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PAINTING MANUAL
FOR
REFINERY & MARKETING TERMINAL
NUMALIGARH REFINERY LIMITED
NUMALIGARH, ASSAM
Central Electrochemical Research Institute
Karaikudi 630 006, India
2007
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CONTENTS
1 Basics of Corrosion and its Forms 4
2 Corrosion Protection by Organics Coatings 17
3 Surface Preparation 31
4 Application of Paints 37
5 Colour Coding 41
6 Recommendation of Paint Schemes 44
7 Specification of Paints 56
8 Testing and Inspection 76
9 Maintenance Painting 83
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PREFACE
Corrosion of metals is prevalent everywhere and its effects are felt and
experienced by everyone in one form or the other. The most widely used way of
corrosion prevention is by the application of organic coatings or paints. Corrosion
prevention is very crucial for a plant/industry since it has a bearing on the life
expectancy of the equipments and consequently on the general condition of the plant.
Paint, though a highly technical product and invariably used by all plants and
industries in large quantities, is the least understood product. Generally it is a tailor
made product specifically suitable to the existing environmental conditions.
This painting manual, prepared by the coatings group of Central
Electrochemical Research Institute (CECRI), Karaikudi, contains recommendations of
specific paint schemes suitable at different locations of the plant and also the
specifications of each individual paints in addition to an insight into the basics of
corrosion, paints, surface preparation, paint application etc. It also contains details of
inspection procedures for surface preparation /paint application and test procedures
for quality control of paints.
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Chapter -1
BASICS OF CORROSION AND ITS FORMS
1.1 INTRODUCTION
Corrosion of metals and alloys poses great national and international problems
from the point of view of conservation of metals, operational costs in industries,
safety of personnel and above all pollution hazards. Metallic corrosion which is due to
the metal - environment reaction involves electrochemical reactions at the metal /
environment interface. Even the phenomenon of high temperature oxidation is
explained on the basis of movement of charged particles across the interface and
hence can be termed as "Electrochemical". Thus, except for dissolution of metals in
certain non-aqueous media, corrosion can be described largely as an electrochemical
phenomenon. The proof for this comes not only from the application of the
electrochemical principles in understanding the corrosion process but also from the
application of various electrochemical techniques in measuring the corrosion rates as
well as in various corrosion control measures.
1.1.1 Mechanism of corrosion
The mechanism of corrosion is explained on the basis of the operation of micro
galvanic cells on the metal surface, leading to the oxidation (dissolution) of metal at
the sites having more negative potential sustained by the same magnitude of a
reduction (hydrogen evolution or oxygen reduction) at the sites having less negative
potential.
At the anode,Fe ------> Fe
2++ 2e
-
At the cathode,
1/2 O2 + H2O + 2e-------> 2 OH
-
Fe3+
+ e-------> Fe
2+
The origin of the micro-galvanic cells can be traced to such factors as metal inclusion,
presence of different phases, presence of grain boundaries, surface defects,
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concentration cells, temperature differences, velocity changes and stresses, both
residual and applied.
1.1.2 Corrosion caused by Petroleum Products
Certain corrosive agents in petroleum products cause corrosion depending on
their concentrations in the products. Crude oil for example, can cause extensive
corrosion damage, especially when water separates from oil into the bottom of the
storage tanks. High concentrations of hydrogen sulphide in crude oil can also cause
high corrosion damage
In the case of installations at petroleum industry, corrosion is mainly due to theatmosphere prevailing in the location like a severely corrosive marine atmosphere or a
corrosive industrial atmosphere nearby or a less corrosive rural atmosphere and hence
we consider below the theory of atmospheric corrosion and other factors that
influence the process of corrosion.
1.1.3 Atmospheric Corrosion
Atmospheric corrosion may well be defined as the destruction of metals /
materials by various ingredients present in the atmosphere. Most of the broad forms of
corrosion occur in the atmosphere and some appear to be largely restricted to it. Since
the corroding metal is not completely immersed in the electrolyte, atmospheric
corrosion takes place at localized corrosion cells as described earlier.
The cells can operate only in the presence of an electrolyte. However, it has not
been clarified what is the minimum thickness of the electrolyte film required for the
operation of a corrosion cell. Various factors in the atmosphere will influence the rate
of corrosion of metals. For e.g. in the salt laden atmosphere prevailing at seashore the
sodium chloride (salt) reacts with steel (iron) as per the following reaction.
4 Fe + 8 Cl-
------> 4 FeCl2 + 8e-
4 H2O + 2 O2 + 8 e- + 8 Na+
------> 8 NaOH
4 FeCl2 + 8 NaOH + O2 ------> 2 Fe2O3 . H2O + 8 NaCl + 2H2O
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The rate of electrochemical corrosion of a metal which can be considered
proportional to the local current is directly dependent on (1) effectiveness of the cathodic
process (2) effectiveness of anodic process and (3) the ohmic resistance.
Many factors are responsible for affecting the corrosion rate of a metal. They
are temperature, relative humidity, solar radiation / sunshine hour, amount of rainfall,
wind direction, velocity and other pollutants like chloride, sulphur-dioxide etc, in the
atmosphere. The most important factors in the atmospheric corrosion over-riding
pollution are moisture, rain, dew and condensation.
Rain may have beneficial effect in washing away atmospheric pollutants that
have settled on exposed surfaces. This effect of rain has been particularly noticeable
in marine atmospheres. If the rain collects in pockets or crevices, it may accelerate
corrosion by causing continued wetness in such area.
Dew and condensation are very bad from the corrosion stand point if not
accompanied by frequent washing with rains which dilute or eliminate contamination.
A film of dew, saturated with sea salt or acid sulphates and acid chlorides of an
industrial atmosphere provides very aggressive electrolyte for the promotion of
corrosion. Also in the humid tropics where condensation appears on many surfaces,
the stagnant moisture film either becomes alkaline from reaction with metal surfaces
or picks up carbon dioxide and becomes aggressive as a dilute acid.
1.1.4 Immersion Corrosion
The factors influencing the immersion corrosion of metals are: pH, dissolved
gases, dissolved salts, presence of bacteria, temperature, velocity and the depth of
immersion (in the case of sea water).
i) Dissolved gases:Gases such as CO2, H2S, NH3, Cl2 and O2 affect the corrosion rate markedly.
CO2 acts by promoting acid-type corrosion and also increasing the solubility of
carbonate responsible for scale formation. H2S attacks the metals by forming the loose
sulphide scales which is non-protective. Ammonia corrodes copper and copper base
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alloys forming soluble complexes. Chlorine forms HCl or HClO which accelerate
corrosion.
Because oxygen plays a predominant role in cathodic reactions in neutral
media, increase in oxygen concentration accelerates corrosion of iron. However at
very high oxygen concentrations, the rate drops due to passivity. In the sea water the
oxygen concentration decreases with depth. It is due to the photosynthesis of marine
plants at the surface. Due to this the corrosion rate of steel decreases with depth. But
the aluminium and stainless steels at higher depths, pitting corrosion takes place due
to lower oxygen availability which evidently reduces the protective effect of oxidefilms.
ii) Dissolved salts:
The presence of calcium and magnesium in the water decreases the corrosion
due to formation of protective scales. The formation of the protective scales can be
assessed by measuring the angler index of water. But alkaline metal salts such as
NaCl, Na2SO4, and KCl accelerates corrosion. Similarly the presence of oxidizing
salts, complexing salts accelerate corrosion. Besides the above salts, H+ ion
concentration in the water influences the corrosion of metals markedly.
iii) Bacteria:The presence of sulphate reducing bacteria, slime forming bacteria, iron
oxidizing bacteria accelerates the corrosion of metals.
iv) Temperature:The rate of corrosion usually increases with temperature. In general, there is a
linear variation with doubling of corrosion rate for rise of 10oC in the water
temperature in sea water.
v) Velocity:
In neutral solution the rate of corrosion depends on the rate of motion of the
corrosive solutions since the motion of electrolyte increases the supply of oxygen to
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the metal surface. Generally carbon steel and zinc corrodes at greater rates with
increased velocity.
vi) Depth:
Oxygen varies with increasing depth, tending to drop at 1000-2000m. and then
rise again. Besides, temperature falls with increasing depth although there appears to
be exceptions. The corrosion rate of mild steel is decreased to 0.2 mmpy for depths
greater than 1000m from 11.1 mmpy. The corrosion rate of zinc rose from 0.015
mmpy at a depth of about 2000m. Both Cu-30Ni and monel 400 also appeared to
increased corrosion at depth.
