CS260488B1 - Anticorrosive heat-stable pigment - Google Patents

Abstract

The solution relates to the use of dicalcium phosphate as anticorrosive a stable pigment which, when used, does not coatings or other protective coatings exhibits marked anticorrosive-inhibitory activity ability. It is thermally stable to high temperatures so it can be used in high temperature corrosion protection wt. Just use it in relatively small quantities and in addition contains a significant proportion low-lime calcium components. Solution Can be used in pigment technology in the paint and high temperature industries wt.

Description

The invention relates to the use of dicalcium pyrophosphate as an anti-corrosion thermally stable pigment.

The most effective anticorrosive pigments are some lead compounds and zinc chromate compounds. Recently, however, they have been replaced by phosphorus compounds for hygienic and ecological reasons. Phosphates suppress oxygen corrosion in a humid, aqueous environment, especially in ferrous materials (steel, cast iron) where they bind iron ions due to corrosion into insoluble phosphate, which then forms a coating that also passivates the metal surface. phosphate cations (the beneficial effect of calcium ions is also known).

Of the phosphorus compounds, resp. designed for use as simple phosphates as anticorrosive pigments. The second group is condensed phosphates. These include the so-called polyphosphoric glasses (higher linear phosphates), which have been proposed so far, and the recently proposed cyclotrophosphates of some divalent metals, and more recently also the pyrophosphates of zinc, manganese, copper, cobalt and nickel.

The most widespread of simple phosphates is zinc phosphate in the form of dihydrate - Zn3 (PO4) 2.2 HžO. It is also known to use CrPO4.3 H2O and phosphates of some alkaline earth metals, especially calcium. In addition, double calcium-zinc phosphate - CaZn2 (PO4) 3.2 H2O has recently been used. Calcium ions will have an effect on corrosion pigments, even if they are in admixture. They effectively increase the alkalinity of the moisture, eventually permeate the coating or generally with an anticorrosive protective layer. 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 pigments of both lead and chromate and need to be applied to paints in relatively large amounts (concentrations) to have their anticorrosive effects satisfactory.

A further disadvantage of simple phosphates is the relatively low thermal stability of these substances resulting from their defined hydrated form, which is essential for their anticorrosive action. Therefore, they cannot be used in anticorrosive coatings for higher temperatures (above 150 ° C). Their lower thermal stability can also complicate the final mechanical-thermal operations of pigment preparation and treatment as well as its dispersion into the anticorrosive mass.

In view of the long-term anticorrosive action of simple phosphates, their partial solubility in aqueous, not completely neutral agents (e.g., the action of so-called acid rain) may also be a disadvantage. Over time, the pigment particles may be washed out of the protective layer, thereby breaking its impermeability to corrosive media.

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 and the content of the metal component, which is less effective than the phosphorus component, is relatively high.

The first group of condensed phosphates designed or used as anticorrosive pigments is called polyphosphoric glass. These higher linear condensed phosphates contain anions in the form of a polymer chain. They are designed with various cations of sodium, potassium and also calcium, magnesium and sometimes also cationic, cadmium, aluminum or ferric.

• Polyphosphoric glasses again do not reach the level of the best lead pigments with their anticorrosive effects. Their thermal stability is considerably higher than that of simple phosphates, but it is also limited as they recrystallize and usually lose the character of higher polymer anions between 400 and 600 ^and C. Therefore, they cannot be used in corrosion protection layers above these temperatures.

Higher linear phosphates are also partially soluble and, if they are powdered. pigment form, even tend to become wet. By the action of moisture they gradually progress to dihydrogen phosphates, which are then readily soluble, can be easily washed out of the protective layers, which thus break up and become permeable to corrosive media.

Higher linear phosphates are not anticorrosive in the long term, despite the fact that their use on a large scale (especially Cd and Zn products) can lead to hygienic and ecological problems.

