BGAS Painting Study Material Chapter 2 continuation

Profile measurement

If a profile requirement is specified, it is the inspector’s duty to ensure that the specification requirements are met. This can be done in two ways.

1. By measuring – using gauges with and without replica tape.
2. By assessing – using surface comparators.

The dial gauges are still very often used.  The dial gauges fall into two categories

1. Surface Profile Needle Gauge
2. Dial Micrometers with Replica Tape.

Surface Profile Needle Gauge.

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

Surface profile needle gauge.

1. We need to zero the gauge when the point of the needle is on the same plane as the flat foot, i.e. on a smooth piece of glass.
2. Applying slight pressure to the foot to ensure that it is perfectly flat on the glass.
3. By loosening the locking screw, the bezel can now be moved.  The bezel should be moved till the zero on the gauge is immediately behind the needle.
4. Then tighten the locking screw and the gauge is ready for use.
5. Several readings are taken, usually more than ten, in random Positions over the substrate, and the average Calculated. This type of gauge is not ideally suited for curved areas such as pipes.
6. It is normal to work to an average figure.

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Painting Defects

Viscosity Drop

Low viscosities may be simply due to incomplete stirring or the addition of too much solvent. The viscosity may decrease on standing in water-borne paints due to enzymic attack on the thickeners used. Modern latex paints use thickeners that are not readily attacked by bacteria. Changes in the orientation of the pigments (for example partial flocculation) may reduce the viscosity.

Wrinkling, Webbing, Frosting and Gas Checking

Wrinkling is the development of wrinkles in the paint film as it dries, usually due to the formation of a skin. Defects similar to wrinkling are webbing, frosting and gas checking. Webbing is the development of wrinkles, usually in a well defined pattern and if it occurs in an oven it is called gas checking. Frosting is the formation a haze which is due to fine wrinkles and it occurs in gas fired drying ovens.

The causes can be:

• Due to the paint's being applied too thick, especially with high oil-length alkyds, varnishes with wood oil and too much cobalt drier, enamels based on alkyd or phenolic resins with drying oils and black enamels containing bitumen.
• Stoving paints containing bitumen.
• Frosting may be due to the products combustion in the oven reacting with the surface of the film or may be due to high humidity.
• Too much cobalt drier.

Sometimes the wrinkle pattern may be induced into the paint to produce films that will hide surface defects.

Another type of defected related to wrinkling are crocodiling or alligatoring where the wrinkle pattern resembles the hide of one of these reptiles.

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Painting Defects

Floatation and Flooding

Floatation or floating occurs when a paint has been incorrectly formulated with two or more different colored pigments when one of the pigments floats to the surface giving different differences. On close examination the surface appears mottled with regular shaped cells.

Flooding is similar to floatation in that one of the pigments migrates to the surface when the paint is produced using two pigments with different densities.

These defects are corrected mainly by better paint formulation.

Gassing

This is the formation of a gas, usually by hydrogen, by the reaction of reactive pigments, like Zinc and Aluminum, with acidic materials in the resin. It can be overcome by better formulation or packaging the paint separate from the pigment and mixing the ingredients prior to application.

Mould

The growth of mould on a paint film causes severe discoloration. Mould is a plant growth that requires moisture, the presence of food and the correct temperature for growth. The defect can occur on most types of paint but is most prevalent in bathrooms, kitchens and exterior walls that are in shady positions. The paints that are most susceptible are soft oil-based paints or varnishes and emulsions, especially if they are low gloss where dirt can be trapped in the film.

Often the mould growth can be killed and color removed by washing with dilute sodium hypochlorite solution taking due care as this preparation is alkaline. Safety glasses and gloves have to be worn. Before repainting, susceptible surfaces should be prepared with anti-mould preparations, like sodium pentachlorophenate and by using either paints prepared with mould inhibiting pigments, like Zinc oxide, or by using high gloss finishes. In extreme cases it may be necessary to remove the high humidity in the room by using exhaust fans.

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Painting Defects

Dirt Retention

This is the deposition of dirt and dust on the paint film. For certain types of paint, the dirt may become entrained into the surface. The paints that resist dirt retention are high-gloss enamels while the low gloss latexes are the most susceptible to this defect.

Fading

Fading is the decrease in the intensity of the color after exposure. It should be tested for after removal of any chalking that may have occurred as this will tend to mask the actual fade of the pigment. In general organic pigments, especially those of low cost, will fade more than pigments that are inorganic. More expensive coatings especially prepared for exterior exposure will resist fading more than less expensive paints.

