot be usedThe paint inspection gauge (p.i.g.) is one such type of Destructive gauge. A small vee shaped channel is cut into the coating at a fixed angle governed by a cutter built into the gauge. The width of the channel is then measured on a graticule scale by means of a microscope, which is again built into the instrumentOther destructive test gauges are the Saberg thickness drill or Erichsen thickness drill which work on a similar principle to the paint inspection gaugeTest panelsTest panels, e.g. metal plates of a known thickness, may be used to measure the d.f.t. indirectly, by coating them in the same way as the work being carried out and measuring the d.f.t. with a micrometer

CalculationThe d.f.t. may be assessed indirectly by measuring the W.f.t. of the paint, and providing the volume solids (v.s.%) content of the paint is known, calculating the d.f.t. is as follows

V.s.% x W.f.t.d.f.t. = -------------------

100ExampleWhat would the d.f.t. if 15litres of paint with a volume solids of 44% is used to cover an area of 12m x 7m?To find d.f.t.

a) D.f.t. v.s.%------ = ------------

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W.f.t. 100

v.s.% x W.f.t.b) D.f.t. = ----------------------

100

44% x W.f.t.c) D.f.t. = ---------------------- 100

To find W.f.t. W.f.t. is not directly given in the question, therefore must be found by calculation

Volume d) W.f.t. = ---------------

Area

15 litres e) W.f.t. = ---------------

12m x 7m

15 x1000cm3 convert all given units to common units, i.e. cmf) W.f.t. = ---------------

1200cm x 700cm

15 cm3 g) W.f.t. = ---------

840

15 x 10,000m convert to m (10,000m to 1 cm)h) W.f.t. = ---------

840

i) W.f.t. = 179m

Return to d.f.t. Formula

44% x 179mj) D.f.t. = ---------------------- 100

k) D.f.t. = 79m

UNIT 15 ADHESIONAdhesion failures more often occur between the uncoated substrate and the primer due to inadequate wetting of the substrate which may be as a result of insufficient surface preparation, insufficient dust removal after surface preparation or contaminationAll paints within a system should have compatibility between coats and with the substrate. It is advisable to obtain all the components for a paint system from one manufacturer otherwise it may not be possible to guarantee a system; when compatibility is lacking it is often the adhesion which suffers

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Adhesive failure between paint films

Cohesive failure

Vee cut testWith a sharp knife, cut a vee using approximately 12mm cuts forming a 300 angle, through the paint film and down to the substrate. Insert the tip of the blade under the tip of the vee and attempt to lever the paint away from the substrate. If the integrity of the coating is sound it should not peel cleanly from the substrateCross cut test (cross hatch test)Using a sharp knife or a multi-bladed cutter, cut 6 lines vertically and horizontally, 2mm apart, to produce 25 squares. Cover with adhesive tape and snatch off; the amount of segments remaining on the tape may be multiplied by four and then given a percentage value or a value may be given in accordance with the applicable specificationThe tapes degree of stickiness will be relevant to this test and the number and size of the squares may vary, therefore always consult the relevant specification for precise instructionsX-cut tape testWith a sharp knife or similar make an x shaped cut with the smaller angle between 300 and 450. The cuts must be made down to the substrate in a single action and are approximately 40mm in length. A piece of specified pressure sensitive tape approximately 75mm long and 25mm wide is placed over the cut and pressed down in the central area first using a finger. An eraser on the end of a pencil is then used to firmly rub the tape so full adhesion is achieved. Within 1-2 minutes the tape is pulled off rapidly at an angle as close to 1800 as possible. The x-cut area is then examined and the adhesion is rated using a scale from 5a= no peeling or removal through to 0a=removal beyond the area of the xDolly testA more technical adhesion test, the pull off adhesion test or dolly test, may show: Adhesive failure between primer and substrate ( most likely ) Adhesive failure between paint films Adhesive failure within a individual paint film

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Adhesive failure between primer and substrate

Substrate

Dolly

Load indicator

Load adjustment

Dolly puller

Paint film

Pull off dolly test

Procedure for carrying out pull-off test1. Clean and degrease the surface to be tested and the dolly contact surface2. Roughen both surfaces with fine/medium grade emery cloth3. Mix regular araldite and stick dolly to surface, leave for 24 hours at 250c4. Cut paint around the dolly down to the substrate using special cutter5. Attach pull-off instrument and apply pull-off force6. Take a reading from position of cursor when dolly detaches. Values will be typically obtained

in either Mpa, N/mm2 or p.s.i.Note alternative adhesives are possible, see test procedure sheetsA minimum pull-off value for the paint type used should ideally be specified in the specifications for the work being carried out. In the absence of such criteria, a minimum pull-off value should be obtained from the paint manufacturer who should also state categorically whether or not all values less than the minimum pull-off value are deemed as a failureHydraulic adhesion testThis test uses a similar principle to the dolly tester, but usually gives more accurate test results. The dollies used are re-usable and contain a hole down the centre through which a hydraulically operated rod applies force directly to the coated surface in order to pull the dolly away from the surface. The opposing force is supplied by the end of the adhesion tester, which grips the top of the dolly

UNIT 16 HOLIDAY DETECTIONHoliday detection or pinhole detection is an operation, which detects any holes/holidays in a coating or wrapping; the instrument used for this is known as a Holiday detector or pinhole detector. Substantial lack of thickness and inclusions in the coating may also be detected in some cases. Visual inspection in addition to holiday detection is still a very important part of inspectionThere are various types of Holiday detector, some used for thin paint coatings, e.g. the wet sponge type, whilst others may be used for coatings over 25mm thick, e.g. high frequency spark testers. For coatings ranging from approximately 0.5mm to 4mm thick AC/DC or pulsed DC Holiday detectors, usually powered by a 6volt battery, would normally be used

