CS257735B1 - Blue-violet thermally stable pigments with anti-corrosion properties - Google Patents

Abstract

Cobalt-calcium double cyclotetraphosphates are used as blue-violet thermally stable pigments with anti-corrosion properties. These compounds are intensely blue-violet and have very good anti-corrosion inhibitory effects even at their relatively low concentration in a paint or other binder. They are also thermally and chemically very stable substances, which allows their use even for high-temperature purposes. The solution can be applied in pigment technology, in the paint industry and in the enamel and ceramic industries.

Description

The invention relates to blue-violet thermally stable pigments with anti-corrosion properties.

Some lead compounds and compounds based on zinc chromates have been considered the most effective anti-corrosion pigments. However, their use brings adverse hygienic and ecological consequences, which is why there is currently an effort to replace them with other compounds. However, these do not yet reach the effectiveness of these pigments, especially lead ones. One group of compounds newly used as anti-corrosion pigments is represented by phosphates.

Phosphates suppress oxygen corrosion in a humid, aqueous environment, especially in ferrous materials (steel, cast iron), where they bind iron ions resulting from corrosion into insoluble phosphate. This then forms a coating that also passivates the metal surface. Phosphate cations can also have a positive effect on anti-corrosion effects. Of the phosphorus compounds, the following are used, or proposed for use, as anti-corrosion pigments: mainly simple phosphates. The second group is represented by condensed phosphates. This group includes, firstly, the so-called polyphosphate glasses (higher linear phosphates), which have been proposed more often so far, and then, recently, the diphosphates and cyclotetraphosphates proposed by the author of this invention *

of some divalent metals. Of the simple phosphates, the most widely used is zinc phosphate in the form of dihydrate - Zn2 (PO^) ^. 2^0. The use of phosphates of some alkaline earth metals, especially calcium, is also known.

In addition, double calcium-zinc phosphate - CaZn2~ (PO^) 2« 1^2°* has recently been used. ^{It is} known that calcium ions affect the action of anti-corrosion pigments even when they are in the form of additives. They effectively increase the alkalinity of moisture possibly penetrating the paint or other anti-corrosion protective layer. However, the use of simple phosphates as anti-corrosion pigments has a number of disadvantages. The main disadvantage of these compounds is that they do not yet achieve the anti-corrosion effects of the best lead and chromate pigments and they need to be applied to paint materials in relatively large quantities (concentrations) so that their anti-corrosion effects are satisfactory. Another disadvantage of simple phosphates is the relatively low thermal stability of these substances, resulting from their defined hydrated form, which is necessary for their anti-corrosion effect.

Therefore, they cannot be used in anti-corrosion protective layers for higher temperatures (usually above 150 °C). Their lower thermal stability can also complicate the final mechanical-thermal operations of preparation and treatment of pigments and also their dispersion into the anti-corrosion mass. From the point of view of the long-term anti-corrosion effect of simple phosphates, their partial solubility in aqueous, not completely neutral environments (e.g. due to the action of so-called acid rain) can also be a certain disadvantage. Over time, pigment particles can be washed out of the protective layer and thus its impermeability to corrosive media can be impaired.

The technology for preparing simple phosphates is also not a simple operation, due to the need to obtain a precisely defined hydrate. It also requires high-quality raw materials, while the content of the metal component, which is less effective than the phosphorus component, is relatively high.

Of the condensed phosphates proposed or used as anti-corrosion pigments, the first group (in terms of chronological sequence) is the so-called polyphosphate glasses. These higher linear condensed phosphates contain anions in the form of a polymer chain.

They are designed with various cations - including calcium. Polyphosphoric glasses do not reach the level of the best lead pigments in terms of anti-corrosion effects. Their thermal stability is significantly higher than that of simple phosphates, but it is also limited, since they recrystallize in the temperature range of 400 to 600 °C and usually lose the character of higher polymer anions.

Therefore, they cannot be used in anti-corrosion protective layers above these temperatures. Higher linear phosphates are also partially soluble and, if they are in powder - pigment - form, they even tend to get wet. Under the influence of moisture, they gradually turn into dihydrogen phosphates, which are then easily soluble, can be easily washed out of protective layers, which are thus destroyed and become permeable to corrosive media. Higher linear phosphates are therefore not effective against corrosion in the long term, not to mention the fact that their use on a large scale can also lead to hygiene and ecological problems. Another disadvantage of these compounds is the high complexity of their preparation, especially from an energy and construction point of view. They are prepared from melts at high temperatures (800 to 1,300 °C), which are quite aggressive and from which aggressive phosphorus fumes are already partially volatilized. The products have a glassy character and therefore the final operations of their processing into powder - pigment - form and dispersion into a coating or other substance are also relatively demanding.

Another group of anticorrosive pigments of the condensed phosphate type are diphosphates of divalent metals, including cobalt and calcium. These substances have high thermal stability and are longer-lasting. At the same time, they contain a smaller proportion of the divalent metal component, which is usually more demanding in terms of raw materials (and also less effective in anticorrosive properties), than was the case with simple phosphates. Their anticorrosive effects are also usually better than those of the corresponding simple phosphates; however, they still do not achieve the effects of the best anticorrosive pigments based on lead compounds.

