Page 172 - CW E-Magazine (24-6-2025)

Special Report

the reason for the failure of a coating able causes, and suggested preventative Prevention methods
due to the many potential factors that measures, are discussed below(6): Ensure that a good anchor pattern
may be involved. These could include exists on the substrate, and if needed
formulation, surface preparation, appli- Adhesion failure increase the surface profile by sanding
cation, drying & curing times and con- An adhesion failure appears as a or abrasive blasting. Next, the surface
ditions, and environmental exposure, paint that is flaking off because it failed to be coated must be properly prepared
with more than one contributing factor to stick to either the substrate or a prior so that it is free of moisture, dust, dirt,
often being involved(3). coating (if there was one). It can be old coatings or any other contaminants
caused by surface contamination, con- such as grease, wax or oil.
Failures and defects can manifest densation on the substrate, applying
themselves at various times in the two different coating products that are Follow the manufacturer’s guide-
life of a coating. Prior to application, incompatible or by exceeding the over lines and apply secondary coatings
they can take the form of settlement coating time. There are two types of only during the specified time window.
and skinning, during application as adhesion failure: inter-coat adhesion Allow the coating to properly cure
runs and sags, shortly after application failure and substrate adhesion failure. according to the manufacturer’s recom-
as solvent popping and orange peel, Figure-1(a) depicts paint failing to ad- mendations.
and during service as blistering and here to substrate or underlying coats of
rust spotting. To determine the cause Alligatoring (crocodiling)
and mechanism of coating failure, all Alligatoring appears as large cracks
possible contributory factors must be or crazing that looks like the scales on
evaluated together with a detailed his- an alligator or crocodile. The cracks
tory from the time of application to the usually do not penetrate down to the
time the failure was first noted. Many substrate but in worst cases may. This
coating failures require further evalu- defect occurs when the upper surface
ation and analysis to be carried out by of the coating shrinks faster than the
a qualified chemist, engineer, or coat- portion of the coating closer to the sub-
ing specialist, often using specialised Fig. 1(a): Adhesion failure strate. The unequal shrinking creates
laboratory equipment. stresses that pull on the surface diffe-
rently than within the body of the coat-
Adherence to the coating specs ing. (Figure-2(a) Figure-2(b)).
takes a lot of guesswork out of a coating
exercise or at least can help the applica-
tor understand what went wrong when
he finds himself with a coating failure.
The sheer numbers of things that can go
wrong when applying a protective coat-
ing to a valuable asset are seemingly
endless. Some problems may be due to Fig. 1(b): Intercoat adhesion Failure
poor technique or selecting an inappro- paint and Figure-1(b) shows an inter-
priate product. Others can be avoided coat adhesion failure.
by adhering to the coating specifica- Fig. 2(a): Alligatoring coating defect
tions provided by the manufacturer or Other coating failures are sometimes
specifications writer. Although there mistakenly called adhesion failures, but
is some leeway for site-specific varia- strictly speaking these are not a proper
tions, it only makes sense to faithfully use of the term. An adhesion failure
follow the coating specs considering occurs when the chemical bond is broken
the amount of effort that went into between the coating and the substrate at
writing them in the first place(4, 5). the bonding interface. To be a true adhe-
sion failure, the bond must not be broken
The examples of coating failures and due to other conditions such as abrasion,
defects, together with descriptions, prob- impact or underlying corrosion. Fig. 2(b): Alligatoring

172 Chemical Weekly June 24, 2025

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