Waterm olecules will eventually reach the coating substrate interface
when acoatedsystem has been exposed to an aqueous solution or ahumid
atmosphere. These moleculesc an interfere with the bonding between the
coating and substrate, eventually resulting in the loss of adhesion and
corrosion initiation if acathodic reaction can take place. Aconstantsupplyof
ac athodic species, such as water or oxygen, is required for the corrosion
reaction to proceed.
Waterpermeation may also resultinthe buildupofhigh osmotic pressure
that are responsible for blisteringa nd delamination.
7.13.7.1 Cathodic Delamination
When cathodic protection is applied to ac oated metal, loss of adhesion
between the substrate and paint film, adjacent to defects, often takes place.
This loss of adhesion is known as cathodic delamination. Such delamination
may also occur in the absence of applied potential. Separation of anodic and
cathodic reaction sites undert he coating results in the same local driving
force as during external polarization. The propagation of ablister because of
cathodic delamination undera nu ndamaged coating on as teel substrate is
schematically illustrated in Figure 7.25.U nder an intact coating, corrosion
may be initiated locallyats ites of pooradhesion.
As imilar situation develops in the caseo fc orrosionu nder ad efective
coating. Whent here is as malld efect in the coating, parto ft he substrate is
directly exposed to the corrosive environment. Corrosion products are
formed immediately that block the damaged site from oxygen. The defect in
the coating is sealedb yt he corrosionp roducts, after which corrosion
propagation takes place, according to the same mechanism as for the initially
undamaged coating. Refer to Figure 7.25 for the sequence of events.
7.13.7.2 AnodicUndermining
Anodic underminingr esults from the loss of adhesion caused by anodic
dissolution of the substrate metal or its oxide. In contrast to cathodic
delamination, the metal is anodic at the blister edges. Coating defects may
causea nodicu ndermining, but in mostc ases it is associated with a
corrosion sensitives ite undert he coating, such as ap article from a
cleaning or blasting procedure, or as ite on the metal surface with
potentially increased corrosion activity (scratches). These sites become
active once the corrodent has penetrated to the metal surface. The initial
corrosion rate is low.H owever, an osmotic pressure is caused by the
soluble corrosion products which stimulates blister growth. Once formed,
the blister will grow because of at ype of anodic crevice corrosion at the
edge of the blister.
Coated aluminumisverysensitive to anodic undermining, whereas, steel
is more sensitivet oc athodic delamination.
Corrosion of Linings and Coatings
480
CAT8247—CHAPTER 7—17/10/2006—15:04—TRGANESH—XMLM ODEL B–pp. 403–485
when acoatedsystem has been exposed to an aqueous solution or ahumid
atmosphere. These moleculesc an interfere with the bonding between the
coating and substrate, eventually resulting in the loss of adhesion and
corrosion initiation if acathodic reaction can take place. Aconstantsupplyof
ac athodic species, such as water or oxygen, is required for the corrosion
reaction to proceed.
Waterpermeation may also resultinthe buildupofhigh osmotic pressure
that are responsible for blisteringa nd delamination.
7.13.7.1 Cathodic Delamination
When cathodic protection is applied to ac oated metal, loss of adhesion
between the substrate and paint film, adjacent to defects, often takes place.
This loss of adhesion is known as cathodic delamination. Such delamination
may also occur in the absence of applied potential. Separation of anodic and
cathodic reaction sites undert he coating results in the same local driving
force as during external polarization. The propagation of ablister because of
cathodic delamination undera nu ndamaged coating on as teel substrate is
schematically illustrated in Figure 7.25.U nder an intact coating, corrosion
may be initiated locallyats ites of pooradhesion.
As imilar situation develops in the caseo fc orrosionu nder ad efective
coating. Whent here is as malld efect in the coating, parto ft he substrate is
directly exposed to the corrosive environment. Corrosion products are
formed immediately that block the damaged site from oxygen. The defect in
the coating is sealedb yt he corrosionp roducts, after which corrosion
propagation takes place, according to the same mechanism as for the initially
undamaged coating. Refer to Figure 7.25 for the sequence of events.
7.13.7.2 AnodicUndermining
Anodic underminingr esults from the loss of adhesion caused by anodic
dissolution of the substrate metal or its oxide. In contrast to cathodic
delamination, the metal is anodic at the blister edges. Coating defects may
causea nodicu ndermining, but in mostc ases it is associated with a
corrosion sensitives ite undert he coating, such as ap article from a
cleaning or blasting procedure, or as ite on the metal surface with
potentially increased corrosion activity (scratches). These sites become
active once the corrodent has penetrated to the metal surface. The initial
corrosion rate is low.H owever, an osmotic pressure is caused by the
soluble corrosion products which stimulates blister growth. Once formed,
the blister will grow because of at ype of anodic crevice corrosion at the
edge of the blister.
Coated aluminumisverysensitive to anodic undermining, whereas, steel
is more sensitivet oc athodic delamination.
Corrosion of Linings and Coatings
480
CAT8247—CHAPTER 7—17/10/2006—15:04—TRGANESH—XMLM ODEL B–pp. 403–485