37

Substrate

Note holiday detection must not be carried out on wet surfaces or in the rainHigh voltage Holiday detectorsVoltage selectionPrior to carrying out holiday detection the correct voltage must be selected because too much voltage may indicate the presence of holidays where they do not exist, or really excessive voltages may even burn a hole into the coating. Not enough voltage may result in holidays not being detectedThe voltmeters or voltage settings on Holiday detectors should be checked for accuracy by using a method recommended by the Holiday detector manufacturer. This may involve using a calibrated volt/multi-meter or proprietary calibration voltmeter supplied by the detector manufacturerNote Holiday detectors should be checked throughout the working day to ensure correct set-up When relatively thin coatings are being tested, e.g. fusion bonded epoxy coatings; it is usually necessary to have a fine scale on the machine, e.g. 0-5kV for accurate voltage selection. For thicker coatings 0-20kV is normalCorrect holiday detection voltage is governed by the thickness and dielectric strength of the coating. The method to use for selecting voltage should be specified for each type of coatingThe correct voltage is ideally determined by detecting the presence of a known pinhole, which has been induced diagonally through the coating to the bare metal. However, the voltage is normally selected by measuring the coating/wrapping thickness and applying a formula, e.g. 125V per 25m of thickness (same as 5kV per mm), or following other specification requirementsNote it is preferable to ensure the coated surface is properly earthed by testing for the presence of a known pinhole. This may not be permitted due to the repair, which will have to be made on the pinholeOperationWhen operating a Holiday detector on a coated surface, an earth wire from the main unit is clipped to the structure or trailed along the ground. If the earth lead is to be trailed along the ground, the structure must be earthed, usually via a crocodile clip to a wire with a metal spike attached which is hammered into the groundThe electrodes (brushes) used, which are attached to the end of an insulated hand stick, are normally of wire brush type although carbon impregnated neoprene brushes also exist but are not as effective, spring wrap around coils are commonly used on pipesThe maximum travel speed for brushes or coils may be quoted in specifications, e.g. 300mm/secWhen the brush or coil comes into contact with a holiday, a spark will jump across between the gap, which completes the circuit. One or more of the following indications will warn the operator of its presence:A. The kV dial will dropB. An alarm will sound e.g. a buzzerC. A light will come onWhen a holiday is detected it should be marked / circled with a waterproof marker, but the marking should be sufficient distance from the holiday so as not to interfere with the adhesion of the repairWet sponge pinhole detectorsOnly low voltages are required for these instruments because water, sometimes containing a wetting agent such as washing up liquid, is used as an electrolyte to conduct the current from an electrode (wet sponge) through a pinhole to the conductive substrateWater is used to wet a sponge, which is connected to the positive terminal on test instrument. When the sponge passes over a pinhole, the water is drawn into it, which allows the DC. Current to pass through to the substrate and back along the return wire to complete the circuitSome wet sponge pinhole detectors have a Variable voltage setting between 9V and 90V, whereas others have only a single setting e.g.9VThere is no hard and fast rule for voltage to use these instruments but it is generally accepted that up to ~300m the 9V setting is adequate; up to ~500m would require the 90V setting. The specification or written instruction should state the voltage to be used

38

UNIT 17 SAMPLINGSampling refers to taking paint samples for analysis/testing and is concerned with taking the correct samples, ensuring correct identification of the samples and maintaining the samples in correct condition until assessedBs3900: Part A1 – methods of tests for paints. Sampling (same as en21512)Generally samples may be taken and analysed at four stages1. During manufacture- taken from the final manufacturing vessel2. During canning(drums, barrels and tanks)3. On delivery to purchaser- in which case the material should be unaltered4. At the point of application- to ensure adherence to manufactures recommendations(data

sheet)For items 3 & 4 an independent laboratory may be used for analysis, e.g. to determine density, viscosity, thinner type used etc.

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When taking samples at the application point, e.g. on site, the sample as supplied is termed sample A; the sample taken at the point of application is termed sample BThe following points should be noted during sampling:a) The paint should be thoroughly mixed to provide a homogenous sampleb) The sample should be truly representative, i.e. not confined to surface areas of the batchc) Two types of apparatus will be needed for mixing and taking the sample. Preferably broad

bladed stirrers of mechanical mixers and special sampling tubes or dip cansd) Containers should preferably be metal or glass. Metal containers should be uncoated

internally with tight closures. Containers should have closures which are not affected by the material e.g. solvent attack

e) All sampling equipment should be scrupulously clean and dry so as not to contaminate the sample

f) Take into consideration safety requirements, e.g. the material under test may be toxic or flammable

g) Method of sampling is related to the type of material, e.g. liquid, highly viscous or powder products. When sampling from bulk storage, several small samples from differing depths and locations should make up a representative sample

h) The number of samples taken upon delivery would largely depend on the amount received. Bs 3900: part 1A recommends n/2

Where n = the number of containers, undamaged, unopened containers would normally be chosen unless specifiedLabelling and sealing of the samples should be done as soon as possible and information on the label should include the following

i. Manufactures name / product descriptionii. Quantity and other delivery of deliveryiii. Batch numbers or other reference, e.g. tank numbersiv. Date of Manufacture and date sample takenv. Total number of samples takenvi. A reference number for the sample (for bulk delivery)vii. The name of the consignorviii. The place where the sample is to takenix. Name of sampler

Sample should be analysed as soon as possible and stored according to Manufactures recommendationsReportingThe sampling report should ideally contain the information given on the label and refer to Bs3900: part 1A or other specification used. Any abnormalities should be noted, for examplea) Container defectsb) Visible foreign matterc) Abnormal coloursd) Abnormal odourse) Errors of labelling etc.f) Presence of skin, settling etcg) Any difficulty in re-incorporationInformation on the tests conducted and the results also requires recording, but the details will depend on the tests conducted and the requirements of the specification

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UNIT 18 WEATHER CONDITIONSThe coating specification should always state the weather conditions in which a coating can or cannot be applied. A typical painting specification extract is as followsIt is not permissible to apply paints when the following conditions apply During rain, snow, or high winds When the air temperature is not at least 30c above the dew point temperature When the air or metal temperature is below 50c When the relative humidity is above 90%Relative humidity RH% and dew pointRelative humidity is the amount of water vapour in the air expressed as a percentage, compared to the amount of water vapour, which could be in the air at the same temperatureThe higher the air temperature the greater the amount of water vapour which can be held in it