Fish Eye

This defect is indicated by small round imperfections in the top coat. The defect is caused by traces of silicone or oil on the surface prior to painting. The remedy is to thoroughly clean the surface and if spray painting, to ensure that there is an oil filter on the air line.

Flaking

Flaking is the lifting of small-to-large sections of the paint and is due to poor adhesion and to the brittleness of the paint. The causes can be varied, for example the defect could be caused by efflorescence or the migration of soluble salts to the paint-media interface which can cause the paint to be forced off the surface. The paint may react with moisture and any traces of alkali to decompose the paint - this is called saponification. It may be due to failure to remove millscale from the steel before painting.

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Painting Defects

Bronzing

This is a defect that was often observed on cars painted red or blue where after a period of time a characteristic red tone developed on the paint surface. The cause was older types of pigments like phthalocyanine or Prussian blues. The defect is not common with the pigments available today.

Chalking

Chalking is the powdery deposit on the surface of the paint which dulls the gloss and appears after exposure. This defect is usually associated with long exposures to sunlight and is a natural degradation of the paint film. Some combinations and types of pigments and resins show more pronounced chalking than others.

Checking

Checking is the appearance of wide splits with round edges that occur in the top coat. The cause is usually due to the surface not being clean (could be old paint) or too high a film build or the materials not being mixed properly. The remedy is to remove the old paint, cleaning the surface and mixing the paint ingredients properly.

Cracking or Crazing

This defect is a series of irregular cracks in the surface of the paint.

The cause of this defect can be:

• Application of the top coat before the previous coat is dried
• Too thick of a top coat
• Impurities on the surface or the effect of impurities on the applied coat.

The remedy is the let the intermediate coats dry before the top coat is applied, clean the surface well, remove the previous coat or ensure that the top coat is not applied too thickly.

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Painting Defects

Blistering

This defect is the appearance of irregular blisters on the paint finish.

This defect can be caused by:

• Not correctly cleaning substrate
• Contamination of the brush, air gun, line etc.
• Using wrong thinner or incorrect amount of thinners
• Old paint surface
• Excess film thickness
• In timber finishes, not allowing the solvent, particularly paint removers , to evaporate before repainting

This defect may be overcome by:

• Cleaning all surfaces free of grease and allowing the solvent to evaporate.
• Using recommended thinner at correct ratio.
• In spray applications, inspect so that water does not build up in the traps, especially in humid weather.
• Check that the new paint is compatible with the old surface.
• Do not apply paint films too quickly and allow solvents to evaporate before re-coating.

Blooming

This defect gives a bloom or white deposit, like the bloom on a grape or plum, after the paint has dried. The cause is the rising of soluble fractions of the pigment rising to the surface on the paint's drying. The remedy for spray paints is to rub the surface down.

Blushing

This is a white deposit appearing on the surface of lacquer films only. The defect is caused by painting with lacquers in high humidity conditions where the water contained in the air condenses on the paint film The remedy is not to paint in humid conditions or to add a strong, active solvent that may stop the blushing.

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Painting Defects

BGAS Painting Inspector Exam Painting Defects Name and Definition

Banding

This defect is found in spray applications where there is heavy application of paint on the outside of the spray pattern with little paint in the canter of the fan.

The cause can be:

• Too much air pressure
• Uneven lapping of the spray gun
• Having the gun too close to the job

The remedy may be found from:

• Use at correct air pressure
• Ensure that the overlap of each stroke is 50% over the previous coat
• Hold gun at the correct distance from the job - about 15 cm for lacquers and 25 cm for enamels. To correct a coat that has been applied, re-coat with double coat using thinner that has been specifically recommended for the paint using indicated solvent ratio and pressure, and ensuring that the gun is held at the correct distance.

Bleeding

This is the migration of the color from a previous coat into the freshly applied top coat. This defect usually occurs when a light color is applied over a dark color, particularly reds and maroons which are prepared by using organic pigments not resistant to solvents or application over a surface contaminated with bitumen where the solvents in the fresh paint dissolve the bitumen.

The remedy is:

• Use a bleed sealer before application of the light color
• Wash the surface with mineral turps if it is contaminated with bitumen.

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BGAS Painting Study Material Chapter 2 continuation

Profile

Surface profile, Anchor pattern, key, Peak to trough height and Amplitude are all expression meaning the cross section of a blasted area, as measured from the top of the peaks to the bottom of the troughs.  The surface profile requirements are given on the specification for the job, e.g. for B. Gas 30 – 75 microns.

