Khanna A.S. (Ed.) High-Performance Organic Coatings: Selection, application and evaluation

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C 5

Non-immersion very

high corrosion

(temperature up to

120°C)

Sa 2.5 Medium durability

Primer: Inorganic Zn silicate Intermediate:

Epoxy MIO

Top coat: 2-pack aliphatic PUR

150 (2 × 75)

Long durability

Primer: Inorganic Zn silicate

Intermediate: High-build epoxy MIO

Top coat: 2-pack aliphatic PUR

160 (2 × 80)

C 3

Non-immersion

medium corrosion

(temperature up to

400°C)

Sa 2.5 Medium durability

Aluminum coating 2 250

Long durability

Primer: Inorganic Zn silicate

Top coat: Silicon-based coating

200

C 3

Non-immersion

medium corrosion

(temperature up to

600°C)

Sa 2.5 Medium durability

Primer: Silicon-based

Top coat: Polysiloxane

250

Immersion service:

tanks for fuel, oil,

crude, chemicals and

process water

Sa 2.5 Solventless epoxies 1 500

Immersion service:

sewage lines, water

treatment facilities

Sa 2.5

epoxies

1 or 2 1000 or more

(Continued)

Glassflake epoxies

Combination of glassflake and reinforced

Environment category

as per ISO-12944-2

Surface preparation

required as per ISO

8501

Coating system No. of

coats

DFT (μm)

Immersion service:

splash zone in sea

water

Underwater

Sa 2.5

HA 2 Water-repellent epoxies

1000

2 × 400

Immersion service:

water jetties, sea

water

Sa 2.5

Novolac epoxy

1 or 2 1500–2000

Immersion service:

freshwater treatment

facilities, locks and

gates

Sa 2.5 Solventless epoxies 1 1000

Underground

structures: gas

pipelines, water

pipelines

Sa 2.5 External oil and gas

3-layer PE (FBE primer, adhesive and PE)

2000–3000

Water pipelines

Coal tar or tar epoxy, tar-urethane

3 3000–5000

Liquid epoxy, elastomeric PU 1 700–1000

Table 5.2 Continued

Polyester glassflake

Glassflake epoxy

Causes and assessment of failures

in organic paint coatings

B P MALLIK,

Asian Paints Ltd, India

6.1 Introduction

The role of high performance coatings in mitigating the menace of corro-

is well established and reported in numerous published articles. Amongst

all possible corrosion control strategies deployed for protecting metals,

application of protective coating systems is the most widely accepted prac-

tice due to several advantages, such as that it is easy to adapt, easy to apply

even on inaccessible areas, easy to rehabilitate, gives simultaneous control

of corrosion as well as aesthetics, and offers possibilities of inserting addi-

tional functionalities through coatings and overall cost-effectiveness. While

a well-formulated, properly applied coating system on a thoroughly cleaned

surface can ensure long life, many times even a well-proven coating system

coating system cannot be derived due to early failure in use. Even a well-

proven coating system behaves differently at different sites. A tremendous

mature failures of paint systems and consequent damage of structures due

to corrosion. According to various published articles, the extent of mone-

tary loss due to corrosion varies between 2% and 4% of national GDP,

depending on the degree of industrialization.

failure is usually manifested either in the form of simple cosmetic failure,

e.g. loss of gloss, colour, etc., or as functional failure like rusting or delami-

coating system with respect to its lifespan. Often the failure looks innocuous

from the surface, but deeper investigation of the underlying condition

failure must be analysed in detail and suitable corrective action deployed

at the right time to prevent all sorts of unforeseen consequences, such as

sion and its economic significance for arresting the loss of national wealth

behaves differently at different sites. As a result the full benefit of the

amount of financial loss is incurred every year in every nation due to pre-

Failure of a coating system may be defined as its inability to maintain the

film integrity and acceptable look over the projected lifetime of applied

coatings on a particular substrate in a specific environment. Paint film

nation leading to non-achievement of the specified performance of the

beneath the paint film often leads to interesting revelations. Therefore any

98 High-performance organic coatings

loss of productivity, early degradation of structures, damage to the environ-

ment, accidents and associated risk related to health and safety.

Coating failures can occur for dozens of reasons but most are attributable

preparation, improper application and environmental stress factors.

Initiation of coating failure may be a slow process but once started it propa-

gates rapidly, thereby defeating the very purpose of using an expensive

coating system. Repair of a failed coating in an operating plant is a cumber-

some task and therefore requires a lot of compromise over and above the

additional cost. Assessment of the fundamental causes behind coating fail-

understanding the underlying causes, so as to formulate appropriate cor-

rective and preventative strategies for dealing with such failures.

