HEMPEL Coatings (Polska) Sp. z o.o.
APPLICATION
AND
COATING DEFECTS
2001
INTRODUCTION
This manual deals with coatings and surface defects, failures that are commonly
found within industrial and marine type painting. The text details the type of failure
along with the description and causes with a selection of photographs for several of the
defects listed. Photographs also illustrate pre-surface conditions.
The aim of this Manual is to give an understanding of coating and surface defects
which will be useful for assessment surveys, future recoating work, writing reports and
specification review etc. It’s not intended to be used as a “Do it yourself manual” for
failure analysis. Additional reading is recommended, e.g. Hess’s Paint film Defects,
where a full description of all the possibilities associated with paint, coating and
surface defects may be listed.
Many coating defects analyses are conducted in conjunction with a qualified Chemist
or Coating Specialist who has access to laboratory equipment. This may not always be
viable due to location, type of work, work schedule etc. and as such, the reader should
make himself/herself familiar with the common defects described within this Manual.
In the main, paint and coating defects are either “immediate” i.e. occur during or soon
after application, or when fully cured and considered to be aged coatings. On this basis
a “process of elimination” may be the first course of action. For example, if a two pack
paint remains soft a full day after application, a checklist of the following items may
eliminate a number of possibilities before the material Supplier is called in to look at
the problem.
1. Have the correct materials been used?
2. Are the products within shelf life?
3. Was the mixing carried out as per Manufacturers data sheets?
4. Were any thinners or solvents mixed with the products, if so what kind?
5. What were the environmental conditions during and after application?
6. Were the overcoating times/temperatures of (if any) previous coat observed?
7. What was the wet/ dry film thickness as compared to those specified?
Investigation of the above may indicate the reasons for the soft coating and would be
essential before further investigation by the Supplier of the product. The same process
of elimination can be conducted for cured and aged coatings, taking into account the
environment to which the substrate is exposed i.e. hot pipework, immersed substrate
etc. The reader must be careful with the description of a “defect” since what may be
considered to be a defect by one Client may be perfectly acceptable to another.
Examples of this can be found with chalking, blooming and wrinkling, to name only
three.
2
INDEX
1. PRE - SURFACE CONDITION……………………………………………….7
1.1.
Virtually Intact (Blue) Millscale
1.2.
Untreated Millscale, 12 Months Exposure
1.3.
Untreated Millscale, 36 Months Exposure
1.4.
Untreated Millscale, 8 Years Exposure
1.5.
Poor Design – Corrosion Trap
1.6.
Weld Undercutting
1.7.
SkipWeld
1.8.
Sharp Edge (Not Dressed)
1.9.
Weld Molten Slag
1.10. Flame Cut Edges
1.11. WeldSpatter
1.12. Oil Contamination
2. APPLICATION AND COATING DEFECTS……………………………...13
2.1. Corrosion Undercutting
Corrosion starting at areas of damage to coating
2.2. Cissing
2.3. Sags/Curtains
2.4. Rust Spotting
Thin paint applied over high profiled surface
2.5. Solvent Popping
2.6. Wrinkling
2.7. Solvent Attack
Topcoat lifting incompatible undercoat (primer)
2.8. Flotation
Cell formation
2.9. Mud Cracking
2.10. Severe Wrinkling
3
2.11. DrySpray
2.12. Bubbling
2.13. Flooding
2.14. Alligatoring
2.15. Stress Cracking
Within coating system
2.16. Adhension Failure
Moisture contamination
2.17. Pinholes
2.18. Incorrect Coating System
Applied to hot surfaces (piping)
2.19. Incorrect Coating System
Applied to bitumen surface
2.20. Blistering
Virtually intact minute blisters to wood surfaces (moisture)
2.21. Rust Nodules
Underside paint system
2.22. Severe Blistering
2.23. Filliform Corrosion
2.24. Efflorescence
Peeling of masonry paint to brickwork
2.25. Efflorescence
Close up
2.26. Rain Damage
Rain spotted damage caused prior to curing of the coating
2.27. Delamination
Intercoat failure (exceeded overcoating times)
2.28. Brittle Paint
Applied to wood surfaces
2.29. Pigment Fading
2.30. Rust Staining
To thermal metal spray
2.31. Corrosion
Due to low dry film thickness
2.32. Adhension Failure
Paint applied to untreated galvanizing
4
2.33. Poor Opacity
Incorrect paint grade, brush applied
2.34. Saponification
Alkyd coatings applied to a zinc rich primer
2.35. Checking
2.36. Poor Spray Application
2.37. Chalking
2.38. Intercoat Adhension Failure
Moisture contamination
2.39. Damp/Moist Concrete
Coating Failure
2.40. Growth
Organic growth to external of paint surface. Damp environments.
2.41. Incompatible Coating Systems
2.42. Erosion
Poor application, brush applied
2.43. Contaminated Surfaces
Prior to painting
2.44. Rust Staining
Danger to the eye!
2.45. Checking
Exposing underlining coats
2.46. Bitty Surface
Foreign contamination on surface of film
2.47. Zinc Carbonate
White salts on exposed zinc coated surface
2.48. Adhension Failure – Thermal Metal Spray
Lack of profile on blast cleaned surface
2.49. Crowsfoot Checking
2.50. Bubbling
Unbroken bubbles
2.51. Shrinkage and Cracking
2.52. Blistering
Ballast tank coating
2.53. Peeling
Non oxidised coating
2.54. Bleeding
5
2.55. Severe Cracking
2.56. Low Dry Film Thickness
Uneven coating/ high peaks
2.57. Solvent Attack
2.58. Over Application
Orange peel/ pinholes
2.59. Intercoat Adhension
Zinc silicate/ topcoat system
2.60. Flaking Topcoat
Hard/ glossy undercoat
2.61. Runs/ Sags
Gross over application
2.62. Misses/ Skips
Leading to rust staining
2.63. Metal Contamination
Metal droppings to painted steelwork
2.64. Flaking
Contaminated substrate
3. PRE – SURFACE CONDITION TABLE…………………………………..45
4. PRE – SURFACE PREPARATION TABLE……………………………….46
5. TABLE OF DEFECTS…………………………………………………...…47
6. COATING DEFECTS……………………………………………………..…53
7. PROTECTIVE COATING CLASSIFICATION - TABLE…………….....72
6
1. PRE-SURFACE CONDITION
1.1.
Virtually Intact (Blue) Millscale
1.2.
Untreated Millscale, 12 Months Exposure
7
1.3.
Untreated Millscale, 36 Months Exposure
1.4.
Untreated Millscale, 8 Years Exposure
8
1.5.
Poor Design – Corrosion Trap
1.6.
Weld Undercutting
9
1.7.
SkipWeld
1.8.
Sharp Edge (Not Dressed)
10
1.9.
Weld Molten Slag
1.10. Flame Cut Edges
11
1.11. WeldSpatter
1.12. Oil Contamination
12
2. APPLICATION AND COATING DEFECTS
2.1. Corrosion Undercutting
Corrosion starting at areas of damage to coating
2.2.
Cissing
13
2.3.
Sags/ Curtains
2.4. Rust Spotting
Thin paint applied over high profiled surface
14
2.4.
Solvent Popping
2.5.
Wrinkling
15
2.7. Solvent Attack
Topcoat lifting incompatible undercoat (primer)
2.8. Flotation
Cell formation
16
2.9.
Mud Cracking
2.10. Severe Wrinkling
17
2.11. DrySpray
2.12. Bubbling
18
2.13. Flooding
2.14. Alligatoring
19
2.15. Stress Cracking
Within coating system
2.16. Adhension Failure
Moisture contamination
20
2.17. Pinholes
2.18. Incorrect Coating System
Applied to hot surfaces (piping)
21
2.19. Incorrect Coating System
Applied to bitumen surface
2.20. Blistering
Virtually intact minute blisters to wood surfaces (moisture)
22
2.21. Rust Nodules
Underside paint system
2.22. Severe Blistering
23
2.23. Filliform Corrosion
2.24. Efflorescence
Peeling of masonry paint to brickwork
24
2.25. Efflorescence
Close up
2.26. Rain Damage
Rain spotted damage caused prior to curing of the coating
25
2.27. Delamination
Intercoat failure (exceeded overcoating times)
2.28. Brittle Paint
Applied to wood surfaces
26
2.29. Pigment Fading
2.30. Rust Staining
To thermal metal spray
27
2.31. Corrosion
Due to low dry film thickness
2.32. Adhension Failure
Paint applied to untreated galvanizing
28
2.33. Poor Opacity
Incorrect paint grade, brush applied
2.34. Saponification
Alkyd coatings applied to a zinc rich primer
29
2.35. Checking
2.36. Poor Spray Application
30
2.37. Chalking
2.38. Intercoat Adhension Failure
Moisture contamination
31
2.39. Damp/ Moist Concrete
Coating Failure
2.40. Growth
Organic growth to external of paint surface. Damp environments.
