Лако-красочные материалы — производство

Anticorrosive pigments can affect the performance of protective coatings in several ways, such as

• Prevention of underfilm corrosion

• Protection of the metal substrate where the paint film is discontinued due to mechanical damage

• Prevention of undercutting in damaged areas

• Improvement of durability per unit of film thickness

• Improvement of durability in thin films making application errors less dam­aging [5.55].

Anticorrosive pigments may be classified by their mode of action, for example chemical and/or electrochemical (Active Pigments), physical (Barrier Pigments), and electrochemical/ physical (Sacrificial Pigments) [5.54-5.56].

1. Active pigments are anticorrosives with a chemical and/or electrochemical ac­tion. They are also described in the literature as inhibitive pigments. These pigments interact chemically, either directly or via intermediates with the metal substrate to reduce the rate of corrosion. Such intermediates can, for example, be formed by reaction with the resin system.

The ability to render a metal surface passive is termed passivation. Those pigments that prevent corrosion of the metal by forming a protective layer on the surface of the substrate are regarded as being active in the anodic areas (anodic passivation). Pigments, which prevent rust formation due to their high oxidation potential, are said to be active in the cathodic areas (cathodic passivation). In general active pigments are able to inhibit one or both of the two electrochemical partial reactions.

Another mechanism of active pigments is neutralization of corrosive sub­stances such as sulfates, acids and chlorides resulting in maintaining a constant pH value in the coating.

2. Barrier pigments act by physically reinforcing the barrier properties of the paint film, which means that they reduce the permeability of the paint film to agents that support corrosion. Normally, they are chemically inert and are said to be inactive or passive.

The barrier effect can be achieved, for example, by using pigments with a platelet-like or lamellar particle shape. This allows them to form a wall of flat particles within a paint film and therefore water and electrolytes have to take an extended/less direct route through the paint film to the substrate.

3. Sacrificial pigments are a special group of active pigments. They are metallic pigments, which act by cathodic protection when applied to ferrous substrates. Such pigments must contain a metal that is higher in the electromotive series of metals (Figure 5.8) than the metal of the substrate to be protected. Under corrosive conditions, the sacrificial pigment, being more reactive than the sub­strate, becomes the anode in an electrochemical corrosion cell (Figure 5.9) in which the substrate is the cathode. This is the meaning of the term cathodic protection. The only sacrificial pigment of commercial importance is metallic zinc, supplied either as zinc dust or zinc flakes [5.55]. In addition to the electro­chemical action of a sacrificial pigment, the chemical reaction of the pigment with the atmosphere results in the formation of insoluble zinc compounds which protect the paint by filling in voids (“cementation”).

There are also anticorrosive pigments described in the coatings literature that are called “film reinforcers” showing “good film forming properties”. The intended meaning of this term might be that some pigments can be incorporated into a coating to improve the overall integrity of the film, because in all cases an inorganic pigment

is not capable of forming films. For example iron oxide red pigments are described as “film reinforcers”. It is discussed that, in addition to their optical properties, iron oxide pigments show excellent resin/pigment bonding properties resulting in an enhancement of the barrier effect of the coating. A second example of a so — called “film reinforcer” is zinc oxide. In the past, zinc oxide was used in oil-based anticorrosive primers to improve hardness. In the protective coatings area it is usually applied in combination with active pigments. It may also protect the coating by acting as an ultraviolet absorber [5.55]. The protective action of zinc oxide is also based on its ability to react with corrosive substances and to maintain an alkaline pH value in the coating [5.56].

5.2.4

Traditional Lead and Chromate Pigments

5.2.4.1