214
Fundamentals of Corrosion
separate thin water layer to form at the interface from water that has permeated the coating. As mentioned previously, all organic coatings are permeable to water to some extent.
The permeability of a coating is often given in terms of the permeation
coefficient P. This is defined as the product of the solubility in water in the
coating (S, kg/cm 3 ), the diffusion coefficient of water in the coating (D, m 2 /s),
and the specific mass of water (p, kg/m 2 ). Therefore, different coatings can
have the same permeation coefficient, although the solubility and diffusion
coefficient, both being material constants, are very different. This limits the
usefulness of the permeation coefficient.
Water permeation takes place under the influence of several driving
forces, including:
1. A concentration gradient during immersion or during exposure to a
humid atmosphere resulting in true diffusion through the polymer
2. Capillary forces in the coating resulting from poor curing, improper
solvent evaporation, bad interaction between binder and additives,
or entrapment of air during application
3. Osmosis due to impurities or corrosion products at the interface
between the metal and the coating
Given sufficient time, a coating system that is exposed to an aqueous
solution or a humid atmosphere will be permeated. Water molecules will
eventually reach the coating/substrate interface. Saturation will occur after
a relatively short period of time (on the order of 1 h), depending on the values of D and S and the thickness of the layer. Typical values for D and S
are 10 −13 m 2 /s and 3%, respectively. Periods of saturation under atmospheric
exposure are determined by the actual cyclic behavior of the temperature
and humidity. In any case, situations will develop in which water molecules
reach the coating/metal surface interface where they can interfere with the
bonding between the coating and the substrate, eventually resulting in loss
of adhesion and corrosion initiation, providing that a cathodic reaction can
take place. A constant supply of water or oxygen is required for the corrosion
reaction to proceed. Water permeation can also result in the buildup of high
osmotic pressures, resulting in blistering and delamination.
7.4.1 Wet adhesion
Adhesion between the coating and the substrate can be affected when water
molecules have reached the coating/substrate interface. The degree to which
permeated water can change the adhesion properties of a coated system is
referred to as wet adhesion. Two different theories have been proposed for
the mechanism for the loss of adhesion due to water:
Fundamentals of Corrosion
separate thin water layer to form at the interface from water that has permeated the coating. As mentioned previously, all organic coatings are permeable to water to some extent.
The permeability of a coating is often given in terms of the permeation
coefficient P. This is defined as the product of the solubility in water in the
coating (S, kg/cm 3 ), the diffusion coefficient of water in the coating (D, m 2 /s),
and the specific mass of water (p, kg/m 2 ). Therefore, different coatings can
have the same permeation coefficient, although the solubility and diffusion
coefficient, both being material constants, are very different. This limits the
usefulness of the permeation coefficient.
Water permeation takes place under the influence of several driving
forces, including:
1. A concentration gradient during immersion or during exposure to a
humid atmosphere resulting in true diffusion through the polymer
2. Capillary forces in the coating resulting from poor curing, improper
solvent evaporation, bad interaction between binder and additives,
or entrapment of air during application
3. Osmosis due to impurities or corrosion products at the interface
between the metal and the coating
Given sufficient time, a coating system that is exposed to an aqueous
solution or a humid atmosphere will be permeated. Water molecules will
eventually reach the coating/substrate interface. Saturation will occur after
a relatively short period of time (on the order of 1 h), depending on the values of D and S and the thickness of the layer. Typical values for D and S
are 10 −13 m 2 /s and 3%, respectively. Periods of saturation under atmospheric
exposure are determined by the actual cyclic behavior of the temperature
and humidity. In any case, situations will develop in which water molecules
reach the coating/metal surface interface where they can interfere with the
bonding between the coating and the substrate, eventually resulting in loss
of adhesion and corrosion initiation, providing that a cathodic reaction can
take place. A constant supply of water or oxygen is required for the corrosion
reaction to proceed. Water permeation can also result in the buildup of high
osmotic pressures, resulting in blistering and delamination.
7.4.1 Wet adhesion
Adhesion between the coating and the substrate can be affected when water
molecules have reached the coating/substrate interface. The degree to which
permeated water can change the adhesion properties of a coated system is
referred to as wet adhesion. Two different theories have been proposed for
the mechanism for the loss of adhesion due to water: