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

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Causes and assessment of failures in organic paint coatings 115

interval allowed between the completion of mixing and the start of applica-

pre-reaction of low molecular weight amine hardener with the epoxy base

material in the can to produce oligomeric molecules having better chemical

homogeneity, intercompatibility and reduced susceptibility to amine blush-

ing. Almost all high-performance epoxy coating manufacturers specify the

induction time required for their coatings to get the best appearance, gloss

and performance.

Due to inadequate understanding or sometimes to push productivity, site

applicators do not pay attention to this factor and start painting immedi-

ately after mixing. Subsequently, when the paint is inspected on the follow-

ing day a hazy layer of whitish material is observed on the surface due to

amine blushing. Further overcoating on this surface may result in intercoat

adhesion failure unless the loose layer of blush is removed by secondary

surface preparation prior to top coating. On the contrary, increasing the

induction time beyond the recommended limit is also not desirable, since

the chances of adhesion failure.

6.7.3 Non-compliance to prescribed overcoating interval

Almost all two-component air-drying thermoset coatings exhibit best per-

formance when overcoating is done within a critical window. The duration

of this window is different for different types of coatings depending on their

reactivity, crosslink density and hardness. The outside temperature also

temperature. It is extremely important that the applicator must check the

recommended overcoating interval in the manufacture’s product data sheet

and take adequate steps to ensure compliance. If the overcoating interval

is too long then the coating becomes too hard for a subsequent coat to

adhere to it satisfactorily. Such types of adhesion-related problems become

more severe in the case of epoxy resin based coatings, which are highly

sensitive to chalking, if left as such in direct sunlight for a long time prior

to overcoating. A longer overcoating interval between two coats in a coastal

environment creates an ideal situation for the entrapment of salty contami-

nants and dust between the sandwiched layers, which later favours the for-

mation of osmotic blisters. A shorter recoating interval likewise has its own

problems, such as lifting and swelling on a soft undercoat during top coating,

blistering due to solvent entrapment. The author witnessed such curing-

tion. The significance of recommending induction time is to allow some

the viscosity of the mix will increase, making spray application difficult with

peel’ on the surface. Poor wetting and improper film formation will increase

cohesive failure due to insufficient curing, solvent popping and osmotic

related problems in an inorganic zinc-rich (IOZ) coating at a refinery site

the resultant effect of incomplete wetting, poor flow out and more ‘orange

plays a critical role, since the rate of reactivity is directly influenced by

116 High-performance organic coatings

where apparently dried IOZ-coated structural plates were moved to the

press shop for tank fabrication almost immediately after application, and

is a slow curing material and requires overnight curing in a humid environ-

ment for the development of full strength. One must ensure proper curing

of the IOZ coat before attempting to handle or overcoat such a painted

structure.

6.7.4 Improper coating thickness

Inability to control the stipulated coating thickness during application can

result in numerous coating failures. The problem appears at both ends of

the thickness scale when the applied coating is either too thick or too thin.

Typical problems encountered with too thick application are slow drying,

sion-related failure. On the contrary, too thin a coating leads to inadequate

to high permeability, susceptibility to pinpoint rusting, blistering, poor

hiding, etc. If shop primers are applied on blasted steel at lower DFT, the

When such shop-primed steel is shipped to the actual job site, pinpoint rust

coated area. Such a type of corrosion failure can be overcome by proper

workmanship and inspection after application. When alkyd or oil based

oxidizing coatings are applied at higher thickness on a wooden surface,

of shrinkage stress. The remedy is to apply these coatings in thinner layers

to ensure thorough drying before application of additional coats. A similar

kind of stress-related crack failure is often experienced if bituminous or

coal-tar based coatings are applied at higher thickness.

6.7.5 Wrong thinner selection

extremely important for controlling the rheology of coatings, which is

Dilution characteristics of different coatings in different solvents vary

present in the coating. In order to obtain the best results it is strongly

advised to use the coating and the thinner from the same supplier. Use of

the wrong thinner can create a solvent shock due to incompatibility which

filled IOZ coating, unless formulated otherwise with a suitable accelerator,

solvent retention within the film, runs and sags, stress cracking and adhe-

coverage at the edges and peak of profiles, reduced barrier properties due

peak of the blast profile does not receive enough material for protection.

develops at the profile peaks which subsequently undercut into the entire

Incorrect selection of thinner can result in both cosmetic deficiencies and

reduced film performance of organic coatings. Use of the correct thinner is

greatly depending on the solubility parameter of the polymeric binders

circular flakes of IOZ subsequently flaked out from the substrate. Highly

failure may result in the form of cracking or even flaking due to build-up

responsible for proper wetting, flow and levelling and film formation.

