Лако-красочные материалы — производство

5.2.5.1

Zinc Phosphate

In order to replace lead and chromate pigments, much attention has been focused upon zinc phosphate. Today, zinc phosphate is one of the most commonly used phosphate-containing anticorrosive pigments. The formulation versatility, due to the extremely lower solubility compared with chromate and other pigments, and therefore the lower reactivity, has led to its economic importance in the market. Zinc phosphate can be used in a wide variety of resin systems [5.53, 5.55].

ISO 6745 — Zinc phosphate pigments for paints — Specifications and methods of test — defines a zinc phosphate pigment as a white corrosion-inhibiting pigment con­sisting either predominantly of zinc phosphate dihydrate (Zn3(PO4)2 -2 H2O) or of a mixture of zinc phosphate dihydrate and zinc phosphate tetrahydrate (Zn3(PO4)2 -4 H2O) or predominantly of zinc phosphate tetrahydrate. Regarding the influence on the corrosion protection behavior, ISO 6754 notes that the corrosion-inhibiting prop­erties of the various types of zinc phosphate pigments may differ [5.61]. The main requirements of the product properties of zinc phosphate pigments are summarized in Table 5.5.

Zinc phosphates are normally produced on an industrial scale by a wet-chemical reaction involving zinc oxide (ZnO) and orthophosphoric acid (H3PO4), followed by filtration, washing, drying and milling processes. Numerous technical publications have gone into detail regarding the mode of action of zinc phosphate [5.62-5.66].

The performance properties of zinc phosphate pigments are attributed to chem­ical effectiveness, and the ability to form adhesion and inhibitor complexes on the surface of the substrate. In addition, in the case of zinc phosphate, electrochemical effectiveness, preferably in anodic areas, is also of note, since small amounts of the zinc phosphate will hydrolyze under moist conditions. The result of this reaction is argued to be the formation of zinc hydroxide and secondary phosphate ions, which are able to build protective layers on the metal surface in anodic areas [5.67, 5.68].

It is also discussed in the literature that, under moist conditions, basic complexes can be formed by reaction of zinc phosphate with inorganic ions or with carboxylic groups of the resin used, which lead, by reaction with metal ions, to so-called adhe­sion, cross-linking and inhibitor complexes [5.67-5.69]. Under the assumption, that the hydrolyzation process is the prerequisite for the effectiveness of zinc phosphates, this means that such pigments need a certain time before becoming active [5.69]. This leads to the conclusion that zinc phosphates do not have the well-recognized electrochemical effectiveness of chromate pigments [5.69].

The formation of hydrolysis products, in the case of zinc phosphate, depends on the permeability of the protective coating. The permeability of the protective coating itself is influenced by the type of resin used, and in particular, by the PVC (Pigment Volume Concentration). This means that the choice of resin, pigments and fillers and the complete formulation have an important influence on the corrosion protection behavior of protective coatings containing zinc phosphate [5.69].

Maybe, the proposed mechanism of the action of zinc phosphate is more related to theoretical considerations than to fully proven knowledge. However, practical experience has shown that zinc phosphate is an active anticorrosive pigment, but the protective effect of lead and chromate pigments can only be achieved in certain systems [5.70].

5.2.5.2