Coating Failures: Richard W. Drisko
Coating Failures: Richard W. Drisko
Coating Failures: Richard W. Drisko
Coating Failures
Richard W. Drisko
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cleaning requirements are similar to those of conven- following treatments may be used:
tional steels, but harder abrasives (e.g., silicon • Chemical treatment such as phosphating
carbide, aluminum oxide, or garnet) may be necessary • Wash priming (good with alkyds)
to produce the desired surface profile. Because of • Blasting with a soft abrasive (e.g., plastic) to produce
their inherent corrosion resistance, they will normally a suitable profile
have less corrosion and undercutting of coating at
film holidays. Aluminum is susceptible to exfoliation, an
advanced stage of intergranular corrosion character-
ized by a delamination of metal along grain bound-
aries. Rolled metal products such as aluminum alloy
plate are especially susceptible to exfoliation due to
their longitudinal grain structure.
Coated aluminum is particularly susceptible to
a form of corrosion called filiform. It is characterized by
threadlike directional growths proceeding away from
damaged areas. In the past, chromate inhibitive
pigments were widely used to control filiform corrosion;
chromate-free inhibitive pigments are now used.
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before painting. An oil-based or water-borne stain- Limitations of Coating Formulations
blocking primer can be used for this purpose. All coating formulations have some limitations
Resinous materials in some trees, such as that restrict their uses to appropriate environments and
lower grades of pine, may seep to the wood surface services. In this section, some of the more important
after painting to cause staining and paint deterioration. limitations are addressed.
This can be minimized by using weathered wood and
sealing it before use.
Woods are very sensitive to moisture so that
they swell during periods of high humidity and shrink
during periods of low humidity. Rigid coatings on wood
may crack when they are unable to expand and
contract with dimensional changes.
Coatings hide wood grain and greatly reduce
water permeability. However, water that enters into the
wood interior may try to escape through impermeable
coatings to cause blistering and/or delamination. For
this reason, latex coatings that permit the passage
of water vapor (sometimes called breathing) may
minimize this problem. Many people prefer to use Figure 3. Chalking.
semi-transparent stains that do not seal the surfaces
of wood. Chalking. Chalking is the formation of loose powder on
the surface of coatings. It is typically caused by
Defects/Failures Associated with the deterioration of the organic coating binder by ultravio-
Coating Itself let light (usually from the sun) to leave a loose residue
Some coating defects and failures are directly of pigment and oxidized binder. All organic coating
related to the coating itself. These include: binders chalk to some extent, but those containing
• Errors by the manufacturer in production of aromatic chemical groups (e.g., epoxies and pheno-
the coating lics) chalk much faster than others.
• Coatings that have exceeded their shelf life Some pigments such as the anatase form of
• Inherent limitations of properly formulated coating titanium dioxide chalk very freely, while other pigments
• Incompatibility of a coating with its substrate or such as rutile, another crystalline form of titanium
undercoat dioxide, are quite chalk-resistant. Opaque pigments,
of course, reduce chalking of underlying organic
Coatings with Errors in Manufacture or that Have binders by shielding them from sunlight. Leafing
Exceeded Their Shelf Life aluminum pigments formulated to float to coating
Errors in coating manufacture do not occur surfaces protect underlying binders especially well.
very often. They can usually be detected in the field Chalking of finish coatings can best be
before use by testing for condition in container, as controlled by proper selection of pigments and binders
described in Federal Test Method Standard 141. If the and by use of additives such as ultraviolet light
viscosity does not appear to be at the proper level, it absorbers.
can be checked in the field using a viscosity cup. Also,
a test patch of coating can be applied to the intended Erosion. Erosion is the gradual loss of coating by wear
substrate to check for such properties as ease of or weathering. Thus, coatings that chalk freely are
application, hiding, leveling, and complete curing. more susceptible to erosion than are coatings that are
If a stored coating has exceeded its shelf life, more chalk-resistant. Erosion may also be caused by
it may have deteriorated to the extent that it can no wind-blown sand or rain.
longer be successfully utilized. Such coatings should Accelerated erosion may significantly
be checked for condition in container before use. reduce coating thickness and even expose under-
coats. Erosion may be minimized by selecting a
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chalk-resistant coating with good leveling properties.