Shot blasted profile

Figure 2.1 Terms relating to preparing surfaces

Grit blasted profile 

Figure 2.2 Grit blasted profile

*Hackle – A small surface lamination, which stands upright like a needle after blasting.  Approximately ≤ 13 mm.  Easily removed.

*Lamination (slivers) – Appears to be a longitudinal ‘crack’, one lip curling back, any laminations found must be referred to engineer for ultrasonic check.

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BGAS Practical Questions Banana Gauge

Practical Assessment 

From the picture shown answer for all questions.

 

1) - What is the name of this equipment?

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2) - What is the purpose of use this equipment?

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3) - On which substrate we can use this equipment?                                                                                                 

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4) - Can we used for measuring DFT of paint which applied on top of Non Ferrous substrate?

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5) - If the painting system containing MIO, and the substrate is Ferrous, this equipment can use or not?

     And why?

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6) - This equipment working in which principles?

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BGAS Painting Study Material Chapter 2 continuation

Other properties of an abrasive have an effect on the resulting substrate also, these being.

A. Size of the particles

B. Hardness of the material

C. Density of the material

D. Shape of the particle

E. Velocity of abrasive

F. Angle of impingement

G. Time & distance

For example steel has a density of approximately 7.6 gm/cc and copper slag, approximately 4.2 gm/ccIf one particle of each material, of identical size, hit a steel substrate, then it would be logical to say that the steel would impinge further into the substrate, resulting in a deeper trough.

A spherical particle would not impinge as deeply because the large smooth surface area would use its energy up in preening or work hardening the surface rather than cutting into it. So a shot blasted surface is different in appearance and texture to that of grit blasted surface.

Sizing of abrasives

G Prefix = Grit (amorphous, points and cutting edges, irregular profile)

S Prefix = Shot (spherical, smoother profile)

The G or S notation is followed by a number, which denotes the particle size.

G-24 or S-330.  BS 2451 the 24 means nominally 24 thousandths of an inch.

SAE (society automotive engineer) USING THE J 444 SIEVE SYSTEM.it represents ¹/₂₄^" = approximately 40 thou.

 

New BS ref. 7079 pt PARTICLE SIZE DISTRIBUTION Uses a different method again, in metric units. G140 would mean a nominal particle size of 1.4mm

Adhesion and Profile

A commonly used definition of Adhesion is: - The force required separating two surfaces in touch.

A newly rolled plate, perfectly smooth, 1m x 1m has an apparent surface area of 1m² and an actual area of 1m².  Abrasive blasting roughens the surface and increases the actual area, (the apparent area is still 1m²), thus increasing the adhesion.

Two theories of adhesion are:

1. Molecular Interference

Because the surface is rough and uneven the paint wets, and locks into the profile, Analogy Velcro Physical.

2. Molecular Attraction

Negatively charged particles attracted to positive areas, and vice versa Analogy Magnet (sometimes called Ionic Bonding) Chemical.

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BGAS Painting Study Material Chapter 2 continuation

In the context of this course we are considering the following: -

a) Sand

It is not permitted to use sand.  SI 1657 states that any mineral used as an abrasive must release less than 1% free silica on impact.  (Silica causes preumonicosis or silicosis).  COSHH REGS does not allow the use of sand containing silica for dry blasting.  Sand itself is perfectly safe, but shattering on impact releases silica which can be inhaled. The amount of copper in the structure is extremely minute.

b) Copper Slag

Copper Slag

1. Minerals melted with the copper,

2. Liquefies and forms a protective cover over the molten copper to prevent reaction with the atmosphere. 

3. When the copper metal is run off the slag is rapidly cooled in cold running water

4. The material is supplied in grit form (random, sharp Edges, amorphous (no definite shape) and is very brittle), shatters into smaller pieces on impact, and should be used only once and then discarded and so classed as expendable.

