I.A. Tolmachev, N.A. Petrenko, K.A. Popelinsky (SPb State Technological Institute), M.D. Sarachuk, E.V. Potapova (LLC "Gamma. Industrial paints", Tikkurila group), A.S. Yankovsky, T.V. Fedorova (LLC NPF "SKAR-LET")
Al-containing pigments have been known for comparatively long time , they are produced and used in the formulations of various anticorrosive coatings [2-5]. It is considered that aluminium salts (phosphates in particular) have the highest chelating ability among all phosphates with respect to Fe2+ ions and the formation of passivating films on steel surfaces. In addition, under conditions of the corrosion process polyphosphate ions depolymerize and generate active phosphate radicals, which contribute to the formation of new passivation sites on the corroding surface.
We have studied the mechanism of action of Al-containing anticorrosive pigment "Fosmet" based on mixed aluminum and calcium phosphate produced by LLC NPF "Skar-Let" (St. Petersburg) , features of pigment use in different formulations (water-dispersible, organ-soluble compositions) and operating conditions of coatings.
The peculiarity of "FOSMET" pigment is its high environmental safety and high whiteness. Lead content is 0.0006% and cadmium content - 0.00007% (Determination was carried out using atomic absorption method of analysis).
The data characterizing the chemical composition, physico-chemical and technical properties of "Fosmet" are given below:
|mass fraction of phosphorus content converted to PO4||55-63%|
|mass fraction of Ca||9-15%|
|mass fraction of Al||7-11%|
|mass fraction of Pb||0,0006%|
|mass fraction of Cd||0,00007%|
|mass loss on ignition||18-20%|
|residue on the sieve with mesh 0063, not more||0,5%|
|specific conductivity of the aqueous extract, not more||0,5мСм/см|
|bulk volume||2,4 см3/г|
|specific surface||5,5 м2/г|
Chemical composition (main ions) and properties of the aqueous extract of the pigment:
mass fraction of phosphate ions
mass fraction of Ca2+
mass fraction of Al3+
pH (10% suspension)
Based on the data of chemical analysis, we can assume that "Fosmet" is a crystalline hydrate of mixed Ca and Al phosphate:
It is known that the anticorrosive action of pigments is due to their ability to create in PC a certain concentration of passivating ions, diffusing to the metal substrate and forming on its surface passivating corrosion films . Therefore, one indication of the effective action of an anticorrosive pigment is the ability of its aqueous drawing to influence the electrochemical behavior of steel. Such research by means of potentiodynamic method (potentiostat P-5848) was carried out for aqueous drawing of pigment "Fosmet", containing ions PO43+, Ca2+, Al3+ and other pigments on a basis of zinc phosphate. Polarization of the working electrode was carried out at potentials close to the stationary one. The figure shows the dependences characterizing the change in corrosion current (ik) on the exposure time of the steel sample in equal-volume mixtures of aqueous extracts of pigments and 6% NaCl solution. As can be seen, the observed dependences are very different. In the case of "Fosmet" initially there is a high activity of the metal, after which within 10 h there is a sharp decrease in activity, then it becomes steadily low. In the case of other phosphate pigments, on the contrary, the metal activity is initially low in all cases, then, depending on the pigment type, it begins to increase and after 50-70 h of contact with the corrosive medium it increases markedly.
Figure - Dependence of ik on exposure time in mixtures of aqueous extracts of various pigments and 6% NaCl solution: 1 - "Fosmet", 2 - Heucophos ZCPP, 3 - Phosphinal PZ-04, 4 - Wacor ZP-BS-M
Visual observation of corrosion changes showed that in the case of using "Fosmet" after a day of exposure there is a formation of a layer of light grayish color on the surface of steel, and in the case of zinc phosphates - reddish-brown color, characteristic of rust.
On the basis of data analysis of the composition of the aqueous extract "Fosmet", pH and visual observations, it can be assumed that the layer observed in the case of "Fosmet" is formed by phosphates of Al, Ca, Fe. Under this friable, easily removed layer we observed the presence of a thin layer colored in the colors of the spectrum, not washed away by water and changing color with a change in the angle of observation. This is usually characteristic of layers having a thickness close to the wavelengths of visible light (300-700 nm). Based on the literature data  describing the behavior of steel and the nature of the layers in systems close to those under study, we can assume that the colored layer is formed by complex compounds involving iron oxide and Ca and Al phosphates. We associate the appearance of such a layer with a sharp decrease and stabilization of the electrochemical activity of the metal.
The high activity of the pigment introduces a number of peculiarities of its use in paintwork materials. In particular, the acidic nature of the surface of pigment particles limits the use of surfactants containing salt groups (-SO3Na, -COONa, -COONH4) which can transform into insoluble or low-soluble H-form in acidic environment, which can reduce the effect of their dispersing, wetting or stabilizing action. Such effects, for example, occur in some formulations using Orotan dispersant additives. Dispersing additives Dispex have been shown to be less sensitive to the acidic nature of the pigment.