1.1.5 Underground Corrosion:
The major importance of corrosion of the underground devices arises from the
fact that there are million miles of buried oil, water and gas pipe lines. The major
factors influence the underground corrosion are porosity, electrical resistance,
dissolved substances, moisture, stray current and bacteria present in the soil. Less
porous less moisture and high-resistant soils (10000FP DUe mildly corrosive.
Dissolved compounds present in the soil influence the corrosion in the same way as
that under immersed conditions.
1.2 FORMS OF CORROSION
Corrosion can manifest itself in many forms and the identification of the forms
of corrosion through careful observation of the corroded parts, is always helpful in
understanding the problem and suggesting ways of solution.
1.2.1. Uniform Corrosion
Uniform corrosion is the most common form of corrosion. This type of attack
(deterioration) is uniformly distributed and proceeds at approximately the same rate
over a metal surface. It is also called general corrosion.
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1.2.2 Pitting
It is one of the most destructive and deciduous forms, as the attack is extremely
localized resulting in holes in metal and causing equipment and pipeline failures. It is
often difficult to identify pits because of their small size and they are often covered
with corrosion products. In the absence of any mechanical stress, the pits formed are
generally saucer shaped. The number, size and depth of pits vary widely. A dotted
appearance of the corrosion product or a surface with well-defined irregularities is
likely to be an indication of pits underneath. Pits usually grow in the direction of
gravity and only rarely do pits grow upward from the bottom of horizontal surfaces.
Pitting usually requires an extended initiation period before visible pits appear. This
period ranges from months to years, depending on the specific metal and the
environment.
1.2.3 Crevice Corrosion
This type of attack is usually associated with small volumes of stagnant
solution caused by holes, gasket surfaces, lap joints, surface deposits and crevice
under bolt and rivet heads. To function as a corrosion site, a crevice must be wideenough to permit liquid entry but sufficiently narrow to maintain a stagnant zone.
Intense localized corrosion frequently occurs within crevice (narrow openings) and
other shielded areas on metal surfaces exposed to corrosive environments. This form
of corrosion is called as crevice or deposit or gasket corrosion. Contact between metal
and non-metallic surfaces can also cause crevice corrosion. Wood, plastics, rubber,
glass, concrete, asbestos, wax and fabrics are example of materials that can cause this
type of corrosion. Stainless steels are susceptible to crevice attack.
1.2.4 Galvanic Corrosion
It often becomes necessary to couple dissimilar metals in the process flow
system and a potential difference usually exists between two dissimilar metals when
in contact with a corrosive or conductive solution. Corrosion of the less corrosion
resistant material in the couple is usually accelerated and that of the more resistant
material is decreased. This form of corrosion whose occurrence is very common next
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to uniform atmospheric corrosion is known as galvanic or bimetallic corrosion, the
driving force for current and corrosion being the potential difference. Bi-metallic
corrosion is readily recognized by the localized attack near the junction. A couple
which has a large cathode and small anode increases the corrosion rate of the anode
considerably.
Bimetallic effect can occur even in humid atmospheric exposure but it is
restricted to an area close to the contact. Intense bimetallic effect can take place when
metals with low over potentials such as copper, silver, carbon etc are coupled to
metals with high over potentials such as zinc, iron, aluminium etc. The preventive
methods of galvanic corrosion are:
a. Metals should be selected in such a way that they should be closetogether in the galvanic series.
b. Insulate, wherever possible dissimilar metals i.e. flanges, washers etc.are to be provided when coupling two dissimilar metals.
c. Application of proper coating materials and giving periodicalmaintenance to those areas where bimetallic couplings are present.
d. Avoid unfavourable area effect of a small anode site to large cathodesite.
e. The aggressiveness of the environment can be brought down by addingproper inhibitors.
f. Welding should be done as far as possible with the same type of alloys.g. Avoid threaded joints for materials which are apart in the galvanic
series.
1.2.5 Inter-granular Corrosion
Corrosion occurring preferentially at grain boundaries, usually with slight or
negligible attack on the adjacent grains is known as inter-granular corrosion. The
alloy disintegrates and / or loses its strength. Inter-granular corrosion can be caused
by impurities at the grain boundaries, enrichment of one of the alloying elements or
depletion of one of these elements in the grain boundary areas.
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1.2.6 Exfoliation or layer corrosion
Corrosion that proceeds laterally from the sites of initiation along planes
parallel to the surface, generally at grain boundaries, forming corrosion products that
force metal away from the body of the material, giving rise to a layered appearance is
called exfoliation or layer corrosion.
1.2.7 Dezincification
Dezincification is the selective removal of zinc in brass alloys and is readily
observed with the naked eye, because the alloy assumes a red or copper colour
contrasting with the original yellow colour. Though overall dimension of the object
does not change appreciably when dezincification occurs, the de-zincified portion will
be weak, permeable and porous. Uniform or layer-type of dezincification seems to be
frequent in high brasses (high zinc content) and in definitely acidic environments. The
plug type (localised) seems to occur in low brasses and in neutral, alkaline or slightly
acidic environments. Stagnant conditions usually favour dezincification.
Dezincification can be minimized by using low zinc brasses, e.g. Red brass
(85% Cu, 15%Zn) or by using better brasses containing small amounts or arsenic,antimony or phosphorous as "inhibitors". e.g. Admiralty brass (70%Cu, 29%Zn,
1%Sn and 0.05%As) and Al brass (76%Cu, 22%Zn, 2%Al and 0.05%As).
1.2.8 Graphitization
Grey cast iron subjected to relatively mild environments as when buried in
water-logged neutral soils can be affected by selective leaching. Grey Cast iron is a
mixture of ferrite and crystals of graphite. These graphite crystals act as cathodes to
iron matrix, the potential difference sometimes being as high as 2V. The iron is
dissolved leaving a porous mass of graphite. Overall dimensions do not change; but
the material loses its mechanical strength and dangerous situations may arise if not
detected. Graphitization is usually a slow process.
1.2.9 Filiform Corrosion
Filiform corrosion is thread-like (filaments) type of corrosion which develops
under protective coatings on certain metals, usually in humid atmospheres. Filiform -
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corrosion has been observed on steel, magnesium and aluminium surfaces covered by
tin, silver, gold, phosphate, enamel and lacquer coatings. It does not weaken or
destroy metallic components but only affects the surface appearance and so it is a
major problem in the canning industry.
1.2.10 Stress Corrosion Cracking
Stress corrosion cracking (SCC) is the spontaneous cracking that may result
from the conjoint action of tensile stress and corrosive media. In the absence of either
stress or corrosion, the failure would not occur. The failure is a type of brittle fracture
of normally ductile metals by the presence of specific environments. During SCC, the
metal or alloy is virtually un-attacked over most of its surface, while fine cracks
progress through it. It is important to differentiate clearly between SCC and stress
accelerated corrosion where structural corrosion is intense even in the absence of
stress and the effect of stress is to rupture the grain boundaries and to promote
penetration of the environment.
SCC in service results from tensile stresses at the surface or subsurface usually
of considerable magnitude acting for prolonged periods of time. Stresses of this natureare usually residual, produced by methods of manufacture (quenching, cold forming,
tube drawing without internal mandrel etc.,) or assembly (welding, press or shrink
fits, wrapping of sheet to fit a structure, joining of poorly fitted parts etc.). Stresses
due to applied loads are seldom met in practice because of design considerations.
Once a crack starts, it is possible for it to continue with no applied or residual stress
but simply to be driven by pressures from corrosion products (of the order of 4000 -
7000 psi). For many alloy systems a "threshold" stress (a stress below which SCC
does not occur in some finite period of time) has been observed. SCC grows in a
plane perpendicular to the operative tensile stress and may take either an inter-
granular or a trans-granular path. The crack propagation is a discontinuous process.
Stress Corrosion Cracking in relation to H2S
Spontaneous brittle failures that occur in steels and other high strength alloys
when exposed to moist hydrogen sulphide and other sulfide environments is known
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by several names, hydrogen sulphide cracking, sulphide cracking, sulphide stress
corrosion cracking etc.
The mechanism of sulphide stress cracking is not fully understood so far. The
following general conditions must be present to have this type of failure.
a. Hydrogen sulphide must be presentb. Water even a trace of moisture must be presentc. High strength steel must be involvedd. The steel must be under tensile stress or loading.
If all these conditions are prevent, sulfide stress cracking may occur after some
period of time. The time to failure may vary from hours to day, or years. The
susceptibility of a material to fail by this mechanism is purely determined by strength
or hardness of the material, stress level, hydrogen sulphide concentration, pH of the
solution and temperature.
