by Sergei STEPIN, Dr. Sc. (Chem.), Head of Chair of Chemistry and Technology of Varnishes, Paints and Paint-work Coatings, Kazan State Technological University; Maya ZIGANSHINA, Cand. Sc. (Tech.), Senior Lecturer of the same Chair; Marina PESHKOVA, Assistant of the same Chair

The main "protectors" of metals we know are what they call undercoatings, or prime coatings. Their effect largely depends on the levels and type of anti-corrosion pigments. But the widely used "protectors" of this kind contain chromium or lead which makes them highly toxic. We at our Institute have investigated the possibility of using as corrosion-proof pigments some natural and also synthetic compounds of manganese-an element whose properties approach those of chromium. Their indisputable advantage is their much lower toxicity.

Our studies of anti-corrosion properties of manganese-containing compounds were conducted in comparison with the commonly used undercoatings of zink tetraoxichromate. The "inhibitor" effect was assessed by the impact upon steel of 08 cp of water extracts of the investigated compounds and pigmented films formed on the basis of alkyd paint-and-varnish materials and also by the time and the degree of damage of the coated material.

In order to assess the anti-corrosive effectiveness of manganites* we measured the electrochemical potential of steel in contact with water extracts and also determined the "impact" of substances washed out from the investigated pigments and those extracted from paint-and-varnish coatings, upon the metal corrosion currents.

We used a sample of granulated pyrolusite - mineral which contains, according to analysis data, some 90 percent of manganese dioxide, which can be regarded as manganese manganate. During contact with steel of water extractions of coatings containing pyrolusite there occurs a notable improvement of the steel potential.

Further studies proved that the ability of extracts of films and pigmented with pyrolusite, to arrest the progress of corrosion could be maintained also in the presence of corrosive agents. For example, increasing the content of pyrolusite in coating to 10 percent of the total volume produces a four-fold reduction of the current of metal corrosion in water extracts containing sodium chloride. It should also be noted that the related values of steel potential remained in the area of sufficiently large negative values. And that means that the products of interaction of pyrolusite with the coating agent slow down not only the anodic, but also the cathodic reactions and can be attributed to pigments-inhibitors of a mixed type. But pyrolusite also has some disadvantages. The main one consists in the fact that the level of manganese dioxide it contains depends on a concrete site and ranges from 50 to 95 percent, and the amounts of other chemical elements in the ore can reach 43 percent. The instability of

* Manganite - salt of permanganic acid. - Auth.

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Dependence of the established steel potential (E_st ) (1) and steel corrosion current i_K (2) in extractions from the level of pyrolusite in paint coating

Chronopotenciometric currents of aquenous extracts of tetraoxichromate and investigated manganites

the composition, and consequently of the properties of the give mineral, complicates the preparation of this or that paint-and-varnish material with some specific characteristics. The situation can be improved by using synthetic manganese dioxide. But producing a pure anhydrous or hydrated compound is accompanied by the formation of manganites-complex oxides of permanent composition.

For the studies of anticorrosive properties of synthetic manganese-containing compounds we synthesized manganites of calcium, zink, strontium and barium. These elements were chosen with the view of the most frequent uses of their salts and oxides as pigments of paint-and-varnish materials. Synthesis was performed by our specialists by the method of precipitation, by the deoxidation of high manganese compounds in the presence of different salts.

The substances thus obtained were high-dispersion powders with different shades of brown color.

As proved by the subsequent experiments, water extracts of synthesized manganites retain the ability to suppress metal oxidation also in the presence of corrosion-active substances. As the criterium of the inhibitive factor of the protective coating produced on steel surface we used what is known as the critical concentration of sodium chloride. Our analysis of the results have proved that in the presence of sodium chloride the tetraoxichoroxichromate of zink loses its anti-corrosive properties. The extract of barium manganite boosts steel corrosion, and substances extracted from calcium and strontium manganites reduce by more than 15 times steel corrosion current.

And one can draw the general conclusion that natural and synthetic manganese-containing pigments, bearing in mind their low toxicity and high anti-corrosion characteristics, offer a promising alternative to the currently used high-toxicity analogues on the basis of lead and chrome.

"Paintwork Materials, Their Application " (Lakokrasochnye materialy i ikh primenenye), No. 3, 2005

Illustrations provided by the authors

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