Coating failure involves different types of coating defects with varying

degrees of impact on the overall coating life. It is important to have a thor-

ough understanding and familiarity with the commonly encountered defects,

causes of their occurrence and possible countermeasures to deal effectively

with coating failures. Failure investigation methodology would require sys-

tematic determination of the type and causes of damage, associated condi-

tions that promote the damage, and potential solutions to either minimize

or eliminate the damage by choosing the best possible solution based on

economics and assessment of impact on safety, health and environment.

Background information on the previously applied coating system, applica-

tion procedure, service environment and physical and instrumental charac-

terization of the failed coating are necessary for identifying the correct

causes of such failures. Finally, while formulating an appropriate remedial

strategy, one must pay due attention to the aspects of practical imple-

6.2 Causes of coating failure

underlying causative mechanisms, requires fundamental understanding of

how a coating system works to meet the primary objective of mitigating

corrosion. A typical coating system functions in several ways, namely by

acting as an impermeable barrier, providing strong adherence with the

service life, resisting weathering and chemical deterioration. The majority

of high performance coatings perform by creating a barrier and therefore

insulate the substrate from the external environment, so that harmful cor-

rosion-promoting ingredients like oxygen, moisture and electrolytes cannot

reach the substrate easily. The rate of diffusion of these corrosive elements

to basic formulation design, incorrect system specification, poor surface

ures is critical not only for assigning financial responsibility but also for

mentability of any proposed solutions in the specified service environment

to confirm their validity.

The subject of coating failure, its investigation and identification of the

substrate, maintaining cohesive and adhesive integrity of film during its

Causes and assessment of failures in organic paint coatings 99

degree of adherence of the coating system with the substrate, which is

indeed a measure of its bond strength. The lifetime of the coating will

depend on its ability to retain its bond strength under all sorts of stress and

strain that it may face during its environmental journey. Higher resistance

to weathering against heat, light and UV radiation helps to impede

the cosmetic deterioration on the surface coating, but it may not affect the

exposed chemicals, the longer is the lifespan of the applied coating

system.

The strength of the initial adhesion of the coating with the substrate and

its retention over the lifetime in the exposed operating environment is the

most crucial factor for preventing premature failure. Adhesive bond strength

is the result of physicochemical interaction between the coating and the

important is the nature of the physical and chemical bond formed with the

substrate. At the substrate coating interface, the force of attraction leading

to adhesion may not result from a single factor but instead is contributed

by the combination of both physical forces and chemical linkages. In

general, if the resultant adhesion comes through primary chemical bonding,

which is ionic or covalent in nature, the bond strength is much higher due

to stronger electrostatic interaction. Such a type of bond formation occurs

in phosphoric acid-catalysed vinyl wash primer, rust converter or inorganic

silicate-based coatings where the coatings form a primary valence bond

with the reactive metal ion. Secondary chemical bonding results from weak

Van der Waals interactions between polar functional groups like hydroxyl

(–OH), amine (–NHR), carbonyl and glycidyl groups of coatings with the

active substrate, in which the adhesion comes through the formation of

hydrogen bonding, long-range interactive forces like ion–dipole and dipole–

dipole interaction and so on. Because of the chemical nature of bonding,

highly polar epoxy resin based coatings show much better adhesion on

clean degreased metal in comparison to other organic binders. Physical

adhesion results from the mechanical interlocking between coating and

substrate which is essentially a function of the contact area. The contact

area can be increased by improving the surface wetting or by creating an

cleaning, or depositon of a thin layer of crystalline zinc phosphate or chro-

mate conversion coating. In all such cases the increased area of contact

ensures better anchorage, leading to stronger physical bond formation.

The precondition for sound bond formation is the intimate contact of the

a process in which one liquid spreads completely over a substrate with zero

through the coating film is governed by the thickness, porosity and perme-

ability of applied films on the substrate. The other important factor is the

corrosion-free service life significantly. The greater the resistance against

additional profile with the help of sandblasting, mechanical emery paper

coating with the substrate which results from wetting. Wetting is defined as

substrate, which is influenced by a number of factors of which the most

100 High-performance organic coatings

contact angles to ensure intimate contact. The wetting process in turn is

guided by the surface tension or surface energy of the liquid and solid

coming in contact with each other. For a coating to wet a substrate com-

pletely and subsequently bond to it, it must have a lower surface tension

than that of the substrate. Materials with higher surface energy cannot

spread completely on a substrate with lower surface energy, but the reverse

is possible. Most of the solvent-borne surface coatings based on organic

binders and solvents have surface tension in the range of 25–40 dyne/cm.