32
2.41. Incompatible Coating Systems
2.42. Erosion
Poor application, brush applied
33
2.43. Contaminated Surfaces
Prior to painting
2.44. Rust Staining
Danger to the eye!
34
2.45. Checking
Exposing underlining coats
2.46. Bitty Surface
Foreign contamination on surface of film
35
2.47. Zinc Carbonate
White salts on exposed zinc coated surface
2.48. Adhension Failure – Thermal Metal Spray
Lack of profile on blast cleaned surface
36
2.49. Crowsfoot Checking
2.50. Bubbling
Unbroken bubbles
37
2.50. Shrinkage and Cracking
2.52. Blistering
Ballast tank coating
38
2.53. Peeling
Non oxidised coating
2.54. Bleeding
39
2.55. Severe Cracking
2.56. Low Dry Film Thickness
Uneven coating/ high peaks
40
2.56. Solvent Attack
2.58. Over Application
Orange peel/ pinholes
41
2.59. Intercoat Adhension
Zinc silicate/ topcoat system
2.60. Flaking Topcoat
Hard/ glossy undercoat
42
2.61. Runs/ Sags
Gross over application
2.62. Misses/ Skips
Leading to rust staining
43
2.63. Metal Contamination
Metal droppings to painted steelwork
2.64. Flaking
Contaminated substrate
44
3. Pre – surface condition of steel substrates is an important element in the
performance of the applied coating:
Pre – Surface Conditions
Weld Locations: (WUSPS)
W=
Weld should be continuous and free from sharp projections.
U=
Undercutting should not be excessive and rough.
S=
Spatter. All weld spatter should be chipped away.
P=
Pinholes. Should be filled.
S=
Slag. Should be removed by chipping so that the underlying weld
may be inspected.
All Locations: (CLEB)
C=
Contamination of the substrate i.e. oil, grease and foreign
contamination must be removed by solvent cleaning or other
suitable cleaner.
L=
Laminations. The surface should be checked before and after
surface preparation for laminations in the steel. Laminations must
be removed.
E=
Edges. Should be radiused to provide a smooth, round surface to
which the paint adheres satisfactorily, providing good protection.
B=
Burrs. Should be removed as above. Check bolt holes and other
areas where cutting has carried out.
45
4.
46
5. Table of Defects
TYPE
DESCRIPTION
CAUSES
Alligatoring
Very large (macros) checking,
usually looks crosshatched.
Coating appears to be cracked,
similar to the skin of an alligator.
Bitty Finish
Paint film which, when applied,
gives a rough and poor
appearance to the surface finish.
Internal stresses where the surface
shrinks faster than the body of the
paint film. Can be found on paint
films having a soft undercoat with
a hard topcoat.
The main cause of a bitty finish on
a paint film is contamination
within or on the surface of the
paint film. This can be airborne
grit, sand etc. or contamination of
the brushes, rollers etc.
“Bleed Through” is generally a
full or partial redissolving of the
previous coat. Bleeding can
happen when strong solvents are
used in the topcoats.
Film contamination
by particles which is
sometimes known as
Bitty Finish
Bleeding
Blistering
Blooming
Brush Drag
Marks
Discolouration of a paint film by
diffusion of the underlying paint
i.e. the underlying coating is
visible through is topcoat. Very
often, this is seen where bitumen
or tar based products are overcoated with alkyd based or other
conventional topcoats.
Areas swollen through pressure
beneath coating.
Blistering is a form of adhesion
failure resulting from a number of
failure possibilities. Foreign
contamination such as grease oil,
etc. rust, trapped moisture,
retained solvent, hydrogen vapour
pressure (on coatings used with
cathodic protection) soluble
pigments etc.
Paint film exposed to condensation
A hazy bloom to the surface of
or moisture during curing,
the paint film. A reduction in
gloss and colour is also generally (common phenomenon with amine
cured epoxies). Incorrect solvent
found.
blend can also contribute to
blooming. Can usually be removed
with a clean cloth.
Uneven and undesirable finish of Viscosity of material may be too
a brush painted item. Sometimes high for brush application.
Incorrect thinners used in the
known as ropiness.
paint, inadequate mixing or poor
application technique. Product
may also be out with pot-life,
(two-pack paints).
47
Bubbles
Similar to pinholes, although
bubbles can be within a paint
film (trapped air/ solvent) and
not visible to the eye or exposed
on the substrate of the paint film.
Surface bubbling can be intact or
broken.
Chalking
A weak film of powdery coating
attached to the surface of a paint
film. A change of colour or
fading is also evident.
Checking
Small cracks within the topcoat
of a paint system. Described as a
surface phenomenon since
cracks do not penetrate all the
way through the paint film.
Some checking can be so minute
that it is impossible to see
without magnification.
Surface breaks within a paint
film exposing the underlying
substrate. Sometimes known as
crawling or fisheyes. The paint is
unable to wet-out the substrate.
This sometimes starts with the
appearance of a pinhole,
expanding over some minutes to
the eventual ciss. Can be very
large i.e. metres.
Paint coatings with visible
cracks or gaps, which have
penetrated down to the substrate.
Cracking comes in several
forms, from minute cracking to
severe cracking and in some
instances to total delamination.
Cissing
Cracking
Delamination
Loss of adhesion between the
applied coats of paint.
48
Can be found on excessively thick
paint films, especially if spray
applied, though sometimes when
applied by saturated rollers or
thick brushes. With the exception
of the above, bubbling is limited
to factory applied coatings where
application is conducted by
dipping, electrodeposition or roller
coated and commonly found on
stoved coating, water thinned or
plastisol coatings.
Disintegration of the paint binder
on exposure to weathering or UV
light. Chalking rates vary with
formulations of the binder.
Chalking is a known characteristic
of certain paints i.e. epoxy paints.
Typically a formulation and/or a
specification problem. As with
cracking, stresses are developed
which cause the surface of the
paint film to become brittle.
Surface contamination by either
moisture or foreign matter such as
oil, grease, silicone etc. Also
known to happen when wrong
solvent blends have been used.
Cracking is a stress related failure
and can be attributed to surface
movement, ageing, absorption and
desorption of moisture. Generally
speaking the thicker the paint film
the greater the possibility of
cracking.
Providing the correct paint
materials have been chosen,
delamination defects are generally
related to poor surface preparation
and application defects:
- Intercoat contamination
- Exceeding overcoating times
Dry Spray
Rough and uneven finish to
surface of paint film.
Incorrect spray application i.e. gun
distance or an application
technique out with environmental
recommendations. Also associated
with fast drying products.
Efflorescence
White (powdery) substance on
the substrate of brick, masonry
and plaster. The efflorescence
forces the paint from the
substrate.
Soluble salts within the substrate.
Moisture brings the salts to the
surface of the substrate resulting in
coating adhesion failure.
Erosion
Selective removal of paint films
to areas or high spots.
The wearing away of the paint
film by various elements such as
rain, snow, winds etc. Found to be
more prominent on brush applied
coatings because of the uneven
finish.
Fading
Discolouration of a paint finish.
Loss of gloss and/ or colour. In
some situations it may resemble
chalking.
Fading is the gradual decrease of
coloured paints when exposed to
sunlight/weather. Fading tends to
accelerate in the presence of
moisture. A number of reasons
exist for the fading phenomena,
i.e. incorrect pigmentation from
the atmosphere or substrate,
porous substrate to name just
three.
Flaking
A form of adhesion failure
where paint literally flakes from
the substrate. A familiar sight on
wood substrate and on
galvanising.
Incorrect paint system utilized.
Either none or incorrect
pretreatment used for certain
substrate i.e. non-ferrous or
galvanised. Also poor application
techniques. May also be attributed
to differential expansion and
contraction of paint and substrate
i.e. wood.
Flocculation
Due to non – uniform pigment
dispersion which then results in
the pigments forming
agglomerations.