1.2.11 Hydrogen embrittlement
Normally metals which absorb large quantities of hydrogen give rise to hydride
formation where as metals which physically absorb small amounts of hydrogen sufferembrittlement. Sources of hydrogen are most commonly for reaction of the metal with
water during melting, casting, hot working, welding, acid treatment and electro-
deposition. Metals most frequently affected are steel, titanium, copper and silver; but
vanadium, zirconium, tantalum, cerium, cobalt and nickel may also get affected.
This is also caused by penetration of hydrogen into a metal, which results in a
loss of ductility and tensile strength. For titanium and other strong hydride-forming
metals dissolved hydrogen reacts to form brittle hydride compounds. The mechanism
is based on slip interference by dissolved hydrogen.
Embrittlement may be prevented by
1. reducing corrosion rate2. altering plating conditions3. baking4. substituting alloys
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5. Practicing proper welding.1.2.12 Corrosion Fatigue
The tendency for a metal to break under conditions of repeated cyclic stressing
considerably below the ultimate tensile strength has been termed fatigue. If in
addition to cyclic stressing, the environment is such as to cause corrosive attack on
the metal, the deterioration will be even marked and this damaging effect due to the
simultaneous action of corrosion and cyclic stress is known as corrosion fatigue.
Corrosion fatigue may be eliminated or reduced by reducing the stress on the
component (by design, by stress relieving, heat treatments or by shot-peening the
surface to induce compressive stresses). Corrosion fatigue resistance can be improved
by coating such as electrodeposited zinc, chromium, nickel, copper and nitride
coatings. (When electro-deposited coatings are applied, plating techniques should not
produce tensile stresses in the coating or charge hydrogen into the metal). Inhibitors
are also effective in some cases. Increasing the tensile strength of a metal or alloy is
detrimental to corrosion fatigue resistance (though it improves ordinary fatigue
resistance)1.2.13 Fretting Corrosion
The accelerated deterioration at the interface between contacting surfaces as
the result of corrosion and slight oscillatory movement between the two surfaces is
called the fretting corrosion.
1.2.14 Cavitation Erosion and Impingement
Cavitation is primarily the wearing away of metal due to repeated impact
blows by the formation and collapse of voids within a fluid. When the voids collapse
with high frequency and violence, a hammer like effect is produced on the metal
surface and consequently
1. the metal is severely deformed
2. in some areas the metal is torn away from the matrix
3. any corrosion resistant film previously formed is rapidly removed.
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If the environment is corrosive, cavitations increases corrosion rate. Discharge side of
turbines, suction side of impeller of flow pumps, discharge side of regulating valves,
high speed marine propellers are mostly affected. The appearance of cavitation
damage is somewhat similar to pitting, except that pitted areas are closely spaced and
the surface is usually considerably roughened.
1.3 CORROSION CONTROL TECHNIQUES
The benefit of corrosion control is always availed by those having a more
intimate knowledge on corrosion. There are several ways to control corrosion as
described below11
1.3.1. Use of Proper Design:
Automobiles are notorious for having inaccessible crevices that could hold
water and dirt leading to the side panels corroding away. Structural steel has been
designed to make it difficult or impossible to coat some surfaces. Some systems
employ different metal combinations, which become large galvanic corrosion cells.
These defects can be overcome by proper design of crevices, metal junctions and
metal non-metal contacts.
1.3.2. Use of corrosion resistant materials:
This can best be included during the design stage. Often simple design changes
can have a big impact on subsequent corrosion control programs such as cathodic
protection.
1.3.3. Use of Cathodic protection:
It is an electrical means of preventing corrosion and can be applied to any
metal immersed / buried in an electrolyte such as soil or water or concrete wherein an
anode can be installed. The DC current being impressed from the anode to the metal
must be continuously applied for this method to be effective. If a less noble metal is
used as anode it an offer sacrificial cathodic protection, without any external DC
current.
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1.4.4. Use of Corrosion Inhibitors:
Chemical control with filming inhibitors or oxygen scavengers can be used to
modify the environment in certain cases to minimize the rate of corrosion inside
vessels and pipelines. Again this program needs to be maintained continuously to be
effective.
1.3.5. Use of Surface Coatings:
The broad category includes all types of coatings such as metallic, organic and
inorganic coatings intended for corrosion protection. Among them, the use of
organic coatings (broadly and frequently mentioned aspaints) is cheap and important.
The wider use of organic/surface/protective coatings to isolate the metal from a
corrosive atmosphere has long been used successfully for reasons of their low cost,
ease of application, suitability to almost all structures and all environments and to
some extent its aesthetic appeal.
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Chapter II
CORROSION PROTECTION BY ORGANIC COATINGS
Among the various methods adopted for corrosion protection, the most
effective, economic and very widely followed technique is protection by organic
coatings or paints which can be very easily adopted, repaired and properly maintained
compared to other techniques in marine, industrial and other polluted atmospheres.
That is why world over 80% of the surfaces needing protection adopt painting for
saving their steel structures. The basic reason is that steel being the most widely used
material of construction under a variety of corrosive atmospheres, is subjected to
severe corrosion and it requires full and fool proof protection which is achieved by
coatings, supplemented by cathodic protection wherever possible. Since the middle of
1950s more sophisticated coating materials and systems have been developed and
today a number of useful and high performance coating systems are available for
corrosion protection, enabling the coating consultant to choose the right material tailor
made for the environment encountered.
Corrosion can be prevented by a coating if it behaves as a perfect barrier
separating the metal from the environment but this seldom happens because paint
films are not perfect barriers and they allow permeation of a reasonable quantity of
water vapour and a small quantity of oxygen through them according to many
researchers in this field. They also add that this permeation sustains the corrosion
process continously. This makes the presence of an inhibitive/sacrificial primer at the
metal/coating interfacenecessary. The introduction of a thick undercoat between theprimer and topcoat helps to reduce the permeation and increases the ionic resistance
to combat the corrosion process.
2.1. MECHANISM OF PROTECTION:
A question uppermost in the minds of the users of protective coatings is that
why steel corrodes even after painting that too in most cases the paint being a multi-
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coat system. Hence the mechanism of protection by coatings has to be well
understood by the user. The protective mechanism of the coatings can be explained by
the following three ways:
2.1.1. Barrier mechanism:
The barrier mechanism works by lowering the permeability to moisture and
offering resistance to the flow of corrosive ions and by enhancing the resistance
against disbondment.
Working of barrier mechanism:
The main barrier effect is provided by the thick undercoat running to 100-150microns of DFT. In certain cases of very severe corrosive atmospheres, this undercoat
may consist of 2 coats with a still higher DFT. This thick undercoat reduces the
quantity of moisture that penetrates through the coating and also being a thick barrier
suppresses reaction between oxygen and the substrate called the cathodic reaction. By
being a cross linked polymer material, it offers resistance to the flow of corrosive ions
like chlorides and sulphates to the substrate. Of course the barrier reduces the
penetration of moisture through the film by possessing a low value Moisture Vapour
Transmission Rate (MVTR). Presence of leafy pigment micaceous iron oxide (MIO)
adds to this resistance.
2.1.2. Inhibitive / Passivation mechanism:
The inhibitive / passivation mechanism works by offering inhibition to
corrosion at the coating-metal interface using anti-corrosive / inhibitive pigments.
Working of inhibitive/passive mechanism:
Even though the coatings offer resistance to transport of moisture, there is still
some amount of moisture reaching the substrate which is bound to react with the steel
substrate in the absence of any primer at the interface. This is the anodic reaction of
the corrosion process as explained in chapter xx. At this condition, if the primer is of
an inhibitive nature, the surface is made passive and prevents corrosion.
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There are many known inhibitive pigments such as red lead, lead silico-
chromate, calcium chromate and zinc phosphate. They leach or dissolve in the
moisture and produce a passive layer over the steel surface thus inhibiting the
corrosion process. But today only the zinc chromate and zinc phosphate pigments are
in use, the others being phased out due to environmental considerations.
2.1.3. Sacrificial mechanism
The sacrificial mechanism works by use of metallic pigments for sacrificial
type cathodic protection in primers
Working of sacrificial mechanism:
In case the primer is of a sacrificial nature, containing fine zinc powder as
pigment in an organic or inorganic medium (called organic or inorganic zinc rich) the
zinc present at the interface being anodic to steel becomes the anode and dissolves
itself preferentially and protects the steel. This is the sacrificial cathodic protection.
The popular primers for such uses are zinc ethyl silicate primer (inorganic zinc rich)
and organic zinc rich primers based on epoxies, polyurethanes and chlorinated
rubbers. The two processes of working of inhibitive and sacrificial primers at thecoating-metal interface can be termed as suppression of anodic reaction.
2.1.4. Working of a Multi-coat Protective Coating System
As it is not possible to realize both-barrier and passivation/sacrificial
mechanism satisfactorily within the same coating layer, multi-coat layers consisting
of primer, undercoat and topcoat are designed for severe corrosive environments.