This is why such coatings cannot adhere satisfactorily on oil-contaminated

steel (surface tension of 20 dyne/cm) or on a very low surface energy mate-

process of degreasing, the oily contaminant layer on steel with the lower

surface tension is removed, thereby allowing the coating to come into direct

contact with the high surface energy substrate, i.e. the degreased steel

(surface tension 400 dyne/cm). This process enables complete wetting of

the coating, leading to more intimate contact for proper bond formation.

A sound coating is expected to resist the onslaught of all sorts of destruc-

tive environmental variables such as wind, rain, sunlight, heat, humidity,

oxygen, salts, chemical contact, microbial degradation and physical abuse.

A strongly adherent coating with low permeability can withstand the stress

created by these destructive forces satisfactorily, but a weakly adherent

coating with high porosity will fail to prevent the ingress of corrosive agents

and yield easily under these stresses, leading to corrosion and consequent

cation of the underlying causes is not a simple task, since often the failure

initiation and propagation process is the result of synergistic interaction of

several environmental variables that differ considerably with respect to

their mode of action and mechanism of degradation. In a multilayer coating

system there can be several causes for premature coating failures, but most

• Improper surface preparation

• Poor coating formulation (design related)

• Improper system selection

• Stress-related failures

• Application-related failures.

6.3 Surface preparation-related failure

Faulty surface preparation is the most critical factor for coating failure since

it affects all three fundamental forces, namely primary chemical bonding,

secondary bonding and the mechanical attachment responsible for coating

adhesion. Even if the coating system is excellent, it may fail if applied over

delamination. Coating-related failure is a complex subject and the identifi-

can be classified into the following broad categories:

rial like well-cleaned Teflon (surface tension of 17 dyne/cm). During the

Causes and assessment of failures in organic paint coatings 101

an unclean surface laden with oily contaminants, loose rust, mill scale, salts,

chippings of old coatings, condensation, etc. It has been estimated that

almost 70–75% of coating failures result from poor or inadequate surface

preparation. It is an established fact that a coating applied over a perfectly

cleaned substrate would survive longer than the same coating applied over

an improperly cleaned surface. Methods of surface preparation adopted in

the industry vary from surface to surface, but the most commonly followed

practices are solvent cleaning, hand and power tool cleaning, sandblasting,

hydroblasting, pickling and conversion coatings like zinc phosphating, chro-

matizing, etc. While sandblasting is more suitable for cleaning the thicker

steel used for fabrication of industrial structures, application of a layer of

conversion coating is more common for pretreating thin gauge ferrous and

non-ferrous metal surfaces. High performance, heavy duty coatings recom-

mended for application in highly corrosive environments such as marine,

immersion and subsoil buried conditions requires white metal blast (SSPC-

SP-5) or near white blast (SSPC-SP-10) cleaning as the minimum accept-

able standard prior to paint coating to ensure long life, whereas a similar

lifespan in a light industrial environment can be achieved by sweep blast

(SSPC-SP-7) or commercial blast (SSPC-SP-6) as the cleaning standard.

The probability of coating failure is directly correlated to the concentra-

tion of residual surface contaminants on the surface. This fact is also corrobo-

rated from the difference in salt spray test results for the same coating at the

same thickness when applied on a surface that has been cleaned by hand

tools compared with the same surface that has been sandblasted in a labora-

tory environment. The nature of the surface contaminants may vary depend-

ing on the nature of the substrate, e.g. oil, grease, loose rust and mill scale

matter, wax and excessive moisture are the common contaminants of con-

crete surfaces. Removal of oily contaminants is extremely important, since

they impair the wetting process and consequent adhesion of coatings on the

surface. Sometimes the accumulation of soluble salts on an unclean surface

creation is also extremely important from the standpoint of increased adhe-

the formation of early blister. A well-prepared, thoroughly cleaned surface

is extremely important prior to application of paint, since it provides the

perfect base for receiving the subsequent coat of paint and thereby ensures

long life to the coating by mitigating the risk of failure.

can promote blistering and subsequent delamination of coatings. Profile

be covered by applying an adequate thickness of paint, which is usually three

sion, since profiles allow more areas for intimate contact between coatings

and substrate. Of course, one should keep in mind that surface profiles must

to four times the profile height. In a highly humid environment a sandblasted

may be more common on metal surfaces, whereas laitance, efflorescence, oily

surface tends to develop flash rust within a short time unless it is quickly

overcoated. The presence of flash rust beneath the coating would facilitate

102 High-performance organic coatings

There are plenty of examples of coating failures due to lack of adequate

care during surface preparation and application. Adhesion-related failures

are quite common when a coating is applied on a surface contaminated with

oily material, rust, mill scale, etc. (Fig. 6.1). Even a thoroughly blast-cleaned

surface can be contaminated with oil droplets coming from a poorly

maintained compressor during air-assisted spray painting. Soluble contami-

nants like dust, salt, etc., coming from an open environment or uncleaned

blasting media, if they get trapped beneath the coating layer or between

two coats, facilitate premature blister formation. That is why the coating

suppliers often recommend fresh water washing, especially after hand tool

cleaning of substrates. Figures 6.2 and 6.3 depict adhesion failure due to

interlayer deposition of contaminants and application over a wet surface

respectively.

