CS247844B1 - Anticorrosive pigment - Google Patents

Abstract

The anticorrosive pigment, which is dicalcium cyclotetraphosphate, when used in coatings exhibits significant anticorrosive - inhibitory properties, its good pigment properties enable easy application in coatings. The concentration of the pigment in the coating, necessary to achieve its sufficient anticorrosive effect, is relatively low. The pigment is very thermally stable, so it can also be used for high-temperature purposes. It contains a readily available calcium component and its preparation is technologically undemanding. The invention can be used in the pigment industry and in the coating industry.

Description

The invention relates to the use of dicalcium cyclotetraphosphate as an anti-corrosion pigment.

Phosphates of various metals are able to suppress oxygen corrosion in aqueous environments, especially in ferrous materials. The phosphate ions released by them react with iron ions formed during corrosion, which bind to form insoluble phosphate.

The coating of this phosphate then anodically passivates the metal surface. Metal cations from the phosphate used can also have inhibitory effects. For example, the inhibitory effect of zinc and calcium ions on steel is well known.

Recently, the use of phosphates as anti-corrosion pigments has been expanding. These are mainly simple phosphates of some metals, which are intended to replace lead-based anti-corrosion pigments. The most widespread is simple zinc phosphate - Zn ₃ /PO ₄ / ₂ .2H ₂ O, chromium phosphate - CrPO ₄ .3H ₂ O is also known.

The inhibitory capabilities of simple phosphates are, however, lower than those of some lead pigments. Zinc phosphate also has a relatively high content of zinc, which is less effective than the phosphate component. The metal cation component is also usually resource-intensive.

The preparation of simple phosphates by precipitation processes is a relatively demanding operation due to the necessity of obtaining them in the form of precisely defined hydrates, moreover, using pure starting materials. Simple phosphates are also partially soluble in aqueous media, which may have adverse hygienic and ecological consequences if they are widely used.

Their use in the form of hydrates also limits the temperature range of their application and does not allow their use in coatings for higher-temperature purposes; it can also complicate the final operations of pigment treatment during its production or during the operation of its dispersion into the coating.

The necessary concentrations of simple phosphates in coatings must be chosen relatively high to achieve sufficient inhibitory effects. The effectiveness of inhibiting phosphate pigments increases if polymeric phosphates are used. The use of so-called phosphate glasses is known, which contain an anion in the form of a polymeric phosphate chain /so-called higher linear phosphates/.

These include, for example, systems containing phosphate chain anions and, in addition to alkali metal cations /Na, K/, cations of some alkaline earths /Ca, Mg/ and other cations Zn, Cd, Al, Fe. However, these glasses also have some shortcomings due to problems arising during their preparation and application.

This is due to the necessity of obtaining a homogeneous melt in the first phase of their preparation and thus the use of high temperatures /800 to 1,300 °C/; furthermore, there is the volatilization of the phosphate component from the melt and the high aggressiveness of the melt, increasing the design demands on the production equipment, and also the relatively difficult grinding of the glassy product, when even with intensive grinding, the particle surface corresponding to the oil consumption of conventional pigments is not achieved.

After application of these glasses to coatings, their wetting ability is adversely affected. In this case, the phosphate chains break down. Due to the content of cations and anions, phosphate glass can be converted relatively quickly into simple phosphates, which mostly correspond to highly soluble dihydrogen phosphates, which is disadvantageous from the point of view of the long-term inhibitory effect of the coating.

•In addition, these phosphates are easily washed out of the paint, which disrupts the paint film, makes it easily permeable to gaseous and liquid media and loses the effect of its protective coating. The proposed use of dizinc cyclotetraphosphate as an anti-corrosion pigment is also known /Czech. copyright certificate No. 245071/, which eliminates most of the shortcomings mentioned for simple phosphates and phosphate glasses.

Its certain disadvantage is the price of the zinc component, even though its content in this pigment is lower than in the case of simple zinc phosphate used in practice. The second disadvantage could be the content of this component in the case of wider use of the pigment in contact with water for supplying the population, although, due to the very low solubility of the pigment and the lower content of this component and the lower necessary concentration of the pigment in the coating, this danger is again several times smaller than in the case of simple phosphate.

The above-mentioned shortcomings are eliminated by the invention consisting in the use of dicalcium cyclotetraphosphate as an anti-corrosion pigment. For use in paints, this substance has

2 beneficial basic pigment properties - density /3.84 g/cm /, specific surface /0.15 m /g/, oil consumption /18.6 g of linseed oil per 100 g of c-Ca ₂ P ₄ O ₁₂ /; it is practically white with approximately 90% reflectance in the entire visible part of the spectrum and easily dispersible into paints.

During its synthesis, it can be produced either in the directly required powder form, or it can be easily converted into this form during its own dispersion into the coating material. The preparation of dicalcium cyclotetraphosphate is not technologically demanding in terms of precise adherence to reaction conditions and the quality of the starting materials, and it is not as energy-intensive as the preparation of phosphate glasses.

Cheap natural limestone, milk of lime (calcium hydroxide) or waste calcium carbonate and lower quality, diluted phosphoric acid can be used. Dicalcium tetraphosphate contains calcium and phosphorus components in a molar ratio of one to two, which is more favorable in favor of the more effective phosphorus component.