A multi-coat protective coating system consists of:
i) An anticorrosive primer containing either an inhibitive or a sacrificialpigment
ii) A thick undercoat of a cross linked polymer system optionally with apigment of leafy structure and
iii) An UV resistant topcoat.The multi-coat system works by a combination of the mechanisms explained
above. When the coated metal is exposed to a corrosive atmosphere, moisture tries to
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penetrate through the coating; the undercoat resists the penetration of moisture to a
greater extent because of its higher thickness and the lower moisture vapor
permeation rate (MVTR) of the cross-linked polymers. The presence of leafy pigment
in the undercoat adds to this resistance. The undercoat, due to its high ionic resistance,
also resists the flow of corrosive ions through it to the substrate. Then whatever
moisture penetrates through the undercoat and primer reaches the substrate, where the
inhibitive or the sacrificial pigment present in the primer provides protection to the
metal either by an inhibitive or sacrificial mechanism. The topcoat protects the bottom
coats from degradation by UV from sunlight.
The total protection provided by a multi-coat system is a cumulative effect of all
the theories and mechanisms explained above.
2.1.5. Some Important Anti-Corrosive Primers
As discussed in the Mechanism of Protection of Coatings, anti corrosive primer
forms an important part of a multilayer protective coating system. There exist a
number of primers widely used for the purpose as described below, belonging to
categories of sacrificial or inhibitive primers. Sacrificial category includes inorganic
and organic zinc rich primers where zinc powder is the pigment whereas the inhibitive
type consists of all organic systems zinc chromate or zinc phosphate. Nowadays
zinc chromate is slowly being replaced due to toxicity by other less toxic pigments.
Zinc Ethyl Silicate Primer:
Inorganic zinc coatings are a class of superior coatings to steel for corrosion
prevention introduced in the 1940s. Even as a single coat applied to a clean surface,
it has proven track records of providing excellent corrosion protection and has
become standard coatings for long term protection against atmospheric corrosion
including marine and offshore atmospheres. Among several versions of inorganic
zincs, the one based on hydrolyzed organic silicates popularly called zinc ethyl
silicate remains the best. This is a two pack system with one pack containing pure
zinc powder and the other pack containing the binder material - partially hydrolyzed
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ethyl silicate with fillers, solvents and additives. The two are mixed at the time of
application and applied on to a thoroughly cleaned (blast cleaned) steel substrate.
This primer coating with excellent corrosion prevention properties can be over-
coated with epoxy, polyurethane and vinyl systems to provide schemes which can
provide protection for very long periods.
Organic Zinc Rich Primers:
Here, the zinc powder as pigment is dispersed in an organic medium more
widely in epoxies and to some extent in other binder systems also. The zinc powder
bound in the organic medium provides cathodic protection to the steel substrate, so
long as the formulation is such that the particle to particle contact of zinc is
maintained. The major advantages of this system are the compatibility to many of the
organic top coating systems and the ability to be repaired easily.
Organic Inhibitive Primers:
Inhibitive primers with zinc chromate or zinc phosphate as pigments are a
widely used organic primer for corrosion protection. The previously used lead based
primers have almost now been replaced. Even zinc chromate is slowly being replacedwith zinc phosphate because of its toxicity. These primers are very popular in epoxies
and also in the alkyds and chlorinated rubber binder.
2.2. PAINT CONSTITUENTS AND PROPERTIES
Paint is a complex liquid coating material composed of pigments, binders and
other additives. When applied to a surface, he liquid is changed to an adherent solid
coating over the substrate. The main constituents of a liquid paint can be grouped into
four broad categories viz. (i) binders (ii) pigments (iii) solvents and (iv) additives.
2.2.1 Binder
Binders otherwise called resins are the materials that hold together the other
ingredients and form the continuous film adherent to the substrate which are complex
organic polymeric materials. They are usually modified with plasticizers and catalysts
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and improve such properties as gloss, durability, adhesion, corrosion resistance,
weather-resistance flexibility etc.
There are many types and systems of protective coatings widely being used.
These systems are known by their generic names like a paint employing chlorinated
rubber as binder being called chlorinated rubber paint and another paint employing
epoxy as the binder being called epoxy paint. Some of those systems used for
corrosion protection are described below.
2.2.1.1 Alkyd resins
Alkyd resins are essentially polyesters of polyhydroxy alcohols and
polycarboxyl acids, chemically combined with acids of various drying, semi-drying
and non-drying oils in different proportions. The oil acids are coupled into the
molecules by esterification during manufacture and become an integral part of the
polymer. The polyester portion contributes hardness and the oil acids contribute
flexibility, adhesion and solubility in inexpensive solvents. The oil modified
polyesters are known as pure alkyds in trade. The extent of oil modification may be
short (45%), medium (45-65%) and long (65%).
Glycerol is the most-commonly used alcohol with pentaerythritol ranking
second. Phthalic anhydride is the principal polycarboxylic acid used with maleic
anhydride ranking second. Linseed, soya, tung, oiticica, castor, coconut, cotton-seed,
safflower, tall and fish oil are used in various proportions to produce drying, semi-
drying and non-drying alkyds of short, medium and long oil varieties.
Alkyd air-dry by the mechanism of auto-oxidation initiated by the driers like
naphthenates and octoates of cobalt, lead, manganese and zinc. Like oil-modified
alkyds, resin-modified alkyds are not available. The widely used synthetic enamel
paints belong to this category.
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2.2.1.2 Phenolic resins
Phenolic resins are made from phenols and para-substituted phenols reacted
with formaldehyde. They have one free methylol group and are of two types of
Novolac and Resole.
Novolac type resin is made by reacting substituted phenols with formaldehyde
and a small excess of phenol such as para-phenyl phenol, para-tertiary butyl phenol
and para-tertiary amyl phenol in presence of acid catalysts. Air-drying paints are
employed in blends with alkyds and other resins to get paints for floors, deck paints,
machinery enamels and other maintenance and industrial finishes.
The resole type is made by reacting substituted phenols with an excess of
formaldehyde in the presence of an alkaline catalyst. Product is a baking finish useful
for electrical insulating varnishes, trade sales coatings, floor varnishes and for
architectural finishes.
The other type (oil free) is made with alkaline catalysts, un-substituted phenols
and an excess of formaldehyde. They also give a baking finish.. Such baking films
have outstanding properties. The paint coatings are unaffected by alcohols, ketones,esters and hydrocarbons. They have excellent resistance to organic and mineral acids
and unusual heat resistance. Resistance to water immersion and weathering is
exceptional. Coatings for interior of storage tanks for wine, beer, milk, cheese and
other food products are of this type.
2.2.1.3 Amino resins
Amino resins are a special type of synthetic resins possessing outstanding
properties. There are two important types-urea formaldehyde type and melamine
formaldehyde type. These resins are seldom used alone. Normally they are blended
with other resins-mostly with akyds to provide improved varieties of binders.
Urea and formaldehyde are reacted to give dimethylol urea, which on reaction
with butanol gives butylated dimethylol urea. Subsequent reaction with amine gives
urea formaldehyde polymer. Melamine on reaction with formaldehyde and butanol
gives melamine formaldehyde.
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Alkyd-amino paints are used for high-grade modern finishes like automatic
topcoats. They provide high build, good gloss, hardness and gloss retention for longer
periods. Urea-alkyd resins have good resistance to moisture, acids, alkalies, grease
and other chemicals and are also durable. They have high resistance to discolouration
and are used for radiators, stoves, washing machines etc.
2.2.1.4. Acrylic resins:
Acrylic resins are of two types: thermoplastic and thermosetting. These are made
from esters of acrylic and methacrylic acids like methyl methacrylate, butyl acrylate,
ethylacrylate, 2-ethyl hexyl acrylate, butyl methacrylate, methyl acrylate etc. by
solution/emulsion polymerization techniques. Thermoplastic resins are air-drying type
and the thermosetting resin contains a functional acrylic monomer containing either a
carboxyl/hydroxyl/nitrogen group.
These can be combined with epoxy or melamine/urea formaldehyde resins and
stoved to get films of improved corrosion-and chemical resistance and superior
mechanical properties.
The outstanding property of acrylics is their high weather-resistance unlikeepoxy and nearly 100% transparency to facilitate fluorescent paint formulations. The
advantage is the polymer can be previously designed to suit the final requirement.
They form very good coatings for cycles, automobiles, washing machines and other
appliances.
