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Fundamentals of Corrosion
7.3.3 Types of Failures
Factors in the atmosphere that cause corrosion or degradation of the coating include UV light, temperature, oxygen, ozone, pollutants, and wind. The
types of failures resulting from these causes include:
1. Chalking. UV light, oxygen, and chemicals degrade the coating,
resulting in chalk. This can be corrected by providing an additional
topcoat with the proper UV inhibitor.
2. Color fading or color change. This can be caused by chalk on the surface
or by breakdown of the colored pigments. Pigments can be decomposed or degraded by UV light or reaction with chemicals.
3. Blistering. Blistering may be caused by:
a. Inadequate release of solvent during both application and drying of the coating system
b. Moisture vapor that passes through the film and condenses at a
point of low paint adhesion
c. Poor surface preparation
d. Poor adhesion of the coating to the substrate or poor intercoat
adhesion
e. A coat within the paint system that is not resistant to the
environment
f. Application of a relatively fast drying coating over a relatively
porous surface
g. Failure due to chemical or solvent attack (when a coating is not
resistant to its chemical or solvent environment, there is apparent disintegration of the film)
4. Erosion (coating worn away). Loss of coating due to inadequate erosion
protection
7.3.3.1 Strength of Paint Film
Paint films require hardness, flexibility, brittleness resistance, mar resistance, and sag resistance. Paint coatings are formulated to provide a balance
among these mechanical properties. The mechanical strength of a paint film
is described by the words “hardness” and “plasticity,” which correspond to
the modulus of elasticity and to the elongation at break obtained from the
stress–strain curve of a paint film. Typical paint films have tensile properties as shown in Table 7.7. The mechanical properties of paint coatings vary,
depending on the type of pigment, baking temperatures, and aging times.
As baking temperatures rise, the curing of paint films is promoted and elongation is reduced. Tensile strength is improved by curing and the elongation
at breaks is reduced with increased drying time.
Fundamentals of Corrosion
7.3.3 Types of Failures
Factors in the atmosphere that cause corrosion or degradation of the coating include UV light, temperature, oxygen, ozone, pollutants, and wind. The
types of failures resulting from these causes include:
1. Chalking. UV light, oxygen, and chemicals degrade the coating,
resulting in chalk. This can be corrected by providing an additional
topcoat with the proper UV inhibitor.
2. Color fading or color change. This can be caused by chalk on the surface
or by breakdown of the colored pigments. Pigments can be decomposed or degraded by UV light or reaction with chemicals.
3. Blistering. Blistering may be caused by:
a. Inadequate release of solvent during both application and drying of the coating system
b. Moisture vapor that passes through the film and condenses at a
point of low paint adhesion
c. Poor surface preparation
d. Poor adhesion of the coating to the substrate or poor intercoat
adhesion
e. A coat within the paint system that is not resistant to the
environment
f. Application of a relatively fast drying coating over a relatively
porous surface
g. Failure due to chemical or solvent attack (when a coating is not
resistant to its chemical or solvent environment, there is apparent disintegration of the film)
4. Erosion (coating worn away). Loss of coating due to inadequate erosion
protection
7.3.3.1 Strength of Paint Film
Paint films require hardness, flexibility, brittleness resistance, mar resistance, and sag resistance. Paint coatings are formulated to provide a balance
among these mechanical properties. The mechanical strength of a paint film
is described by the words “hardness” and “plasticity,” which correspond to
the modulus of elasticity and to the elongation at break obtained from the
stress–strain curve of a paint film. Typical paint films have tensile properties as shown in Table 7.7. The mechanical properties of paint coatings vary,
depending on the type of pigment, baking temperatures, and aging times.
As baking temperatures rise, the curing of paint films is promoted and elongation is reduced. Tensile strength is improved by curing and the elongation
at breaks is reduced with increased drying time.