Garnet

c) Garnet

A natural mineral classed as being “of a diamond type Hardness” can be either expendable or recyclable. Cleansing units are available to extract contamination so that the material can be reused, usually up to three times.  Doesn’t shatter on impact but does suffer some “Wear” Supplied in Grit form. 

d) Metallic Grit

Metallic Grit

Steel and Iron are both metallic.  Steel grit being the Softer of the two to round off on impact and loses its sharp edges.  Angular Chilled Iron chips off small slivers on impact to produce sharp cutting surfaces on its next cycle.  Metallic abrasives are recyclable because the particles reduce in size slowly. Hence it can be re-used many times and still perform a useful function in a '‘working mix’. A working mix is an accepted ratio of large and small particles, where the large particles cut the profile and the smaller particles clean out the troughs.

e) Metallic Shot

Metallic Shot

Shot is spherical and doesn’t shatter (otherwise it would form grit).  When supplied the particles are virtually uniform in size and shape, (not a working mix) But like the grit they wear down slowly in size. The particles are worn down eventually to finings, and are drawn out of the system during cleansing.

f) Metallic Shot and Grit Mixed;

A mix of shot and grit results in a more uniform profile. 

1. The grit cuts the profile

2. The shot, being unable to enter the troughs produced, controls the peak height and so greatly reduces the number of ‘rogue peaks.’

Metallic Grit & Shot Mix

A rogue peak; is one, which is well proud of the acceptable profile range, and if painted over due to contraction of the paint, will leave bare metal in contact with the atmosphere, thus allowing corrosion to occur.  When rogue peaks are in concentrated area the effect is of a rash, hence rust rashing or rust spotting.

A typical mix ratio of Shot to Grit as used in a pipe coating mill would be 70 – 80 % shot to 20-30% grit.

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BGAS Painting Study Material Chapter 2

SURFACE PREPARATION METHODS & STANDARDS

If paint is applied over the corrosion reactions, and other contaminants,  

1. The poor adhesion of the coating and thus the coatings life would be far from satisfactory. 

2. A good surface preparation grade (degree of cleanliness) along with a suitable surface profile can give 10 years life from a typical four-coat paint system.  The same system applied over a substrate with little or no profile and contaminant remaining might give four to six years, or even less.

Surface Preparation

Involves removing these contaminants, and in some instances increasing the area available for adhesion by roughening up the substrate.

Therefore two factors need to be considered when inspecting a surface preparation.

1.         Degree of cleanliness

2.        Surface Profile (degree of roughness)

Surfaces can be prepared for paint application in several different ways; each one varies in cost, efficiency, ease and suitability.

a)      Dry Abrasive Blast Cleaning

b)      Water Blasting

c)      Hand and Power Tool Cleaning

d)      Flame Cleaning

e)      Pickling

f)      Vapour Degreasing

g)      Weathering

Dry abrasive blast cleaning

A. Dry abrasive blast cleaning involves compressing air and forcing it along a hose and out of a small aperture (gap) called a nozzle.

B. A pressure of 100 psi results in the air speed exiting the nozzle at approximately 450 mph.

C. Abrasive particles are mixed in with the air and travel at the same speed; they will carry a lot of work energy.  This energy is used in chipping away mill scale and other detritus from the substrate.  And shattering into small pieces and with others all the energy is used in impinging into the steel surface, roughening the surface and increasing the surface area to increase adhesion properties.

Because all standards refer to the amount of contamination remaining on the surface, (The longer the time spent on this operation, the higher the degree of cleanliness.)

Abrasives

Abrasives come in many forms and can be classified in several different ways, as shown below.

None metallic (Mineral) Expendable	Metallic (Recyclable)	Agricultural by-product
Copper Slag  Nickel Slag  Boiler Slag  Glass Bead  Aquamarine  Garnet  Sand	ACI (Angular Chilled Iron) Steel Grit Steel Shot Grit and Shot Mix Garnet	Walnut Shell Coconut Shell Eggshell Corn Cob Husk Peach Husk

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BGAS Painting Study Material Chapter 1 Corrosion

Corrosion can be generally defined as;” Degradation of a metal (material) by chemical or Electro-chemical means. It is obvious that two mechanisms are involved, Firstly an Electrical Circuit and secondly a Chemical Reaction.

Electrical Circuit

In corrosion circuit the current is always D.C. (Direct Current). For corrosion circuit to exist three things are needed: Anode, Cathode and Electrolyte.

1. An Anode

Is a positively charged area?  (It becomes positively charged because the atoms release two     electrons), the iron atom has 26 of each, 26 protons and 26 electrons, in its passive state

When the two electrons are released the atom still has its 26 protons, but now only 24     electrons. (In this state the atom is now an ion, positively charged by two units and written as Fe⁺⁺) (An ion is a charged particle, and can be positive or negative, a single atom or a group of atoms, known as a molecule.)