Another group of anti-corrosion pigments of the condensed phosphate type can be called cyclotetraphosphates of some divalent metals, including cobalt and calcium (Čsl. AO 247 844). They eliminate most of the shortcomings mentioned for simple phosphates and for higher linear phosphates and alleviate the shortcomings mentioned for diphosphates. Cyclotetraphosphates are thermally very stable substances, up to their melting temperatures (e.g. cobalt product up to 1,060 °C, calcium up to 900 °C). Cyclotetraphosphates are also chemically very stable, with very low solubility in aqueous and not entirely neutral environments, so their anti-corrosion effect is long-term, especially in humid, aggressive environments. However, the slow and very slow solubility of some pure cyclotetraphosphates (especially the cobalt product) can be a certain disadvantage of their use in some cases, especially when faster release of phosphate passivating anions is required.

In that case, it may be more effective to use cyclotetraphosphates with a somewhat slower solubility, such as the calcium product. Cyclotetraphosphates have a high proportion of the phosphorus component, which is more effective in terms of corrosion protection, which is up to three times higher than in simple phosphates and almost twice as high as in diphosphates. However, in the case of some products (e.g. cobalt), this component can also be very, very demanding in terms of raw materials? in other cases (e.g. calcium product), it is on the contrary very cheap. The preparation of cyclotetraphosphates is not as technologically and structurally demanding as the preparation of most of the other phosphates mentioned above. However, for some products, their preparation is more difficult to manage in order to achieve sufficient yield.

For example, this is the case with pure dicalcium cyclotetraphosphate, while these problems do not exist with the cobalt-based product.

Among the double products that have been proposed for anti-corrosion use so far are, for example, the above-mentioned zinc-calcium phosphate - CaZn2 (PO^) 2 · 21^0, then zinc-manganese and zinc-manganate-calcium and also some linear condensed phosphates (e.g. hl i.nito-zinc, ci. sodium-calcium). Cyclotetraphosphates as double salts have not yet been proposed.

However, most phosphate pigments are white or colorless. A pigment that would be produced by high thermal or chemical stability, have anti-corrosion properties and be intensely colored in blue or purple hues was not known until recently. Although the use of some cobalt compounds (oxide, silicate) as thermally stable pigments in blue and purple hues was known, from the point of view of anti-corrosion effects they are practically inert, and their main disadvantage is the relatively high content of deficient cobalt (especially in the case of oxide) and the technological and energy requirements of their preparation.

The present inventors have recently proposed the use of dicobalt cyclotetraphosphate and dicobalt diphosphates as blue-violet thermally stable pigments with anti-corrosion properties.

The proportion of cobalt in these compounds is lower than in the above-mentioned oxide, but the consumption of cobalt raw material for these pigments is still a significant item for their potential manufacturers. The anti-corrosion effects of these pigments suffer in some applications from their very low solubility, which, however, can be an advantage in other applications; it is also necessary to take into account the high complexing ability of cobalt ions.

The above-mentioned shortcomings resulting from the use of phosphates, higher linear phosphates and diphosphates as anticorrosive pigments, as well as the disadvantages resulting from the content of raw material-intensive cobalt in dicobalt cyclotetraphosphate and especially in dicobalt diphosphate, are eliminated, or rather most of the individual advantages resulting from the use of pure cyclotetraphosphates - dicobalt and dicalcium - are combined by the invention, consisting in ^the use of double cobalt-calcium cyclotetraphosphates as blue-violet thermally stable pigments with anticorrosive properties. The composition of these pigments according to the invention is expressed by the formula ^{c Co} 2-x ^Ca x ^p 4°i2' ^{where x varies from} a value approaching zero to a value of one, preferably in the range of 0.4 to 1.

Double cyclotetraphosphates have some basic physical properties that are advantageous for pigmentary use - density, specific surface area and oil consumption. They are intensely blue-violet and easily dispersible in paints. Their preparation is a technologically easy-to-manage process that does not require high-quality starting materials, as is the case with the preparation of non-condensed phosphates. Waste cobalt compounds, cheap calcium compounds and lower-quality phosphoric acid can also be used. In addition, compared to pure c-C^P^O^, the consumption of cobalt-containing raw materials is not as high.

The synthesis provides a good yield of products in microcrystalline form, which is higher than in the case of pure c-Ca2p^O^2’. It is also not as energy-intensive as the preparation of condensed phosphates of the polyphosphoric glass type. The double products are mostly formed directly in the powdered, easily dispersible form of solid cycles consisting of four bonded (-PO4-) tetrahedra, which are the main carriers of the pigment and anti-corrosion properties of the products. Their thermal stability is determined by their melting temperatures, which range between 1,060 and 885 °C (they decrease in this interval with an increasing proportion of the calcium component).

