particularly moisture and water.All organic polymersand organic coatings
are permeable to water.T hey differ only in the degree of permeability.
Table 7.13 provides typical values for the permeability and the diffusion
coefficient for water in an umberofd ifferent polymers.
When moisture permeates into the paintsubstrate interface, blisters form.
There are fourmechanisms whereby blisters can form:
1. Vo lume expansion because of swelling
2. Gas inclusion or gasf ormation
3. Osmoticp rocess becauseo fs olublei mpuritiesa tt he film–
substrate interface
4. Electroosmotic blistering
Of the fourm echanisms, osmotic blister formation is the most important.P ast experience has indicatedt hat 70% of all paint coating failures
are the result of poor or inadequate surfacep reparation prior to application of the coating. Types of osmotically active surfaces at film-substrate
interfaces are impurities such as sand particles from incomplete washing
after wet sanding, water solubles alt residues from phosphating, or
surfacen ests in ar ust layer.I ti sa lso possible to entraph ydrophilic
solvents to create blisters.
Regardless of the specific driving force, the blister formation process is
shown in Figure 7.24 using an exampleo fa ni ntact coating on steel.
When the coating is exposed to an aqueous solution, water vapor molecules
(andt oalessere xtent,o xygen), diffuse into the film toward the coatingsubstrate interface. Eventually,w ater may accumulate at the interface
forming at hin water film of at least several monolayers. The accumulation
takesp lace at sites of poora dhesion or at sites where wet adhesion
problems arise.The degree that permeated water may change the adhesion
properties of ac oated system to is referred to as wet adhesion. No initial,
poorly adhered spots need be present for chemical disbondmentt ot ake
place. Ac orrosion reaction can be initiated by the presence of an aqueous
electrolytew itha ne lectrochemicald ouble layer, ac athodics pecies
(oxygen) and an anodic species (metal). When the corrosion reaction has
started at the interface, the corrosion products can be responsible fort he
buildup of osmotic pressure, resulting in the formation of macroscopic
blisters (see Figure 7.24). Depending on the specificm aterials and circumstances, the blisters may grow out because of the hydrodynamic pressurein
combination with one of the chemical propagation mechanisms sucha s
cathodic delaminationa nd anodic under-coating.
Oxygen migration through ap aint film coating is generally much lower
than that of water.I ns omei nstances, the flow of oxygen increases with
the water contento ft he film, probably because the water acts to swell
the polymer.
Corrosion of Linings and Coatings
478
CAT8247—CHAPTER 7—17/10/2006—15:04—TRGANESH—XMLM ODEL B–pp. 403–485
are permeable to water.T hey differ only in the degree of permeability.
Table 7.13 provides typical values for the permeability and the diffusion
coefficient for water in an umberofd ifferent polymers.
When moisture permeates into the paintsubstrate interface, blisters form.
There are fourmechanisms whereby blisters can form:
1. Vo lume expansion because of swelling
2. Gas inclusion or gasf ormation
3. Osmoticp rocess becauseo fs olublei mpuritiesa tt he film–
substrate interface
4. Electroosmotic blistering
Of the fourm echanisms, osmotic blister formation is the most important.P ast experience has indicatedt hat 70% of all paint coating failures
are the result of poor or inadequate surfacep reparation prior to application of the coating. Types of osmotically active surfaces at film-substrate
interfaces are impurities such as sand particles from incomplete washing
after wet sanding, water solubles alt residues from phosphating, or
surfacen ests in ar ust layer.I ti sa lso possible to entraph ydrophilic
solvents to create blisters.
Regardless of the specific driving force, the blister formation process is
shown in Figure 7.24 using an exampleo fa ni ntact coating on steel.
When the coating is exposed to an aqueous solution, water vapor molecules
(andt oalessere xtent,o xygen), diffuse into the film toward the coatingsubstrate interface. Eventually,w ater may accumulate at the interface
forming at hin water film of at least several monolayers. The accumulation
takesp lace at sites of poora dhesion or at sites where wet adhesion
problems arise.The degree that permeated water may change the adhesion
properties of ac oated system to is referred to as wet adhesion. No initial,
poorly adhered spots need be present for chemical disbondmentt ot ake
place. Ac orrosion reaction can be initiated by the presence of an aqueous
electrolytew itha ne lectrochemicald ouble layer, ac athodics pecies
(oxygen) and an anodic species (metal). When the corrosion reaction has
started at the interface, the corrosion products can be responsible fort he
buildup of osmotic pressure, resulting in the formation of macroscopic
blisters (see Figure 7.24). Depending on the specificm aterials and circumstances, the blisters may grow out because of the hydrodynamic pressurein
combination with one of the chemical propagation mechanisms sucha s
cathodic delaminationa nd anodic under-coating.
Oxygen migration through ap aint film coating is generally much lower
than that of water.I ns omei nstances, the flow of oxygen increases with
the water contento ft he film, probably because the water acts to swell
the polymer.
Corrosion of Linings and Coatings
478
CAT8247—CHAPTER 7—17/10/2006—15:04—TRGANESH—XMLM ODEL B–pp. 403–485