This losing of electrons can be shown as: - Fe→Fe⁺⁺+2e (The Fe⁺⁺ is called a positive iron ion). 

2. A Cathode

Is a negatively charged area (where there are more electrons than needed in its   passive state).   At the cathode the electrons enter into the electrolyte to pass back to the anode.

3. An Electrolyte

Is a substance, which will conduct a current and be broken down by it, (dissociate into ions). Water, Acids, alkalis and salts in solution are very efficient electrolytes.

As the electrons pass into the electrolyte it is dissociated or (separated) into positive and negative ions, as shown by the formula: -2eH₂O→2H⁺+2OĦ.

The couple electrons back with the Hydrogen ions to form two full Hydrogen atoms, which join together to form Hydrogen gas.  The hydroxyl ions return to the anode through the electrolyte carrying the electrons.

Corrosion Triangle

Osmotic or Hygroscopic Blisters

MATERIAL	KNOWN POTENTIAL AV. VALUES
Graphite	+ 0.25 v
Silver	- 0.1 v
Nickel 200	- 0.15 v
Copper	- 0.35 v
Mill Scale	- 0.4 v
Mild Steel	- 0.7 v
Aluminium Alloys	- 0.9 v
Zinc	- 1.0 v
Magnesium	- 1.6 v

The Chemical Reaction

Only the chemical reaction, (the formation of corrosion products), occurs at the Anode.The positive iron ions, Fe++, receive the returning hydroxyl ions and ironically bond together to form iron hydroxide, which is hydrous iron oxide, rust, and is shown by the formula: Fe⁺⁺ + 2OĦ →Fe (OH)₂

Corrosion only occurs at the Anode, never at the Cathode.

The corrosion triangle shows the three elements needed for corrosion to occur, Anode, Cathode and Electrolyte. If any one of these three is removed from the triangle, corrosion cannot occur. The one most commonly eliminated is the electrolyte.  Placing a barrier between the electrolyte and the anodic and cathodic areas, in the form of a coating or paint system does this. If electrolyte is not in direct contact with anode and cathode, there can be no circuit, and so no corrosion.

Certain factors can increase the reaction rate, listed below are some of these.

1. Temperature.

Steel, is thermodynamically unstable metal.

The hotter steel is faster in corrosion than the other cooler one.

2. Hygroscopic Salts

(Hygroscopic-tending to observe)

A hygroscopic salt is one, which will attract water and dissolve in it.

When salts are present on a substrate (top of the surface) and a coating is applied over them, water will be drawn through the film and the resulting solution builds up a pressure under the film.

Eventually the film is forced up to form blisters.

These blisters are called osmotic or hygroscopic blisters, and are defined as ‘pinhead sized water filled blisters’.

Sulphates and Chlorides are the two most common salts, chlorides predominant in marine environments, and sulphates in industrial areas and sometimes agricultural.

3. Aerobic conditions

(Presence of oxygen) By introducing oxygen into the cathodic reaction the number of Hydroxyl ions doubles.

This means that double the number of iron ions will be passivated and therefore double the corrosion rate.  Shown by:  2H₂O + O₂ + 4e → 4OH-

4. Presence of some types of bacteria

On the metal surface, for example Sulphur Reducing Bacteria, better known as (SRB), or MEMs, Metal Eating Microbes.

5. Acids and alkalis

6. Bi-metallic contact.(corrosion) Otherwise known as Bi-Metallic Corrosion.

Metals can be listed in order of nobility. A noble metal is one, which will not corrode.  In descending order, the further down the list the metal is, the more reactive it is, and so, the more anodic it is, the metal loses its electrons to become reactive ions. The degree of activity can be expressed as potential, in volts. The list can be called Galvanic List, Electro Motive forces series or the Electro-Chemical series.

Mill scale

Is immediately above steel on the galvanic list.

This means that mill scale is Cathodic to steel, and if left on the surface of steel will accelerate the corrosion of the steel substrate.

Mill scale is formed during the rolling operation of steel sections e.g. RSC, RSA, RSJ.

The oxides of iron form very quickly at temperatures in excess of 580°C

The first oxide formed is FeO, iron oxide, the next is Fe3O4 and last of all Fe₂O₃. Common names in order are Wustite, Magnetite and Haematite.

These oxides are compressed during the rolling operation to produce blue mill scale.

The thickness of mill scale varies from 25 to 100 um(Microns)

When it has been removed by any surface preparation method, it can never re-cur.

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