would adversely affect the substrate wetting, flow and levelling, gloss and

Causes and assessment of failures in organic paint coatings 117

spray characteristics. The wrong thinner can cause rundown in some

cases and dry spray ‘orange peel’ in other cases, depending on the boiling

range and evaporation characteristics of the thinner. In some situations

the wrong thinner can play havoc, e.g. if a high-build coating meant for

immersion services or a potable water lining is mixed with water-miscible

solvent coming from a wrong thinner, the chances of osmotic blistering

Similarly, if alcohol-containing thinner is used for diluting polyurethane

paint, it interferes with the normal curing reaction between isocyanate and

acrylic hydroxyl, thereby affecting both the physical and the chemical

properties of the urethane coating. Sometimes such undesirable side-

reactions of isocyanate, especially with water, can cause micro-pinhole

6.7.6 Overbaking/underbaking of stoving paints

Stoving paints such as alkyd-melamine, acrylic-melamine, epoxy ester-

Any deviation from the standard baking schedule would adversely affect

the desired metal temperature inside the oven, due to the absence of proper

control or poor housekeeping. If the applied coating is underbaked the

gloss.

6.7.7 Control of spray process

During spray painting, whether by air-assisted spray or airless spray, appro-

priate care must be taken to control all parameters in order to achieve the

shape and diameter, and the distance between the object and the gun tip.

Overspray in the form of dry mist is a common problem encountered fre-

quently during outdoor painting. It result from poor application practices

and too long a distance between object and gun tip. Around the overspray

ity, leaving behind imperfections or tiny voids in the form of pinholes.

Pinpoint rusting can occur when such a coating is exposed outside due to

easy permeation of moisture and oxygen through the voids. A longer

distance between gun and object increases the intensity of ‘orange peel’

increase dramatically in the eventuality of solvent retention within the film.

defects in the film.

melamine, etc., render best performance within a specified baking schedule.

the film properties of these coatings. Sometimes it is difficult to achieve

film does not achieve adequate resistance against solvent, chemical and

weathering. Conversely, overbaked films are too hard and brittle, and

proper film formation. The critical parameters responsible for controlling

mist the deposited paint cannot penetrate efficiently due to its high viscos-

in the film.

display poor flexibility and impact resistance, more yellowing and reduced

the spray process are air pressure, paint viscosity and flow rate, nozzle size,

118 High-performance organic coatings

6.7.8 Inappropriate control of application environment

The outside environment plays a very important role during the application

by the relative humidity and temperature. If the ambient temperature is

higher than 40°C then the coating mist is deposited in the form of dry spray

life is considerably reduced. On the other hand, if the ambient temperature is

much lower, surface drying is severely affected along with drastic reduction

in the curing rate. The remedial measure to control such situations is to use

accelerated products. If the temperature is lower than the dewpoint of the

site location, condensation of water takes place on the surface. Application of

amine-cured epoxy or coal-tar coatings on such a surface leads to amine

blush reaction, resulting in paint peeling problems during overcoating. This is

why most epoxy coating manufacturers do not recommend applying 2K

epoxy material if the relative humidity is above 85% and the ambient tem-

perature is not exceeding the dewpoint temperature by at least 5°C. During

the application of water-borne latex wall coatings, if the ambient tempera-

ings, the emulsion micelles cannot coalesce properly, resulting in improper

High humidity is congenial for moisture-cured coatings such as inorganic

zinc-rich primer and epoxy polysiloxane to ensure proper curing. Excessive

relative humidity retards the drying rate of coatings and increases the ten-

internal areas of storage tanks.

6.8 Common types of coating failures and

remedial measures

When dealing with surface coatings we come across a variety of coating

defects, some of which are related to liquid coatings, a few appearing in the

wet or dry coating immediately after application, and several others devel-

oping afterwards when coatings are exposed to the actual operating envi-

ronment. The defects usually encountered with liquid coatings in the can

the defects visible in the wet or dry coatings are cissing, cratering, orange

gloss dulling, and so on. Discussion of these defects is beyond the purview

of this chapter. Most of the major defects observed during on-site perfor-

mance of applied coatings belong to the following categories:

of coatings, since both film formation and the curing process are influenced

ture is lower than the minimum film formation temperature of the latex coat-

film formation with powdery material deposition on the surface.

fier for humidity control is advised during sandblasting and painting of

rence of ‘orange peel’ and pinholes in the film. In addition, the working pot

due to faster evaporation of solvent. The net effect is poor flow out and occur-

dency to flash rusting after sandblasting. This is why the use of a dehumidi-

are settling, syneresis, flotation, streakiness, colour separation, etc., whereas

peel, pinholing, solvent popping, flooding, brush marks, dry spray, haziness,

Causes and assessment of failures in organic paint coatings 119

• Corrosion in the form of rusting, pitting, edge rupture

• Delamination from substrate or from intercoat due to complete loss of

adhesion

•

• Colour fading, gloss reduction.