Due to unsuitable thinners or
inadequate mixing. Could also be
a manufacturing or formulation
problem.
Flooding
A defect, which appears soon
after application. The visual
appearance is a deepening of the
colour, which dries to a deeper
shade than the remainder of the
surface.
A common occurrence with gloss
finish coats having a minimum of
two different types of
pigmentation with some difference
in chemical compositions.
49
Flotation
A close visual resemblance to
flooding although a mottled
effect is produced. Hexagonal
cells are formed (invisible to the
eyes) as pigmentation appears to
float to the surface.
The mottled effect, which is
visible to the eye is caused by
differences in the pigment
concentration between the edges
and centers of the hexagonal cells.
Filiform
(corrosion)
Random small threads of
corrosion that develop beneath
thin lacquers and other semipermeable films. Often seen on
old tin food cans, with thin
electro plating.
Contamination on the metallic
substrate allowing corrosion cells
to be developed.
Grinning
Exposure of the underlying
surface (paint of substrate)
through paint film. This is
sometimes expressed as
“grinning – through”.
Low film thickness of penultimate
coat. Poor opacity and covering
power of previous coat. Strong
colour of primer/undercoats.
Growth
(on surface of
film)
Growth and attachments of
natural and foreign bodies to
surface of finished products.
These vary in form, size and
lifespan, e.g. algae, mosses etc.
Generally, natural bodies within
moist, wet or immersed
conditions. Growth may continue
after attachment.
Misses/Skips/
Holidays
Exposed areas of substrate,
where the intention was to coat
the entire area. Could be
confined to a single coat in a
multi – coat paint system.
Poor application techniques.
Mudcracking
The dried paint film has the
appearance of a dried – out mud
bath. Cracks vary in size and
amount, from minute to the easy
visible.
Generally over application of
heavily pigmented primers such as
inorganic zinc silicates or water
based coatings.
Orange Peel
Where the surface of the paint
film resembles the skin of an
orange.
Failure of the paint film to flow
out. Usually caused by poor
application techniques or by
incorrect solvent blend.
Peeling
Similar to flaking, although
peeling tends to produce soft and
pliable coatings, which can be
pulled away from the substrate
or from between coats.
Peeling is the reduction in bond
strength of the paint film due to
contamination or incompatibility
of coats.
50
Pinholes
Small holes found within the
paint film during application and
drying.
Solvent or air entrapment within a
paint film. A common problem
when coating porous substrate
such as zinc filled primers, metal
spray coatings etc. Pinholes can
also be caused by incorrect spray
application or wrong solvent
blend.
Rot (wood)
Damp and rotten wood exposing
friable substance, splitting,
flaking and cracking paint.
Generally poor maintenance.
Limited cases of incorrect paint
system.
Runs
Movement and tears of paint
soon after application of paint to
vertical substrate. Also know as
sags or, in severe situations,
curtains.
Over application of paint,
excessive thinners, wrong (lack) of
curing agent or just poor
workmanship. Could, in extreme
circumstances, be formulation
problem.
Rust Rashing of
Thermal Spray
Coatings
A coating of ferrous oxide
developed within the pores of
the sprayed metal coating.
Similar to rust spotting or
premature failure of a paint film.
A phenomenon, which can
develop on aluminium thermal
sprayed coatings soon after
application. Moisture is absorbed
into the metal coating and reacts
with the ferrous substrate. Early
paint sealing prevents such events.
Rust Staining
A light coating/ staining, which
is the precipitation of ferrous
oxide.
Rust staining is found where rust I
wetted-out and the water
contaminates and discolours other
items or locations. Usually more
of an eye – sore than a defect.
Could be run off from a rusty
surface above a soundly coated
surface. Assuming a defect in this
instant is dangerous.
Rust Spotting
Fine spots of rust, which appear
on a paint film, usually a thin
primer coat.
Low film thickness, voids and
holidays, also defects in the steel
i.e. laminations. Too high a
surface profile may cause
penetration of “peaks” through a
paint and cause rust spotting. May
also occur from metallic
contamination of a coated surface
by, say, grinding dust etc.
51
A soap formation reaction between
esters, alkali and water. Often
found in alkyd and oil coatings.
Notably found on concrete coated
structures and coating used in
conjunction with cathodic
protection.
Saponification
The coating appears to be soft
and sticky. Can be sporadic
isolated spots or an overall
effect.
Settlement
Old stock, heavily pigmented
A term used to describe the
settled pigment/ solids in a liquid paint, wrong formulation or
contamination of product.
prior to application. Found
during paint mixing.
Solvent Lifting
Eruption of the surface of the
paint film Wrinkling and
blistering which leads to a weak
surface and ultimate breakdown.
Incompatible paint system used.
Topcoats with a strong solvent
blend can attack previous and
weaker solvent blended coatings.
Tackiness
Soft film of applied coatings.
Although beyond the wet and
liquid stage, the paint film
remains as a tacky and soft
surface. Sometimes only
apparent on touching the
substrate.
There are various reasons why a
paint will remain tacky:
• Over thickness
• Excessive thinners
• Wrong (lack of) curing agent
• Low temperatures
• Use of coating beyond pot life
or shelf life.
Undercutting
Visual corrosion beneath a paint
film. Corrosion travels beneath
the paint film and pushes the
paint from the substrate. Severe
cases can show blistering,
flaking, cracks and exposed rust.
Application of paint to corroded
substrate. Can be found in areas of
poor design or access where
inadequate preparation and coating
thickness was applied. Could also
be due to lack of maintenance.
52
6. COATING DEFECTS
1. Coatings are much like human, animal, or plant life. Sooner or later failure occurs and
„life” ceases. According to this concept, all coatings are doomed to failure. Paint failure
may be defined as deterioration of the paint system, or corrosion of the coated structure,
more rapidly than would normally be expected, under the service conditions.
2. The painters are pushed by all concerned (the owner, the shipyard and their supervisors) to
complete the work as rapidly as possible. The resulting attitude is thus “get the job done
fast.” This operating philosophy can only results in lack of proper care during application
and thus eventual coating failures.
3. It has been estimated from past experience that 70% of all coating failures have resulted
from poor or inadequate surface preparation.
4. Care in surface preparation, application, and inspection is the best insurance against
coating failure.
5. While such close attention to the physical application of the coating may initially seem
costly, it is much less costly than continuing repair over the life of the structure.
6. The objective of surface preparation is to create proper adhesion of a coating over the
substrate. Adhesion is the key to coating effectiveness, and it determines whether the
coating is merely a thin sheet of material lying on the substrate or whether it becomes an
actual part of the substrate.
7. Adhesion becomes an even more critical condition for coatings applied in corrosive areas.
Thus, proper surface preparation is vital to the long life and effectiveness of a coating
applied in corrosive service.
8. It has been stated that paints fail in direct proportion to their lack of adhesion. Stating this
in a positive way, paints (or anticorrosive coatings) are successful in direct proportion to
their bond strength to the substrate.
9. The purpose of surface preparation is to insure that the maximum bond strength will
develop at the interface between the substrate and the coatings. The weakest area across
the coating should be within the adhesive or organic coating layer and not at the interface
of the coating and the substrate.
10. Failure within the coating is referred to as cohesive failure; failure at the interface
between the substrate and the coating is referred to as adhesion failure. If any failure is to
be tolerated, the cohesive – type failure is preferable.
11. The goal of surface preparation should never be less than the level sufficient to insure
cohesive failure of the coating. From a surface preparation standpoint, the ideal type of
coating failure is 100% cohesive.
12. Figure 1 indicates adhesive failure where the coating separates from the substrate cleanly
and does not leave any coating attached to the substrate.
53
Figure 1
13. Figure 2 indicates cohesive failure in which the coating breaks within itself and leaves a
continuous layer of coating on the substrate, even though the coating surface may be
completely removed.
Figure 2
14. Such cohesive failure is easy to understand with some of the soft coatings such as asphalt,
coal tar, and similar materials where there is practically no cohesion within the coating.
15. The majority of the coatings dealt with by corrosion engineers are hard, tough, and, in
many cases, extremely strong; yet the adhesive bond must still be greater than the
cohesive strength of the coating. Figure 3 shows type of adhesion failure where the
substrate itself fails rather than the coating. Such failure is not uncommon on concrete.
54
Figure 3
16. The type of adhesive bond depends on both the substrate and the coating. The three types
of adhesive bonds are: (1) chemical, (2) polar, and (3) mechanical.