They can therefore also be used in various anti-corrosion coatings for high-temperature purposes. The solubility of double cyclotetraphosphates, similar to that of pure cyclotetraphosphates (with one cation) in aqueous environments is gradual and only gradual. (However, it is faster than that of a pure cobalt product). In the case of moisture passing through the coating and the pigment particle being attacked by water molecules (hydroxyl or hydroxonium ions), the tetraphosphate cyclic anions are first slowly hydrolytically broken down in the first stage. In this process, half of the passivating phosphate ions are released in a practically controlled manner, depending on the degree of corrosion.

The second half remains bound in the pigment particle in the form of diphosphate, to which the remaining solid phase is converted partly by topochemical processes. This preserves the original shape of the pigment microparticles, so that undesirable micropores in the paint film do not form, which would facilitate further penetration of various media and thus corrosion. In the second stage of possible dissolution of the remaining pigment particle in the paint due to moisture, the diphosphate is converted to simple phosphate, releasing another third of the passivating phosphate anions.

Its particles, remaining in the paint, also have certain anti-corrosion effects, as they are able to further release passivating anions. During these dissolution processes, the blue-violet shade of the pigment is practically preserved, as the particles of cyclotetraphosphate, diphosphates and simple phosphate have this shade.

In all these processes, cations are also gradually released, which either have partially passivating effects themselves or effectively increase the alkalinity of the aforementioned moisture penetrating the coating.

The following are examples of some pigment properties and anti-corrosion inhibitory capabilities of three products of the type of cobalt-calcium cyclotetraphosphates C-COJ, ,4 ^P 4°i2 / c-Co^ 2 ^Ca 0 8 ^P 4°12 ^and c-CoCaP^O^ · 3 ^e also documented their intense color in a blue-violet shade and thus their very good prospects for use as blue-violet thermally stable pigments with anti-corrosion properties.

Example

Some properties of cobalt-calcium cyclotetraphosphates have been determined, related to their pigmentary use and inhibitory action:

x = 0.4 x = 0.8 x = 1 2 density (g/cm ) 3.22 3.13 3.00 linseed oil consumption (g/lOOg pigment) 19.1 20.8 21.9 pH of aqueous extract 5.81 6.05 6.36 - 8 days after inserting the steel sheet 6.38 6.53 6.89 - 8 days after removing the steel sheet 6.01 6.22 6.52 inhibitory properties of aqueous extract - corrosion losses of steel after 8 days of immersion in leachate (mg/g) 0.682 0.602 0.660

Example 2

The anticorrosive effects of coatings prepared using oil-based paints containing C-CO2 as anticorrosive pigment (t ^x = 0.4', 0.8 and 1) were assessed. The paint had a composition (wt. %) of 29% linseed oil, 53% iron red pigment, 7% talc, % siccatives (1% cobalt octanate in gasoline) and 10% ^c-Co 2-x ^Ca x ^P 4°i2'

With coatings prepared according to ČSN 673 004 on 0.6 mm thick rolled steel sheet

cold, corrosion tests were performed, which established: ^{c Co} 2-x ^Ca x ^P 4 °12 x = 0.4 x = 0.8 x = 1 corrosion losses of steel sheet (mg/g) in a condensation chamber after 28 days (ČSN 038 131) 1.18 1.10 1.12 corrosion losses of steel sheet (mg/g) in a chamber with hydrochloric acid vapors after 8 days (ČSN 673 094) 2,022 1,923 1,931 2 area of damaged paint (mm ) in accelerated immersion test of resistance to undermining corrosion - according to Mach and Schiffman (ČSN 673 087) 17 16 . 14 Furthermore, classification tests of oil products were carried out paint mass with c-Co according to ČSN 673 004: - exam A: x = 0.4 x = 0.7 x = 0.95 2 area of damage to the slag (mm ) in the condensation chamber with the contents 18 17 18 SO ₂ /points/ /1/ /1/ /1/ - exam B: 2 area of damaged paint (mm ) 19 23 18 in NaCl solution with H ₂ O ₂ /points/ /1/ /2/ /1/

2-x

- exam C:

relative mass loss (%) of steel sheet in paint leachates

of films related to the decrease in dest. 15.2 19.3 9.8 water /points/ /5/ /4/ /4/ classification class of oil paints CSN CSN CSN mass with ^c “ ^Co 2- _x ^Ca x ^P 4°i2 673 004 673 004 673 004 Fe-1 Fe-1 Fe-1

When assessing the thermal stability of double cobalt-calcium cyclotetraphosphates by calcination in an electric furnace at various temperatures and analysis of the calcinates by instrumental analysis methods and leaching procedure with an aqueous solution of 0.3N HCl (according to Czechoslovak author's certificate 232 090), it was shown that up to the melting temperatures, they do not undergo chemical, structural or color changes. Melting temperatures ^c-Co 2-x ^Ca x ^P 4 ^CJ i2 3 ^SOU P ^{ro x =} 4:900 °C, for x = 0.8:890 °C and for x = 1:885 °C.

Claims (1)

The use of cobalt- ^calcium - ^calcium double cyclotetrophosphates, the composition of which is represented by the formula ^Co 2 -x ^Ca x ^P 4 ⁰ τ 1 x de x, ranges from greater than zero to one, preferably in the range 0.4 to 1, as blue-violet thermally stable pigments with anticorrosive properties.