PHOSPHATE PIGMENTS: ADVANCES IN THE USE IN ANTICORROSION COATINGS

Economic losses caused by corrosion of metals (3-4% of the world gross domestic product annually) make it urgent to search for means of their anticorrosion protection, which includes the application of paint and varnish coatings. An important component of the latter are anticorrosive pigments of inhibiting type, the most effective of which (containing lead, chromium) are characterized by high toxicity. Studies have shown that a promising non-toxic alternative to these pigments are salts of phosphoric acid. Zinc phosphate, the level of toxicity of which is 50 times lower than that of chromates, was the first to find practical application. The mechanism of its anticorrosive action is the formation of a protective layer of basic iron (III) phosphate on the cathodic and anodic sites of the steel surface with the participation of zinc hydroxide and secondary phosphate ions formed as a result of hydrolysis of the pigment. The identified disadvantages of zinc phosphate, limiting its widespread use, include low anti-corrosion efficiency at the initial stages of operation of painted products. The positive influence on anticorrosive efficiency of zinc phosphate pigments of increasing dispersibility, especially to the nano level, as well as their immobilization in nanopores of zeolite was found. In case of increased solubility of the phosphates obtained, in order to prevent rapid depletion of inhibitory substances in the coating during operation under conditions of high humidity, resort to the encapsulation of pigments. High inhibitory ability of polyphosphate pigments has been noted, in particular, the highest anticorrosion efficiency in alkyd coatings was demonstrated by zinc pyrophosphate, and in epoxy coatings - by tripolyphosphate. Inhibiting pigments of the “core/shell” type, in which the core is a cheap mineral and phosphates are deposited on its surface by various methods, are considered promising. Certain problems in the use of zinc phosphate created a decision of the European Community, limiting the use of zinc, in connection with which it is proposed to use as anticorrosive pigments phosphates with different salt-forming metals and their combination. The above allows us to conclude that the development of phosphate, including modified pigments as anticorrosive components of paint primers is far from being completed and has not lost its relevance up to the present time.