17. The chemical bond, which is created by a chemical reaction between the coating and the
substrate, is undoubtedly the most effective bond. One example of this type of bond is hot
dip galvanizing, where the steel and the zinc metal amalgamate or dissolve within
themselves.
18. Inorganic zinc coatings are chemically bonded between the silicate molecule and the steel
substrate. Such bonding is called primary valence bonding, where the chemical groups on
the coating actually react across the interface with complimentary groups on the substrate,
forming a chemical compound. (Figure 4).
Figure 4
19. It is also possible that, epoxy molecules are bonded to the metal surface by metal
hydroxide groups through a condensation reaction (Figure 5). In both of these examples, a
new chemical compound would have been formed, joining the coating and the substrate.
55
Figure 5
20. Polar adhesion is a more common type of adhesion than the chemical adhesion, especially
with organic coatings. Polar adhesion or bonding is the attraction of the resin molecule to
the substrate. The polar groups are positively and negatively charged portions of the
coating molecule that are attracted to oppositely charged areas on the metal or substrate.
21. Chemically, polar adhesion is considered secondary valence bonding where the adhesion
occurs by way of physical physiochemical attractions between the resin molecules and the
substrate surface molecular structure.
22. Figure 6 shows the secondary valence bonding of hydroxylated coating, such as an epoxy,
to the metal hydroxyl groups of a metallic surface by way of hydrogen bonds.
Figure 6
56
23. The primary valence bonding, or the chemical bond is significantly stronger than
secondary valence bonding.
24. Mechanical adhesion is the type of adhesion that is associated with surface roughness or
anchor pattern. Anchor pattern is the surface roughness formed by peaks and valleys on
the substrate. These can vary over a relatively wide range of depth; however, of most
importance to coatings is the number of hills and valleys, which increase adhesion by the
increase in surface area and by the actual roughness.
25. Some coatings, because of the relatively poor adhesion within the coating itself or because
of thickness, require good surface roughness and a deep anchor pattern in order to obtain
adequate adhesion.
26. Strong adhesion is the key to coating performance and long life. If adhesion is weak, the
coating will gradually fail by blistering, under film corrosion, or chipping and flaking.
With strong adhesion, the coating can withstand most of the environmental resistance
requirements that otherwise would affect its integrity.
27. The primary objective of surface preparation is to provide maximum coating adhesion.
The actual mechanism of surface preparation has a twofold purpose.
28. The first purpose is to remove any extraneous, loose materials from the surface of the
substrate, as well as to eliminate chemically bonded scales, oxide films, and similar
surface reaction products that cover active adhesion sites on the metal surface. The
removal of such materials exposes the reactive sites so that the primers can have contact
with them and develop the maximum adhesion possible.
29. The second purpose of surface preparation is to increase the surface area by increasing the
roughness and anchor pattern of the surface. By this means, the actual exposed surface
area per unit of actual area is greatly increased. By increasing this effective surface, many
additional reactive sites on the metal surface are exposed, allowing for additional polar or
chemical adhesion of the primer to surface.
30. This is extremely important, since increasing the opportunity for either primary or
secondary valence bonding with the coating system is the key to the best possible
adhesion of any coating.
31. Figure 7 shows the effect of even lightly contaminated and unprepared surfaces, and how
the monomolecular film of coating adjacent to the surface has the polar sites on molecule
insulated from the reactive metal sites on the metal substrate. The metal reactive sites and
the coating reactive sites are separated at some distance by the contamination, thus the full
effect of either chemical or polar bonding is prevented.
57
Figure 7
32. Primers are applied directly to a surface. If this surface includes dirt, dust, scale, rust, oil,
moisture, or other contamination, the adhesion of the protective coating to the surface can
only be as good as the bond of the primer to the contamination and the bond of the
contaminated material to the actual substrate. The contamination provides a secondary
interface between the coating and the actual substrate. Primers are formulated to adhere to
metal, concrete, wood, and other structural surfaces rather than to the surface of
contaminants.
33. It is important that the metal surface be entirely new in order for the maximum number of
metal reactive sites to be available to the coating. It can be safely stated that the cleaner
the metal surface, the better the bond or adhesion of the coating to the surface. Stated in a
negative way, the adhesion of a coating is inversely proportional to the amount of
contamination on the surface. The greater the contamination, the less adhesion the coating
will have.
34. Painters of all types – industrial, marine, commercial, or even weekend painters –
generally have one thing in common. That is, they tend to neglect the most important part
of coating job: surface preparation. The time and expense required to take this step is
generally resented, creating a psychological barrier. Physical application of the coating
seems to be a much more productive and therefore satisfying activity than sandblasting,
chipping, scraping, or even thoroughly washing a surface. Unfortunately, if these activities
are omitted (i.e., proper surface preparation methods), the whole coating program is
doomed to be a waste of both the time and expense involved.
35. The advancement of surface preparation methods paralleled the development of high –
performance protective coatings.
58
36. Experience also indicates that heating surfaces, particularly those that cannot be
thoroughly prepared, improves the wetting of the surface by the coating and increases the
life span of the coating itself. Coating a warm surface lowers the viscosity of the coating
and increases its penetrating and wetting ability over either rusted or clean metal surfaces.
37. Most of the structural steel that is used today for heavy construction, either in the marine
industry or in chemical, petroleum, paper, and similar industries, is hot-rolled steel which
contains varying amounts of mill scale on the surface. Some of lighter shapes have a thin
mill scale on the surface, while heavy plate and heavy shapes generally are covered with a
heavy mill scale. Mill scale forms on the hot-rolled steel because in the process of rolling,
the steel is suitably heated below the melting point, yet at temperatures where the steel is
plastic and can easily be shaped by the rolling mechanism. The steel is first heated in open
furnaces where oxygen in the furnace atmosphere combines with the hot metal to form
oxides on the iron surface. Much of the oxide formed in the furnace is broken off during
the rolling process, and usually it is the oxide that is formed by the latent heat after the
shape has been milled and is allowed to cool that remains on the surface. Since the rolls
do not break it from the surface after the rolling is completed, it forms as a tight oxide on
the surface.
38. The mill scale itself is quite brittle. It has a lower thermal expansion than steel and cracks
on cooling. The cracks can extend through the scale and may be aggravated by any flexing
of the steel during handling. The cracking of the scale allows access of oxygen and
moisture through the scale down to the metal itself.
39. When corrosion does occur in this manner, the corrosion products are of a considerably
greater volume than the original metal. The increasing volume tends to create a wedge and
pops the mill scale from the surface of the metal. This occurs rather easily when mill scale
is weathered; however, it can also occur underneath a coating.
40. When a coating is applied over mill scale, the coating does not adhere to the steel itself.
Therefore, when corrosion takes place under the mill scale, the scale has no adhesion to
the steel, allowing a loose opening between the coating and the steel surface. Moisture
vapor then penetrates the coating and condenses in the area of loose adhesion. This creates
blistering and eventual corrosion, which loosens a greater amount of the mill scale as well
as the coating.
41. Rust tubercules and heavy corrosion scale build up under the coating.
42. The amount of time, work, and effort required to achieve any particular degree of surface
preparation depends to a great degree on the initial condition of the surface to be cleaned.
43. It is necessary to take into consideration the amount of rust, old paint, contamination, and
active corrosion or pitting on the surface to be protected.
44. This is extremely important since it is the degree of contamination that is the key to
coating adhesion.
45. In the marine industry minute quantities of chlorides can remain on the surface,
particularly in rough and pitted areas, to the extend that within a short period of time after
blasting in humid areas, the steel begins to rapidly change color because of corrosion from
the retained chlorides or sulfates.
46. If the surface is pitted, slight residues of rust or paint are found in the bottom of the pits.
59
47. Power tool cleaning is a method of preparing metal surfaces for painting by removing
loose mill scale, loose rust, and loose paint with power wire brushes, power impact tools,
power grinders, power sanders, or by a combination of these methods. It is not intended
that, all mill scale, rust, and, paint be removed by this process, but loose mill scale, rust,
paint, and other detrimental foreign material shall be removed.
48. Hand tool cleaning is a method of preparing metal surface for painting by removing loose
mill scale, loose rust, and loose paint by hand brushing, hand sanding, hand scraping, hand
chipping, or other hand impact tools or by a combination of these methods. It is not
intended that all mill scale, rust, and paint be removed by this process, but loose mill
scale, loose rust, loose paint, and other detrimental foreign matter present should be
removed.