41

The dew point is the temperature at which water vapour in the atmosphere would form condensation. Therefore, if the temperature dropped to the dew point temperature the Relative humidity would rise to 100% and condensation would be formed on any objects at, or below that temperatureNote the capacity of air to hod water doubles every 110c rise in temperatureMeasuring R.H.% AND Dew point Both Relative humidity RH% and dew point are measured using a hygrometer of which there are many types1. Aspirated hygrometersa) The screen hygrometers and masons hygrometers are static types which rely on a natural

airflow over a wet wickb) Assman and psychrodyne hygrometers are also static types which work by a fan driven

airflow over a wet wickc) Whirling hygrometers is a portable and dynamic type which operates by physically moving air

through the air2. Dial hygrometers come in two main forms: hair and paper. Hair hygrometers operate by

expansion and contraction of hair, usually human (treated), and are extremely accurate and fast in operation. Paper hygrometers also work on absorption but this time the absorption properties of paper

3. Digital hygrometers are split into two categories:i. Rh meters which give digital readouts of R.H. and D.P. onlyii. Thermo-hygrometers which give digital readouts of R.H.,D.P. and ambient dry bulb

temperaturesThe Whirling hygrometer or psychrometer, is the most common type used by coating inspectors consisting of two mercury-in-glass thermometers set side by side in a frame which is provided with a handle and spindle so that the frame and thermometers can be rotated quickly about a horizontal axis. The bulb on one of the thermometers, called the wet bulb thermometer, is covered with a closely fitted cylindrical cotton wick, the end of which dips into distilled water or clean rainwater contained in a small cylinder attached to the end of the frame Note the transport of mercury by air is not permitted, therefore coloured alcohol- in-glass thermometers may be specified for work, which involves equipment being transported by airThe frame is rotated by hand as fast as possible for at least 90 seconds, or otherwise specified, so that the bulbs pass through the air at least 4ms-1. This causes the water to evaporate from the wet bulb. The wet bulb cools down to a constant wet bulb temperature due to the evaporation rate of water from the wet wick. Always read the wet bulb temperature before the dry bulb temperature immediately after rotationNote the dry bulb temperature is the air temperature with a wind chill factorRepeat the operation until consecutive readings of each bulb temperature agree to within0.20cIf it is 100% Relative humidity the wet bulb will be the same as the dry bulb, because no evaporation can occur i.e. the air is saturated. If the wet and dry bulb temperatures are the same, the current temperature is dew pointThe Relative humidity and dew point cannot be read directly from the apparatus, hygrometric tables or special slide rules must be used. Hygrometric tables are more accurate in the 90% R.H region and aboveMetal temperatureThe metal temperature is measured with a magnetic temperature gauge, sometimes known as a limpet gauge, or electrical contact thermometer.

42

UNIT 19 PAINT APPLCATIONBrush applicationThere are many types of brush, which may be used to apply paint; the flat paintbrush is the most common type encountered in the UK and USA. The oval type tends to be favoured in continental EuropeBrushes may have fillings of the following types; Bristle, I.e. hogs hair Horsehair Natural fibres Synthetic fibres mixture of above

Bristles have a natural taper allowing the brush to maintain its form; they also have scales along the length of each hair, which allow the brush to hold more paint. Bristle brushes are usually high quality brushes and are quite expensive

43

Brush applied coatings often do not have the uniformity of thickness usually encountered with spray coatings, but they do not produce spray for or overspray, i.e. there is less of an environmental hazard, less wastage and less spotting etc. on nearby structuresBrushing also works the paint into a substrates surface, which gives optimum coverage and mixes in any dust particles, thus helping to achieve optimum adhesionRoller applicationRoller application is a quicker method than brush application and is useful for large flat areas, but unlike brush application, the paint does not get worked into the surface to the same extent and there is also a lack of uniformity in film thickness. Roller application is not permitted in certain specifications for certain workCurved roller exist i.e. for pipes; some types of roller have a paint fed via a fluid line connected to the handleSpray applicationSpray application produces a production rate well in excess of that achieved by brushing and is therefore a very common paint application methodFor a paint to be sprayed successfully it must first be atomised, i.e. it must be broken down into very fine droplets. Atomisation is achieved by both the conventional spray and airless spray application methods but by different methods in each caseConventional sprayThe paint is held in a container attached to the top of the gun in the case of gravity feed; underneath the gun in the case of suction feed; or remote from the gun in the case of pressure feedPressure fed conventional spray guns are able to cover much greater areas without the need to continually refill the container and therefore the most common type of conventional spray systems encountered. Two lines feed the spray gun; one carrying the paint at low fluid pressure, usually under 20 p.s.i. and the other carrying an air supply at a pressure of approximately 40-75 p.s.i.. The air supply leads to an air cap on the gun and blows onto the paint stream as it exits the nozzle resulting in very fine atomisationAirless sprayOne line feeds the spray gun carrying the paint at a pressure typically between 2000 and 4000 p.s.i.. There is no air cap on the gun, hence the term airless. The paint is atomised by forcing it through a small orifice at high pressure; when the paint meets the air it splits up into fine droplets due to the air resistanceThe unpressurised container containing the paint is remote from the gun; the paint is sucked up using a fluid pump and fed to the gun by way of a special reinforced high pressure fluid line. A compressor supplies the air up to approximately 100p.s.i. to the fluid pump; this air inlet is adjustable. The fluid pump multiplies the pressure by a ratio governed by the pump, e.g. 35:1 ratio pump supplied with an air pressure of 100 p.s.i. would lead to a fluid pressure of 3500 p.s.i In addition to the conventional compressors supplying compressed air, other methods exist to achieve the fluid pressures necessary for airless spray, e.g. the diaphragm pump and the electric pump, both these supply hydraulic pressureThe spray tips on airless spray guns are usually lined with tungsten carbide; some have the advantage of being reversible by turning a lever 1800 to clear out any blockages in the small orifice. There is also a tip known as the titian tip which has an adjustable hole sizeThe orifice size governs the throughput of paint whilst the orifice angle governs the size of the patternTypical requirements for tip orifice sizes used with airless spraying are shown in the table