The investigation of properties of PC, formed from water-dispersion compositions has shown, that at a certain content of "Fosmet" at test in corrosive active medium formation of black rash takes place, that is a consequence of the increased activity of pigment and is connected with formation of particles of phosphates under PC (Table 1).
Table 1. Recipes and protective properties of coatings based on primer paints, containing different amounts of "Fosmet"
It follows from Table 1, that the maximum "Fosmet" content in the formulation of water-dispersion paint, at which the black rash effect does not occur, is about 6 %, which corresponds to the mass content of "Fosmet" in PC not more than 10 %.
In Table 2 there are given the data, characterizing protective effect of PC, containing minimum quantity of "Fosmet", received from latex-based primer paint Main Coat PR-71 in comparison with the indicators of coatings, containing zinc phosphate Wacor ZP-BS-M.
Table 2. Recipes and protective properties of coatings based on the primer paint, containing a minimum amount of "Fosmet" from zinc phosphate
In this formulation, the amount of "Fosmet" providing maximum protective action in any of the studied corrosive environment lies in the range of 2.2-4.4 %. At the optimum dosage the anticorrosive action of "Fosmet" is considerably superior to the anticorrosive action of zinc phosphate.
Tests within 1000 hours showed the similarity of the anticorrosive effect of pigments.
Comparative assessment of "Fosmet" with a series of Halox anticorrosive pigments was carried out in PC, obtained from compositions of the following composition (mass %):
Corrosion inhibitor SER-AD-FA 179
Thickener Acrysol RM 1020
Corrosion protection pigment
Micro Mica W1
TiO2 (Kemira 650)
Latex Finndisp RSD-20
Результаты испытаний после 7 суток выдержки в 3%-ном растворе NaCl:
|Точечная коррозия, % поверхности||Пожелтение, балл (10-максимальное пожелтение)||Сыпь, % поверхности|
As can be seen, the anti-corrosive effect of Fosmet is at the level of the best versions of these pigments.
In organically soluble paintwork materials Fosmet was tested in the formulation of epoxy primer in comparison with zinc phosphate (in the salt fog chamber) in JSC YarnII LKP and primer GF-0119 in comparison with the mixture of zinc phosphate and zinc tetraoxychromate and phosphate-calcium crown (in water and 3% NaCl solution) in LLC VDM Pigment Plant (Saint-Petersburg). Tests in all cases showed the equivalence of protective properties of PC. The essence of the test consisted in application of a scratch on the surface of the plate used and with the subsequent test of VOC in a chamber of a salt fog. The results are presented below.
Alkyd varnish PF-053
Titanium dioxide (Du Pont R706)
Talcum powder MT-1099
Fixative (LB "Biolar")
In organically soluble coating materials, Fosmet pigment has a high hydrophilicity of the particle surface, which is typical for all sedimentary pigments. In some cases the increased hydrophilicity of the pigment can lead to partial flocculation and aggregation of the particles. This reduces the effectiveness of anticorrosive action of pigments, because it reduces their specific surface area and, accordingly, decreases the leaching rate of passivation ions. To eliminate this drawback was developed a brand hydrophobic pigment "Fosmet" with a technology in which the hydrophobic effect is evenly distributed on the surface of the particles in the form of a nanolayer 5-10 nm thick. As a result the pigment particles retain their individuality in organic medium, the leaching rate of passivation ions increases, which provides an increase in the anti-corrosive effect of the pigment and the service life of the protective PC. This is illustrated by the data in the table below, which shows the test results of coatings (thickness 40-50 microns), obtained from alkyd compositions of the following composition (mass %):
It should be noted that, as expected with hydrophobization, there was no phenomenon of flocculation and aggregation of pigment particles. Due to the change in surface activity of anticorrosive pigment a high dispersibility and uniform distribution in the whole volume of the paint-and-lacquer composition were noted.
On the basis of the obtained results it was decided that further work in this direction is expedient. Intensive experiments are being conducted to increase the anticorrosive effect of "Fosmet" pigment in organorosoluble paintwork materials. In particular, it was possible to approach the widely used in this area, crown zinc. To date, research and development work aimed at increasing the efficiency of Fosmet anticorrosive effect to the level of zinc crown in organor soluble paintwork materials is being carried out.
List of references
Drinberg A.S., Itsko E.F., Kalinskaya T.V. Anticorrosive primers. Saint-Petersburg: OOO Niproins LKM & Pcop, 2006, 168 p.
Patent 2287544 RF.
Application 95102638/25 RF.
Application 97111107/25 RF.
Indekin E.A., Leibzon L.N., Tolmachev I.A. Pigmentation of paint and varnish materials. L.: Chemistry, 1983, 160 p.
Verenkova E.V., Ilyina L.K., Seryanov Y.V. et al. Paint and varnish materials and their application, 2005, № 4, pp. 12-15.