Corrosion of Paint
207
7.3.1 Surface Preparation and application
Coating failures typical of surface preparation and application problems
include:
1. Cracking, chalking, alligatoring. These types of failure develop with
the aging of the paint film. Shrinkage within the film during aging
causes cracking and checking. Alligatoring is a film rupture, usually caused by application of a hard, brittle film over a more flexible
film.
2. Peeling, flaking, delamination. These failures are caused by poor adhesion. When peeling or flaking occurs between coats, it is called
delamination.
3. Rusting. Failure of a coated surface may appear as (a) spot rusting in
minute areas, (b) pinhole rusting in minute areas, (c) rust nodules
breaking through the coating, or (d) underfilm rusting, which eventually causes peeling and flaking of the coating.
4. Lifting and wrinkling. When the solvent of a succeeding coat of paint
too rapidly softens the previous coat, lifting results. Rapid surface
drying of a coating without uniform drying throughout the rest of
the film results in a phenomenon known as wrinkling.
5. Failures around weld areas. Coating adhesion can be hampered by weld
flux, which can also accelerate corrosion under the film. Relatively
large projections of weld spatter cause possible gaps and cavities
that may not be coated sufficiently to provide protection.
6. Edge failures. Edge failures usually take the form of rusting through
the film at the edge where the coating is usually the thinnest. This is
usually followed by eventual rust seepage under the film.
7. Pinholing. These are tiny holes that expose the substrate, and are
caused by improper spray atomization or segration of resin in the
coating. If practical during application, brush out the coating. After
application and proper care, apply additional coating.
7.3.2 atmospheric Effects
Polymeric coatings are exposed to environmental constituents. The primary
factors promoting degradation are thermal, mechanical, radiant, and chemical in nature. Polymers can also be degraded by living organisms such as mildew. Any atmospheric environment is subject to dry and wet cycles. Because
water and moisture have a decided effect on the degradation of a coating,
the duration of wetness of a coating is important. Moisture and water that
attack organic films derive from rain, fog, dew, snow, and water vapors in the
atmosphere. Relative humidity is a particularly important factor. As expo-
207
7.3.1 Surface Preparation and application
Coating failures typical of surface preparation and application problems
include:
1. Cracking, chalking, alligatoring. These types of failure develop with
the aging of the paint film. Shrinkage within the film during aging
causes cracking and checking. Alligatoring is a film rupture, usually caused by application of a hard, brittle film over a more flexible
film.
2. Peeling, flaking, delamination. These failures are caused by poor adhesion. When peeling or flaking occurs between coats, it is called
delamination.
3. Rusting. Failure of a coated surface may appear as (a) spot rusting in
minute areas, (b) pinhole rusting in minute areas, (c) rust nodules
breaking through the coating, or (d) underfilm rusting, which eventually causes peeling and flaking of the coating.
4. Lifting and wrinkling. When the solvent of a succeeding coat of paint
too rapidly softens the previous coat, lifting results. Rapid surface
drying of a coating without uniform drying throughout the rest of
the film results in a phenomenon known as wrinkling.
5. Failures around weld areas. Coating adhesion can be hampered by weld
flux, which can also accelerate corrosion under the film. Relatively
large projections of weld spatter cause possible gaps and cavities
that may not be coated sufficiently to provide protection.
6. Edge failures. Edge failures usually take the form of rusting through
the film at the edge where the coating is usually the thinnest. This is
usually followed by eventual rust seepage under the film.
7. Pinholing. These are tiny holes that expose the substrate, and are
caused by improper spray atomization or segration of resin in the
coating. If practical during application, brush out the coating. After
application and proper care, apply additional coating.
7.3.2 atmospheric Effects
Polymeric coatings are exposed to environmental constituents. The primary
factors promoting degradation are thermal, mechanical, radiant, and chemical in nature. Polymers can also be degraded by living organisms such as mildew. Any atmospheric environment is subject to dry and wet cycles. Because
water and moisture have a decided effect on the degradation of a coating,
the duration of wetness of a coating is important. Moisture and water that
attack organic films derive from rain, fog, dew, snow, and water vapors in the
atmosphere. Relative humidity is a particularly important factor. As expo-