SO x gas is the most aggressivep ollutant for anodic films. The corrosive
effect is dependentonthe concentration, with the corrosion area increasing
linearlyw ith concentration.
7.13 OrganicC oatings
Organic coatings are widely used to protect metal surfaces from corrosion.
The effectiveness of suchcoatings is dependent not onlyonthe properties of
the coatings, that are related to the polymeric networkand possibleflaws in
this network,b ut also on the character of the metal substrate, the surface
pretreatment, and the applicationprocedures. Therefore, when considering
the applicationofacoating, it is necessary to take into account the properties
of the entire system.
There are threeb road classes of polymeric coatings: lacquers, varnishes,
and paints. Varnishes are materials that are solutions of either aresin alone
in asolvent (spirit varnishes) or an oil and resin together in asolvent (oleoresinous varnishes). Alacquer is generally considered to be amaterial whose
basic film former is nitrocellulose, cellulose acetate–butyrate, ethyl cellulose,
acrylic resin, or another resin that dries by solvent evaporation. The term
paint is applied to more complex formulationsofaliquid mixture that dries
or hardens to form ap rotective coating.
Organic coatings provide protection either by the formation of ab arrier
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 organic coatings are permeable to water and oxygent os ome
extent.T he average transmission rate of water through ac oating is about
10–100 times larger than the water consumption rate of af reely flowing
surface, andi nn ormal outdoor conditions, an organic coating is saturated
with water at least half of its service life. For the remainder of the time,
it containsaquantity of water comparable in its behaviortoanatmosphere
of high humidity.T able 7.13 shows the diffusion data for water through
organic films.
It has also been determined that, in mostc ases, the diffusion of oxygen
through the coating is large enough to allow unlimited corrosion. Taking
these factors into account indicates that the physical barrier properties alone
do not account for the protective action of coatings. Table 7.14 shows the flux
of oxygenthrough representative free films of paint100 m mthick.
Additional protection may be supplied by resistance inhibition that is
also ap art of the barrier mechanism. Retardation of the corrosion action
is accomplished by inhibiting the charge transport betweenc athodic and
anodicsites. The reaction rate may be reduced by an increase in the electrical
resistance and/ort he ionic resistance in the corrosion cycle. Applying
an organicc oating on am etal surface increasest he ionic resistance.
Coatings
459
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485
effect is dependentonthe concentration, with the corrosion area increasing
linearlyw ith concentration.
7.13 OrganicC oatings
Organic coatings are widely used to protect metal surfaces from corrosion.
The effectiveness of suchcoatings is dependent not onlyonthe properties of
the coatings, that are related to the polymeric networkand possibleflaws in
this network,b ut also on the character of the metal substrate, the surface
pretreatment, and the applicationprocedures. Therefore, when considering
the applicationofacoating, it is necessary to take into account the properties
of the entire system.
There are threeb road classes of polymeric coatings: lacquers, varnishes,
and paints. Varnishes are materials that are solutions of either aresin alone
in asolvent (spirit varnishes) or an oil and resin together in asolvent (oleoresinous varnishes). Alacquer is generally considered to be amaterial whose
basic film former is nitrocellulose, cellulose acetate–butyrate, ethyl cellulose,
acrylic resin, or another resin that dries by solvent evaporation. The term
paint is applied to more complex formulationsofaliquid mixture that dries
or hardens to form ap rotective coating.
Organic coatings provide protection either by the formation of ab arrier
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 organic coatings are permeable to water and oxygent os ome
extent.T he average transmission rate of water through ac oating is about
10–100 times larger than the water consumption rate of af reely flowing
surface, andi nn ormal outdoor conditions, an organic coating is saturated
with water at least half of its service life. For the remainder of the time,
it containsaquantity of water comparable in its behaviortoanatmosphere
of high humidity.T able 7.13 shows the diffusion data for water through
organic films.
It has also been determined that, in mostc ases, the diffusion of oxygen
through the coating is large enough to allow unlimited corrosion. Taking
these factors into account indicates that the physical barrier properties alone
do not account for the protective action of coatings. Table 7.14 shows the flux
of oxygenthrough representative free films of paint100 m mthick.
Additional protection may be supplied by resistance inhibition that is
also ap art of the barrier mechanism. Retardation of the corrosion action
is accomplished by inhibiting the charge transport betweenc athodic and
anodicsites. The reaction rate may be reduced by an increase in the electrical
resistance and/ort he ionic resistance in the corrosion cycle. Applying
an organicc oating on am etal surface increasest he ionic resistance.
Coatings
459
CAT8247—CHAPTER 7—17/10/2006—15:03—TRGANESH—XMLM ODEL B–pp. 403–485