Paint type Tip size Airless pressure (p.s.i.)Chlorinated rubber 13-21 thou” 2400High build epoxy 17-23 thou” 3000Zinc rich paints 17-23 thou” 2800

Using airless spray equipment can be very dangerous due to the high fluid pressure; operatives must always adhere to the following

use tips designed for airless spray

44

use fluid lines in good condition designed for airless spray ensure there are no kinks in the line ensure the safety catch is on when the gun is put down never point the gun at anybody or yourself never attempt to clean or change nozzles when the fluid is pressurised ensure the equipment is earthed to prevent static shock wear an airfed helmet

Comparison of airless and conventional sprayingFactor Airless spraying conventional spraying

atomisation Usually coarser atomisation Fine atomisation for high quality finishes

Method of atomisation Pressure drop when fluid exits small orifice

By air jets on the nozzle

Air pressure Up to 100p.s.i. to the fluid pump

40 to 75 p.s.i. to the gun

Fluid pressure 600 to 6000 p.s.i. Less than 20 p.s.i.Fluid delivery Medium to high

higher productionLow to medium

Lower productionAir contamination More overspray but less fog

and reboundless overspray but More fog

and reboundMaterials sprayed Uniform fine grinds necessary,

although can handle relatively high viscosity’s

Basically any materials that flow well can be sprayed

Maintenance More required especially tips, equipment is more

sophisticated

Less require; equipment is more basic

Electrostatic sprayThis method of paint application requires the use of a special spray gun, which applies a charge to the paint when applied. The article to be coated is earthen so that the charged particles are attracted towards it; when an area on the component has been coated to a particular thickness there is less of an attraction, due to an insulating effect the coating has, although the paint may still be attracted by unquoted areas on the article which may not be in the direct line of the application nozzleThis type of application results in uniform coating thickness with a substantial reduction in overspray. Electrostatic spray and other Electrostatic application methods are widely used in factories for coating all types of components including pipe, fridge’s, washing machines etc using both liquid and powder coatings

Other methods of paint applicationOther methods of paint application include dip coating padding hot spraying spin rotating flow coating/curtain coating aerosol

GalvanisingUsed for structures, fitting and cladding. The components are degreased, e.g. with warm caustic soda, acid cleaned, washed, fluxed with ammonium chloride then immersed in a bath of molten zinc between 420-4500c to achieve a coating thickness of approximately 85-130m

45

Sheardising Used for fittings, fasteners and small items, which are likely to be distorted by, hot dip Galvanising.Sheardising is particularly suited to threaded components where only a small change in dimension is acceptable. The items are first degreased and pickled then tumbled for a few hours in hot zinc dust at a temperature just below the melting point of zinc. Coating thicknesses of approximately 15-30m are typically achieved

CalorisingSame principle as Sheardising but using aluminium powder

Anodising An electrolytic method of coating aluminium with a dense oxide. The component to be Anodised is dipped into a bath of weak acid usually sulphuric and oxidation is induced electrically. For adhesion of subsequent paint films, etching may be required

ElectroplatingThe plating of small parts by the electrolytic deposition of metal, e.g. zinc, from metal salt solutions, coating thickness of up to 25m are usually applied

Hot metal sprayingMetallisation, or the method of hot spraying one metal with another, is a widely used system for preventing corrosion of metal structures. The most common sprayed metals used are aluminium and zincApplication is normally carried out using one of the following methods1) Powder fed system the coating metal is supplied in fine powder form, and blown through a

heat source onto the substrate. This is a very wasteful method which often requires a recycling facility

2) Electric arc system the wire is used as one electrode (similar to the welding process), the energy from the arc melts the wire and resulting molten metal is blown onto the substrate by means of an air jet. The arc method of metal spraying is now widely used for mechanical applications, e.g. production lines, where thousand of components of identical shape are to be coated

3) Wire and pistol system this is the most common method of metal spray application in use. It consists of a hand held gun with an air-powered motor, which draws wire through knurled feed rollers into an oxygen/fuel gas flame where the metal is melted. This molten metal is then projected onto the prepared surface by the products of combustion augmented by an air blast from the compressed air driven motor

Metal-sprayed coatings are often sealed to prolong their life due to their porous nature

UNIT 20 COATING FAULTS

The following have been extracted from BS2015 glossary of paint termsBittiness: The presence of particles of gel flocculated material or foreign matter in a coating material or projecting from the surface of a filmNote 1: the term seedy specifically denotes the presence of bits that have developed in a coating material during storageNote 2: the term peppery is sometimes used when the bits are small and uniformly distributedBleeding: The process of diffusion of a soluble coloured substance from, into or through a coating material from beneath, thus producing an undesirable staining or discolourationBlistering: The formation of dome shaped projections or blisters in paints in the dry film of a coating material by local loss of adhesion and lifting of the film from the underlying surface

46

Note: such blisters mat contain liquid, vapour, gas or crystalsBloom: A deposit resembling the bloom on a grape that sometimes forms on the gloss film of a coating, causing loss of gloss and dulling of colourChalking: 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 weatheringNote: chalking can be considerably affected by the choice and concentration of pigmentCissing: The formation of small areas of the wet film of a coating material where the coating material has receded leaving holidays in the filmCracking: Generally, the splitting of a dry paint or varnish film, usually the result of ageing. The following terms are used to denote the nature and extent of this defect1) Hair cracking: cracking that comprises of fine cracks which may not penetrate the top coat;

they occur at random2) Checking: cracking that comprises of fine cracks which may not penetrate the top coat and

are distributed over the surface giving the semblance of a small pattern3) Cracking: specifically, a breakdown in which the cracks penetrate at least one coat and which

mat be expected to result ultimately in complete failure4) Crazing: cracking that resembles checking but the cracks are deeper and broader5) Crocodiling or alligatoring : a drastic type of crazing producing a pattern resembling the hide

of a Crocodile or alligator6) Mud cracking: a network of deep cracking 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 high pigmented water borne paints