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be prevented by avoiding the application of moisture- greater film thickness.
sensitive coatings on humid days.
Orange Peel. Orange peel is similar to brush marks in
that it is caused by insufficient leveling of the wet film.
However, this defect occurs with spray rather than
brush application of coatings.
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with the coating. having significant water solubility. This is especially
likely to occur during water immersion service. These
High-Temperature Attack on Coatings. High- pigments can be detected during a laboratory failure
temperature attack is likely to occur to organic coat- analysis of the water (sometimes colored) in filled
ings even during intermittent high-temperature service. blisters.
Thus, heat-resistant inorganic coatings are usually Osmotic blistering may also occur if soluble
used at temperatures above 450°F (230°C). salt contaminants are not completely removed from
substrates during surface preparation.
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alligatoring, other intercoat cracking, and mud crack-
ing. Cracking usually occurs all the way through the
coating to expose the substrate.
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incompatibility between coatings are described here. forms of cracking.
Topcoat Solvent Attack on Undercoat Binder. Topcoat Saponification (hydrolysis). Saponification may occur
solvent attack on undercoat binder may cause the to an alkyd or other drying oil applied over a zinc-rich
latter to soften, swell, or disbond. In any case, the primer. As with concrete, the surface alkalinity on the
intercoat adhesion is significantly diminished. An zinc-rich primer causes this chemical degradation.
example of this is a chlorinated rubber or an epoxy
coating with a strong solvent being applied over a vinyl
dispersion (latex) coating.
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steel-to-water potentials are regularly monitored. uniformly thick layer to form areas of little, if any,
thickness. It occurs when the surface tension of a
Electroendosmosis. Electroendosmosis is a mecha- coating is greater than the surface tension of the
nism of coating deterioration in which excessive substrate. Crawling is caused by substrate
cathodic potentials causes electrolyte to penetrate contamination with oil or some other low surface
rapidly through a coating film. It normally results in energy contaminant.
coating blistering and peeling.
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surface preparation may result in osmotic blistering of detection of defects and their immediate correction. It
coatings subsequently applied. These salts are usually is much easier to prevent coating problems associated
not readily visible and so must be removed and with improper spray application than to correct them
analyzed using special techniques.6 The adverse after application.
effects of soluble salts are much greater on coatings in When applying two-component thermosetting
immersion service than in atmospheric service. coatings, careful attention must be paid to the
manufacturer’s recommendations for induction, pot
Improper Surface Profile life, and recoat times. If this is not done, catastrophic
Each primer has a profile height recom- failure may occur.
mended by its manufacturer for best performance. Any
significant deviation from this recommendation may Mixing Coatings
result in reduced coating system performance. In Although coatings are prepared ready to
general, recommended profile heights vary directly apply, settling of the heavier pigment portion may
with the primer film thickness. Thus, primers with occur during storage. Thus, all paints should be
greater film thickness usually have higher recom- thoroughly mixed before application to ensure that the
mended surface profiles. material being applied is the homogeneous blend
originally manufactured. Improper mixing can lead to
Insufficient Profile Height. Insufficient profile height of uneven color in cured paint, inadequate film thickness,
cleaned surfaces may provide insufficient bonding poor coating adhesion, and checking or cracking of the
areas for adequate coating adhesion. This, in turn, paint film.7
usually results in early coating loss by disbonding Coatings should not be overmixed to avoid
and peeling. entrapping air into them. Thus, a mechanical mixer
should be used at a speed set so that a small rather
than a large vortex or depression on the paint surface
is created in the center of the can. Use of paint shak-
ers is not recommended. Allowing stirred paint to set
for several minutes before application may permit the
release of entrapped air.
Two-component coatings such as thermoset-
ting epoxies and polyurethanes are normally supplied
in kits composed of Component A and Component B.