Surface preparation is equally critical before applying repair material

over a previously applied aged coating, for which both intercoat compatibil-

ity as well as proper surface cleaning are of the utmost importance.

Application of a coating over improperly cleaned non-ferrous materials like

galvanized iron or aluminium shows early failure if white corrosion prod-

6.1 Detachment of coating from oil-contaminated substrate.

6.2 Failure due to intercoat contamination.

ucts of zinc and aluminium oxide are not first removed by brush-off blast

Causes and assessment of failures in organic paint coatings 103

cleaning (SSPC SP 7). Before painting old concrete the surface laitance,

light blasting and subsequent cleaning with fresh water to prevent adhe-

sion-related failures.

6.4 Design-related failures

Several types of coating failures can occur due to the basic formulation of

the coating. If the coating formulation is not customized with proper selec-

use and exposure environment, then the overall purpose of the coating may

be defeated. Formulation-related failures are manifested in the form of

several types of coating defects such as chalking, checking, cracking, discol-

oration, blistering, adhesion failure, etc. Corrosion engineers and applica-

tors have little control over formulation-related defects.

Many coating systems fail because of cracking or splitting within the

single coat when exposed for some time. Such types of failures, often

referred to as cohesive or intralayer failures, occur due to incorrect optimi-

zation of design variables such as pigment/binder (P/B) ratio, PVC–CPVC

Usually, anticorrosive coatings exhibit the best performance within an

optimum PVC range. Coatings become more permeable when formulated

at much lower PVC as well as above CPVC due to onset of porosity.

matrix and hence become susceptible to intralayer splitting, as exhibited

commonly in inorganic zinc silicate coating, coal-tar based coatings, block

connected networks such as cured epoxy show excellent cohesive strength

since they require considerable force to pull them apart. Such coatings, if

they fail at all, show adhesive failure at the intercoat or base metal due to

6.3 Adhesion failure due to excessive condensation on substrate.

tion of ingredients and design structure, keeping in mind the specific end

distance, crosslink density of binder system and inadequate film flexibility.

Coatings above CPVC are cohesively weak due to insufficient binder in the

filler etc. On the other hand, highly crosslinked polymer systems with inter-

insufficient film flexibility. High molecular weight linear or lightly branched

efflorescence and powdery material must be removed by acid etching or

104 High-performance organic coatings

sively strong. This is why, while formulating epoxy primer for direct applica-

tion on base metal, solid epoxy resin (Araldite 6071) is preferred over liquid

epoxy (Araldite GY 250).

mers are to some extent permeable. The effectiveness of a coating depends

on its ability to form an impermeable barrier against the ingress of corro-

sion-propagating elements like oxygen, moisture and salts to the steel

coatings formulated with plate like pigments with high aspect ratio, like

vapour transmission characteristics of such coatings are attributed to the

longer path length for diffusion through the thin plates oriented parallel to

the surface (Fig. 6.4). A highly crosslinked thermoset binder matrix, e.g.

epoxy–polyamide, is less permeable than a linear thermoplastic polymer

matrix, due to less space being available for diffusion through these materi-

universally true that higher thickness is always good for long-term protec-

tion. Quite often thicker coatings create more internal stress than thinner

ones, leading to stress cracks or even entrapment of solvents that may result

in pinholes or even blisters. Zinc-rich coatings tend to mud crack if applied

characteristics of spherical zinc particles, such coatings face tremendous

Substrate

spherical pigments,

increased permeability

Substrate

flaky pigments,

reduced permeability

performance of coatings is the film permeability. Almost all organic poly-

surface. The important factors that affect the film permeability are PVC, type

of pigment used, crosslink density, cured film T and overall DFT. Generally

barrier properties than those with spherical fillers. The reduced moisture

als (Fig. 6.5). With increase in film thickness a greater barrier is created for

diffusion, provided the applied film is compact and holiday free. It is not

polymer materials exhibit greater flexibility and cohesive strength than

densely crosslinked rigid materials, which are less flexible but still cohe-

The second important influencing variable that controls the anticorrosion

micaceous iron oxide, glass flake and aluminium flake, help to achieve better

6.4 Permeability difference between spherical and flaky pigment.

\at higher thickness. Being a highly filled system with poor reinforcing