CS259926B1 - Anti-corrosion thermally stable pigments - Google Patents

Abstract

Calcium manganate double cyclotetraphosphates are used as thermally stable anticorrosive pigments. These substances have very good anticorrosive inhibitory effects even at their relatively low concentration in a coating or other binder. They are also thermally and chemically stable substances, which allows their use even for high-temperature purposes. The solution can be applied in pigment technology and in the coating industry.

Description

The invention relates to anti-corrosion thermally stable pigments.

The most effective anti-corrosion pigments have so far been considered to be some lead compounds and compounds based on zinc chromates. However, their use brings adverse hygienic and ecological consequences, which is why efforts are currently being made 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 corrosion by oxygen 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 (in addition to zinc ions, manganese and calcium ions are also known to have a positive effect). 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. It includes, firstly, the so-called polyphosphate glasses (higher linear phosphates) that have been proposed more frequently so far, and secondly, the diphosphates and cyclotetraphosphates of some divalent metals proposed recently by the author of this invention. Of the simple phosphates, the most widespread is zinc phosphate in the form of dihydrate - Zn ₂ (PO ₄ ) ₂ -2 H ₂ 0. The use of phosphates of some alkaline earth metals, especially calcium, is also known. In addition, double calcium-zinc phosphate - CaZn ₂ (PO^) ₂ .2 H ₂ 0 has also been used recently. 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 or penetrate the paint or other anti-corrosion protective layer. Among the simple manganese phosphates, certain anticorrosive properties have been confirmed for Mn ₂ (PO ₄ > ₂ and also for hydrogen and dihydrogen phosphate - MnHPO^ and Mn(H ₂ PO ₄ ) ₂ . It is also known that the anticorrosive effects of zinc phosphate can be increased by adding manganese ions. However, the use of simple phosphates as anticorrosive pigments has a number of disadvantages. The main disadvantage of these compounds is that they do not yet achieve the anticorrosive effects of the best lead and chromate pigments and they need to be applied to paints in relatively large quantities (concentrations) in order for their anticorrosive effects to be satisfactory. Another disadvantage of simple phosphates is the relatively low thermal stability of these substances, resulting from their defined hydrate form, which is necessary for their anticorrosive effect. Therefore, they cannot be used in anticorrosive protective layers for higher temperatures (usually above 150 °C). Their smaller Thermal stability can also complicate the final mechanical-thermal operations of pigment preparation and treatment, as well as its dispersion into the anti-corrosion compound.

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 corrosion-causing media can be impaired. The technology of 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 time sequence) is the so-called polyphosphoric glasses. These higher linear condensed phosphates contain anions in the form of a polymer chain. They are proposed with various cations - mostly also with calcium. (The case with manganese cations is not yet known.) Polyphosphoric glasses do not reach the level of the best lead pigments in terms of their 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 mostly lose the character of a higher polymer anion. 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 become wet. Under the influence of moisture, they gradually transform into dihydrogen phosphates, which are then easily soluble and 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 in the long term as anti-corrosion agents, not to mention the fact that their use on a large scale can also lead to hygienic and ecological problems. Another disadvantage of these compounds is the high complexity of their preparation, especially from an energetic and structural point of view, since they are resistant to high temperatures (800 and 1,300 °C).

which are quite aggressive and from which aggressive phosphoric fumes are partly already emitted. The products have a glassy character and so the final operations of their processing into powder - pigment form and dispersion into paint or other material are also relatively demanding.

Another group of anti-corrosion pigments of the condensed phosphate type are diphosphates of divalent metals, including manganese 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 anti-corrosion), than was the case with simple phosphates. Their anti-corrosion effects are also usually better than those of the corresponding simple phosphates; however, they still do not achieve the effect of the best anti-corrosion pigments based on lead compounds.

Another group of anticorrosive pigments of the condensed phosphate type can be identified as cyclotetraphosphates of some divalent metals, including manganese (Čsl. AO 248 540) 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. manganese oil 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 media, so their anticorrosive effect is long-term, especially in moist, aggressive media. However, the slow and very slow solubility of some pure cyclotetraphosphates (including the manganese 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 (including manganese), this component can also be very demanding in terms of raw materials; in other cases (e.g. calcium product), it is 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 in the case of manganese products.

Among the double products that have been proposed for anti-corrosion use, for example, the above-mentioned zinc-calcium phosphate - CaZn ₂ (P0 ₄ ) ₂ .2 H ₂ 0, then zinc-manganese and zinc-manganate-calcium, as well as some linear condensed phosphates (e.g. aluminum-zinc or sodium-calcium). Cyclotetraphosphates have not yet been proposed as double salts.

The above-mentioned shortcomings resulting from the use of phosphates, higher linear phosphates and diphosphates as anticorrosive pigments are eliminated, or rather most of the individual advantages resulting from the same use of pure cyclotetraphosphates - dimanganese and dicalcium are combined by the invention, consisting in the use of double cyclotetraphosphates - calcium manganate as thermally stable anticorrosive pigments. The composition of these pigments according to the invention is expressed by the formula c-Mn _{2 χ} Οθ _χ Ρ ₄ Ο^ ₂ , where x ranges from a value approaching zero to a value of one, preferably in the range of 0.6 to 0.9. Double cyclotetraphosphates have some basic physical properties that are advantageous for pigmentary use - density, specific surface area and oil consumption. They are white and easily dispersible into coatings. Their preparation is a technologically easy process that does not require high-quality starting materials, as is the case with the preparation of non-condensed phosphates. Waste manganese compounds, cheap calcium compounds, and lower-quality phosphoric acid can also be used.

