200
Fundamentals of Corrosion
Additional protection may be supplied by resistance inhibition, which is
also a part of the barrier mechanism. Retardation of the corrosion action
is accomplished by inhibiting the charge transport between cathodic and
anodic sites. The reaction rate may be reduced by an increase in the electrical resistance and/or the ionic resistance in the corrosion cycle. Applying an
organic coating on a metal surface increases the ionic resistance. The electronic resistance can be increased by the formation of an oxide film on the
metal. This is the case for aluminum substrates.
Corrosion of a substrate beneath an organic coating is an electrochemical
process that follows the same principles as corrosion of an uncoated substrate. It differs from crevice corrosion because the reactants often reach the
substrate through a solid. In addition, during the early stages of corrosion,
small volumes of liquid are present, resulting in extreme values of pH and
ion concentrations.
The total corrosion process takes place as follows:
1. Migration through the coating of water, oxygen, and ions
2. Development of an aqueous phase at the coating/substrate interface
3. Activation of the substrate surface for the anodic and cathodic
reactions
4. Deterioration of the coating/substrate interfacial bond
7.2 Composition of Paint
Paints are broadly classified as primers and topcoats. Primers are applied
directly to a metal surface. They contain pigments of zinc and perform the
primary job of corrosion protection. Topcoats are applied over the primer
mainly for the sake of appearance. However, they also provide a diffusion
barrier and close the pores in the primary coat. Pores, or “holidays,” are the
starting point of paint failures. The application of topcoat minimizes these
potential points of failure. Three to five topcoats are often recommended for
industrial and marine atmospheres.
The composition of a coating (paint) determines the degree of corrosion
protection that will be supplied. A paint formulation is made up of four general classes of ingredients: vehicle (binder, resin component), pigment, filler,
and additive. It is the combination of these ingredients that imparts the protective properties to the coating.
7.2.1 binder
The binder (or resin) forms the matrix of the coating, the continuous polymeric phase in which all other components can be incorporated. The resin is
Fundamentals of Corrosion
Additional protection may be supplied by resistance inhibition, which is
also a part of the barrier mechanism. Retardation of the corrosion action
is accomplished by inhibiting the charge transport between cathodic and
anodic sites. The reaction rate may be reduced by an increase in the electrical resistance and/or the ionic resistance in the corrosion cycle. Applying an
organic coating on a metal surface increases the ionic resistance. The electronic resistance can be increased by the formation of an oxide film on the
metal. This is the case for aluminum substrates.
Corrosion of a substrate beneath an organic coating is an electrochemical
process that follows the same principles as corrosion of an uncoated substrate. It differs from crevice corrosion because the reactants often reach the
substrate through a solid. In addition, during the early stages of corrosion,
small volumes of liquid are present, resulting in extreme values of pH and
ion concentrations.
The total corrosion process takes place as follows:
1. Migration through the coating of water, oxygen, and ions
2. Development of an aqueous phase at the coating/substrate interface
3. Activation of the substrate surface for the anodic and cathodic
reactions
4. Deterioration of the coating/substrate interfacial bond
7.2 Composition of Paint
Paints are broadly classified as primers and topcoats. Primers are applied
directly to a metal surface. They contain pigments of zinc and perform the
primary job of corrosion protection. Topcoats are applied over the primer
mainly for the sake of appearance. However, they also provide a diffusion
barrier and close the pores in the primary coat. Pores, or “holidays,” are the
starting point of paint failures. The application of topcoat minimizes these
potential points of failure. Three to five topcoats are often recommended for
industrial and marine atmospheres.
The composition of a coating (paint) determines the degree of corrosion
protection that will be supplied. A paint formulation is made up of four general classes of ingredients: vehicle (binder, resin component), pigment, filler,
and additive. It is the combination of these ingredients that imparts the protective properties to the coating.
7.2.1 binder
The binder (or resin) forms the matrix of the coating, the continuous polymeric phase in which all other components can be incorporated. The resin is