49. In application where hand or power tools are used, a highly penetrating coating with a
strong wetting action for steel and iron oxide should be used.
50. Without a proper surface preparation, high – performance coatings cannot provide the
corrosion resistance for which they were intended.
51. If the surface preparation is improper, the protection provided by the coating is shortlived.
52. A less effective yet well – applied coating can provide better and longer lasting protection
than the best coating material poorly applied.
53. If the application is poor or careless, coating protection will not be achieved, regardless of
the strength of the material and surface preparation.
54. If a thick, overall coating is desired, the oil-base material must be applied in several coats.
55. Thickly applied coats of these materials tend to react on the surface, which may cause a
number of coating problems, e.g., wrinkling, checking, or cracking.
56. Good workability is the overriding characteristic of oil – base coatings.
57. Easy workability is undoubtedly the outstanding characteristic of the water-base materials.
58. Coatings were applied with at least half of the pigment remaining in the bottom of the
container undispersed and later thrown away with the container.
59. This procedure does not allow for the maximum performance of coating properties and
normally leads to rapid coating failure.
60. The mixing should be done in such a manner that splashing is avoided.
61. Two – component materials are designed to react chemically, so that if they are not
thoroughly mixed, the chemical reaction may not take place properly.
62. Straining prior to spraying often eliminates considerable downtime due to gun clogging by
small particles that restrict the orifice in the gun.
60
63. Keeping the material clean in the field is essential, not only to prevent gun stoppages, but
also to prevent imperfections in the coating due to particles of dirt that were allowed into
the coating material.
64. If the surface is used and pitted, brushing of the first coat is recommended.
65. Spray equipment must be cleaned with a suitable solvent. If the equipment is not
thoroughly cleaned, old, dried paint from the interior of equipment and hoses may be
removed by the new solutions, causing gun stoppage and unsatisfactory results.
66. This is strictly the result of a lack of attention, care, and good application procedures.
67. The lifting of previous coatings usually occurs during coating repair and where one
coating is applied over a previously dried coating. The previous coating may be corroded
or damaged, and if coating of different type is applied (particularly one which has a rather
strong solvent base), it may swell the previous coating around the break and cause it to lift
and curl.
68. The answer to these situations is to thoroughly sand the coating smooth the damaged
area, feathering the broken edge sufficiently far from the damaged area so that the coating
adhesion is sound. Apply a thin coating so that it can dry quickly. This can be followed by
several other thin coats, each well dried before another is applied, to build up the required
thickness. Thick coats can cause the undercoat to wrinkle and swell and generally lose
adhesion to the surface.
69. Irrespective of the type of coating job, a proper specification is the key to its success.
70. A poor specification, i. e., provides for poor selection of materials, is an invitation to
disagreements, and, above all, early coating failure.
71. Apply coating as soon as possible to prevent blasted surfaces from rusting. Keep moisture,
oil, grease, or other organic matter off surface, before coating. Spot reblast to remove any
contamination. Solvent wiping is not satisfactory.
72. Exposure to direct sunlight for more than 24 hours between coats will result in intercoat
delamination.
73. To repair coating, or to add thickness, clean area and remove all dust. Spray material on
larger areas, brush may be used on smaller areas.
74. If film is not up to required thickness, add additional material.
75. Much of the coating may be lost due to the wind creating dry spray, which is blown, or it
may create a serious overspray condition on the finished coating or on adjacent structures.
76. All difficulties and improper work should be called to the attention of the applicators as
quickly as possible so that they can be remedied prior to the application of the following
coat.
77. Causes for coating failures:
a) Failures due to the basic formulation of coatings. Such failures can be chalking,
checking, cracking, discoloration, and similar phenomena.
61
b) Failures due to improper coating selection. A coating that was designed for steel
surfaces is often extended out over a concrete or wood surface, rapidly failing over these
other surfaces.
c) Failures resulted from the nature of the substrate; i.e., a coating can be incompatible
with the surface over which it is applied.
d) Failures due to improper or poor surface preparation.
e) Application and surface preparation together form a substantial part of the
difficulties, which create coating failure. Inadequate thickness, pinholes, overspray,
improper drying, improper curing – all are causes for rapid coating failure.
f) Adhesion-related failures are numerous and generally catastrophic. Such failures
include blistering, flaking, peeling, and intercoat contamination.
g) The design of a structure itself is often the cause for severe failure. Sharp edges,
crevices, skip welds, back – to – back angles – all are focal points for failure.
h) Exterior forces are forces such as chemical exposure, abrasion, reverse impact, severe
weathering – all of which can cause rapid coating failure.
78. If the coating is formulated poorly and that coating is selected, the coating will fail in spite
of any effort that is made to insure its proper application.
79. The formulation-related types of coating failure are as follows:
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FORMULATION – RELATED FAILURES
Organic
Coating
Failure
Failure Appearance
Cause of Failure
Remedy
Chalking
Surface soft and powdery.
Easily removed by wiping
surface.
Surface disintegration by actinic
rays of sun on the organic resin
binder, improper pigmentation.
Erosion
Similar to chalking. Surface
removed on high spots and
brush marks to base coating or
primer.
Surface phenomenon-uneven,
small, non – continuous
fissures in coating, which do
not penetrate to the substrate.
Chalking mechanism with coating
surface removed by weathering.
Select coatings formulated with
radiation – resistant resins
(acrylics) and noncatalytic,
nonchalking pigments.
Select chalk – resistant coating
with good flow out to a smooth
film.
Surface stressed caused by
shrinkage due to weathering and
continued surface polymerization
and oxidation.
Select coating formulated with
weather – resistant resins and
inert reinforcing pigments in
addition to noncatalytic colored
pigments.
Alligatoring
Very large macrochecking,
generally crosshatched pattern.
Internal stresses where surface
shrinks more rapidly than body of
coating. Hard topcoat applied
over soft undercoat.
Apply thin coats and
thoroughly dry before adding
additional coats. Never apply
hard topcoats (epoxy) over soft
undercoats (asphalt).
Cracking
Small breaks in coating to
substrate. May be linear, crosshatched, or curved. Cracks may
or may not be continuous.
Stress set up in coating due to
continued polymerization and
oxidation; Improper
pigmentation.
Select coating formulated from
nonreactive weaher-resistant
resins, reinforcing pigments,
and nonreactive colored
pigments.
Mud
Cracking
Large macrocracking. Coating
may curl at cracks and lose
adhesion.
Rapid drying of highly filled
coatings, especially water-based
materials (water emulsion paints).
Wrinkling
Furrows and ridges in. Wrinkle
may be fine or quite large.
Biological
Failure
Softening or on of coating.
Blotchy brown or black spots
on coating surface causing
poor, dirty appearance.
Surface reaction where surface of
coating expands more rapidly
during drying than does the body
of the film.
The biodegradation of the coating
by bacteria or fungi. The coating
is used as a source of
nourishment.
Discoloration
Yellowing, graying, or
darkening of coating.
Resin or pigment color change
due to weather or chemical action.
Use coatings with strong
adhesion. Apply coatings under
proper drying conditions and
prevent sags, puddles, or areas
of excess thickness.
Choose coatings with even,
thorough drying characteristics.
Apply evenly; avoid excessive
thickness.
Select oil – base coating which
contains permanent fungicides
or bacteriacides. Nonoil
coatings should use
nonbiodegradable modifiers.
Select coating formulated with
both color stable resins and
pigments.
Inorganic
Coating
Failure
Failure Appearance
Cause of Failure
Remedy
Checking
Usually fine visible or
microscopic checks. Do not
penetrate to the substrate.
The zinc to binder ratio is high;
rapid drying conditions cause
surface checking.
Mud
Cracking
Fine to fairly large segments
( ¼ in.) flaking from surface
Application of coating too heavy.
Rapid drying conditions.
Pinpoint
Rusting
Pinpoint spots of corrosion
progressing from a few per
square feet to almost
continuous. Early failure can be
catastrophic.
Zinc pigment mask by other
pigmentation or improper
zinc/binder ratio. Uneven coating
thickness; thin coated areas show
first failure.
Formulation should include
reinforcing pigments. Apply
coating as thin as
recommended. Second coat, if
necessary. Apply under
favorable drying conditions.
Apply coating at no more than
recommended thickness. Apply
under favorable drying
conditions.