2.2.1.5. Vinyl resins:
Though there are many types of vinyl resins, the copolymer of vinyl acetate is
of much interest to us. Polyvinyl chloride is a hard and tough material difficulty
soluble in organic solvent at room temperature; by co-polymerising with 10-15% of
vinyl acetate, a useful resin results. The ratio ranges from 85-15%to 90-10. Some
include 1% maleic anhydride to improve adhesion to metals. It is formed by emulsion
polymerization by free radical polymerization mechanism to get fine powdery
copolymer. Solvents are blends of 50:50 ketone, toluene/Xylene.
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Coatings obtained from these resins are unaffected by strong or weak acids
(except formic and acetic acids); by strong or weak alkalies (except ammonia which
diffuses through the film); by many corrosive liquids such as mixtures of sulfuric aid
and potassium dichromate, aqua regia etc. or by oxidizing or reducing agents; by
petroleum hydrocarbons, alcohols, grease or water. Coating is highly resistant to
corrosion by chlorides (salt spray). Limitations are: (i) a low-solid solution is only
possible (around 30%) making the covering area per litre of paint low and hence paint
is costlier per unit area of surface; (ii) Poor adhesion to metals (improved by adding
maleic anhydride in the structure or by a blast primer); (iii) decomposition possible at
elevated temperatures.
Vinyl paints are highly corrosion resistant and applied to ships, hulls,
immersed structures in marine environments and in industrial areas, where all types of
chemical fumes are met with. Also suitable as linings for beer cans, juice can etc. as
inner food can coatings, for concrete swimming pools and other masonry surfaces.
2.2.1.6. Chlorinated rubber:
Chlorinated rubber is an important film-forming resin that is available in awide range of molecular weights, ranging from 3500 to 20,000. It is prepared by
chlorinating rubber latex to the extent of 65-67% chlorine. The resin is in powder
form and soluble in aromatic hydrocarbons. Films made from these resins are very
brittle and so a large amount of plasticizers have to be used in the formulation.
Normally chlorinated paraffins are used. Now drying alkyds can also be used as
plasticizers.
Paints formulated from these resins possess very high corrosion and chemical
resistant properties and have high durability. These paints are used successfully for
ships hulls, immersed structures, polluted industrial atmospheres and other marine
environments; also used for buildings, masonry, swimming pools and road markings.
These are thermoplasitc coatings and are cheaper than epoxies as protective coatings.
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2.2.1.7. Epoxy resins:
Most of the epoxy resins used in surface coatings is made from bisphenol A
and epichlorohydrin in varying proportions depending upon the property desired. The
most popular type epoxy resin is the DGEBA-diglycidyl ether of bisphenol-A.
Coatings based on epoxy resins are two-pack systems one containing the resin
part and the other the hardener or curing agent part. Just before application, these two
are thoroughly mixed and applied within the given pot lifetime. After application,
they cross-link at room temperature to give dry films of good thickness per coat and
outstanding properties. In addition to surface coatings, epoxies are used in plastics,
electrical insulation, adhesives and laminates for paper, glass, cloth and other fabrics.
Curing agents for epoxy resins
Many chemicals and resins containing functional groups with active hydrogen
atoms (or other functional groups) can be used for curing epoxy resins. They cause
polymerization by cross-linking of epoxy molecules. These curing reactions take
place at room temperatures:
1. Amines:Amine catalysts in small percentages cure epoxy resins. Tertiary and some
secondary amines catalyse the curing reaction. Typical examples are
diethylenetriamine and triethylene tetramine.
2. Polyamide resins:
Polyamide resins contain free primary and secondary amine groups which can
react with the epoxy resin.
Polyamide resins are used with epoxy resins as two-component, air-drying or
baking coatings. They have longer pot life and slower curing rate. They produce
smooth, high-gloss films with good hardness, toughness and flexibility. They have
much better water-resistance than amine-cured films and are widely used in
undercoats and special finishes such as swimming pool paints.
The outstanding properties of epoxy polyamide systems may be explained by
their structure. The very stable carbon-carbon and ether links in the backbone
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contribute to chemical resistance, while their toughness and superior mechanical
properties are due to the wide spacing between the reactive epoxide groups and in turn
the hydroxy groups. The polar hydroxy groups, some of which may always remain,
also assist adhesion by hydrogen bonding which explains the excellent adhesion to
metals possessed by these coatings. The aromatic ring structure enhances thermal
stability and rigidity. Though these properties are very attractive, the inherent
weakness of the system is their poor resistance to UV, restricting its use to primer and
undercoats (where the structure is exposed to sunlight), where adhesion and corrosion
resistance are very valuable and also to can coatings due to their good one-coat
performance. This problem can be overcome by having a overcoat of polyurethane or
acrylic over epoxy when it is exposed to direct sunlight.
2.2.1.8. Polyurethane resins:
Isocyanate group is reactive at medium and room temperatures, reacting
exothermically (40 kl/mol) with many groups containing active hydrogen atoms like
alcohols, amines, phenols, amides and water. The reaction initially produces an
unstable carbamic acid (RNHCOOH) which decomposes to a primary amine andcarbon-di-oxide. This reaction is exploited in the case of urethane foam formations
and avoided in the case of urethane coatings. Hence, systems to be reacted with
isocyanates should be dehydrated and low moisture content urethane grade solvents
are used. Low molecular weight di-isocyanantes used in coatings are mainly toluene
diisocyanate (TDI) (aromatic) and hexamethylene diisocyanate (HMDI)(aliphatic).
These are blocked in alcohols or phenols and adducts are prepared and used in
coatings.
Two-pack polyurethane coatings are based on hydroxy functional resins and
isocynate adducts. These are mixed at the time of use and the resultant finish is a
tough solvent, chemical and corrosion-resistant coating, curable at room temperature
like epoxy. The hydroxy functional resin may be alkyd, polyester, polyether, epoxy or
acrylic. Alkyd-based coatings find use in wood finishing. Polyester-based finishes are
applied for high durability transport finishes including marine and aircraft and
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automobiles. Acrylic-based polyurethanes find application in car refinishing,
appliance finishes and for corrosion protection in industrial atmosphere.
2.2.1.9. Silicone resins:
Silicone resins are based on an inorganic silicon-oxygen structure which has
organic radicals attached to the silicon atoms. Silicon-Oxygen and Silicon-Carbon
bonds are particularly stable and this has a beneficial influence on the behaviour of
semi-organic silicone resins making them exceptionally resistant to thermal
decomposition and oxidation and hence they are very much used in heat resistant
coatings of very high temperature ranges in the order of 1000o
F. These silicone resins
are also used to modify other film-formers such as alkyds, polyesters, acrylic or
epoxy, to enhance their thermal stability and durability, the range of modification
being 15% to 40%, though higher level of modification of alkyds is possible for
special heat-resistant applications. Pure silicone surface coatings are used, where very
high heat-resistant properties are required.
Silicone resins are made from intermediates known as silanes, which are
monomeric chemicals containing silicon combined with various organic groups andusually one to three atoms of chlorine. The most commonly used silanes are the
methyl and phenyl chlorine compounds. The organo chloro silanes are liquids soluble
in aromatic hydrocarbons and are easily hydrolyzed to hydroxyl-bearing monomers
known as silanols. Silicone resins are sold as 50% or 60% solution in xylene or
toluene. They are nearly water white in colour. Some are air-drying and some are
baking type (1-3 hrs at 300-480 F). The raw materials chlorosilanes and silanols are
also available in market to make tailor-made resins.
2.2.2 Pigments
Pigments are finely divided, insoluble solid particles that are dispersed in the
binder and remain suspended in the binder after film formation. The role of pigments
in the paint coatings is
a. to provide color and reinforcement
b. to hide the surface
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c. to decrease the permeability of the film
d. to provide rust inhibiting characteristics and
e. to protect the film from the effect of ultraviolet light and weather.
Pigments used in the paint industry may be divided into two broad groups viz.
i. Main pigments and ii. Extender pigments. The main pigments once again may be
divided into three categories viz. i. Inhibitive pigments, ii. Inert-pigments and iii.
Metallic pigments.
2.2.2.1 Inhibitive pigments
The main inhibitive pigments used in paints are red lead, lead silicochromate,
zinc chromate, zinc phosphate, calcium plumbate etc. They are mainly used in
primers. The mechanism of inhibition of corrosion differs from one pigment to
another.
2.2.2.2 Inert Pigment
The main inert pigments used in the paint industry are red iron oxide,
micacious iron oxide, titanium dioxide etc. They will have very good hiding property.
2.2.2.3 Metallic pigmentsNormally used metallic pigments are aluminium pigment and zinc dust.
Aluminium pigments are used because of their excellent properties such as brilliance,
durability, high covering and hiding power and high resistance to atmospheric
corrosion. It is normally used in top coats.
