6. Coating Principles

6.3 Coating Systems

Types of Coating Systems

A coating system may use different types of pigments depending on their attributes such as their impermeability, their inhibition capabilities and cathodic protection.


1. Impermeable systems

In these systems, the coating must be impervious to air, oxygen, carbon dioxide as to the penetration of ionic species. It must have good dielectric properties. A common example is the application of coal tar enamel. The primer must be impermeable.


2. Inhibitive systems

The success of this system depends on the nature of inhibitive films which absorbs the penetrating moisture and forms a passive film on the surface, thus blocking any atmospheric intervention with the substrate to cause corrosion. Red lead and lead chromates are good examples. These coatings are suitable for application in the marine, industrial or atmospheric condition.


3. Cathodically protective systems

The success of this system depends on the Zinc primer which are highly adherent to the surface. The zinc protects the substrate by breakdown, releasing electrons to the substrate and protecting it. Intermediate and top coats of polyurethane, or vinyl have been successfully used for protection of underground pipes.


Components of Coating Systems

The chief components of coating systems and their functions are:


They provide a strong bond to the substrate for adhesion. It includes rust inhibitors and wetting agents. They also provide a compatible base for the top coats. They also prevent corrosion.

2. Bodycoats

The bodycoats is applied to increase the total thickness of coating. It provides strong adhesion, strong chemical resistance and strong bond to primer and top coat.

3. Topcoats

They provide a sealing for the coating systems and prevent the ingress of chemicals, water and other corroding species. They also provide a pleasing color.


Paints and Coating Systems Categories

Basically there are two categories:

  1. Convertible

  2. Non-convertible

Convertible paints and coatings: They require a chemical reaction or polymerization to form the protective coating film. Example: oil based points, alkyd paints, epoxy coatings, etc.

Non convertible paints and coatings: A two component paint of polyvinyl chloride, zinc chromate and phosphoric acid. These coatings are applied to enhance the adhesion of top coatings.