Usual remedy: remove coating
and reapply more satisfactory
zinc coating. Apply
maintenance coat at first sign
of pinpoint failure.
Checking
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Organic coatings
a) Chalking – the mechanism is essentially one where the coating binder tends to
gradually disintegrate, leaving the surface covered with the pigments that have been held
on the surface by the binder. Powdering or chalking of coating is due to the exposure of
the coating to the actinic rays of the sun and the action of the radiation on the organic
binder.
b) Erosion – is essentially a wearing away of the coating surface in a gradual manner
similar to the chalking mechanism.
c) Checking – can be describes as small breaks in coating surface that are formed as the
coating ages and becomes harder and more brittle. Checking is a surface phenomenon
and does not penetrate the full depth of the coating. It is a combination of the resins and
pigments, which are not properly combined, so that, as the coating dries or continues to
react, the surface becomes hard and brittle, and surface stresses develop in the coating.
d) Alligatoring – can be considered a checking reaction in which the surface of a
coating hardens and shrinks at a much faster rate than the body of the coating itself.
Alligatoring usually does not penetrate through the coating, but is a surface reaction. The
reaction occurs where a hard, tough coating may be applied over a softer, extensible
coating. The harder material tends to shrink and float on the surface of the underlying
material, with the surface cracking in large segments.
e) Cracking – it is different from checking and alligatoring in that it is not a surface
problem, but one where the breaks in the coating extend from the surface through to the
substrate. Both checking and cracking are the result of stresses in the coating film, which
exceed the strength of the coating. Checking results from stresses on the surface, while
cracking is caused by stresses throughout the film and between the film and the substrate.
f) Mud cracking – goes directly through to the substrate and therefore is a source of
immediate corrosion, along with possible chipping and flaking of the coating from the
surface. Mud cracking occurs when highly filled coatings are applied rather heavily,
which may be the case in runs and sags in the coating. This type of failure is usually
found in overly thick areas of coating. Water-base coatings that are highly filled often fail
by mud cracking. The more rapidly the solvent or water evaporates, the greater the
chance that mud cracking will occur. The prevention of mud cracking is a combination of
selection and proper application.
g) Wrinkling – is a peculiar phenomenon and is usually associated with coatings that
are applied at too great a thickness, many times occurring in sags and puddles. In place of
shrinkage, as is the case with checking and cracking, wrinkling is the result of the
swelling of a coating where the surface of the coating expands more rapidly during the
drying period than the body of the coating. Wrinkling occurs most often with oil-base
coatings, alkyds, and similar materials.
h) Biological failure – there are two types of action. One is the activity of a
microorganism due to dirt and contamination on the coating. In this case, the bacteria or
fungi merely live on the surface of the coating and do not necessarily affect its resistance.
The second type is where the microorganisms actually use the coating for food and
64
derive their energy from it. Under certain conditions, coatings can be rapidly
disintegrated by this type of action.
i) Discoloration – the discoloration of a coating may not seem like a major failure
mechanism; however, appearance is a part of coating’s purpose. If its appearance is poor,
the coating is generally considered unsatisfactory. Thus, coatings that fade, discolor, or
become unsightly a short time after application, can be considered to have failed.
Inorganic coatings.
One of the principal causes of formulation problems with inorganic zinc coatings is
the fact that they are made with silicate vehicles, either alkali silicate or organic
silicate such as ethyl silicate. Neither of these materials is a good film former in the
sense of film formation by organic materials. When unpigmented, these materials
usually form a brash, clear deposits on the surface which tends to check and crack
and to have little coating strength. It is only after the reaction with the zinc
pigmentation that these materials form a film, which has both adhesion and strength.
Under these conditions, it is obvious that formulation is critical, and small variations
in additives or pigmentation can cause the coating to be soft, poorly adherent, or to
have generally poor film characteristic.
a) Checking – the ratio of pigments to binder is high, and under these conditions,
particularly where drying is rapid, fine surface checking can occur. Checking can be
prevented by proper application techniques. Thin, multiple-pass coats are often helpful.
b) Cracking – or mud cracking occurs with inorganic zinc coatings. It often occurs
rapidly and is related to the rapid drying of the coating as well as the coating thickness.
Many times, the cracked area is curved up from the surface in chips or flakes. Primarily,
the elimination or prevention of this problem is in the application of the coating i.e.,
applying a relatively thin film, with additional passes if necessary, in order to obtain the
thickness recommended by the manufacturer.
c) Pinpoint rusting – usually occurs where a coating has been in service for a long time
and is nearing the end of its useful life. However, if a coating is improperly formulated,
pinpoint rusting can begin even a few days after application. This has occurred in a
number of instances with some of the zinc coatings with a low zinc loading.
80. Failures due to coating selection:
a) Mixed surfaces. One area where coating selection can be difficult is where a coating
extends from one type of surface to another, e.g., structural steel in a concrete foundation.
In this case, the selection of a coating for one surface would not be adequate for the
second surface.
b) Chemical exposure. Selection of a coating that is inadequate for the subsequent
chemical exposure of the film is a common occurrence.
c) Tank linings. While a coating may be entirely satisfactory for one of the cargoes,
when a second loading follows the first, the coating may fail.
d) Repairing and recoating. Coating selection is even more of a problem when
recoating is being considered. In this case, not only is it necessary to determine the
properties of the coating from the standpoint of the exterior exposure, but also the coating
must be satisfactory over the existing coating on the structure.
81. Substrate-related failures.
65
SUBSTRATE – RELATED FAILURES
Coating
Failure
Failure Appearance
Cause of Failure
Remedy
Previously Used
Steel
Blistering, rust, tubercles, loss
of adhesion in areas where steel
was previously exposed to
corrosive conditions.
Retention of minute amounts
of corrosion product or
contaminant along grain
boundaries of the steel
surface, even though blasted
to white metal.
Galvanized or
Metallic Zinc
Surface
Aluminum
White zinc corrosion product
forming under the coating or
actually breaking through the
coating.
White corrosion product
causing pinpoint failure in
coating; loss of adhesion
because of very smooth
surface. Possible blistering.
Formation of zinc salts
(oxide, sulfide, oxychloride,
zinc soap) underneath
coating.
The very smooth aluminum
oxide surface. No physical
adhesion.
Copper
Grey-green corrosion product;
loss of adhesion.
Very smooth copper oxide
surface. No physical
adhesion.
Wood
Checking and cracking of
coating. Flaking from hard
winter grain. Dense coatings
with a low MVT rate may
blister due to absorption and
evaporation of moisture from
the wood.
Expansion and contraction of
wood due to varying
temperature and humidity.
Differential expansion
summer and winter grain
combined with soft porous
summer grain, causing a
variation in coating
adhesion.
Concrete
Blistering of coating.
Formation of calcium salts
under coating, forcing from the
surface. Loss of adhesion and
peeling.
The chemical reactivity and
moisture content of concrete.
Its nonhomogeneous very
porous structure. Pinholes,
water, and air pockets in
poured concrete surfaces.
Wash-blasted surface with
water or dilute phosphoric acid
solution and reblast. Apply an
anticorrosive primer with
strong adhesion. Where
applicable, an inorganic zinc
primer may provide a good
base coat by reacting with the
minute surface corrosion after
the first blasting.
Brush blast zinc surface or treat
with commercial zinc
treatment. Apply a nonoil base,
inert, strongly adherent primer.
Very lightly dust blast the
aluminum surface, or where
applicable, treat with
commercial aluminum
treatment. Apply a primer with
known compatability and
strong adhesion to aluminum
surface.
Brush blast copper surface or
etch with commercial copper
treatment . Apply a primer with
known high adhesion to
copper.
Start with a clean, newly
sanded wood surface. Apply
low molecular weight highly
penetrating paint, preferably oil
base, with sufficient elasticity
to expand and contract with the
wood surface. Paint should
have relatively high moisture
porosity to allow wood to
breathe.
The concrete should be clean
and the surface dry. It may be
acid etched or lightly blasted to
obtain proper surface condition.
Use a low molecular weight
highly penetrating primer with
strong alkali resistance (liquid
epoxy). Primer should be heavy
bodied and thixotropic to fill
imperfections in concrete
surface.
