CS253098B1 - Anticorrosive pigment - Google Patents

Abstract

Dimagnesium cyclotetraphosphate as a pigment with inhibitory - anti-corrosion effects in coatings has better capabilities than commercial anti-corrosion pigments based on simple zinc phosphate. The concentration in coatings necessary to achieve sufficient anti-corrosion effects can be relatively low, the pigment is chemically and thermally highly stable and is therefore also suitable for special high-temperature purposes.

Description

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

Phosphates of some metals have the ability to inhibit deep corrosion. They are able to suppress oxygen corrosion in a humid, aqueous environment, especially in ferrous materials.

Their phosphate anions react with iron ions released by corrosion and bind them into insoluble phosphate, the coating of which then anodically passivates the metal surface. Metal cations from the phosphate used may also have certain inhibitory effects. For example, the beneficial effect of zinc and calcium ions is known, as well as manganese ions, which allegedly increase the effects of ions in zinc.

In accordance with the beneficial effects of calcium ions and with some of the data given below, certain inhibitory abilities can be expected for magnesium ions as well. In recent years, the use of simple phosphates of some metals as anti-corrosion pigments has become widespread, with the aim of replacing very effective, but hygienically and ecologically problematic lead pigments. However, so far, their inhibitory anti-corrosion abilities do not reach the effectiveness of lead pigments.

The most widespread is simple zinc phosphate - Ζη ₂ (ΡΟ^) ₂ ·2 H ₂ O - possibly with admixtures of other divalent metals (Ca, Mn). It has a relatively high content of zinc, which is raw material-intensive and is less effective than phosphorus.

Other anti-corrosion pigments of the type of simple phosphates are also known - chromium and alkaline earth metals. The production of simple phosphates is not a technologically simple operation, because they need to be prepared in the form of hydrates with a precise content of crystal water, as they significantly affect the final anti-corrosion properties of the products. The demands on the quality of the starting raw materials are also relatively high. Anti-corrosion pigments of the type of simple phosphates are also partially soluble in aqueous environments, which could bring adverse hygienic and ecological consequences if they are widely used. The necessity of their use in the form of precisely defined hydrates also limits the temperature range of their application; it does not allow their application in coatings for coatings resistant to high temperatures and can also complicate the final operations of pigment treatment or their dispersion. When using these compounds, it is also necessary to choose their relatively high concentrations in coatings to achieve sufficient inhibitory effects.

It is also known to use so-called polyphosphate glasses as anti-corrosion agents that contain an anion in the form of a polymeric phosphate chain (so-called higher linear phosphates). These are systems containing phosphate chain anions and, in addition to alkali metal cations (Na, K), also cations of some alkaline earths (especially Ca, but systems containing Mg are also known), possibly also other cations - Zn, Cd, Al, Fe. However, these glasses also have some shortcomings due to problems arising during their preparation and application. This is the need to obtain a homogeneous melt in the first phase of their preparation and thus the use of high temperatures (800-1,300 °C); further, there is the volatilization of the phosphate component from the melt and the high aggressiveness of the melt, increasing the design requirements for 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 common pigments is not achieved.

After application of these glasses to paints, 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 relatively quickly convert to simple phosphates corresponding to mostly highly soluble dihydrogen phosphates, which is disadvantageous from the point of view of the long-term inhibitory effect of the paint. 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. Their higher solubility can again lead to certain hygienic and ecological problems, which could also arise with their widespread use as in the case of simple phosphates.

Recently, there have also been proposed uses of cyclotetraphosphates - dizinc (Čs. AO 245 071), dimanganese (Čs. AO 248 540) and dicalcium (Čs. AO 247 844), which eliminate most of the shortcomings mentioned for simple phosphates and phosphate glasses. A certain disadvantage of cyclotetraphosphate dizinc and dimanganese is the price of the zinc or manganese component, although their content in these pigments is more than twice lower than in the case of simple phosphates.

Another minor drawback could also be manifested in the wider use of these pigments in contact with water for direct supply to the population, where the presence of zinc and manganese ions could be problematic. However, this drawback is negligible compared to simple phosphates, primarily due to the several times lower solubility of cyclotetraphosphates and further due to the lower content of the aforementioned components in these pigments and also due to the possibility of using lower necessary concentrations of pigments in paints.

In the case of dicalcium cyclotetraphosphate as an anticorrosive pigment, another minor disadvantage may in some cases be its somewhat higher solubility than in other cyclotetraphosphates. However, only in some cases of use, because on the one hand its solubility is significantly lower than that of simple phosphates and phosphate glasses and, on the other hand, in most cases it is favorable that calcium ions increase the alkalinity of the environment. In the case of dicalcium cyclotetraphosphate, it is a certain problem to manage the conditions of its preparation so that its yield is sufficient and there are no losses of the phosphate component, which is more demanding in terms of raw materials.

The thermal stability and thus the temperature range of use of the three mentioned cyclotetraphosphates is limited by temperatures of 800 °C (c-Zn2P^O-£2), 900 °C (c-Ca2P^O^2) ^and 950 °C (c-Mn^P^O.^) These are temperatures that allow their use as anti-corrosion pigments for high-temperature purposes and significantly exceed the temperature ranges of possible use of simple phosphates and phosphate glasses; however, for some special applications it would be advantageous to achieve even higher stability temperatures.