A further disadvantage of these compounds is the high demands on their preparation, especially from an energy and construction point of view. They are prepared from melt at high temperatures (800 to 1300 ° C), which are considerably aggressive and from which some aggressive phosphorus products are already volatile. The products have a glassy character and so the final operations of their preparation into powder - pigment form and dispersion into paint or other mass are also quite demanding.

The most recently proposed for use as anticorrosive pigments of cyclo-tetraphosphates of some divalent metals (including calcium) eliminates most of the drawbacks mentioned for both simple phosphates and higher linear phosphates. They are thermally very stable up to their melting point (calcium product up to 900 ° C). Above this temperature, however, they decompose.

β

Cyclo-tetraphosphates are also chemically very stable, with very low solubility in aqueous and not completely neutral compositions, so that their anticorrosive action has a long-term character, especially in wet, aggressive environments; however, this may be a disadvantage, since a faster release of phosphoric passivating anions is needed.

Cyclotrophosphates have a high proportion of phosphorous anticorrosive active ingredient, which is 2 to 3 times higher than that of simple phosphates. However, for some products, this component may be more material intensive than the cationic component. This is especially true for dicalcium phosphate, where the calcium component is very inexpensive and it would be advantageous to increase its proportion in the pigment, while maintaining the character of the condensed phosphate-anion (eg as in pyrophosphate). It is not as technologically and structurally demanding as the preparation of the latter phosphates, but some products are more difficult to manage to achieve a sufficient yield, including in the case of dicalcium phosphate.

Recently, zinc, manganese, copper, cobalt and nickel pyrophosphates have also been proposed as anticorrosive pigments, in addition to the advantages mentioned for cyclotrophosphates. They have an even higher thermal stability, are somewhat faster acting and have a slightly lower content of raw material-intensive, albeit anti-corrosion, very effective phosphorous components. On the other hand, the content of the cationic - divalent metal - component, which, in most cases, of the pyrophosphates proposed hitherto, is higher in terms of raw materials. From this point of view, it seems very effective to use pyrophosphate of some cheap metal - eg calcium.

The use of dicalcium pyrophosphate as an anticorrosive thermally stable pigment therefore removes the drawbacks mentioned for single phosphates and for higher linear phosphates, complements (or removes some of the drawbacks) of the advantages mentioned for the cyclotrophosphates and for the second pyrophosphates.

Ca2P2O7 has suitable basic pigment properties - density, specific surface, oil consumption, is completely white and easily dispersible into organic and inorganic binders. The solubility of dicalcium pyrophosphate is higher than dicalcium phosphate pyrophosphate and thus releases phosphoric passivating ions more quickly. Again, however, in a gradual and practically regulated way, depending on the degree of corrosive environmental impact.

In the first step, half of the anions are slowly released and the solid residue corresponds to a single phosphate. Therefore, the impermeability of the coating film, respectively, is still not impaired at this stage. another protective layer to which the pyrophosphate has been applied. The remaining phosphate then has an anti-corrosion effect again, so that Ca2P2O2 generally has longer-lasting effects. It is also completely stable up to its melting point of 1250 ° C and is still stable even after reaching this temperature.

The diphosphate has a P / Ca molar ratio equal to one. This is a higher value and more favorable from an anticorrosive point of view than for simple phosphates and, on the contrary, it is lower and thus less expensive in terms of raw materials than for cyclotetaphosphates. The content of a very cheap calcium component is twice as high as dicalcium cyclophosphate.

In the process for preparing dicalcium phosphate, a relatively high yield of pure product is easily achieved, which is practically in pigmented form, while not requiring high quality raw materials. Inexpensive natural limestone, lime milk (calcium hydroxide) or waste calcium carbonate and lower quality (extractive) dilute phosphoric acid can be used.

The following are examples of some of the determined pigment properties of Ca 2 P 2 O 7, which roughly correspond to the most common inorganic pigments. The following are examples of established anticorrosion inhibition properties of dicalcium pyrophosphate, which illustrate its superior abilities in this respect than commercial anticorrosive pigments based on zinc phosphate dihydrate.