82. Surface preparation-related failures. There is no substitute for proper surface preparation
if long life is expected from a coating. When applying a repair coat over a previous
material, even greater care is necessary to make sure the surface is prepared properly to
accept the repair coating. Compatibility of the repair material, as well as proper cleaning
of the surface, is the key to a satisfactory repair job. There are coatings, which are
specially formulated for application to rusty hand – cleaned surfaces. Some of these do a
good job, but only where used in a light to very moderately corrosive atmosphere.
66
83. Application-related failures. Almost all application – related failures are due to
carelessness and poor workmanship. Poor workmanship refers to improper coating
application by being too close to or too far away from the surface, or by spraying the
surface at angles, which cause coating imperfections. All application – related failures are
due to lack of care, lack of an understanding of coating fundamentals, or poor
workmanship, which may be the result of poor training in the application of high –
performance coatings or little pride in proper workmanship.
APPLICATION – RELATED FAILURES
Coating Failure
Poor Workmanship
Runs, Sags, Curtains
Brush Marks
Improper Coating
Thickness
Overspray
Pinholes
Holidays
Spatter Coat
Cratering
Failure
Appearance
Cause of Failure
Remedy
Dirt, dust, grime on surface
and in coatings. Holidays,
overspray, pinholes, runs,
sags.
Heavy areas in coating
which flow down vertical
surface in streaks or curtains.
Lack of care in application.
Better training for workmen.
Instill sense of pride in
work.
Lack of care in application.
Linear hills & valleys in
coating. Considerable
difference in coating
thickness from hills to
valleys. Rusting starting in
valleys.
Areas of pinpoint corrosion
between areas of solid
coating. Where coating is
over thick. Possible
checking and cracking.
Very rough coating surface.
May appear like sand in
coating. Some dry coating,
like dust, on surface.
Pinpoint corrosion
throughout rough areas.
Small, visible holes in
coating (1/32 in.). Holes
generally appear in
concentrations with a
random distribution.
Pinpoint corrosion in
pinholes.
Poor workmanship. Very
heavy bodied (thixotropic)
coating.
Remove runs and sags with a
brush prior to initial set of
coating. Smooth area with
light spray coat.
Train workmen to brush
smoothly. Brush coating out
well, finishing by light
brushing in one direction.
Thin areas, spatter coating,
holidays. Runs, puddles,
excessive number of spray
passes in areas where
coating is difficult.
Improper spraying technique
Uneven spray passes with
gun too far from surface.
General corrosion in bare or
thin areas of surface which
were uncoated by the
painter. Most often in
difficult areas to coat.
Pinpoint rusting in area of
thin coating, usually at end
of spray pass or around a
complex section of structure.
Small spots of coating which
are noncontinuous over
substrate. In poor light may
seem continuous.
Pinpoint rust forming in thin
areas of bug eyes, fish eyes,
or craters randomly
dispersed over coated area.
May be more prevalent in
thicker sections.
Careful application. Even
spray passes with each pass
overlapped 50 %. Use cross
spray technique.
Apply coating with care and
with even wet spray passes
overlapped 50 %. If
overspray occurs, remove
before overcoating.
Improper spray technique.
Apply coating with care with
Spray gun too close to
spray gun at the optimum
surface with air bubbles
distance from surface. Make
being forced into coating.
sure gun is properly
Spray pot pressure too high
adjusted. If pinholes already
with atomizing air pressure
exist, apply coating by
too low. Pinholes may exist
brush, working it into
in the substrate (concrete).
surface.
Poor, inconsistent application.
Apply coating in careful,
Lack of care.
consistent manner, making
certain that no areas remain
uncoated. Overlap each pass
50 %.
Discrete coating droplets,
Apply coating with care. Use
which are not continuous
even, wet spray with each
over surface. Inconsistent
pass overlapped 50%. Use
spray passes not overlapped
cross spray technique.
50 %. Spray gun flipped at
end of spray pass.
67
Improper solvent mixture,
oil in atomizing air, surface
contamination, particulate
fallout during application,
high surface tension.
Once cratering occurs, sand
or roughen crater area.
Apply second coat by brush,
working coating into
cratered area.
a) Brush marks – the failure, which occurs when pronounced brush marks are left in a
surface is due to the hills and valleys left in the coating by the brushing technique. These
can be overcome through the use of proper application techniques. The failure starts as
pinpoint rusting in the low, thin areas of the coating, which soon undercut, and because
the low areas of the coating are rather close together, rapid total failure results.
b) Runs and sags. Failures, which are due to runs and sags, are usually a result of too
great of a coating thickness. Many of the internally reacted or catalyzed coatings
continue to cure over a considerable period of time, and, in areas where the coatings is
heavy (e.g., runs and sags), they often check and crack, leaving the substrate open to
rapid coating failure. Runs and sags are also a result of careless application. With
spraying, improper gun adjustment can cause sags or runs, or merely the overly heavy
application of the coating at any one spot. This can be caused by the gun being held too
close to the surface and not moved rapidly enough over the surface in order to obtain a
uniform film.
c) Improper coating thickness. There are two types of failure, which occur from
improper coating thickness. One, as runs and sags, is a result of coating being too thick,
and the other is a result of the coating being too thin. Where a coating is applied too
thickly the coating may not dry properly and may have an excessive retention of solvent
in the coating. Retained solvents may cause blistering and poor adhesion to the substrate
because the underlying coating is softer and because of the retained solvents close to the
substrate. Where the coating is catalyzed or internally cured, the coating may have
internal shrinkage because of the thickness, which causes checks, cracks, and even
scaling. Where this occurs, rapid undercutting of the coating may result from the poor
adhesion of the overly thick film. In the case of thin coating, failure is primarily due to
rapid application, which does not leave enough material on the surface. The type of
failure, which occurs in these areas is due to pinpoint rusting, with the pinpoints
gradually becoming larger until the entire coated area is undercut in the thinnest spot.
d) Holidays. Holidays are areas where the applicator has missed coating the surface, or
where an extremely thin spot remains after the coating is applied. They are more often
found in difficult – to – coat areas. The type of failure resulting in these areas is an early
pinpoint rusting of the area, followed by the formation of rust scale and the undercutting
of the coating adjacent to the holiday.
e) Overspray. A failure because of overspray is due to pinpoint rusting of the film in
and around the overspray particles. When overspray is allowed to remain on the coating,
and additional coats are applied, the coating generally will not penetrate around the
overspray particles, but will leave imperfections in the coating at this point. Overspray is
common type of coating failure, particularly where fast – drying coating are used.
Overspray is a result of poor application techniques, a result of either poor gun
adjustment or application of the coating at too great a distance from the surface and not
applying an even, wet film to the surface.
f) Pinholes. Pinholes in a coating can result from overspray. For the most part they
result from heavy application of a coating, with air being blown into the coating surface
during the application. As with overspray, pinholing can be caused by improper gun
adjustment, where the gun does not atomize properly and air is entrapped in the coating.
It can also be caused by the application of the coating too close to the surface, creating an
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area in the center of the fan where too much material is applied and where air bubbles are
entrapped in the wet coating.
g) Spatter coat. It is an area where the coating is not applied as a continuous film, but is
applied sufficiently fast and in a small enough volume so that the coating reaches the
surface not as a wet film, but as a series of discrete droplets with bare areas in between.
The coated area may look like it is covered, but on close inspection, the droplets are
visible, as are the bare substrate areas in between.
h) Cratering (or bug – eyeing or crawling) – may be either an application problem, a
material problem, or both. Some materials, which have a high surface tension have a
greater tendency to behave in this manner that those with a lower tension. Much creating
is due to contamination and dirt that may be on the surface or that falls into the wet
coating during application. When a vehicle has some cratering tendency, minor
contamination can seriously aggravate the problem. The coatings most susceptible to this
problem are those which thermoset, e.g., epoxies, phenolics, etc. The areas of a coating
where a crater forms are thin and are therefore focal points for coating failure. Without
repair during application, pinhole corrosion can occur.
