Corrosion of Paint
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Water and gases pass through the film and dissolve some ionic material
from either the film of the substrate, causing an osmotic pressure greater
than that of the external face of the coating. This produces a solution
concentration gradient, with water building up at these sites until the
film eventually blisters. Visual blistering standards are found in ASTM
D–714–56.
Electrochemical reactions also assist in the formation of these blisters.
Water diffuses through a coating also by an electro-osmotic gradient. Once
corrosion starts, moisture is pulled through the coating by an electrical
potential gradient between the corroding areas and the protected areas that
are in electrical contact. Therefore, osmosis starts the blistering, and once corrosion begins, electro-endosmotic reactions accelerate the corrosion process.
The addition of heat and acidic chemicals increases the rate of breakdown.
Temperatures of 150 to 200°F (66 to 93°C) accelerate the chemical reaction.
Under these conditions, steel will literally dissolve in a chemical environment. Moisture is always present and afterward condenses on the surface
behind the blister. This condensation offers a solute for gaseous permeants
to dissolve. When the environment is acidic, the pH of the water behind the
blister can be as low as 1.0 or 2.0, thus subjecting the steel to severe attack.
7.5.2 Second Stage of Corrosion
After observing the initial one or two rust spots, or after having found a few
blisters, a general rusting in the form of multiple rust develops. This rusting
is predominantly Fe 2 O 3 , a red rust. In atmospheres lacking oxygen, such as
in sulfur dioxide scrubbers, a black FeO develops. Once the unit has been
shut down and more oxygen becomes available, the FeO will eventually convert to Fe 2 O 3 .
7.5.3 Third Stage of Corrosion
This advanced stage of corrosion is the total disbondment of the coating from
the substrate, exposing the substrate directly to the corrodents. Corrosion
can occur at an uninhibited rate because the coating is no longer protecting
the steel.
7.5.4 Fourth Stage of Corrosion
Attack of the metal substrate after removal of the coating is not usually of a
uniform nature but rather that of a localized attack, resulting in pitting.
7.5.5 Fifth Stage of Corrosion
Deep pits formed in the substrate during the fourth stage of attack can eventually permeate completely to cause holes. Within the corrosion cell, pitting
219
Water and gases pass through the film and dissolve some ionic material
from either the film of the substrate, causing an osmotic pressure greater
than that of the external face of the coating. This produces a solution
concentration gradient, with water building up at these sites until the
film eventually blisters. Visual blistering standards are found in ASTM
D–714–56.
Electrochemical reactions also assist in the formation of these blisters.
Water diffuses through a coating also by an electro-osmotic gradient. Once
corrosion starts, moisture is pulled through the coating by an electrical
potential gradient between the corroding areas and the protected areas that
are in electrical contact. Therefore, osmosis starts the blistering, and once corrosion begins, electro-endosmotic reactions accelerate the corrosion process.
The addition of heat and acidic chemicals increases the rate of breakdown.
Temperatures of 150 to 200°F (66 to 93°C) accelerate the chemical reaction.
Under these conditions, steel will literally dissolve in a chemical environment. Moisture is always present and afterward condenses on the surface
behind the blister. This condensation offers a solute for gaseous permeants
to dissolve. When the environment is acidic, the pH of the water behind the
blister can be as low as 1.0 or 2.0, thus subjecting the steel to severe attack.
7.5.2 Second Stage of Corrosion
After observing the initial one or two rust spots, or after having found a few
blisters, a general rusting in the form of multiple rust develops. This rusting
is predominantly Fe 2 O 3 , a red rust. In atmospheres lacking oxygen, such as
in sulfur dioxide scrubbers, a black FeO develops. Once the unit has been
shut down and more oxygen becomes available, the FeO will eventually convert to Fe 2 O 3 .
7.5.3 Third Stage of Corrosion
This advanced stage of corrosion is the total disbondment of the coating from
the substrate, exposing the substrate directly to the corrodents. Corrosion
can occur at an uninhibited rate because the coating is no longer protecting
the steel.
7.5.4 Fourth Stage of Corrosion
Attack of the metal substrate after removal of the coating is not usually of a
uniform nature but rather that of a localized attack, resulting in pitting.
7.5.5 Fifth Stage of Corrosion
Deep pits formed in the substrate during the fourth stage of attack can eventually permeate completely to cause holes. Within the corrosion cell, pitting