198
Fundamentals of Corrosion
considerations (such as thickness variances for application condition, substrate condition), and finally scheduled inspection and maintenance.
Organic coatings can protect metal structures against a specific or otherwise corrosive environment in a relatively economical way. The degree of
protection depends on a number of properties of the total coated system,
which consists of paint film, metal substrate, and its pretreatment.
Paints and coatings are based on naturally occurring compounds, synthetic materials, or a mixture of both. The natural type systems are based on
asphaltic, bituminous materials or on natural oils, such as those produced
from rice, fish, etc. The latter group is composed of the original “oleo-resinous” paints although the term has a much broader meaning today. The older
systems are much more tolerant of poor surface preparation and contamination than the more modern synthetic paints.
Painting systems are classified according to the generic type of binder or
resin, and are grouped according to the curing or hardening mechanism
inherent in that generic type. Although the resin or organic binder of the
coating material has the predominant effect on the resistances and properties of the paint, the type and quantity of pigments, solvents, and additives
have an influence on the application properties and protective ability of the
applied film. In addition, systems can be formulated that are crosses between
categories. For example, the acrylic monomer or prepolymer can be incorporated with practically any other generic resin to produce a product having
properties that are a compromise between the acrylic and original polymer.
Modern synthetic coatings are based on a variety of chemistries. They usually require more sophisticated surface preparation and application than the
natural type systems.
Today’s paints and coatings must be in compliance with volatile organic
compound (VOC) restrictions and U.S. OSHA regulations. Some states and
local municipalities have imposed even stricter limits.
7.1 Mechanisms of Protection
Organic coatings provide protection either by formation of a barrier action
from the layer or from active corrosion inhibition provided by pigments in
the coating. In actual practice, the barrier properties are limited because all
coatings are permeable to water and oxygen to some extent. The average
transmission rate of water through a coating is about 10 to 100 times larger
than the water consumption rate of a freely flowing surface, and in normal
outdoor conditions, an organic coating is saturated with water at least half of
its service life. For the remainder of the time, it contains a quantity of water
comparable to its behavior in an atmosphere of high humidity. Table 7.1
shows the diffusion data for water through organic films.
Fundamentals of Corrosion
considerations (such as thickness variances for application condition, substrate condition), and finally scheduled inspection and maintenance.
Organic coatings can protect metal structures against a specific or otherwise corrosive environment in a relatively economical way. The degree of
protection depends on a number of properties of the total coated system,
which consists of paint film, metal substrate, and its pretreatment.
Paints and coatings are based on naturally occurring compounds, synthetic materials, or a mixture of both. The natural type systems are based on
asphaltic, bituminous materials or on natural oils, such as those produced
from rice, fish, etc. The latter group is composed of the original “oleo-resinous” paints although the term has a much broader meaning today. The older
systems are much more tolerant of poor surface preparation and contamination than the more modern synthetic paints.
Painting systems are classified according to the generic type of binder or
resin, and are grouped according to the curing or hardening mechanism
inherent in that generic type. Although the resin or organic binder of the
coating material has the predominant effect on the resistances and properties of the paint, the type and quantity of pigments, solvents, and additives
have an influence on the application properties and protective ability of the
applied film. In addition, systems can be formulated that are crosses between
categories. For example, the acrylic monomer or prepolymer can be incorporated with practically any other generic resin to produce a product having
properties that are a compromise between the acrylic and original polymer.
Modern synthetic coatings are based on a variety of chemistries. They usually require more sophisticated surface preparation and application than the
natural type systems.
Today’s paints and coatings must be in compliance with volatile organic
compound (VOC) restrictions and U.S. OSHA regulations. Some states and
local municipalities have imposed even stricter limits.
7.1 Mechanisms of Protection
Organic coatings provide protection either by formation of a barrier action
from the layer or from active corrosion inhibition provided by pigments in
the coating. In actual practice, the barrier properties are limited because all
coatings are permeable to water and oxygen to some extent. The average
transmission rate of water through a coating is about 10 to 100 times larger
than the water consumption rate of a freely flowing surface, and in normal
outdoor conditions, an organic coating is saturated with water at least half of
its service life. For the remainder of the time, it contains a quantity of water
comparable to its behavior in an atmosphere of high humidity. Table 7.1
shows the diffusion data for water through organic films.