The possible causes and remedial measures for commonly encountered

6.9 Conclusion

Analysis of coating failure is a complex subject since no single factor can

be relatively easy because of obvious visibility, but establishing the real

when the history of the applied paint systems is not available. Very often,

information based on memory or a hunch creates more confusion due to

lack of authenticity. An absence of a portable characterization tool on site

But one thing is for sure, that the subject of failure analysis requires a

thorough understanding of the basics of paint chemistry, formulation,

surface preparation and application, including knowledge of substrate char-

acteristics, science of adhesion, corrosion control and instrumental charac-

terization techniques of various defects.

For failure mitigation the famous golden rule ‘prevention is better than

cure’ is more apt. Failure, once it has occurred, may be corrected by deploy-

ing suitable remedial measures but the process is cumbersome, time con-

suming and expensive and retards productivity. Failure management of a

coating system requires careful planning involving all stages of the painting

process right through from system selection, material procurement and

surface preparation to coating application, inspection and future mainte-

nance. Therefore a preventive strategy is the best way to mitigate failure.

This is possible when all the interested parties, namely the paint supplier,

contractor, applicator and end customer, collaborate with each other by

sharing all relevant information and knowledge in a transparent manner

and by adopting an integrated approach towards the installation of the

coating system.

Blister – microblister, dense blister or water-filled osmotic blister

field failures are described in Table 6.1.

conclusively pinpoint the exact cause of failure. Identification of failure may

cause and the underlying mechanism is extremely difficult and more so

and supportive data stands in the way of making a definitive conclusion.

Checking, chalking, cracking, flaking

Table 6.1 Failures commonly encountered in the protective coating segment

Type of failure Possible causes Remedial measures

Corrosion Soluble salt contamination Mechanical cleaning followed by soft water

washing

Permeation of moisture, oxygen, electro-

mechanically damaged points

Feather off edge of damage and apply extra coat

of paint

Mill scale or remnants of rust Abrade or blast clean loose mill-scaled or rusty

areas

Grit inclusions Remove by chipping or abrading

Pittings

sible to coat pittings totally by spray application

and sometimes even by brush)

Increase DFT to stop pinpoint rusting

Contact of aggressive chemicals/solvents Stop splash and spillage or choose right paint

system

Film delamination Intercoat contamination within latest

applied system due to long overcoating

interval

Check and remove defective paint. Ensure good

secondary surface preparation or sweep blasting

if primer is overaged

Incompatibility of paint coats between

latest applied system and old system.

Amine blush due to exudation

Attack by aggressive solvents to old system

(swelling)

Change paint system, remove old paint

Internal stresses leading to detachment

from bare steel. Too thick coat or quick

temperature change

Check DFT of total system, remove old paint and

apply new paint at correct DFT

Detachment from bare steel initiated by

cracks and moisture

Check extent of undercreep, remove defective

paint and repaint with correct coating

lytes, etc., through pinholes, porous film or

Investigate cause. Grind and fill pittings (impos-

Too low dry film thickness

Check specification, change paint system

Blistering Soluble salts present under or between

paint coats

Check and if still present wash with soft water

Cathodic overprotection Adjust CP and/or change paint system

Contamination or grease present under or

between coats

Thorough degreasing before painting

Trapped moisture due to painting over

damp substrate

Application on dry substrate

Permeation with chemicals, solvents and

water (condensation)

Take preventive measures or change paint system

Allow thorough drying between coats

Too short overcoating time or application at

too low temperature

Follow instructions as per product data sheet and

control application

Checking–cracking Hard coating on top of soft coating (check-

ing). Sometimes caused by too short over-

thickness

Check if soft material is still present

underneath, abrade defective paint layer

completely, adjust paint system

Hard coating on top of soft old coating

(cracking)

Check if soft material is still present

underneath, abrade cracked paint layer

completely, adjust paint system

Chalking, colour

fading

Use of pigments and binders which are not

lightfast, UV stable (colour fading)

Abrade with emery paper and repaint with

new coating

White powdery deposit on surface due to

attack of binder by UV

Radiation (chalking)

Solvent trap in case of thick film application

coating interval or application at too high film

122 High-performance organic coatings

6.10 Bibliography

1. Clive H. Hare, Paint Film Degradation – Mechanisms and Control, 2001, SSPC,

Pittsburgh, PA.

2. Charles G. Munger, Corrosion Prevention by Protective Coatings (2nd edition),

1999, NACE International, Houston, TX.

