GB2099415A - Water-soluble glass for use in anticorrosion paints - Google Patents

Abstract

A corrosion-inhibiting material comprises a water-soluble glass of the zinc oxide/calcium oxide/ phosphorus pentoxide type which, when in contact with water, releases phosphate ions which inhibit corrosion of adjacent metal surfaces. The glass solubility may be controlled by the incorporation of one or more glass-modifying oxides (Al2O3, alkali metal oxide). Advantageously the glass is finely ground and then dispersed in a resin carrier to form a paint.

Description

SPECIFICATION Improvements in paint compositions This invention relates to coating compositions for inhibiting corrosion of a metal surface to which they may be applied, to glasses for use in said coating compositions and to processes for the preparation and use of the compositions.

One of the major problems involved in the use of metals as structural materials is that of corrosion of the metal, ferrous metals being particularly susceptible. The mechanism of corrosion is incompletely understood, but it is well known that the process is accelerated under hostile conditions, typically in industrial and marine environments. The standard technique for reducing corrosion is to apply to the metal surface a primer coating containing one or more corrosion inhibiting materials. Such primer coatings generally comprise a resinous binding medium in which finely ground pigments are dispersed, the purpose of these pigments being either to provide opacity and colour or to provide corrosion inhibition, these latter being known as active pigments. Themost commonly used active pigments are red lead and calcium plumbate, but these materials are highly toxic.Zinc chromate is also employed as a corrosion inhibitor, but it does not possess the level of performance of the lead pigments and can also cause colour bleeding of a subsequent paint coat. Futhermore hexavalent chromium salts are suspected of having carcinogenic activity.

More recently zinc phosphate has been employed as a non-toxic alternative to lead and chromate pigments. Compositions employing this material are described in U.K. Patent Specifications Nos. 904,861 and 915,512. It is claimed that this material is almost as effective as the previously employed toxic pigments, but its performance is poor in certain binder media and under conditions of exposure where the atmospheric sulphur dioxide level is low, typically marine conditions. Furthermore where a primed steel surface is to be welded, the use of conventional zinc phosphate paints, which have a relatively high loading of zinc, should preferably be avoided. The intense heat generated in the welding process can cause vaporisation of the paint producing toxic fumes of zinc oxide and/or free zinc.

The aforementioned UK Patents Nos. 904,861 and 915,512 also describe the use of calcium phosphate (tricalcium phosphate, calcium hydrogen phosphate and mono-calcium dihydrogen phosphate) which avoid the toxicity problem experienced with zinc based paints. However these calcium salts do not possess the optimum values of water solubility and pH for effective corrosion inhibition over a range of paint media and environmental conditions. Also it will be clear that, as the compounds are stoichiometric, these properties are not subject to control.

Our co-pending patent application Nos. 23790/77 (C.F. Drake-58), No. 8036718 (C.F.

Drake-A. Maries-P.F. Bateson 73-2-1) and No. 810776 (C.F. Drake-A. Maries-P.F. Bateson 74-3-2) describe the use, an anti-corrosion materials, of various zinc/phosphorus oxide and calcium/phosphorus oxide glass pigments. These materials are more effective than the conventional zinc orthophosphate pigments in that they provide zinc or calcium and phosphate ions at predetermined optimum rates and ratios under a variety of corrosion conditions.

We have now discovered that glass materials based on the zinc oxide/calcium oxide/phosphorus pentoxide system provide even more effective anti-corrosion pigments than those comprising primarily zinc oxide or calcium oxide together with phosphorus pentoxide.

According to one aspect of the invention there is provided a glass composition for inhibiting corrosion of a steelsurface by releasing corrosion inhibiting ions when in contact with water, said composition comprising a glass including phosphorus pentoxide as a glass forming oxide, and a mixture of zinc and calcium oxides which together comprise the glass modifying oxide content of the glass.

According to another aspect of the invention there is provided a paint formulation adapted to ihibit corrosion of a metal surface to which it is applied, said formulation including a particulate water soluble glass composition dispersed in a resin binder, said glass composition including phosphorus pentoxide as the glass forming oxide and a mixture of zinc and calcium oxides which together comprise the glass modifying oxide content of the glass.