Cratering: The formation of small bowl shaped depressions in the film of a coating materialCurtaining, sagging: A downward movement of a paint film between the times of application and setting, resulting in an uneven coating 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; hence the synonymous term curtainingDry 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 coatEffloresenceNOT A PAINT DEFECT. It is the development of a crystalline deposit on the surface of brick, cement, etc., due to water containing soluble salts, coming to the surface, and evaporating so that the salts are deposited. In some cases the deposit may be formed on the top of any paint film present, but usually the paint film is pushed up and broken by the Effloresence under the coatFlakingLifting of the coating materials from the substrate in the form of flakes of scalesFlocculationThe development of loosely coherent, pigment agglomerates in a coating materialGrinning The showing through of a substrate due to inadequate hiding power of the coating materialHolidaysA 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 substrateLiftingSoftening, swelling or separation from the substrate of a dry coat as the result of the application of a subsequent coatOrange peel effectThe 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

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Note see also spray mottle and pock-markingPinholingThe formation of minute holes in the wet film of a coating material that form during application and drying due to the air or gas bubbles in the wet film which burst, giving rise to small craters that fail to coalesce before the film has setResidual tackThe degree of stickiness remaining in a film of a coating material which, although set, does not reach the true tack-free stageRopinessPronounced brush marks that have not flowed out because of the poor levelling properties of the coating materialSaponificationNot specifically a paint defect term. The formation of a soap by the reaction between a fatty acid ester and an alkaliNote: 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. Saponification films may become tacky and discoloured. In very severe cases film may be completely liquefied by SaponificationWrinkling, rivellingThe development of wrinkles in a film of a coating material during drying, usually due to the initial formation of a surface skinNote: see also crinkling and finish

UNIT 21 DEFINITIONSCompliant coatingA coating which complies with the environmental protection act of 1990 E.P.A.ContractorA person, firm or company which enters into a contractLong term protectionTypically 10 yearsMedium term protectionTypically 5 yearsShort term protectionTypically 2-3 yearsDamp surfacesTemperature below dewpoint but on which water is not readily detectableMoist surfacesWhere standing water and droplets have been removed but which still displays a noticeable film of water

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Wet surfacesSurfaces on which standing water or droplets are present (when these may be in evidence final surface preparation is not normally carried out)New galvanisingA galvanised steel surface upon which a cohesive oxide layer has not yet formed (bright and shiney, less than three months old)Fully weathered galvanisingA galvanised steel surface upon which a cohesive oxide layer has formed by natural weathering (dull and lacking in sheen)DewpointThe temperature at which condensation would form on a substrateT WashAn etch primer for zinc metal surfaces. Blue in colour it turns black upon drying if it has been successfully appliedHot duty surfacesMetal surfaces that will reach a temperature in excess of 990c when in useTest areasMay be requested in order to demonstrate that the selected system is capable of meeting its requirements. This refers to the method and standard or preparation, equipment used, paint specified and on the same substrateAccess equipmentA green tag identifies safe scaffolding. Unsafe scaffolding by a red tag. Scaffolding is inspected by a competent at least every seven days and always after bad weatherSheeting for protectionSheeting of a non-flammable nature must be used to protect areas not being prepared or painted from contamination, (in particular tarpaulins must not be used)Metallic zinc paintsSpecial care must be taken not to allow zinc rich paints to contaminate stainless steel nor to be applied within 75mm of weld end preparationsTie coatWhere adhesion problems, solvent stripping or bleeding might occur, a tie coat would be employed to prevent these particular problems arising

UNIT 22 DUTIES OF A PAINT INSPECTORThe job of a paint inspector is to inspect and report. He must act with integrity at all times and be true to the specification requirementsThe following is a list of points, which form the basis of typical activities performed by painting inspectors

1 General Obtain or gain access to the specification Learn the specification Ensure your instruments are In proper working order Get to know the plant Get to know the personnel Check work for conformance to the specification

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Keep the engineer/supervisor informed at all times especially if there is any departure from the specification

Make written reports at an agreed frequency Attend site meetings when required

2 At the beginning of the day Check the environmental conditions Check the equipment (preparation, application and inspection for conformance to

specification) Check materials Check the previous days work where applicable ( film contamination and dft if wet films have

been left overnight) Establish with the contractors senior personnel the days work programme

3 During the day Check environmental conditions Check equipment Check usage of materials (when specifically required to do so) Check each operators work

4 On completion of the work at each stage Ensure that the work meets the specification by carrying out or witnessing tests Check for any application faults

5 At the end of the day Check the days work Check housekeeping Complete reports

6 On completion of the job Ensure as objectively as possible, that the specified requirements have been satisfied Write a summary report if required to do so

Ideally, painting inspection personnel should be issued with relevant procedures and work instructions to enable them to carry out inspection and associated activities in accordance with the clients or organisations requirements. The procedures should leave the inspector in no doubt as to what is to done. Unfortunately, this documentation rarely exists!

Typical inspectors duties

Before work commences1. Determine your duties and responsibilities. Duties may include those, which relate to health

and safety aspects talking into consideration mandatory requirements. You may also be required to check that rejected paint or used abrasive is disposed of correctly

2. Ensure the contractors supervisor is aware of your duties and authority3. Ensure you have correct applicable specifications and data sheets. Also ensure you have at

least have access to relevant referenced normative documents4. Determine the order of precedence for narrative documents if the specification does not make

it clear5. Learn the specification, procedures, work instructions etc.6. Approach the senior inspector or supervisor if you are not sure of what is intended of any

requirement7. Ensure you have copies of any applicable documentation, e.g. correspondence, minutes from

meetings, concessions etc.