2.2.3 Solvents
Solvents are organic liquids which dissolve or disperse the film former.
Solvents reduce the solution to the proper solids and proper viscosity. For different
binders, different solvents will be used; normally a mixture of solvents will be used.
In addition to the solvents already in the paint, so called diluent solvents (thinners) are
used to give the paint a viscosity that makes it easier to apply.
They are seldom used singly; a mixture of two or more are used in a paint
formulation, the combination being optimized due to solvency power, cost of each
solvent and the required drying time of the paint.
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2.2.4 Additives
Additives are ingredients that are added to the paint in small quantities in order
to improve certain properties to the desired level e.g. dryers to improve drying;
plasticizers to give flexibility to the film; anti skinning agents to prevent skinning
during storage; wetting agents to permit easier grinding and to improve wetting ability
of the surface to which the paint is applied, anti-flooding agents to reduce flooding
and floating of some pigments etc.
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Chapter III
SURFACE PREPARATION
The adhesive bond between the metallic substrate and the coating plays a key
role in the success of the coating. More the adhesion, greater would be the protection.
Surface preparation is one of the important preliminary steps to ensure proper bonding
of the coating. Investigations have shown that the best system applied over an
unprepared surface performs poorly than a moderate system applied over a well
prepared surface. The total adhesive value of organic paint coatings to steel surfaces is
contributed partly by the adhesive property of the polymer and partly by the
mechanical bonding to the substrate. A proper surface preparation i.e. the removal of
visible elements like rust, oil, grease, dust, dirt etc. and invisible contaminants like
weld-flux and ions of chloride and sulfate ensures the maximum polymeric adhesion.
The mechanical bonding can be improved to a greater extent by creating surface
roughness called surface profile.
Surface profile/anchor pattern is defined as the maximum average peak to
valley depth caused by the impact of the abrasive on to the substrate during blast
cleaning. These pits and peaks called surface profile increases the surface area to
which the coatings adhere providing a mechanical anchor and resulting in good
adhesion. As a general rule, the depth of surface profile required for good adhesion is
proportional to the thickness of the coating i.e. thicker the coating deeper the surface
profile. Consultants while specifying surface preparation and paint schemes normally
specify the level of surface profile also considering the total proposed thickness of thepaint scheme.
3.1. METHODS OF SURFACE PREPARATION:
The general methods of surface preparation can be classified as mechanical
methods and Chemical methods.
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3.1.1. Mechanical methods:
The various mechanical methods employed are, hand cleaning, power tool
cleaning, dry abrasive blasting and wet blast cleaning.
3.1.1.1. Hand cleaning:
Hand cleaning removes loose rust, loose mill scale, dirt and non adherent oil
paint. This is generally done with wire brushes, scrappers, scrubbing with bristle
brush and in some instances emery and sand paper.
Before hand cleaning is undertaken the surface should be examined to
determine the amount of nature of the contaminants. If detrimental amount of oil or
grease are present, solvent cleaning must precede hand cleaning.
There are limitations for hand cleaning. They are:
a. Properly shaped tools may be necessary
b. Tough mill scale cannot be removed
c. Very slow and impracticable for large areas
3.1.1.2. Power tool cleaning
Power tool cleaning operation implies the removal of loose rust, mill scale andold paint by power tool chipping, descaling wire brushing etc. without excess
roughening leading to the formation of ridges and burrs.
It does not remove the tight mill scale or all traces of rust on pitted steel. It is
employed in places where blast cleaning is impracticable. Power driven tools include
pneumatic chippers, chisels, descaling tools and needle hammers, rotary scalers,
rotary wire brushes and abrasive wheels. Auxiliary equipment includes the air or
electric power supply dust brushes and safety equipments as required.
3.1.1.3. Abrasive blast cleaning:
Abrasive blast cleaning is one of the best methods available for surface
preparation. Many varieties of abrasives are used, the most common being iron grit or
shot, malleable iron grit or shot, hardened steel shot and chopped steel wire among the
metallic and aluminium oxide, sand or silica, slag from steel plants among the non
metallic materials. The abrasives are classified based on their shape as shot or grit.
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This operation is normally done at an air flow rate of 60-300cft/minute at a pressure
of 100psi
.Sand / Iron grit / Iron shot
For fast cleaning, graded flint or silica sand or iron grit or iron shot of 16-30
mash are being used. The particle should pass through 100% when sieved with 16
mesh and nothing to pass through when sieved with 30 mesh i.e. size below 16 and
above 30 mesh is not recommended.
The shape of abrasive can be sharp, semi-sharp, spherical or near spherical.
Semi-sharp means, some sharp as well as round edge in one particle grit.
Abrasives commonly used for white blasting of steel are sand flint, synthetic
abrasives, steel or cast iron grit. Generally a stream of compressed air in combination
with a blasting unit is the means of driving the abrasive on to the surface. The blasting
unit places the abrasive on the air stream in measured quantity.
Sand blast nozzles
The three common sizes of sand blast nozzles for general maintenance painting
are 16 mm, 8 mm and 9 mm.The nozzles made up of Tungsten carbide are superior to the other types of
nozzles. These hard nozzles have a life of 800 hrs of continuous blasting.
Air supply pressure and volume
Usually 100 pounds per sq. inch is considered to be the ideal working pressure
for the operation.
Wet Blast cleaning
Due to increased emphasis on reduction of air pollution, a new form of
cleaning steel work has come into existence replacing dry blasting. This is
comparatively new form of cleaning for steel work and includes a number of different
methods. It is normally done at a pressure of 3000-6000psi. at 8-10gpm water volume.
Basically water is used, generally with abrasives, at various pressures, to clean off rust
and to remove soluble deposits and dampen down the dust. The major advantage of
this cleaning process is the absence of dust pollution leading to silicosis of the
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operator. It enables inclusion of an inhibitor as part of the process to prevent rusting
for several hours so that the primer can be applied to clean steel. A water-tolerant
primer can be applied before the drying of steel.
Removal of mill-scale:
Though hot rolled steel sheets coming out of the steel mills possess an oxide
coating named mill-scale in bluish green colour which itself is a protective layer
offering a limited protection to the steel surface for a short period of time, this layer
has to be removed in order to ensure proper performance of the paint coating applied
on the steel once the fabrication /erection is completed and before the structure is put
to use. This is particularly very critical when expensive multi-coat systems are applied
for protection against corrosive environments.
3.2. INSPECTION OF BLASTED STEEL SURFACE
There are many standards prescribed by various bodies for surface preparation
which are indicated below;
Steel Structure Painting Council SSPC
National Association of Corrosion Engineers NACE
Swedish Standards SA
Some of the important standards relevant to our purpose of the above bodies
are reproduced here.
Inspection has to be carried out by visual comparison with the original
standards. These pictorial standards are available with paint inspection tools suppliers.
They are also available in the book Steel Structures Painting Manual (Vol.I), Good
Painting Practice in pages 185 & 186. Surface profile gauge can be used to measure
surface profiles obtained during surface preparation.
White metal blast (SSPC 5-63, NACE No.1, SA 3)
This is defined as removing all rust, scale paint etc. to a clean white metal
which has a uniform grey white appearance. Streaks and stains of rust or other
contaminants are not allowed:
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Near white metal (SSPC 10-63, NACE No. 2, SA - 2 )
This provides a surface about 95% as clean as white metal. Light shades and
streaks are not allowed.
Commercial blast (SSPC 6-63, NACE No.3, SA 2)
This type of blast is more difficult to describe. It essentially amount to about
2/3 of a white metal blast which allows for very slight residues of rust and paint in the
form of staining.
Brush off blast (SSPC 7-63, NACE No.4 SA-1)
This preparation calls for removal of loose rust, paint, scales, etc. Tightly
adherent paint, rust scale is permitted to remain.
This provides a surface about 95% as clean as white metal. Light shades and
streaks are not allowed.
The type of surface preparation technique to be followed i.e. blast cleaning /
power tool cleaning etc. and the specification of the surface status to be achieved (SA
1/SA 2 or SA 2 etc.) are specified in the recommendations.
The following are some of the general precautions to be observed duringsurface preparation and subsequent painting:
(1)All fabrication activities such as welding, grinding, cleaning and finishingshall be completed before the surface preparation for painting.
(2)Weld spatter, slag/flux, surface deformities etc, shall be completelyremoved from the weld metals and surrounding areas by scrapping,
chipping and grinding.
(3)Sharp edges shall be made rounded. Gas-cut edges shall be ground to asmooth finish.
(4)Threaded holes and externally threaded parts shall be protected by suitablemeans during surface preparation and painting
(5)Compressed air used for blasting shall be dry and completely free frommoisture and traces of oil. Moisture traps/filters with adequate capacity
shall be fitted in the compressed air line near the exit.
