Page 174 - CW E-Magazine (24-6-2025)

Special Report

coating is applied over mill scale, including improper mixing of compo-
which in the presence of water and nents (especially with oil-based coat-
oxygen reacts to cause a loss of adhe- ings or alkyds that contain driers), not
sion and progressive corrosion. Less allowing sufficient time for the un-
frequently, it may be caused by an dercoat to dry before applying a sub-
incompatibility between the coating sequent coat or top-coat, applying the
and the substrate. coating with a too-high film thickness
(this is especially common with alkyd
Basically, it takes place due to the coatings), or applying the coating in
application of paint to corroded sub- hot ambient temperature or high humi- Fig. 6: Abrasion
strate, presence of rust creep from areas dity conditions. Baking the object at removal to expose the substrate by con-
of mechanical damage. Missing primer too high a temperature will cause the tact with another object, such as the
coat can also cause this defect. It is gene- coating’s surface to cure faster than the use of metal chains for lifting, or the
rally found in areas of poor design or body of the coating and lead to wrink- grounding of a ship (Figure-6).
access and where inadequate prepara- ling. Wrinkling may also be caused
tion and coating thickness was applied. Prevention methods
It can also be caused due to lack of Use of abrasion-resistant coatings
maintenance. formulated with particular regard to
resins and extender pigments will help
Prevention methods prevent abrasion of the coating. With
Ideally, improve the asset’s design severe cases of abrasion, the effects
or access. Perform proper maintenance will be reduced by an abrasion-resistant
procedures throughout the service life coating.
of the asset to avoid corrosion in loca-
tions that will be difficult to rectify later. Bleeding
Perform proper surface preparation Fig. 5: Wrinkling Bleed through is generally a full
(e.g., abrasive blasting or water jetting by applying the coating over wax or or partial re-dissolving of the previous
and cleaning) before applying the coat- grease. Figure-5 indicates wrinkling coat or an ingredient of a previous coat
ing. The substrate’s surface must be coating defect appearing as ridges and and can occur when strong solvents are
free of old corrosion products. Consider furrows. used in the topcoats. Staining of a paint
inorganic zinc or other suitable primer. film takes place by diffusion of a solu-
Use adequate coating specifications Prevention methods ble coloured substance from the under-
and maintenance procedures. Apply a Use a wax and grease remover to lying paint giving undesirable discolo-
suitably formulated primer. Finally, wash away contaminants before coat-
select a coating with very good adhesion ing. Select an appropriate coating sys-
characteristics. tem and follow the manufacturer’s
recommendations regarding environ-
Wrinkling mental limitations and compatibility of
Wrinkling appears as wrinkles in the primers and paints. Follow any in-
the coating film while it is drying. The structions regarding formulating/mix-
coating’s surface may look like a piece ing of the components. Apply the coat-
of cloth or plastic film with furrows ing thinly and avoid thicker sections or
and ridges, or like a diamond-patterned puddles. Use several coats if necessary.
steel floor (although not in a regular Use correct coating specification and Fig. 7: Bleeding
pattern). materials, and ensure adequate mixing, ration. Often seen where bituminous
application, and curing by following or tar-based products are overcoated
Wrinkling is initiated when a skin the paint supplier’s recommendations. with coats in which the tar or bitumen
quickly forms on the coating’s surface is soluble. This defect also occurs with
while the body of the coating that is Abrasion emulsion paints. Figure-7 shows stain-
closer to the substrate remains liquid. Removal of a portion of the surface ing of paint film by diffusion of soluble
It is caused by many possible causes, of the coating or, in severe cases, coloured substance from the underlying

174 Chemical Weekly June 24, 2025

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