The components of each kit must be properly propor-
tioned for mixing together to achieve proper curing and
Figure 20. Pinpoint rusting. optimum coating performance. Therefore, use of
complete kits rather than partially filled kits are recom-
Excessive Profile Height. Pinpoint rusting may occur mended. Each component should be mixed separately
on coated steel structures where abrasive blast and then mixed together in the order specified by the
cleaning has produced so high a profile that it is not coating manufacturer.
adequately protected by a relatively thin primer. Plural-component spray application systems
Pinpoint rusting may also occur when erosion signifi- combine Components A and B together automatically
cantly reduces coating film thickness. in a specific ratio. However, the proportions should be
checked before beginning coating application to be
Coating Defects/Failures from Improper sure that the proportions are those specified by the
Coating Application manufacturer. When spraying with plural-component
As with surface preparation, the best way to equipment, it is common practice not to use the
avoid coating failures resulting from improper coating triggering technique commonly used with other spray
application is by (1) carefully following the coating equipment because the ratio of components may vary
manufacturer’s recommendations for application and significant at the start and stop of each trigger stroke.
(2) carefully inspecting the work to permit early Skilled applicators are required for the successful use
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of plural-component application equipment. Insufficient Coating Thickness. If a coating is applied
with less than the specified minimum thickness, its
Thinning barrier protection will be lessened, and thus its service
Coatings are manufactured for application as life will be reduced. As discussed earlier, a thinner than
received without thinning. However, low temperatures desired coating may contribute to pinpoint rusting on
or other conditions may necessitate thinning to reduce steel surfaces.
the viscosity for effective application. When necessary
to use a thinner, it should be of the type and in the Excess Coating Thickness. If a coating is applied too
amount recommended by the coating manufacturer. thickly, its weight may cause the wet coating to flow
Thinner should be added to the coating slowly and downward to form sags, runs, or curtains. Such
with thorough mixing to avoid overthinning one portion defects should be detected and corrected as soon as
of the paint and the possibility of curdling the coating observed.
or flocculation of the pigment. Excessive coating thickness may lead to the
acceleration of common cracking, mud cracking, and/
or disbondment of relatively rigid coatings. As de-
scribed earlier, thicker films have more rigidity than
thinner films and thus are less able to expand and
contract with substrate dimensional changes.
Excess coating thickness may be gradually
built up by application of additional coats to an existing
coating system during periodic maintenance painting.
When the total stress built up in the coating system
exceeds the adhesion at its weakest point (usually
primer to substrate), disbondment will occur.
Disbondment may take the form of chipping, flaking,
peeling, or delamination.
As described earlier, wrinkling occurs more
often with thicker than thinner coatings that cure by
oxidation of drying oils. Excess thickness may also
result in other types of incomplete or improper curing.
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Figure 23. Pinholing.
Figure 22. Dry spray. the topcoating of inorganic zinc-rich coatings on warm
days. Topcoat solvent that enters the naturally porous
Dry Spray. Dry spray is a rough, powdery, non- film of the inorganic zinc-rich coating evaporates in the
coherent film produced when an atomized coating warm environment, and the resulting vapors rise to the
partially dries before reaching the intended surface so surface of the uncured topcoat to form pinholes. This
that the coating cannot flow to form a uniform continu- phenomenon is somewhat similar to outgassing of wet
ous film. This condition most commonly occurs with coatings on concrete, in which, during periods of rising
fast drying coatings. Holding the spray gun too far from temperature, interior air and solvent vapors rise to the
the substrate may also contribute to dry spray. Dry concrete surface to form small bubbles in the topcoat.
spray film have little, if any, protective value. Cratering is a special form of pinholing caused
Dry spray should not be confused with by foreign matter in or deposited on the wet film.
overspray. Overspray consists of atomized paint
particles that deflect from or miss the surface being Coating Holidays. A holiday is a pinhole, skip, disconti-
sprayed and fall on unintended surfaces. nuity, or void in a coating film that exposes the sub-
strate. Unless detected and corrected, holidays
Pinholing. Pinholing is the formation of small holes that constitute a source of early electrolyte penetration and
extend through the entire thickness of a coating. It coating deterioration. Holidays in coatings are best
occurs most often with lacquers and other coatings discovered using holiday detectors, as described in the
that contain fast evaporating solvents. Solvent imbal- chapter of this book on coating inspection.
ance is an important source of pinholing.