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8. Liase with the contractors supervisor to determine whether the contractors personnel are familiar with the work requirements

9. When required, confirm that the contractors operators are properly trained and conversant with the equipment, materials and application techniques being used

10. Agree with the client /supervisor the level of liaison that is required and determine reporting/recording requirements

11. Ensure you have test instruments etc. that are required and that they are properly calibrated and in correct working order

Surface preparation

1. Check the specification, procedures and/or work instructions to establisha) Standard against which work will be measuredb) Methods by which work is to be assessed, e.g. surface comparatorc) Degree of surface cleanliness requiredd) Surface profile requirements (where required)e) Any special test to be carried outf) Requirements regarding equipment and consumablesg) Ambient conditions requiredh) Recording/porting requirements

2. Check the condition of the substrate before cleaning and make a note of rust grade, general contours (sharp edges, burrs etc.), spatter or flux residue on welds, algae or mould growth, zinc salts etc. any areas suspected to be defective e.g. cracked, laminated or mechanically damaged, should be reported immediately to the supervisor or client

Note do not allow surface laminations, cracks and similar to be dressed without the permission of the supervisor/client

3. Ensure ambient conditions allow surface preparation to take place. The following may have to be assessed/measured

a) Air temperatureb) Steel temperaturec) Relative humidityd) Dew pointe) Moisture on substratef) Potential sources of contamination, i.e. chemicals, salt spray, fumes, etc.g) Potential changes in the weather to adverse conditions

4. Identify areas being prepared

5. Check that the correct materials and equipment are being used, e.g. correct type, correct size, consumables are free from contamination, etc. examples

a) Abrasive type, size and cleanliness. No reuse of expendable materialb) Correct wire brushesc) Correct needles in needle gunsd) Presence of carbide tips on scraperse) Correct chemicals for cleaning

6. Carry out inspection of prepared surfaces as required by the specification

7. Record the results of the inspection. The areas inspected must be identified in the report ensuring that it is clear what has been accepted and what has been rejected. The reasons for any rejections should be clearly identified

8. Ensure that all concerned are clear about the reasons for any rejections

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9. Where remedial work has been necessary, re-inspect for conformance to the specification

Paint Material

1. Check the specified requirements

2. Check that the paints delivered to the work place correspond to the requirements of the specification and data sheets. The specification may require certain information to be displayed on each paint container

3. Check that all paints to be used on a surface are supplied by one paint manufacture (unless otherwise specified)

4. Check that the paint is the correct type for application method being used, i.e. brush grade or spray grade

5. Check that the paint storage conditions are correctNote: any warranty on the material is likely to depend upon proper handling and storage

6. Determine whether the paint is being withdrawn from the store in proper rotation

7. Ensure paint is not being used beyond its shelf life

8. Monitor material usage to determine whether there is sufficient paint in storage for the completion of the job (or part job). This is not always the responsibility of the inspector

9. Check that the paint is being mixed and stirred correctly. Any permitted addition of thinners must be monitored to ensure correct type and amount. For two pack paints

a) Check that the materials are mixed strictly in accordance with the paint manufactures data sheets

b) Confirm that any induction time is strictly adhered to or time is allowed for gas bubbles to escape (if applicable)

c) Confirm that mixed material is not used after pot life

10. Conduct all necessary paint sampling procedures and tests; or confirm that such tests have been carried out prior to the commencement of work. Record batch numbers of paints tested

Paint Application

1. Check the specified requirements

2. Check that the surface to which the paint is being applied is free from contamination, i.e. oil/grease, dust, spent abrasive. Corrosion products etc. any areas suspected as being defective, e.g. cracked, laminated or mechanically damaged, should be reported immediately to the supervisor or client

3. Ensure that the ambient conditions allow painting to take place. The following may have to be assessed/ measured

a) Air temperatureb) Steel temperaturec) Relative humidityd) Dew point temperaturee) Moisture on the substratef) Potential sources of contamination, i.e. chemicals, salt spray, fumes, dust etc.g) Potential changes in the weather to adverse conditions

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Note: check that the particular paint being applied does not have any special restriction on its application

4. Confirm that paint is not being applied to coated substrates either before or beyond the specified overcoating times for the existing coating

5. Check that the correct application method is being used

6. Identify areas being painted

7. Confirm that stripe coats have been applied correctly if specified

8. Carry out inspection of painted surfaces as required by specification. For examplea) Check each coat of paint is uniformly applied and is free from curtains, sags, runs, holidays or

other visible defectsb) Measure the wft immediately after applicationc) Measure the dftNote: it there are significant differences between the dft calculated from the wft and the measured dft, check the material to ensure that only permitted additions of thinners were made to the paint if possible

9. Ensure that any areas of defective coating are identified for remedial work

10. Ensure that all concerned are clear about the reasons for any rejections

11. Re-inspect any remedial work carried out to ensure that it conforms to the specified requirements

12. Check that the completed work is uniform in colour and finish

13. Unless the specification states otherwise, check that the handling of recently coated items is carried out in such a way that the coatings are not damaged

14. Record the results of the inspection. The areas inspected must be identified in the report ensuring that it is clear what has been accepted and what has bee rejected. The reasons for any rejections should be clearly identified

Typical contractor malpractice’sTypical contractor malpractice’s which the inspector should be aware of are as follows

1. Use of unskilled operators. This may relate to surface preparation, application of paint or safety considerations e.g. unsafe scaffolding

Note: the painting inspector cannot normally report on unskilled operator as something, which does not conform, to specification

2. Use of unsuitable equipment, which may be worn brushes, poorly maintained and leaking compressors, damaged ladders contaminated equipment from previous contract etc.