84. Design – related failures.
a) Edges. The failure of coatings on edges is important since this is the area where
corrosion usually first begins. The edge provides a sharp break in the coating and is also
subject to more damage than flat area. Edges invariably seem to be problem, unless
special care is taken during the application of the coating to these areas.
b) Corners. Exterior corners can be considered the same as edges, and the same
corrosion occurs on exterior corners as would be expected on edges.
c) Welds. The failure of coating on welds is very common. There are several causes, the
primary cause being that the weld is a rough and discontinuous surface area on plane
surface. On the other hand, many welds are tested for leaks, after which a soap solution is
applied over the weld. In many cases, the coating is applied over such a surface, with
early failure of the coating resulting over the soaped surface. Many welds are cleaned
less than perfectly, and weld slag and similar contamination can cause early coating
failure.
d) Nuts, bolts, and rivets. Corrosion on nuts, bolts, rivets, and similar construction
fasteners is almost inevitable, unless extra care is taken during the application of the
coating. This is due to the many edges and corners that are present in a bolted structure.
e) Overlapping joints or plates. There are always critical corrosion areas whenever
bolted or riveted structures are used. Failure of the coating in these areas is, in many
cases, rapid and is due to the crevice between the two plates and the rapid change in
direction of the coating in the joint. The coating in the joint often crack, with moisture,
salts, and similar contamination penetrating the overlap and causing rapid corrosion.
f) Construction aids. Construction aids, hold downs, and similar areas are areas where
corrosion is usually initiated and coating breakdown quite rapid. The cause is that these
aids to construction are temporary and are usually welded rapidly, without particular care
during the welding process. The welds are also usually quite rough-cut and may be skip
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welded or only welded on one side. This allows easy access to moisture and salts with
resulting corrosion. The temporary aids are too often left on the surface as well.
g) Pipe structures. The failure of coating on pipe structures is usually due to two
principal causes. One is the welding of the pipe itself. The second is due to holidays left
in the coating where the coating has not been properly overlapped during the coating
process.
85. Adhesion – related failures.
The coating must adhere to the substrate if the substrate if the substrate is going to be
protected from corrosion.
a) Blistering. One of the most common types of failure related to adhesion is that of
blistering. The usual cause for blistering is the penetration of moisture through the
coating into areas of poor adhesion. The moisture vapor may condense and form a liquid
blister, or the vapor pressure of the moisture at the interface between the coating and the
surface may be sufficient to lift the coating away from the substrate. There are generally
two types of blisters: (1) those that are formed from the substrate, and (2) those that are
formed between coats and where the topcoat separates from an undercoat, forming
circular blisters. Blistering can be caused by a number of different conditions:
Soluble pigments in the primer – have been a cause for many blistering failures. The
soluble pigments absorb moisture vapor as it passes into the coating, creating a rather
concentrated solution. At that point, the phenomenon of osmosis occurs, pulling water
trough the coating into the area where the pigments have been dissolved. This is the
reason why impervious types of coatings are recommended for highly humid or
immersion service. This same phenomenon can occur if there are soluble salts
contaminating the substrate or contaminating the surface between coats of the coating.
Osmosis is the transfer of moisture through the coating in the direction of the most
concentrated solution. When there are soluble materials on or within the coating,
blistering is almost inevitable. Blisters, in this case, would be liquid filled from the
osmotic action. Blistering can also be caused by contamination of surface by materials
(e.g., oils, waxes, dust, etc.) that will not allow proper adhesion of the coating. Again,
the moisture vapor transferring through the coating is the cause for the blistering. The
blisters are so-called “dry” blisters.
Shop primers. Blistering can also be caused by the selection of an improper primer for
the surface. This can often be the case when a shop primer is used on surface, which are
later overcoated with a high-performance coating.
Incompatibility. Incompatible coatings often have poor adhesion, which causes
blistering between coats.
Retained solvents. Solvent retained by the coating act like plasticizers, making the
coating softer and more flexible, and, depending on the water sensitivity of the solvent,
can increase the water absorption and moisture vapor transmission of the coating. Where
there is solvent retention and the coated surface changes in temperature, the solvent itself
may create a sufficient vapor pressure to cause blisters to form in the coating. Blisters of
the solvent type may also occur where a topcoat is applied over a porous substrate or a
porous undercoat.
Cathodic protection. Blistering is often caused where cathodic protection and coatings
are used to supplement each other. This type of blistering is most often caused by
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hydrogen gas being formed on the metal substrate underneath the coating in sufficient
volume so that the hydrogen vapor pressure pushes the coating off of the surface.
Previously corroded surfaces can also cause blistering where salts or oxides remain in
the surface pores of the substrate, creating focal points for rusting and blistering.
b) Peeling. It includes peeling from the surface or peeling between coats. It is generally
caused where the tensile strength of the coating is higher than the adhesive strength or
bond strength. This may be caused by poor surface preparation, which reduces the bond
of the coating to the surface. It can also be caused by contamination between coats,
where the topcoat then peels from the surface; or it may occur when the thickness of the
coating is too great, causing a film that is stronger than its adhesive characteristics.
c) Flaking. Is much like peeling, with the exception that the flaked coating is usually
hard and brittle and tends to pull itself away from the surface. Once the coating cracks,
the edges may tend to curl away from the surface, creating the flaking tendency.
d) Intercoat delamination. Delamination is the lost of adhesion between coats and is a
common type of coating failure. The failure occurs most often where repair or
maintenance coatings are being applied over existing coatings that have been in service
for some period of time. Under these conditions, the original coating usually is chalky,
has dirt embedded in surface, or has oxidized or cured on the surface to the point of
complete insolubility and impenetrability. What – ever the cause, the repair coating has
difficulty wetting and establishing bond areas over the previous surface. There are many
causes for intercoat delamination. Incompatibility of two coating materials is one. This is
an area where mixing coating systems or mixing suppliers can cause some serious
problems. Causes of intercoat delamination:
Contamination. Coatings, which have been in service for some time and are being
repaired accumulate dust, dirt, grease, chemical, and similar contamination which must
be thoroughly removed prior to the application of an additional coat.
Chalking. A chalky surface is a difficult one over which to apply any coating. On a
chalky surface, it is recommended that it be thoroughly scrubbed prior to the application
of any additional coats.
Overcured surfaces. Overcured surfaces occur with many coatings, particularly those
that react with air or are internally cured. The application of too great a thickness of a
coating can also lead to delamination. It can cause delamination not only down to the
substrate, but also between coats. The shrinkage stresses in a heavy coating can increase
until they exceed the force of the adhesion between coats, and the heavy coat will
actually pull itself apart and away from the surface. Chemical attack can also cause
delamination.
e) Undercutting. Undercutting of a coating is the action of rust under the coating,
usually forming around a small break in the coating. A coating that has been applied over
mill scale is susceptible to undercutting. Undercutting is a serious problem, and very
deep pitting can result from the corrosion, which builds up underneath a coating.
86. Failures due to exterior forces. There are certain types of exterior forces that cause coating
failure which are over and above the ordinary atmospheric types of exposure to which the
coating is expected to be resistant. Chemical exposure is one of the principal types of
exterior forces that can cause coating failure.
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Reverse impact. Reverse impact is an area, which can be particularly damaging to
coating. Reverse impact is a difficult problem for many coatings, and in some cases it is
used as a test for coating adhesion. Actually, because the coating is heavily stressed, it is
not only a test of adhesion, but a test of extensibility as well.
Abrasion. In order to resist such forces, the coating must have the ability to resist
abrasion, but, at the same time, have sufficiently strong adhesion to the substrate so that,
as the coating is worn away, it does not flake or chip, but wears down to an even,
feathered edge.
Faying surfaces or joint movement. Failures can occur where joints in a steel structure
are fastened by riveting or by the use of high-strength bolts. Faying surfaces are designed
to resist movement. Inorganic zinc coatings are satisfactory for use on these surfaces and
are an answer in protecting the overlapping surfaces from corrosion.
7. TABLE – PROTECTIVE COATING CLASSIFICATION
Basic Coating Formation
Generic Coating Material
Natural Air – Oxidizing Coatings
Drying Oils
Tung Oil Phenolic Varnish
Alkyds
Vinyl Alkyds
Epoxy Esters
Silicone Alkyds
Uralkyds
Nitrocellulose
Polyvinylchloride – acetate
Copolymers
Acrylic Polymers
Chlorinated Rubber
Coal Tar Cutback
Asphalt Cutback
Epoxy
Coal Tar Epoxy
Polyurethane
Polyesters
Silicone
Vinyl Acetate
Vinyl Acrylic
Acrylic
Epoxy
Pure Phenolic
Epoxy Phenolic
Coal Tar Enamel
Asphalt
Polyesters
Synthetic Air – Oxidizing Coatings
Solvent Dry Lacquers
Coreactive Coatings
Emulsion – Type (Coalescent)
Coatings
Heat – Condensing Coatings
100 % Solid Coatings
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