Pinholing is sometimes caused by holding the Topcoating Outside of Recommended Recoat
spray gun too close to the surface with excessive Window
atomization pressure or a combination of a low atomi- Manufacturers of two-component thermoset-
zation pressure and excessive material pressure. ting coatings specify a window of time during which
A special case of pinholing often occurs during
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their coatings can be successfully topcoated. If References
topcoated too soon, the curing of both coats may be 1. Protective Coatings Glossary; Richard W. Drisko,
adversely affected. If topcoated too late, the topcoat ed; SSPC: Pittsburgh, 2000.
will have limited adhesion to the undercoat. 2. Drisko, Richard W.; Jenkins, James F. Corrosion
and Coatings; SSPC: Pittsburgh, 1998.
Coating Defects/Failures from Improper 3. Munger, Charles G.; Drisko, Richard W. A Review of
Curing Common Failures of Paint Coatings: Part I, Design
Most coatings require special conditions for Factors. Journal of Protective Coatings and Linings;
proper curing. These include ranges of temperature July 1989, pp 36-41.
and relative humidity. Curing wet coatings at tempera- 4. Munger, Charles G.; Drisko, Richard W. A Review of
tures significantly above or below the recommended Common Failures of Paint Coatings: Part II, Factors of
range may result in improper or incomplete curing. Uncoated and Coated Substrates that Affect Coating
Moisture-curing polyurethanes and alkyl Performance. Journal of Protective Coatings and
silicate inorganic zinc-rich coatings cure to a solid film Linings; May 1990, pp 62-66.
by reaction of their binders with moisture from the air. 5. The Fundamentals of Cleaning and Coating
They must cure within a specific relative humidity Concrete; Randy Nixon and Richard W. Drisko,eds.;
range to achieve complete and proper curing.8 Mois- SSPC: Pittsburgh, 2001.
ture-blushing of coating surfaces during periods of 6. SSPC-TU 4. Field Methods for Retrieval and
high humidity was discussed earlier in this chapter. Analysis of Soluble Salts on Substrates; SSPC:
Coatings applied in confined spaces such as Pittsburgh.
storage tanks may require both heating and ventilating 7. Skinner, Jim. Applicator Training Bulletin: Mixing and
to remove coating solvents and permit complete curing Thinning; Technology Publishing Company: Pittsburgh,
or curing to the extent required for topcoating. Other- 1992, pp 65-68.
wise, osmotic blistering may be caused by the en- 8. Hare, Clive H. Protective Coatings, Fundamentals of
trapped solvent. Chemistry and Composition; Technology Publishing
Company: Pittsburgh, 1994.
Summary
There are many causes of coating deteriora-
tion. In order to avoid or minimize deterioration, the About the Author
following actions should be taken:
• Proper selection of a high-performance coating Dr. Richard W. Drisko
system appropriate for the particular environment and Dr. Richard W. Drisko has been the senior technical
service advisor to SSPC: The Society for Protective Coatings
• Preparation of a job specification that includes all since January 1995. Prior to this, he was employed for
requirements necessary to achieve long-term coating over 40 years at the Naval Civil Engineering Labora-
performance tory, Port Hueneme, California, where he conducted
• Appropriate surface preparation for the environment, research, evaluation, and testing, and served as the
service, and coating system, as recommended by the Navy’s center of expertise on coatings for shore
coating manufacturer structures. He is a professional engineer in the state of
• Appropriate application of the coating system, as California, an SSPC certified protective coatings
recommended by its manufacturer specialist (PCS), and a NACE International certificated
• Thorough inspection of all phases of the work to corrosion specialist. Dr. Drisko received his BS, MS,
ensure that all specification requirements are met and PhD degrees from Stanford.
• Rapid corrective actions to address any deviations
from recommendations or early signs of coating
defects.
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