We have found that glass compositions incorporating phosphorus pentoxide as the glass forming oxide and both zinc and calcium oxides as the glass forming modifiers have a relatively low solubility. This means that the glasses can be used in a very finely divided form, typically below 10 microns in average diameter, to produce a paint having a sufficiently high quality finish for use in applications where a cosmetic effect is required in addition to the corrosion protection provided by the paint. Such applications include the corrosion protection of vehicle bodies.

It is important to be able to adjust and control the dissolution rate, the ionic ratio and the solution pH of anti-corrosion pigments to optimise the pigment performance under various conditions, e.g. marine, neutral or industrial, and in different paint media, e.g. natural or synthetic resins, chlorinated rubbers or cellulose derivatives. The glasses described herein are designed to release calcium, zinc and phosphate ions into solution at predetermined rates, and the glass composition is therefore tailored to provide the desired dissolution rate. The dissolution rate of the glass is determined primarily by the proportion of the acid glass forming oxide (phosphorus pentoxide) present in the composition. An increase in this proportion increases the glass dissolution rate correspondingly and, conversely, a decrease in this proportion decreases dissolution rate.Another technique that can be employed to determine glass dissolution rate is to incorporate in the glass a proportion of additional glass modifying oxides, in particular metal sesquioxides such as alumina (Al203). Thus, the addition of a quantity of alumina decreases the dissolution rate of the glass. Conversely the dissolution rate of the glass is enhanced by the incorporation of one or more alkali metal oxides.

Techniques of glass pH and dissolution rate control are more fully described in our co-pending application No. 7930041 (C.F. Drake 70).

The term 'glass pigment' as employed herein is understood to include not only homogeneous glasses but also partially devitrified and partially phase separated materials which have been reduced to a powder of pigmentary grade by successive stages of comminution and milling. All the glass compositions referred to herein are described, for simplicity, in terms of the molar proportions of their constituent oxides although of course those oxides are not necessarily present in their free form.

Embodiments of the invention will now be described with reference to the accompanying drawing in which the single Figure is a composition diagram illustrative of the zinc oxide/calcium oxide/phosphorus pentoxide glass forming system.

Refering to Fig. 1, the triangular composition diagram refers to the zinc oxide/calcium oxide/phosphorus pentoxide system, the glass-forming region of the system being shown shaded. Corrosion inhibiting compositions can be made from glasses throughout this glasforming region, but we prefer to employ compositions in the range 1 5 to 60 mole % zinc oxide and 5 to 40 mole % calcium oxide, the remainder comprising phosphorus pentoxide. An even more advantageous composition range in terms of corrosion inhibition is 20 to 60 mole % zinc oxide and 5 to 40 mole % calcium oxide and 28 to 50 mole % calcium oxide.

Corrosive inhibiting materials, including paint pigments, based on glasses in the zinc oxide/calcium oxide/ phosphorus pentoxide system (Fig. 1) offer a number of advantages over glasses in which either zinc oxide or calcium oxide is the principal glass modifier. In particular, although the mechanism of corrosion inhibition is not well understood, it is apparent that the modes of action of zinc and calcium ions are different, and we have in fact found that where these ions are released into solution in comparable concentrations their corrosion inhibiting effects become synergistic. Thus the presence of the two cations provide an enhancement of corrosion protection above that which would be obtained by a purely additive effect.

Futhermore, the glass forming region of the ZnO/CaO/P20s system is considerably larger than those of the ZnO/P205 or the CaO/P205 systems thus offering a much wider range of compositions. This facilitates the selection of a suitable glass for a particular application. In addition we have found that a fine control of the dissolution rate of the glass can be achieved by corresponding variation of the proportion of calcium oxide. In many applications a sufficiently low dissolution rate can be achieved without the addition of alumina to the glass but, where alumina addition is required, this can be used to provide a coarse dissolution rate control. The final fine adjustment of dissoluton rate can then be effected by adjustment of the calcium oxide content of the glass.