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(6)Surface roughness profile on the blasted surface shall be maintained as perspecification
(7)7After completion of blasting, abrasive dust shall be carefully cleaned fromthe surface using a brush and by blowing dry compressed air.
(8)The blasted surface shall be primer painted immediately after blasting. Thefirst coat of painting shall be completed within 4 hours of blasting. In case
of any brown rust formation the blasted surface, the area shall be locally
sweep blasted and cleaned before the primer painting.
(9)Blast cleaning shall not be adopted in locations, where the abrasive materialcan contaminate the subject surface or during weather conditions with
R.H.>80-85%. The supervising personnel from NRL side shall confirm the
R.H. from laboratory.
3.3. SURFACE PREPARATION FOR MAINTENANCE PAINTING:
All intact areas shall be roughened by sweep blasting wherever possible; or
such areas shall be cleaned by manual means; all damaged and rusty areas shall be
treated as under:
i) Remove all loose paints and rustii) Treated surface must be completely dry and free from contamination.
Note: Refer chapter No.9 on Maintenance Painting for application procedure
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Chapter IV
APPLICATION OF PAINT
The liquid coating material is a highly technical product containing volatile
and non volatile parts as described elsewhere in this manual and hence it has to be
applied with utmost care by a skilled applicator.
There are a number of application methods by which coating can be applied.
Though there are other methods such as electrostatic spray, electro-deposition, curtain
coatings, roller coatings and dip coatings, the two principal and popular methods
widely used for field application are by brush and spray and they are described here.
4.1. BRUSH APPLICATION
Brushing should be used mainly for small areas and edges around rivets, in
corners, along welds and similar areas, prior to the application of a general spray coat.
Brushing is also useful to improve wetting of primers, particularly on the surfaces
which are difficult to coat such as those just mentioned.
The two general designs of brush which may be used for steel are conventional
wall type brush and flat brush made of nylon or hog bristles. The proper application of
a coating by brush depends on the proper handling of the brush. The brush should be
held with the fingers, like holding a pencil. Brushes should not be dipped deep into
the paint. Dipping into the paint to a depth of approximately 2.5 cm is adequate.
4.2. SPRAY APPLICATIONAir spray is a process in which compressed air and the coating liquid are
brought together in a way that forms a fine spray. The spray gun is the important
component in the spray system as is the machine which brings the air and liquid
together.
Compressed air is used to atomize the coating liquid at the tip of the gun and
also to apply pressure to the liquid coating material and force it through the nozzle on
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efficiency reported also being higher. The droplets formed are larger than in air spray
and hence produces a heavier coat in a single pass. Highly viscous paints and paints
meant to produce a high level of thickness in a single coat can be successfully applied
by this technique.
The normal rates of application of paint by different techniques are given
below:
Method of application Area in sq.meter covered in one day
i) Brush 93
ii) Air spray 372-744
iii) Airless spray 744-1115
4.4. PRECAUTIONS TO BE OBSERVED:
Normally paints have high density pigments dispersed in low-density polymer
binders and hence the pigments and extenders settle at the bottom leading to caking
on storage for long periods, in spite of using anti-settling agents in the formulation.
This should be checked while opening the drum and corrected before application. The
lumps so formed if any should be broken up by rubbing against the sides of the
container and thoroughly mixed with the liquid portion so that a consistent smooth
flowing liquid paint material is obtained. If the pigment lump does not break and has
set as a hard mass, the paint should be discarded.
It is probable that a small amount of skin may be formed on the top of the paint
material in some cases. The skin may be removed before application and the most of
the material may be stirred well. If the quantity of skinning is high or the skin may
thick, the paint may be rejected.
4.5. MIXING OF TWO COMPONENT PAINTS:
The base paint shall be stirred and made homogenous. The hardener shall be
added to the base paint in the specified mixing ratio and thoroughly blended using a
power stirrer. Viscosity of the mixed paint shall be adjusted to the required level by
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adding thinner and thorough mixing so as to maintain the WFT as explained in
Chapter VIII, where monitoring of DFT through WFT during application is explained.
The quantity of paint that can be applied well within the pot life of the mixed paint
should only be mixed.
Health and Safety Factors:
Most of the volatile organic solvents used in paints causes health and fire
hazards. They are harmful for the respiratory system, eyes and skin and also are
potentially explosive. Hence, the following general precautions are necessary while
handling and application of paints.
Inhalation of solvent vapours or paint mist, contact with liquid paint with skin
and eyes should be avoided.
Suitable approved respirators or face masks should be used during paint
application and handling.
Since solvent vapours are heavier than air, they tend to accumulate at the
bottom of tanks or of confined spaces. Care should be exercised while entering areaswhere this might have happen.
Food and drink should not be stored or consumed where paints are stored and
smoking should be prohibited in such areas.
All fire safety precautions are to be followed in areas where paints are stored
and where paint application is done.
Cans holding liquid paints are to be handled carefully and should not be kept
near or above any hot surface or object.
All scaffoldings, etc. erected for painting must be to the satisfaction of NRL
safety requirements. Safety belts with lifelines are mandatory requirements, while
painting at elevated locations.
Solidarity working in confined space must be avoided.
Apart from these, the general safety norms for the refinery & marketing
terminal shall be adhered to.
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Chapter V
COLOUR CODING5.1. The following colour coding scheme shall be generally followed:
Sl.No Service Ground
Colour
1st
Colour
Band
2nd
Colour
Band
1 Drinking water Sea Green French blue Signal red
2 DM water Sea Green Orange ---
3 Cooling water Sea Green French blue ---
4 Boiler Feed Water Sea Green Gulf red ---
5 Fire water Fire Red -- ---6 Steam condensate Sea Green Light brown ---
7 HP Steam Aluminium Signal red ---
8 MP Steam Aluminium French blue ---
9 LP Steam Aluminium Canary yellow ---
10 Plant Air Sky blue French blue ---
11 Instrument Air Sky blue Gulf red ---
12 Hydrogen Canary Yellow Signal red French blue
13 Nitrogen Canary Yellow Black ---
14 Flare Gas Aluminium Signal red ---
15 ATF Aluminium Black ---16 SKO Aluminium Golden yellow ---
17 HSD Aluminium French blue ---
18 Naptha Aluminium Dark brown ---
19 Reformer Naptha Aluminium Dark brown French blue
20 VGO Aluminium Golden yellow Signal red
21 Liquid LPG Aluminium Signal red Green
22 Vapour Aluminium Signal red Black
23 Crude Oil Aluminium Black French blue
24 RCO Aluminium Black Brown
25 Fuel Gas / sweet gas Canary Yellow Silver grey26 Sour gas Canary Yellow Grey Violet
27 Amine Admiral Grey Signal red Lt. brown
28 Acid Violet Canary yellow
29 Other chemicals White Violet Signal red
30 Caustic White Violet
31 Phosphate Violet Orange White
Note : All LPG service PSCs shall be painted Blue.
All drains & Vents shall be painted in Main line colour
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The colour code scheme is for identification of piping service group, it consists
of a ground colour and colour bands.
5.2 Ground Colour.On un-insulated pipes, the entire pipe has to be painted in ground colour, and
on metal claded insulated lines, minimum 2M long portion should be painted.
5.3 Colour Bands:
Location of colour bands: At Battery Limits
Intersection points & change of direction points in
piping
Midway of piping section, near valves, across
culverts
At 50 M interval on long stretch pipes
At the starting and termination points
Minimum width:
NB Width
------------------------------------------------3 and below 75 mm
Above 3 to 6 NB x 25 MM !!! Note :
Above 6 to 12 NB x 18 MM For insulated pipes, NB indicates OD
Above 12 NB x 15 MM of the insulation
Sequence :
Colour bands shall be arranged in sequence showing Table above and the
sequence follows the direction of flow. The width of the 1st
band to 2nd
band is 4:1.
Note : Wherever deemed required by Process Department of Safety, pipes handling
hazardous substances will be given hazard marking of 30 mm wide diagonal stripes of
Black and Golden Yellow as per IS: 2379.
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5.4 Special Camouflage Painting for Un-insulated Crude and Product Storage
Tanks
Paint specification shall be as per standards.
Camouflage painting scheme for Defense requirement in irregular patches will
be applied with 3 colours.
Dark Green : Light Green : Medium Brown
5 : 3 : 2
The patches shall be irregular and asymmetrical and inclined at 30 to 60
Degrees.
Patches should be continuous at surface meeting lines / points
Slits / holes shall be painted in dark green shade
Width of patches shall be 1 to 2 meters.