It contains anions in the form of tetraphosphate cycles, formed by four bonded tetrahedra /P0 ₄ /. These are very stable anions, advantageous in terms of pigment and anti-corrosion properties of cyclotetraphosphate. This results in its thermal stability /up to a melting point of 900 °C/, which allows for advantageous application in anti-corrosion coatings for high-temperature use, and also in its difficult and slow solubility in aqueous environments.

When dissolving c-Ca ₂ P^O ₁₂ , hydrolytic cleavage of tetraphosphate cycles must first occur. In the case of moisture passing through the coating and attack of cyclotetraphosphate particles by water molecules, phosphate ions then pass into soluble form by a gradual process. Thus, these passivating ions are released only slowly and practically regulated according to the degree of corrosive action of the environment.

In the first, slowest stage, only half of them is gradually released, as the other half remains bound in the form of the resulting dicalcium diphosphate, to which c-Ca ₂ P ₄ O ₁₂ gradually converts. The conversion is largely a topochemical process, so the shape of the original pigment microparticles remains preserved.

This prevents the formation of undesirable holes - micropores - in the paint film, which would further promote corrosion. The second stage of possible dissolution of the pigment in the paint is the gradual transition of the resulting diphosphate particles, with the gradual release of another third of the phosphate inhibitory anions, to simple calcium phosphate, which also has anti-corrosion properties.

The following are examples of some of the pigmentary and inhibitory properties of dicalcium cyclotetraphosphate and their comparison with commercial phosphate pigments.

c-CagP^Oj^g shows favorable pH values of aqueous leachates and very good inhibitory abilities of this leachate towards steel sheet. Steel sheets coated with dicalcium cyclotetraphosphate showed lower weight losses due to corrosion during tests in a condensation chamber /CSN 038 131/ and in a chamber with hydrochloric acid heels /ČSN 673 094/ than sheets coated with commercial phosphate pigments.

The areas of damaged coating with ο-032Ρ^0^ ₂ were also smaller in the accelerated immersion test of resistance to undermining corrosion /according to Mach and Schiffman - ČSN 673 087/ and the same in the classification test of the coating material /ČSN 673004/ in a condensation chamber with sulfur dioxide vapors and in a sodium chloride solution with hydrogen peroxide; the relative weight losses of steel sheets in paint film leachates /ČSN 673 004/ were also smaller.

Example 1

Some properties of dicalcium cyclotetraphosphate have been determined, related to its pigmentary use and inhibitory action:

specific gravity linseed oil consumption pH of aqueous extract

- 8 days after inserting the steel sheet

- 8 days after removing the steel sheet, the inhibitory properties of the aqueous extract

- corrosion losses of steel after 8 days of immersion in the leachate /mg/g/

3.84 g/cm^

18.6 g oil/100 g c-Ca^P^O^j

5.12

5.98

5.82

1,033

Example 2

The inhibitory capabilities of coatings prepared using three oil-based coatings containing as an anticorrosive pigment were compared.

- dicalcium cyclotetraphosphate /c-Ca ₂ P ₄ O ₁₂ /

- commercial core pigment formed from simple zinc phosphate precipitated on iron oxide particles /Zn^/PO^/^FejO^/

- a commercial core pigment consisting of simple zinc phosphate precipitated on titanium dioxide particles /Zn ₂ /PO^/ ₂ -TiO ₂ /.

The coating material with c-Ca ₂ P^O ₁₂ had a composition (wt. %) of 29% linseed oil, 43% iron red pigment, 10% zinc white pigment, 7% talc, 1% siccative /1% cobalt octanate in gasoline/ and 10% c-Ca^P^O^j.

The coatings with core pigments contained: 29% linseed oil, 7% talc, 1% siccatives and 63% core pigment; the core pigments always contained 16% zinc phosphate, which corresponded to 10% simple zinc phosphate in the coating.

Corrosion tests were performed with coatings prepared according to ČSN 673 004 on cold-rolled steel sheet 0.6 mm thick /table/.

Table

Coatings with commercial. Coating with c-Ca_P.0,„ nuclear pigments Z ⁿ 3/P ^O 4/ ₂ F ^e 2°3 ^Zn3 / _PO4 /2-Ti° ₂ Corrosion losses of steel sheet in a condensation chamber after 28 days /ČSN 038 131/ 10.65 mg/g 7.39 mg/g 1.19 mg/g Corrosion losses of steel sheet in the chamber with hydrochloric acid vapors after 8 days /ČSN 673 094/ 3.30 mg/g 2.58 mg/g 1.04 mg/g Areas of damaged paint in accelerated immersion test for resistance to underbarking - according to Mach and Schiffman /ČSN 673 087/ 28mm 18mm 11mm Classification tests of paint according to ČSN 673004: - exam A: areas of damaged paint in condensation chambers containing S0_ 38 mrn 52mm 27mm 2 /6 points/ /7 points/ /4 points/ - exam B: areas of damaged paint in solution 9 2 NaCl with H-O, 32.5mm 32mm 15mm /6 points/ /5 points/ /3 points/ - exam C: relative mass losses of steel sheet in paint film leachates relative to losses in distilled water 14.7% 17.9% 9.75% /4 points/ /4 points/ /3 points/ Paint classification class CSN CSN CSN 673004 Fe-3 5673004 Fe-3 673004 Fe-2

SUBJECT OF THE INVENTION

Claims (1)

OBJECT OF THE INVENTION The use of dicalcium cyclotrophosphate as an anticorrosive pigment.