3. Painting or preparing surfaces during inclement weather conditions such as rain, snow, fog, mist etc.

4. Hand mixing paint which should be mechanically mixed

5. Painting before inspection of substrate preparation or previous coat

6. Applying two paint coats or more to the same area in one day assuming the specification does not allow this

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7. Missing out a coat

8. Use of wrong solvent or an excessive amount of solvent

9. Use of incorrect paint type or mixing different manufactures products

10. Storing paint incorrectly, e.g. where the specification requires paint to be stored In a temperature controller environment

11. Paint used outside expiry date, suspected by condition of can, excess binder on top of freshly opened can, pigment settlement. Check the date by way of the manufactures coding: inform engineer

12. Re-using expendable abrasives

13. Insufficient blasting or painting in difficult areas such as under pipes

14. Cleaning surfaces with contaminated cotton waste or rags or using materials for cleaning which are not permitted

15. Applying a thickness of paint which is less than the specified minimum

Note: using diesel in the paint (as a plasticiser or thinner) has been done on many occasions! This will prevent proper drying

QUESTION PAPER 11. Describe why and how corrosion occurs?2. Describe what is meant by Bi-metallic corrosion?3. Describe the following

a) The galvanic series?b) Millscale?c) Rust grades to SIS 05-59-00?d) Blast cleaning grades to SIS 05-59-00?

4. Describe the difference between water blasting and pressure washing?5. Name two tests used to determine the presence of hygroscopic salts left on a blast cleaned

surface?

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6. Name four expendable abrasives and state the difference between an expendable and a reusable abrasive?

7. Name the three basic constituents of a solvent carrying paint and the functions that each performs in the paint?

8. Name six pigments and there respective colours?9 Describe what qualities a laminar pigment gives to a paint and name four laminar pigments?10 Describe

a) A barrier paint system?b) A sacrificial paint coating?

11 Describe what a tie coat is and give three reasons why we may require one?12 Describe a typical paint manufacturing process and give two types of paint mill, which could

be used?13 Describe what you know of viscosity and outline one method of testing?14 Describe what you know of density?15 Can you use a banana gauge over

a) An M.I.O coating?b) An aluminium substrate?c) A galvanised surface?

16 Describe a destructive test for determining the dry film thickness of a paint?17 Give four artificial weathering devices and describe what they simulate?18 What is the name of an instrument, which measures opacity and gives two examples?19 Describe a dolly test for adhesion?20 How do the following work

a) Conventional spray set-up?b) Airless spray system?c) Electrostatic spray system?

21 How is B.S. 4800 used in the construction industry?22 What are the essential differences between a blister and an osmotic blister?23 How do the following display their characteristics

a) Bleeding?b) Blooming?c) Chalking?d) Cissing?e) Grinning?f) Flocculation?g) Lifting?h) Ropiness?i) Solvent stripping?j) Oxidation?k) Chemical curing?l) Overspray?

24 Define the followinga) Dewpoint?b) R.H.?c) Induction period?d) Pot life?e) Flash point?f) Shelf life?g) Batch number?h) Masking out?i) Rust blooming?j) Water borne coating?k) B-SA2 ½? l) Feathering?m) A-ST3

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QUESTION PAPER 21. What kind of process is corrosion?2. What particular problems relate to maintaining of an effective coating system in the splash

zone?3. What factors influence the rate of corrosion?4. What is millscale and why is it a particular problem when attempting to protect steel?5. What is the number of the Swedish standard contained within BS 7079 and list the rust and

blasting grades together with their brief titles?6. Name three tests for locating hygroscopic salts and mill scale on a blast cleaned substrate?7. What factors influence how clean and how rough the surface becomes after blast cleaning?8. Regarding hand and power tool cleaning state what always concludes this process and to

what standards?

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9. What three basic ingredients make up a traditional solvent carrying paint?10. What are the functions of each of these three in a solvent carrying paint?11. Name five items that can be included in paint to improve its performance or suitability for a

certain use?12. Identify six paints by their binder names?13. Name four binder /solvent combinations?14. Name six opaque pigments together with their respective colours?15. Describe what a laminar pigment is?16. What is the difference between a convertible and a non-convertible coating?17. Define the term induction period?18. Define the term pot life?19. Define a barrier system of coatings?20. Define the term tie coat and give an example of its use?21. What is the difference between thermosetting and thermoplastic?22. Name two types of paint mill and give two examples of each?23. What is the title of BS3900a) How many tests are set in BS3900?b) How are the tests identified (name, number, letter) which?24. What does a fineness of grind gauge measure?25. What is flashpoint and in what apparatus is it determined?26. What is viscosity and what equipment is used to measure the viscosity ofa) Free flowing paintb) Thrixotropic paint?27. What is a density cup and give an example of its use?28. Calculate the volume solids of a paint it the w.f.t. was 186m and the d.f.t. was 93m?29. Calculate the w.f.t. of a paint if the vs% was 66% and the d.f.t. was 88m?30. Calculate the d.f.t. of a paint if the vs% was 44% and the w.f.t. was 234m?31. Calculate the density of a paint if a 5ltr tin weighed 15 kg?32. Calculate the density of a two pack material if part A paint was 1.5g/cm3 and part B catalyst

was 0.5 g/cm3 assume a mix ratio of 1:1?33. Name an example of a non-destructive d.f.t. gauge?34. Name an example of a destructive d.f.t. gauge?35. Name four artificial weathering devices and say what they are designed to simulate?36. Name two drying / curing test and stare hoe they are operated?37. What does a cryptometer measure and give two examples of a cryptometer?38. What determine the degree of gloss a paint may possess?39. Name three adhesion tests and describe how one of them is carried out?40. Name eight duties of a paint inspector?41. List eight-contractor malpractices?42. Name three methods of applying paint and compare the advantages and disadvantages of

each in terms of quality and cost effectiveness?43. What does BS2015 glossary of paint terms refer to and give three examples?44. What is BS 4800 and how is it used in the construction industry?

45. Define the following a) Long term protectionb) Medium term protectionc) short term protectiond) new galvanisinge) weather galvanising46. What is the procedure for removing oil and grease from a surface before preparation

commences?47. What is the procedure for removing oil and grease from a surface if found after preparation

has taken place?48. What is the procedure for removing algae and mould growth from pipework?49. Under what conditions must paint be stored?