5.5 Identification Markings on Equipment / Piping
Equipment tag Numbers shall be stenciled / neatly painted using normal Arial
lettering style on all equipment and piping (Both insulated & un-insulated) after
completion of all paint works. Lettering colour shall be either BLACK or WHITE,
depending upon the background, so as to obtain good contrast.
Operations Group shall specify location:
Sizes shall be:
Columns, Vessels, Heaters 150 mm
Pumps and other M/c 50 mm
Piping OD / 2 with Maximum 100 MM
Storage Tanks (As per Drawings)
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Chapter VI
RECOMMENDATION OF PAINT SCHEMES
The specific paint schemes recommended to be applied for different areas of
the plant are given in the following tables which are grouped according to the
corrosive nature. The individual paints are code named as P1, P2, B1, B2 etc. The
corresponding names are indicated below and the detailed specifications of individual
paint are provided in the next chapter.
CODE NAMES FOR DIFFERENT PAINTS:
P1 Zinc ethyl silicate primer
P2 Epoxy zinc phosphate primer
P3 Epoxy mastic primer-I
P4 Epoxy red oxide primer
U1 Epoxy MIO undercoat
L1 Tank-liner
B1 Phenolic bitumen black paint
B2 Bituminous aluminium paint
B3 Graphite based bituminous paint
C1 Coal tar epoxy coating
F1 Acrylic PU top-coat
F2 Synthetic enamel
F3 Aluminium finish paint
H1 Heat Resistant AluminiumH2 High Temperature Silicone Coating
H3 Epoxy phenolic thermal coating
CR1 Chlorinated rubber zinc phosphate primer
CR2 Chlorinated rubber MiO undercoat
CR3 Chlorinated rubber topcoat
T1 Tie coat
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6.1. CDU / VDU / H2U / PUMP HOUSES / FLARE STACKS /MS PLANT
Sl.
No. Equipment Group
Max.
Operating
Temp.
0
C
Surface
Preparation Pain
1 Bare columns / vessels / reactors / heat
exchangers / towers / pipelines & piping
supports / pumps / blowers / compressors / CV /Motors / Instruments & Electrical Mounting
80 Manual cleaning One coat of P
2 Structural steel works for heaters (including
railings / platforms etc.)
80 Manual cleaning One coat of P
3 Other structural steel works
Gratings
80 Manual cleaning
Manual cleaning
One coat of P
One coat of P
4 Fin Fan coolers (Except 03-EA-27) 120 Manual cleaning One coat of P
5 Heater Shell, ducting, manholes, foundationbolts etc., flare stacks, flare lines, KOD, and
piping requiring Al ground colour
150 Manual cleaning Two coats of
6 Reformer Shell, Convection boxes, box covers,
Hot ducting, 03-EA-027 header and other itemsof high temperature
150 300 Manual cleaning Two coats of
7 Grab crane, Flare Line Supporting Structure 80 Manual cleaning One coat of P
8 Lifts / HOT / EOT etc. Manual cleaning One coat of P
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6.2. HYDROCARBON STORAGE TANKS AND TANK FARMS
Sl.
No. Equipment Group
Max.
Operating
Temp.0C
Surface
Preparation Painting Sy
EXTERNAL PAINTING1 Tank shell, tank roof
For un-insulated tanks
For insulated tanks
80
80 250
Blast cleaning
Manual cleaning
Manual cleaning
One coat of P1 +One coat o
One coat of P2 +One coat o
Two coats of H3
2 Stair case, railing gratings etc. for
both insulated as well as un-
insulated tanks
Gratings
Blast cleaning
Manual cleaning
Manual cleaning
One coat of P1+One coat o
One coat of P2+One coat o
One coat of P2 + Two coat
3 Tank farm piping, supports
including fire water etc
Manual cleaning One coat of P2 + Two coat
4 Piping where Aluminium Finish is
required
Manual cleaning One coat of P2 + Two coat
INTERNAL PAINTING1 For ATF services (submerged
area, floating roof etc.)
Blast cleaning One coat of P1 + Two coat
2 Non ATF services (bottom, 1st
shell, vapour zone, pontoon inside)
Blast cleaning One coat of P1 + Two coat
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6.3. DCU/HCU and CCU Painting:
Sl..
No. Equipment Group
Max.
Operating
Temp.0C
Surface
Preparation Pain
1 Silos, Feeders, Conveyor covers, BunkerHouse, Un-insulated piping, pipe supports,
structures of silos, Electrical & Instrumentation
Mounting
80 Blast cleaningOR
Manual cleaning
One coat of PTwo coats of
2 Kiln Structures
Platform, grating, ladder etc.
80 Blast cleaning
OR
Manual cleaning
One coat of P
+ Two coats o
One coat of P+ One coat of
3 Kiln (Girth gear to Dish end) 300 Blast cleaning/
Manual cleaning
Two coats of
4 Kiln Feed end to Girth gear, Cooler, Feed
Hood, Kiln to Combustion Chamber duct
300 Blast cleaning/
Manual cleaning
Two coats of
5 Comb Chamber, Chamber to Stack, drain
Chute, cooler hood, disch chute, Kiln Structures& railings
150 Blast cleaning/
Manual cleaning
Two coats of
6 Structures & railings other than kiln Blast cleaning/
Manual cleaning
Two coats of
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6.4. SRU Painting:
Sl.No. Equipment Group
Max.Operating
Temp.0C
SurfacePreparation Pain
1 Combustion Chamber 300 Blast cleaning/
Manual cleaning
Two coats of
2 Incinerator and Stack 150 Blast cleaning/
Manual cleaning
Two coats of
3 Un-insulated Piping, columns, Vessels,
reactors, heat exchangers, Pipe supports,
base frames, Control valves,
Instrumentation mountings
80 Manual cleaning One coat of P
U1 + Two co
4 Structures including Platforms, railings,
gratings etc
.
80 Manual cleaning One coat of P
U1 + Two co
5 Tankages in SRU, External 80 Manual cleaning One coat of P
U1 + Two co
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6.5. DM Plant, ETP, Caustic Storage, RWTP, CNP:
Sl.
No. Equipment Group
Max.
Operating
Temp.0C
Surface
Preparation Pain
1 General Un-insulated Piping, Gratings etc. 80 Manual cleaning One coat of P
F1
2 Structures including Platforms, Railings,
Gratings etc.
80 Manual cleaning One coat of P
B1
3 Underwater steel structures of ETP &RWTP
80 Manual cleaning One coat of P
4 CNP cold box, cold converter. N2 bottles,
Freon condenser
80 Manual cleaning One coat of P
+ One coat o
5 Tankages including Caustic plant tankages 80 Manual cleaning One coat of P
U1 + One co
6 DM Plant Acid tanks, Pipelines carryingacid, ion-exchange tanks and pipes & valves
around them (external painting). Sulphuric
acid tanks in cooling water plant and ETP
(external painting)
80 Manual cleaning One coat of C
CR2 + One c
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6.6. Cooling towers & Utility Compressor House:
Sl.
No. Equipment Group
Max.
Operating
Temp.0C
Surface
Preparation Paint
1 Un-insulated Piping, Vessels, reactors,
Pipe supports, base frames, Compressors
& Motors, Control Valves,
Instrumentation Mountings
80 Manual cleaning One coat of P3
2 Hot piping, Compressors drums, InterCoolers
200 Manual cleaning Two coats of H
3 Structures including Platforms, railings
Gratings & Platforms
80 Manual cleaning
Manual cleaning
One coat of P2
One coat of P2
4 Vessels, Drums, equipment handling
dosing chemicals in cooling towers
80 Manual cleaning One coat of P3
5 Piping & Structures and equipments near
cooling towers
80 Manual cleaning One coat of P2
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6.7. CPP:
Sl.
No. Equipment Group
Max.
Operating
Temp.0C
Surface
Preparation Pain
1 Boiler & HRSG shell, ducting, steam piping
under insulation
150 Manual cleaning Two coats of
2 Boiler Drum (wherever required), wall
tubes, ducts
100 Manual cleaning Two coats of
3 Steel Structures, gratings, Stair case,
monkey ladders
100 Manual cleaning Two coats of
4 Un-insulated piping, Compressors, Pumps,
Blowers, Panels, Instrumentation &
Electrical Mountings, Filter Skids
80 Manual cleaning One coat of P
F2
5 GTG By Pass Stack, UB & HRSG startup
silencers
600 Manual cleaning Two coats of
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6.8. Loading Gantries:
Sl.
No. Equipment Group
Max.
OperatingTemp.
0C
Surface
Preparation Pain
1 Piping and Steel structures, fittings,
Instrumentation, pipe supports
80 Manual cleaning One coat of P
2 Structures including Platfor
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