In addition, compared to pure c-Mn ₂ P ₄ O ₁₂ , the consumption of manganese raw material is not so high. The synthesis provides a good yield of products in microcrystalline form, which is higher than in the case of pure Ca-product. It is also not as energy-intensive as the preparation of condensed phosphates of the polyphosphate glass type. Double products are mostly produced directly in powder, easily dispersible form. The anions of double cyclotetraphosphates are in the form of solid cycles consisting of four bonded (-PO^-) 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 950 and 840 °C (they decrease in this interval from 950 °C for almost pure c-Mn ₂ P^O^ ₂ i.e. x -t 0 with an increasing proportion of the calcium component up to a minimum of 840 °C for x = 0.75, then increase again up to 907 °C for x = 1. They can therefore also be used in various anti-corrosion coatings for high-temperature purposes. The solubility of double cyclotetraphosphates, similarly to pure cyclotetraphosphates (with one cation) in aqueous media is gradual and only gradual. (However, it is somewhat faster than in the case of a pure manganese product.) In the case of moisture passing through the coating and the pigment particle being attacked by water molecules (hydroxyl or hydroxonium ions), in the first stage they are first slowly hydrolytically cleaved tetraphosphate cyclic anions.

In this case, half of the passivating phosphate ions are released in a practically regulated manner, depending on the degree of corrosion. The other half remains bound in the pigment particle in the form of diphosphate, to which the remaining solid phase is converted partly by a topochemical process. This preserves the original shape of the pigment microparticles, so that undesirable micropores do not form in the paint film, which would facilitate further penetration of various media and thus corrosion. In the second stage of possible dissolution of the remaining pigment particles in the paint due to moisture, the diphosphate is converted to simple phosphate, releasing another third of the passivating phosphate anions. Its particles, which remain in the paint, also have certain anti-corrosion effects, as they are capable of releasing passivating anions. In all these processes, cations are also gradually released, which either have passivating effects themselves (manganate ions) or effectively increase the alkalinity of the moisture penetrating the paint (calcium ions).

The following are examples of some pigment properties and anti-corrosion inhibitory capabilities of three products of the type of calcium manganate cyclotetraphosphates c-Mnj ₅ Ca _Q 5^4 ⁰ ]^ ' ^C-Mn i 25 ^Ca 0 75 ^P 4°12 ^{and c-MnCaP} 4°i2· ^D °^ ^ument their very good prospects for use as anti-corrosion pigments.

Example 1

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

density (g/ ^cm3 ) x = 0.5 3.0 ^c - ^Mn 2-x ^Ca x ^P 4°12 x = 0.75 2.91 x = 1 2.85 linseed oil consumption (g/100 g pigment) 18.6 18.55 18.5 pH of aqueous extract 5.81 6.15 6.30 - 8 days after inserting the steel sheet 6.22 6.78 6.92 - 8 days after removing the steel sheet 6.25 6.32 6.45 inhibitory properties of aqueous extract - corrosion losses of steel after 8 days of immersion to the extract (mg/g) 0.665 0.543 0.576

Example 2

The anticorrosive effects of coatings prepared using oil-based paints containing the anticorrosive pigment c-Mn ₂ _ _x Ca _x P^O^ ₂ (x = 0.5; 0.75; 1.0) were assessed. The paint had a composition (wt.%) of 29% linseed oil, 53% iron red pigment, 7% talc, 1% siccative (1% cobalt octanate in gasoline) and 10% ^c_Mn 2-x ^Ca x ^P 4°12'

Corrosion tests were performed with coatings prepared according to ČSN 67 3004 on cold-rolled steel sheet 0.6 mm thick, which determined:

x = 0.5 ^C ' ^Mn 2-x ^Ca x ^P 4°12 x = 0.75 X = 1 corrosion losses of steel sheet (mg/g) in condensation chamber after 28 days (CSN 03 8131) 1.38 1.24 1.34 Corrosion losses of steel sheet (mg/g) in a chamber with hydrochloric acid vapors after 8 days (CSN 67 3094) 1.17 1.12 1.25 9 area of damaged paint (mm ) in accelerated immersion test of resistance to undermining corrosion - according to Mach and 13.7 12.9 9.8

Schiffman (CSN 67 3087)

Furthermore, classification tests of oil coatings with c-Mn2_ _x Ca _x P^O^2 were carried out according to

ČSN 67 3004: - exam A: x = 0.5 x = 0.75 x = 1 2 area of damaged paint (mm ) in the condensation chamber with the contents 18 14 15 S0 ₂ (points) (1) (1) (1) - exam B: 2 area of damaged paint (mm ) 18 16 18 in NaCl solution with HjOg (points) (1) (1) (1) - exam C: relative mass loss (%) of steel sheet in paint film extracts related 18.3 16.4 13.2 for loss in distilled water (points) (4.8) (4,6) (4.3) classification class of oil paints CSN CSN CSN wt. s ^c -Mn ₂ _ _x Ca _x P ₄ O ₁₂ 67 3004 67 3004 67 3004 Fe-1 Fe-1 Fe-1 OBJECT INVENTION

Claims (1)

The use of double-manganese-calcium cyclo-tetraphosphates, the composition of which is represented by the formula c-Mnj-Ca P 2 O 4 ^x, ranges from values greater than zero to one, preferably in the range of 0.5 to 0.90 as thermally stable anticorrosive pigments.