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50. What is ‘t’ wash, what is it used for, what colour is it, and how is it checked for correct reaction when using it?

51. What distance must be left at areas to be welded when painting?52. How much must overlap old repairs?53. What is the maximum allowed time for priming to take place following preparation and by

what method is primer applied?54. What must all pneumatically and electrically equipment operated power tool be?55. What type of sheeting may and may not be used for protection against spillage and spotting?56. What must not be used to clean out paint kettles or to dispose of rubbish?57. Give the rules for the mixing of paints regarding sizes?58. What are osmotic blisters and what can cause them?59. Some metals are difficult to prepare name three and describe the way you might prepare

them?60. Where a pipeline leaves the ground what problems would you find and how might you treat

this?61. Name five paints and state their drying or curing method?62. Name a black pigment in common use apart from coal tar?63. What is a holiday detector what does it detect and how does it work?64. If you worked on a contract where frequent stoppages occurred for poor weather, what

additional information would you record?65. What is important about selecting a paint system suitable for certain environment?66. State the principles of being able to use a banana gauge for taking d.f.t. readings?67. Give one advantage of using testex tape and disposable w.f.t. combs?

ANSWERS PAPER 2Example answers

1. Corrosion is an electro chemical process

2. Problems include: growth of algae, never dries properly, excess electrolytic solution

3. Oxygen content on the surface, R.H. above 60%, temperature, presence of impurities and higher nobility metals

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4. Millscale is loosely adhering and will flake off and also higher nobility

5. Sis-05-59-00 Rust grades Blast gradesa) Millscale and no rust sa1 light blast cleanb) Rust and little millscale sa2 thorough blast cleanc) Rust and light pitting sa21/2 very thorough blast cleand) Heavy pitting and rust sa3 blast to visually clean steel

6. Potassium ferrocynaide, silver nitrate and copper sulphate for millscale

7. Abrasive – size, shape, hardness and density of blastHuman – speed, angle, distance and time of the nozzle

8. Sis 05-59-00St2 manual wire brushingSt3 mechanical wire brushing

9. Solvent, binder and pigment

10. Binder- hold particles together, gives finish, gives adhesion, gives flexibility and dryingPigment- gives colour and opacity, resistance to acids/alkalisSolvent- evaporating part of process, thins paint, cleans equipment and degreaser

11. Extenders- improve opacity help flow and lower costAnti-skinning- to stop paint skinning overStabiliser- to stop paint separating in canThrixotropic agent- to enable high buildDryer- to improve drying

12. Acrylic, cellulose, epoxy, emulsion, polyurethane, chlor rubber and alkyd

13. Epoxy – acetoneChlor rubber – xyleneAlkyd – white spiritEmulsion – water

14. Red lead- redZinc chromate – yellowCarbon – blackTitanium dioxide – whiteCalcium plumbate – whiteCoal tar – black

15. A pigment that has leaf shaped particle that interlock together to form an impervious coating when dry

16. A non-convertible coating undergoes a chemical change upon drying and cannot be damaged by its own solvent, a convertible coating can be damage by its own solvent as this change does not happen

17. The period when a two pack paint must be left after mixing to allow for reactions and escape of air

18. The time 2 pack paint remains usable after mixing

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19. A method that isolates the substrate using a low permeable coating

20. Used to tie two incompatible systems together used for when there are adhesion problems, solvent stripping or bleeding

21. Thermosetting cannot be changed once set thermoplastic can be heated and changed

22. Direct charge – ball mill, high speed disperserPre mix – sand mill, pearl mill

23. Method of test for paints, eight tests, lettered with title

24. Fineness of the pigment grind in paint

25. Is the point at which vapours will ignite measured in an able type cup

26. Viscosity is the measurement of a liquids resistance to flow free flowing paint is measured in ford flow cup and thrixotropic is measured using a rotational viscometer

27. Small alloy cup holding 100ml which can be used for checking if 2 pack paint has been mixed correctly

Dft x 100 93 x 10028. Vs% = ---------------- ------------ 50%

Wft 186

100 x dft 100 x 8829. Wft = --------------- ------------ 133.35m

Vs 66

Vs x wft 44 x 23430. Dft = ----------------- ----------- 103m

100 100

31. weight 15--------- --- 3g/cm3

volume 5

32. a 1.5 ---- 2/2 = 1 g/cm3

b 0.5 2.0

33. non-destructive test gauge is a banana gauge

34. destructive gauge is a paint inspectors gauge (P.I.G.)

35. water soak test – to test permeability

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Cold box – to test for cold crackingSalt spray – to test under marine environmentTemperature box – to check under hot and cold conditions for flexibility

36. Ballotini, which are tiny glass balls, are dropped onto a wet painted panel. The time in hours is given at the side of the panel and where the Ballotini fail to stick to the painted panel drying has occurred. Stylus test, the stylus employs a series of trailing needles which pass over the wet painted panel, because the needles are set at different tensions it can be established when the paint is tack dry, hard dry and fully cured

37. Measures opacity, hiding power charts and pfund cryptometer

38. Fineness of pigment grind, amount and type of binder and application skill

39. Cross cut, cross hatch and dolly test Dolly testa) Clean and degrease the surface to be tested and the dolly contact surfaceb) Roughen both surfaces with fine/medium grade emery clothc) Mix regular araldite and stick dolly to surface, leave for 24 hours at 250cd) Cut paint around the dolly down to the substrate using special cuttere) Attach pull-off instrument and apply pull-off forcef) Take a reading from position of cursor when dolly detaches. Values will be typically obtained

in either Mpa, N/mm2 or p.s.i.

40.a) Learn specificationb) Check all consumablesc) Check temperatures and humidityd) Assess condition of substratee) Check wft and dftf) Witness work going ong) Make reportsh) Ensure safe working is carried out

41.a) using unqualified personnelb) using unsafe plantc) incorrect paintd) incorrect thickness appliede) re-using expendable shotf) incorrect storage of paintg) using paint after specified pot lifeh) incorect use of thinners

42.

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