Primers
Primers are universal for most anticorrosive coating systems and are considered the most important component of the system. The primer is the first coat of a paint system and the base on which the rest of the coating system is applied. As a base, it must have strong adhesion to the substrate surface and must form a good basis for the next layer of the coating system. The most important properties of primers are listed as follows:
- Adhesion (strong bonding to substrate).
- Cohesion (high internal strength in the film).
- Inertness (strong resistance to corrosion & chemicals).
- Inter-coat bond (high bonding to intermediate coat).
- Appropriate flexibility.
Most primers for steel contain anti-corrosive pigments such as red lead, zinc powder (zinc dust), zinc chromate, zinc phosphate, calcium phosphate etc. Due to their toxicity, red lead and zinc chromate are used less frequently nowadays and certainly not in prefabrication (shop-priming) as they will release noxious or toxic fumes during welding and flame-cutting.
If the coating system is an inhibitive one, it must contain the inhibitive pigments and be capable of using these pigments in a way which will passivate the metal surface and reduce its tendency to corrode. In cathodically protective primers, zinc silicates to a certain extent react with the steel surface in order to obtain an even greater and stronger adhesion.
The primer must also provide a proper and compatible base for the intermediate coat. It must provide a surface that can be thoroughly wetted by the subsequent coat. The generally flat, non-glossy surface must also provide some physical adhesion to the next coat.
Where primers are used for immersed conditions or tank linings they must have a chemical resistance equivalent to the remainder of the coating system in order to satisfactorily protect against the chemical cargo / substance in which it is immersed.
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Variations
A coating system need not to be composed of the three different parts; even a single coat can provide a coating system, depending upon the requirements of the coating. Inorganic zinc coatings, for example, provide an excellent one-coat system for the storage of refined oil products and many solvents. A single coating formulation applied in two or more coats may provide the best answer to a specific problem. In most barrier coating systems special intermediate coats are omitted. The surface tolerant mastics can be applied directly on to steel and therefore serves both as a primer and an intermediate coat.
As for all paint systems the properties of these products must satisfy a number of requirements, like:
- Expected lifetime (design life)
- Future maintenance
- Classification society requirements
- International standard requirements
- Health, safety and environmental requirements
- Cost for the total design life