3. Dwight G. Weldon, Failure Analysis of Paints and Coatings, 2001, John Wiley &

Sons Ltd.

4. D.H. Deacon, Surface Coatings International, 1999, No. 8, pp. 403–405.

5. Percy E. Pierce and Clifford K. Schoff, Coating Film Defects (2nd edition), 1994,

pp. 11–19, Federation of Societies for Coatings Technology, Blue Bell, PA.

6. Mark Weston, Journal of Protective Coating and Lining, Janvan 2000 pp.

56–57.

7. www.Paint_worx.com/PaintFailure.htm

8. www.npl.co.uk

9. www.paintquality.com

10. www.specialchem4.com

123

Mechanical degradation of organic paint coatings

S ROSSI and F DEFLORIAN,

University of Trento, Italy

7.1 Introduction: paint damage produced by

mechanical action

The use of organic coatings is one of the most common methods for corro-

sion protection of industrial components. In the past few years, demand for

better protective performance and coating integrity has increased. In addi-

tion, the protective properties against corrosion, in many applications,

require several other properties including aesthetic aspects, resistance to

solar exposure, etc. In other applications, good resistance to abrasion and

particulate impacts is required. This is particularly important for the auto-

motive industry, naval applications, military and agricultural uses, and all

those applications that involve the possibility of abrasion due to the friction

between the coated component and other components, or in case of the

presence of dust, sand or grains of hard materials.The effect of the impact

of stones or damage due to abrasion is of primary importance in the opti-

mization of paint. In fact, mechanical damage can locally result in reduction

of the protective properties, hence the need to restore the integrity of the

paint. In the case of limited damage without a reduction of corrosion pro-

tection properties, it is possible to have a detrimental effect on the aesthetic

second reason for the application of an organic coating is aesthetic appear-

ance, it is also very important to understand this damage phenomenon, as

well as to evaluate with accuracy the resistance to abrasion of the coating

system to obtain information about the service life of the protection

system.

In the case of the surface of two components that are in contact, recipro-

cal action produces a damage wear phenomenon. There are four wear

mechanisms but only two are of interest in paint applications. Adhesive

wear is present when two bodies are in contact and there is plastic deforma-

tion and adhesion phenomena between the surfaces. It is possible to obtain

the formation of junctions and the resulting breakage causes the damage.

aspect of the paint, with gloss or colour modification. Considering that the

124 High-performance organic coatings

This phenomenon is characteristic of the interaction of two components

made of the same material. Considering the paints this aspect could be

present when both components are protected with an organic coating.

Considering that the contact of two painted components is not very fre-

quent and that the damage is limited due to the similar surface hardness,

the second damage method is more important. Abrasion wear is observed

when the abrasion action is produced by the roughness of one body or, in

case of the presence of three bodies, by the action of a third body entrapped

between the other two surfaces. Considering the ductile materials the abra-

sion wear could be observed in the case of one surface having more hard-

ness (20–30%) than the second one. In case of a coupling of a painted

component with a brittle material, such as a ceramic material, the damage

could be produced by the ceramic. The third body could be produced during

wear action or could be present just at the beginning. The last case is the

presence of sand or abrasive ceramic particles present in the environment

masonry ash, mineral handling, seashores or deserts. The effect of winds,

tornadoes and desert storms can move great quantities of abrasive sands.

Moreover, winds can transport the lightest sands to areas very distant from

the origin. These phenomena can cause abrasion of organic coatings, due to

the direct action of transported sand, or to cases of deposited sands, which

act as third bodies.

In the case of abrasive particles, the dimension of particles, their geome-

try and hardness are very important. In cases where the abrasive particles

phenomenon erosion. Quite frequently there is only the painted component

present, without contact with another part. The damage effect is produced

by the action of free abrasives. Considering the organic coating systems

there are two types of mechanical damage of a paint. Mar is damage which

occurs within a few microns of the surface. In this case the corrosion protec-

tion properties remain and only a change in appearance is seen. The second

type of damage produces a reduction or loss of protection properties. This

phenomenon is more dangerous. In this case a removal of organic matter

from the paint surface is present with a decrease in protection properties.

The abrasion resistance is connected with the hardness of the coating and

abrasion behaviour. A material with elastic deformation properties that

recover normally shows a good abrasion resistance; materials which show

a lower tearing resistance present a lower abrasion resistance.

In the evaluation of mechanical damage of paints there are several criti-

mechanical damage. In fact, it is not easy to reproduce actual damage

with a natural or artificial origin. Presence of abrasive sand can come from

with the friction coefficient, nevertheless this relation is not obvious. In the

first instance, the higher hardness and lower friction coefficient gives better

cal aspects. The first important point is how possible it is to simulate

are transported by the air or by a fluid it is possible to call the damage