Painting Defects

Settling

This is the separation of the pigments and occurs to a certain extent in all paints. It becomes a serious defect when the pigment is difficult to reincorporate into the paint by stirring.

The defect occurs due to the high densities of some pigments and can be accelerated by a drop in viscosity, the paint being stored at high ambient temperatures or by being subjected to vibration for example on long transportation by rail.

The control of settling lies in selection of suitable pigments and the addition of additives that increase the viscosity of the paint.

Slow Drying

Slow drying occurs when the paint remains tacky for an extended period of time. This will result in the film picking up insects or dirt before it is hard and will make repainting difficult.

The main causes are:

• Too thick of an application of the paint when using air-drying paints. As these paints require oxygen to penetrate the film to produce dying, if the film is too thick oxygen will not penetrate.
• The viscosity of the paint is too high for the application. This can occur in cold weather and can be overcome by reducing the viscosity with the recommended thinners for the paint.
• The paint was applied at too cold a temperature. This will cause the chemical reaction that takes place to cause the film to cure slowly.
• High humidity due to rain or the like will reduce the evaporation of the solvent, the first step in drying.
• The surface to which the paint is applied is not clean and has traces of wax or paint removed on it.
• The coat to which the paint is being applied has not dried when the next coat is applied.

Staining

Contamination of many surfaces with water, soot, smoke, tannins and tobacco can result in color coming through the paint surface to cause stains. Stains caused by water will leave a tide mark and after drying, the paint around the stain can be removed and the surface repainted. If the surface may become damp again, remove the source of the water or paint with chlorinated rubber or a hard varnish. Patches of soot or smoke should be removed before coating. Use of an insulating paint before the final coat can help. Nicotine should be removed with an alkaline cleaner (bleach) before coating. Remember to wash off all the alkali before attempting to paint.

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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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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

continue......

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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.

Will continue…

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