Лако-красочные материалы — производство

Zinc dust is the only sacrificial anticorrosive pigment (see Section 5.2.3) on the market with significant economical importance. Zinc dust is a blue-gray free-flowing powder composed of spheroidal particles [5.56]. In the past, the zinc dust for use in protective coatings was a by-product of zinc metal production. Today, zinc dust pigments for application in protective coatings are specifically produced to offer different qualities for varying uses [5.105].

Zinc dust is produced from primary or secondary zinc metal by evaporation. The generated zinc vapor is condensed under controlled conditions under exclusion of air followed by sieving processes in order to obtain different particle size fractions [5.105]. The particle sizes of zinc dust are normally in the range 2 to 8 pm [5.105]. A product classed as fine quality shows a typical particle size of 3 pm whereas standard quality has particle size 8 pm [5.105].

The anticorrosive properties of zinc dust in protective coatings are based upon sealing effects and electrochemical reactions. Due to the high reactivity of zinc, it reacts with atmospheric components (water, oxygen, carbon dioxide), that diffuse into the binder, resulting in the formation of voluminous water-insoluble zinc salts, mainly basic zinc carbonates. The increase in volume connected with the filling of existent voids in the paint film decreases the permeability, and hence increases the barrier properties [5.56, 5.105]. The electrochemical effectiveness of zinc dust arises because zinc has a high position in the electromotive series of metals (see Section 5.2.3). Zinc becomes a sacrificial anode under corrosive conditions when in contact with ferrous metals. The sacrificial anode protects the ferrous cathode, as described before. This protection can be efficient as long as the zinc is able to go into solution. Weathering, together with zinc removal, should be avoided in any case by application of a topcoat [5.105].

While the correct loading level of zinc dust in primers is still a matter of debate, these primers normally have a zinc dust content of more than 80% in the dry paint film [5.53, 5.105]. The requirements of test methods for zinc dust pigments suitable for use in protective coatings are specified in ISO 3549 — Zinc dust pigments for paints — Specifications and test methods [5.106].

Zinc dust coatings are utilized in large quantities for the protection of structural steel, also including underwater steel construction and ship building [5.56]. Zinc dust is used in organic and inorganic binder systems (alkali silicates, ethyl silicates) [5.56, 5.105].

Zinc as a pigment for application in protective coatings is also available in the form of zinc flakes with a lamellar particle shape. Zinc flakes are mainly used in so-called

micro-layer corrosion protection systems. These systems are accepted in the industry as standard surface protection for threaded parts [5.107].

5.2.15