Corrosion of Paint
215
1. Chemical disbondment resulting from the chemical interaction of water
molecules with covalent hydrogen, or polar bonds between polymer
and metal (oxide)
2. Mechanical or hydrodynamic disbondment as a result of forces caused
by accumulation of water and osmotic pressure
For chemical disbondment to take place, it is not necessary that there be
any sites of poorly bonded coating. This is not the case for mechanical disbonding, where water is supposed to condense at existing sites of bad adhesion. The water volume at the interface may subsequently increase due to
osmosis. As the water volume increases under the coating, hydrodynamic
stresses develop. These stresses eventually result in an increase in the nonadherent area.
7.4.2 Osmosis
Osmotic pressure can develop from one or more of the following:
1. Pressure of soluble salts as contaminants at the original metal
surface
2. Inhomogeneities in the metal surface such as precipitates, grain
boundaries, or particles from blasting pretreatment
3. Surface roughness due to abrasion
Once corrosion has started at the interface, the corrosion products produced
can be responsible for the increase in osmotic pressure.
7.4.3 blistering
Various phenomena can be responsible for the formation of blisters and the
start of underfilm corrosion. These include the presence of voids, wet adhesion problems, swelling of the coating during water uptake, gas inclusions,
impurity ions in the coating, poor general adhesion properties, and defects
in the coating.
When a coating is exposed to an aqueous solution, water vapor molecules
and some oxygen diffuse into the film and end up at the substrate interface.
Eventually, a thin film of water may develop at the sites of poor adhesion or
at the site where wet adhesion problems arise. A corrosion reaction can start
with the presence of an aqueous electrolyte with an electrochemical double
layer, oxygen, and the metal. This reaction will cause the formation of macroscopic blisters. Depending on the specific materials and circumstances, the
blisters may grow out because of the hydrodynamic pressure in combination
with one of the chemical propagation mechanisms such as cathodic delamination and anodic undermining.
215
1. Chemical disbondment resulting from the chemical interaction of water
molecules with covalent hydrogen, or polar bonds between polymer
and metal (oxide)
2. Mechanical or hydrodynamic disbondment as a result of forces caused
by accumulation of water and osmotic pressure
For chemical disbondment to take place, it is not necessary that there be
any sites of poorly bonded coating. This is not the case for mechanical disbonding, where water is supposed to condense at existing sites of bad adhesion. The water volume at the interface may subsequently increase due to
osmosis. As the water volume increases under the coating, hydrodynamic
stresses develop. These stresses eventually result in an increase in the nonadherent area.
7.4.2 Osmosis
Osmotic pressure can develop from one or more of the following:
1. Pressure of soluble salts as contaminants at the original metal
surface
2. Inhomogeneities in the metal surface such as precipitates, grain
boundaries, or particles from blasting pretreatment
3. Surface roughness due to abrasion
Once corrosion has started at the interface, the corrosion products produced
can be responsible for the increase in osmotic pressure.
7.4.3 blistering
Various phenomena can be responsible for the formation of blisters and the
start of underfilm corrosion. These include the presence of voids, wet adhesion problems, swelling of the coating during water uptake, gas inclusions,
impurity ions in the coating, poor general adhesion properties, and defects
in the coating.
When a coating is exposed to an aqueous solution, water vapor molecules
and some oxygen diffuse into the film and end up at the substrate interface.
Eventually, a thin film of water may develop at the sites of poor adhesion or
at the site where wet adhesion problems arise. A corrosion reaction can start
with the presence of an aqueous electrolyte with an electrochemical double
layer, oxygen, and the metal. This reaction will cause the formation of macroscopic blisters. Depending on the specific materials and circumstances, the
blisters may grow out because of the hydrodynamic pressure in combination
with one of the chemical propagation mechanisms such as cathodic delamination and anodic undermining.