The above-mentioned shortcomings are eliminated by the invention consisting in the use of dimagnesium cyclotetraphosphate as an anti-corrosion pigment. This substance has favorable basic pigment properties for use in paints - density, specific surface area and oil consumption; it is practically white with approximately 90% reflectance in the entire visible part of the spectrum and is easily dispersible in paints.

Obtaining cyclo-dimagnesium tetraphosphate is not a technologically demanding process for exact adherence to reaction conditions and the quality of starting materials as the preparation of some simple phosphates and is not as energy-intensive as the preparation of phosphate glasses. Cheap natural magnesium carbonate or some waste magnesium compounds - hydroxides, carbonates - and lower quality (extraction) diluted phosphoric acid can be used. During its synthesis, c-MgP^O.^ can be produced either in the directly required powder form, or it can be easily converted into this form during its own dispersion in the coating paste.

Dimagnesium cyclotetraphosphate contains magnesium and phosphorus components in a molar ratio of one to two; this is more favorable in favor of the more effective phosphorus component. It contains anions in the form of tetraphosphate cycles, formed by four linked tetrahedra (PO^).

These are very stable anions, advantageous in terms of pigment and anti-corrosion properties of cyclotetraphosphate. This results in its high thermal stability (up to a melting point of 1,160 °C) - the highest of all cyclotetraphosphates proposed so far. This allows for advantageous application also in anti-corrosion coatings for special high-temperature use.

Another important property is its very difficult, very slow and gradual solubility in aqueous environments. When dissolving ^C “ ^M ^2 ^P 4°12 ^{, that} is, in the first stage, hydrolytic cleavage of tetraphosphate cycles occurs. In the case of moisture passing through the coating and attack of cyclotetraphosphate particles by water molecules, phosphate ions then come into soluble form by a slow process. In this way, these passivating ions are released in a practically regulated manner according to the degree of corrosive action of the environment.

In the first, slowest stage, only one half of them is gradually released, while the other half remains bound in the form of the resulting dimagnesium diphosphate, to which c-Mg ₂ P ₄ 0g ₂ 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 facilitate further corrosion.

The second stage of possible pigment dissolution in the coating is the gradual transition of the resulting diphosphate particles, with the gradual release of another third of the phosphate inhibiting anions, to simple magnesium phosphate, which also has certain anti-corrosion properties.

The following are examples of some pigment and inhibitory properties of dimagnesium cyclotetraphosphate and their comparison with commercial phosphate pigments. c-Mg ₂ P ₄ 0g ₂ shows favorable pH values of aqueous solutions and very good inhibitory properties of this solution against steel sheet. Steel sheets coated with dimagnesium cyclotetraphosphate showed lower corrosion weight losses in tests in a condensation chamber (ČSN 03 8131) and in a chamber with hydrochloric acid vapors (ČSN 67 3094) than sheets coated with commercial phosphate pigments. The areas of damaged coating with c-Mg ₂ P ₄ 0g ₂ were also smaller in the accelerated immersion test of resistance to undermining corrosion (according to Mach and Schiffman - ČSN 67 3087) and in the classification test of the coating material (ČSN 67 3004) 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 67 3004) were also smaller.

Example 1

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

density specific surface area 2.85 g/ ^cm3 0.27 ^m2 /g linseed oil consumption 23.3g oil/100g pH of aqueous extract c-Mg ₂ ^p 4 ^O 12 - 8 days after inserting the steel sheet 4.87 - 8 days after removing the steel sheet 5.58 inhibitory properties of aqueous extract 5.10 - corrosion losses of steel after 8 days immersion in leachate (mg/g) 2.71

Example 2

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

- dimagnesium cyclotetraphosphate (c-Mg ₂ P ₄ 0g ₂ )

- commercial core pigment formed from simple zinc phosphate precipitated on iron oxide particles (Zn^(PO^) ₂ ~Fe ₂ 0g)

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

The coating material with c-Mg ₂ ^p 40g ₂ 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-Mg ₂ P ₄ 0gg.

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 in the coating.

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

Table

Coatings with commercial cores pigments Coating c-Mg ₂ ^p 4 ^O i2 ^Zn 3 < ^p0 4)2 ^-Fe 2°3 Zn ₃ ( ^po 3)2- ^{TiO 2} Corrosion losses of steel sheet in a condensation chamber after 28 days (ČSN 03 8131) Corrosion losses of steel sheet in a chamber with hydrochloric acid vapors 10.65 mg/g 7.39 mg/g 2.35 mg/g after 8 days (ČSN 67 3094) Areas of damaged paint in accelerated immersion resistance test 3.30 mg/g 2.58 mg/g 3.02 mg/g against undercutting - according to Mach and 2 2 Schiffman (ČSN 67 3087) Classification tests of paint according to ČSN 67 3004: - test A: 28mm . 18 minutes 13 mm' ⁵ areas of damaged paint in condensation- o o o start the chamber with the content of S0 _o 38mm 52mm 31mm from (6 points) (7 points) (5 points) - exam B: areas of damaged paint 2 2 2 in NaCl solution with H _o 0 38.5mm 32mm 14.5mm - test C: relative weight losses of steel sheet in paint film leachates based on (6 points) (5 points) (2 points) losses in distilled water 14.7% 17.9% 9.41% (4 points) (4 points) (3 points) Paint classification class matters ČSN 67 3004 ČSN 67 3004 ČSN 67 3004 Fe-3 Fe-3 Fe-2

SUBJECT OF THE INVENTION

Claims (1)

SUBJECT OF THE INVENTION Use of magnesium di-cyclophosphate as anticorrosive pigment.