Page 176 - CW E-Magazine (24-6-2025)

Special Report

cobwebbing
Too high a viscosity with some
types of polymer solutions is the main
cause of cob-webbing. It will occur
with chlorinated rubber at almost any
viscosity. Figure-16 illustrates cob-
webbing where fine filaments instead of

Fig. 12: cathodic disbonding Fig. 14: checking
nation of a coating system around bare ing, stresses are developed that cause
steel areas & coating defects associated the surface of the paint film to become
with CP on buried pipelines, immersed brittle and crack. Checking produces
structures, and the hull of ships. fine cracks that do not penetrate the top-
coat of a paint system, and some cracks
Prevention methods can be so minute that it is impossible to
Use a well-designed CP system and see without magnification as illustrated
regularly monitor it with well-placed in Figure-14. Fig. 16: cobwebbing
reference electrodes; and apply an alkali- normal atomized particles are produced
resistant coating system. Prevention methods when some solutions of high molecular
Use a correctly formulated coating weight polymers are sprayed.
chalking system.
Chalking in coating takes place due Prevention methods
to disintegration of the paint binder on cissing Reduce the spraying viscosity.
exposure to weathering and/or ultra- Cissing takes place when surface Select a more suitable solvent blend.
violet light. Figure-13 shows a powdery is contaminated by either moisture Change the spraying conditions.
layer on the surface of a paint film. or foreign matter such as oil, grease,
or silicone, or when incorrect solvent cracking
blends have been used. Cissing of Cracking is generally a stress-re-
coating causes surface breaks in a wet lated failure and can be attributed to
paint film where the paint has receded surface movement, aging, absorption
to expose the underlying substrate as

Fig. 13: chalking
A change of colour or fading is also seen.
Chalking rates vary with pigment
concentration and choice of binder. It is Fig. 17: cracking
common with epoxy paints.
Fig. 15: cissing
and desorption of moisture, and general
Prevention methods depicted in Figure-15. In this coating lack of flexibility of the coating. The
Apply a suitable topcoat with high failure the paint is unable to wet-out the thicker the paint film, the greater is the
resistance to chalking and with high substrate. possibility that it will crack. Figure-17
ultraviolet resistance. shows the splitting of a dry paint film
Prevention methods through at least one coat to form visible
checking Ensure surface is clean and free cracks, which may penetrate down to
It is typically a formulation and/or from grease, oil, and other contami- the substrate. The cracks may be
a specification problem. As with crack- nants prior to application of coating. minute to severe.

176 Chemical Weekly June 24, 2025

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