{22922622-E805-4028-9241-057C05EAB20F}_NRL Painting Manual

8/6/2019 {22922622-E805-4028-9241-057C05EAB20F}_NRL Painting Manual

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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.)


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


U1 + Two co

4 Structures including Platforms, railings,

gratings etc

.


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.


B1

3 Underwater steel structures of ETP &RWTP


4 CNP cold box, cold converter. N2 bottles,

Freon condenser


+ 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


5 Piping & Structures and equipments near

cooling towers



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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


2 Boiler Drum (wherever required), wall

tubes, ducts


3 Steel Structures, gratings, Stair case,

monkey ladders


4 Un-insulated piping, Compressors, Pumps,

Blowers, Panels, Instrumentation &

Electrical Mountings, Filter Skids


F2

5 GTG By Pass Stack, UB & HRSG startup

silencers



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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


2 Structures including Platfor

{22922622-E805-4028-9241-057C05EAB20F}_NRL Painting Manual

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