4. The coating has defects (mechanical damage,p ores, insufficient
coverage of localized areas, air bubbles).
Filiform corrosionunderorganic coatings is commononsteel, aluminum,
magnesium, and zinc (galvanized steel). It has also been observed under
electroplated silver plate, gold plate, and underphosphate coatings.
This form of corrosion is more prevalent undero rganic coatings on
aluminum than on other metallic surfaces, being as pecial form of anodic
undermining.
Ad ifferential aeration cell is the basic driving force. The filaments have
considerablel ength but little width and depth and consisto ft wo parts: a
head and at ail. The primary corrosion reactions and, subsequently,t he
delaminationp rocess of the paintfi lm takesp lace in the active head,
whereas, the tail is filled with the resulting corrosionproducts. As the head
of the filiform moves, the filiform grows in length.
7.13.8C auses of Coating Failures
Coating failures result from two basic causes:
1. Poor or inadequate surfacep reparation and/ora pplication of the
paint to the substrate
2. Atmospheric effects
Coatingf ailurest ypicalf roms urface preparationa nd application
problems include:
1. Cracking, checking, and alligatoring. Thesetype of failures develop
with the aging of the paintfi lm. Shrinkage within the film during
aging causes cracking and checking. Alligatoring is afilm rupture,
usually caused by application of ah ard brittle film over am ore
flexible film.
2. Peeling, flaking, and delamination. These failures are caused by
poor adhesion. When peeling or flaking occurs betweencoats, it is
called delamination.
3. Rusting.Failure of acoated surfacemay appear as (a) spot rusting
at minute areas, (b) pinhole rusting at minute areas, (c) rust
nodules breaking through the coating, and (d) underfilm rusting,
that eventually causes peeling and flakingo fthe coating.
4. Lifting and wrinkling. When the solvent of as ucceedingc oat
of paintt oo rapidly softens the previous coat, lifting results.
Rapid surface drying of ac oating without uniform drying
throughout the rest of the film results in ap henomenon known
as wrinkling.
Corrosion of Linings and Coatings
482
CAT8247—CHAPTER 7—17/10/2006—15:04—TRGANESH—XMLM ODEL B–pp. 403–485
coverage of localized areas, air bubbles).
Filiform corrosionunderorganic coatings is commononsteel, aluminum,
magnesium, and zinc (galvanized steel). It has also been observed under
electroplated silver plate, gold plate, and underphosphate coatings.
This form of corrosion is more prevalent undero rganic coatings on
aluminum than on other metallic surfaces, being as pecial form of anodic
undermining.
Ad ifferential aeration cell is the basic driving force. The filaments have
considerablel ength but little width and depth and consisto ft wo parts: a
head and at ail. The primary corrosion reactions and, subsequently,t he
delaminationp rocess of the paintfi lm takesp lace in the active head,
whereas, the tail is filled with the resulting corrosionproducts. As the head
of the filiform moves, the filiform grows in length.
7.13.8C auses of Coating Failures
Coating failures result from two basic causes:
1. Poor or inadequate surfacep reparation and/ora pplication of the
paint to the substrate
2. Atmospheric effects
Coatingf ailurest ypicalf roms urface preparationa nd application
problems include:
1. Cracking, checking, and alligatoring. Thesetype of failures develop
with the aging of the paintfi lm. Shrinkage within the film during
aging causes cracking and checking. Alligatoring is afilm rupture,
usually caused by application of ah ard brittle film over am ore
flexible film.
2. Peeling, flaking, and delamination. These failures are caused by
poor adhesion. When peeling or flaking occurs betweencoats, it is
called delamination.
3. Rusting.Failure of acoated surfacemay appear as (a) spot rusting
at minute areas, (b) pinhole rusting at minute areas, (c) rust
nodules breaking through the coating, and (d) underfilm rusting,
that eventually causes peeling and flakingo fthe coating.
4. Lifting and wrinkling. When the solvent of as ucceedingc oat
of paintt oo rapidly softens the previous coat, lifting results.
Rapid surface drying of ac oating without uniform drying
throughout the rest of the film results in ap henomenon known
as wrinkling.
Corrosion of Linings and Coatings
482
CAT8247—CHAPTER 7—17/10/2006—15:04—TRGANESH—XMLM ODEL B–pp. 403–485