Corrosion of Paint
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the coating is sealed by corrosion products, after which corrosion propagation takes place according to the same mechanism as for the initially damaged coating. See Figure 7.3 for the sequence of events.
7.4.5 anodic undermining
Anodic undermining results 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 cause
anodic undermining, but in most cases it is associated with a corrosion-sensitive site under the coating, such as a particle from a cleaning or a blasting
procedure, or a site on the metal surface with potentially increased corrosion activity (e.g., scratches). These sites become active once the corrodent has
penetrated to the metal surface. The initial corrosion rate is low. However, an
osmotic pressure is caused by the soluble corrosion products that stimulate
blister growth. Once formed, the blisters will grow due to a type of anodic
corrosion at the edge of the blister.
Coated aluminum is very sensitive to anodic undermining, while steel is
more sensitive to cathodic delamination.
7.4.6 Filiform Corrosion
Metals with semipermeable coatings or films may undergo a type of corrosion resulting in numerous thread-like filaments of corrosion beneath the
coatings or films. Conditions that promote this type of corrosion include:
1. High relative humidity (60 to 95% at room temperature)
2. Coating is permeable to water
3. Contaminants (salts, etc.) are present on or in the coating, or at the
coating/substrate interface
4. Coating has defects (e.g., mechanical damage, pores, insufficient
coverage of localized areas, air bubbles)
Filiform corrosion under organic coatings is common on steel, aluminum,
magnesium, and zinc (galvanized steel). It has also been observed under
electroplated silver plate, gold plate, and phosphate coatings.
This form of corrosion is more prevalent under organic coatings on aluminum
than on other metallic surfaces, it being a special form of anodic undermining.
A differential aeration cell is the driving force. The filaments have considerable
length but little width and depth, and consist of two parts: a head and a tail. The
primary corrosion reactions, and subsequently the delamination process of the
paint film, take place in the active head, while the tail is filled with the resulting
corrosion products. As the head of the filiform moves, the tail grows in length.
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the coating is sealed by corrosion products, after which corrosion propagation takes place according to the same mechanism as for the initially damaged coating. See Figure 7.3 for the sequence of events.
7.4.5 anodic undermining
Anodic undermining results 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 cause
anodic undermining, but in most cases it is associated with a corrosion-sensitive site under the coating, such as a particle from a cleaning or a blasting
procedure, or a site on the metal surface with potentially increased corrosion activity (e.g., scratches). These sites become active once the corrodent has
penetrated to the metal surface. The initial corrosion rate is low. However, an
osmotic pressure is caused by the soluble corrosion products that stimulate
blister growth. Once formed, the blisters will grow due to a type of anodic
corrosion at the edge of the blister.
Coated aluminum is very sensitive to anodic undermining, while steel is
more sensitive to cathodic delamination.
7.4.6 Filiform Corrosion
Metals with semipermeable coatings or films may undergo a type of corrosion resulting in numerous thread-like filaments of corrosion beneath the
coatings or films. Conditions that promote this type of corrosion include:
1. High relative humidity (60 to 95% at room temperature)
2. Coating is permeable to water
3. Contaminants (salts, etc.) are present on or in the coating, or at the
coating/substrate interface
4. Coating has defects (e.g., mechanical damage, pores, insufficient
coverage of localized areas, air bubbles)
Filiform corrosion under organic coatings is common on steel, aluminum,
magnesium, and zinc (galvanized steel). It has also been observed under
electroplated silver plate, gold plate, and phosphate coatings.
This form of corrosion is more prevalent under organic coatings on aluminum
than on other metallic surfaces, it being a special form of anodic undermining.
A differential aeration cell is the driving force. The filaments have considerable
length but little width and depth, and consist of two parts: a head and a tail. The
primary corrosion reactions, and subsequently the delamination process of the
paint film, take place in the active head, while the tail is filled with the resulting
corrosion products. As the head of the filiform moves, the tail grows in length.