Example 1

Some properties of dicalcium phosphate related to its pigment use and inhibitory action have been determined:

density specific surface consumption of flaxseed oil pH of aqueous extract - 8 days after insertion steel. - 8 days after removal of steel. sheet · inhibitory properties of aqueous · leachate - corrosion loss of steel after 8 days immersion in Ca2P2O7 leachate

3.1 g / cm ^{3 of} 1.05 m / g, 17.5 g of oil per 100 g of Ca2P2O7 6.85

7.08

6.89

11.73 mg / g

Example 2

The ability of coatings prepared with three oil paints (a, b, cj containing as anticorrosive pigment) was compared:

(a) dicalcium phosphate (Ca2iP2O7)

2B34I

(b) a commercial core pigment consisting of simple zinc phosphate precipitated on ferric oxide particles (ferric red) [Zn3 (PO4) 2.2 H2O - Fe2O3]

(c) commercial core pigment consisting of simple zinc phosphate on titanium dioxide particles (titanium dioxide) [Ζη3 (Ρθ4) 2.2Η2θ —TIO2]

The paint with Ca2P2O7 had a composition (wt% J:

% linseed oil, ° / o ferric red pigment,% zinc white pigment,% talc,% siccative (1% cobalt octanate in gasoline) and

Table 0 / o Ca 2 P 2 O 7.

Coatings with core pigments contained:

% linseed oil,% talc,% siccatives and% core pigment;

the core pigments each contained 16% zinc phosphate, which corresponded to 10% single zinc phosphate in the paint.

Corrosion tests were performed with coatings prepared according to ČSN 67 3004 on a steel sheet thickness of 0.6 mm cold rolled (table).

Coatings with commercial core pigments

Zn3 (PO4) 2.2 H2O - Zn3 (PO4) 2.2 H2O - Coating with Ca2P2O7 - Fe2O3 - T1O2

Corrosion loss steel. sheet (or areas of damaged paint around 100 mm cut)

in the condensation chamber after 21 days (ČSN 03 8130) 43.8 g / m ² (38 mm ² ) 31.6 g / m ² (52 mm ² ) 16.35 g / m ² (37 mm ² ) Corrosion loss steel. sheet in a chamber with vapors of 18% hydrochloric acid after 8 days 15.2 g / m ² 11.9 g / m ² 11.45 g / m ² Surfaces of damaged paint even during accelerated immersion test against corrosion - acc Mach and Schiffman (ČSN 67 3087) 28 mm ² 18 mm ² 15 mm ² Surfaces of damaged paint (around 100 mm longitudinal section) after 14 days immersion in 1000 ml of an aqueous solution containing 50 g of NaCl and 10 ml H2O2 38.5, mm ² 32 mm ² 24.5 mm ² Relative mass steel drop. sheet after 21 days immersion in aqueous extracts of the coating film (10% by weight of the coating coating, film after 14 days of leaching) - based on steel losses. sheet after 21 days in dest. water 14.7% 17.9% 17,63%

Example 3

Steel plates with prepared coatings. According to Example 2, with 10% oil paints. Ca2P2O7 (or 63% of the core pigments) were exposed to weather conditions of the East Bohemian chemical-industrial agglomeration for two years (or one year). The weight loss of platelets due to corrosion (CSN 03 8140) ranged from 16.2 to 23.1 g / m ² after two years with CazP2O7 coating, while with commercial core pigments it was 25 to 28 after one year g / m ² . Example 4

The thermal stability of dicalcium pyrophosphate was calcined in an electric furnace to various temperatures up to 1400 ° C and the analysis of calcinates by instrumental analysis. Ca2P2O7 has been shown to melt at 1250 ° C, but is stable up to 1400 ° C in composition. Once cooled and solidified, it again represented crystalline pyrophosphate.

Claims (1)

Use of dicalcium phosphate as an anti-corrosion thermally stable pigment.