The glasses may be formed by fusing mixtures of the constituent oxide, or precursors thereof, for a sufficient period of time to obtain a homogeneous melt which is then cast into cold water or on to a cold steel plate. The solidified material is crushed and ground to a particle size typically below 100 microns and is then dispersed in a paint resin as part of the total solid pigment. Advantageously the solid pigment content of the paint comprises e.g. 40 vol % and of this a total of typically 20 weight % may comprise the corrosion inhibiting glass material.

The ultimate particle size to which the glass is ground is determined by the particular application for which the glass is required. It will also be appreciated that the particle size and the glass dissolution rate are interrelated. Thus relatively small particles have a high total surface as compared weight for weight, with coarse particles. For equivalent effective release rates low dissolution rate glasses are fine ground and high dissolution rate glasses are coarse ground.

Where the glass is to be employed in an anti-corrosion paint, the glass particle size is, in general, dictated by the intended use of the paint. Thus, for structural steel applications where appearance of the paint is not a primary requirement, relatively coarsely ground glass pigment can be employed. In other applications, for example in the corrosion protection of vehicle bodies, it is essential that the paint provide a smooth surface for the subsequent application of one or more high quality top coats and for this purpose the pigment should be ground to a fineness of 10 microns or less. The ZnO/CaO/P2O5 glasses described herein are particularly advantageous for this latter application as their dissolution rates are ideally matched to their use at such fine particle sizes.

We have found that suitable dissolution rates are obtained when the proportion of phosphorus pentoxide in the glass is below 50 mole % (Fig. 1) and preferably below 45 mole % or even more advantageously below 40 mole %.

It will of course be appreciated by those skilled in the art that the composition diagram comprising the accompanying drawing is an indication rather than an exact definition of the composition ranges envisaged. Thus the difficulties of providing an exact chemical analysis of any glass system preclude an accuracy of better than + 1 mole % for any given constituent.

The soluble glass pigments may be present in a paint formulation either as the entirety of the active pigment volume, or as a replacement for certain conventional pigments when they may exhibit a synergistic effect on the inhibition of corrosion. In other applications glass pigments of different colubilities may be blended in the same paint medium to provide corrosion inhibition both in the short term and over an extended period. This technique may also be employed to optimise the performance of a coating which may be subject to exposure in environments of different degrees of aggression.

The use of the glass is not of course limited to paint compositions. Thus they may also be incorporated, for example, in reinforced concrete to prevent corrosion of steel reinforcing rods, or in water repellent grease compositions. In such applications the glasses may be provided in the form of fibres, granules, blocks, powders, stoving enamels etc. They may also be applied to various substrates by plasma spraying, flame spraying, electrostatic coating, electrodeposition, etc.

The glass compositions are prepared by fusing a mixture of the constituent oxides, or compounds which on heating decompose to form the respective oxides, for a sufficient period of time to form an homogeneous melt. For example one or more of the metal oxides may substituted by the metal carbonate, acetate, citrate or mixtures thereof. The phosphorous content of the glass may be added as phosphorus pentoxide, ammonium dihydrogen phosphate, aqueous phosphoric acid or mixtures thereof. Advantageously a slight excess of the phosphorus compound may be provided in the mix to compensate for the loss by evaporation of phosphorus pentoxide during the fusion process. The melt so formed is quenched rapidly to solid material by pouring either on to a cool steel plate or on to water-cooled rollers. Quenching may also be achieved by pouring the molten glass into a bath of water or an oil.We have found that although the glass is water soluble, its solubility is sufficiently low that only a small proportion is lost by dissolution when water quenching is employed as the glass is in contact with water only for a short time.

The quenched material, which may be in the form of flakes, granules or slabs, is then comminuted to a fine powder by one or more stages of crushing or grinding. Typically the glass is jaw crushed, or dry milled in a pestle and mortar or pin disc mill, or wet ground in a rotary or vibratory ball mill followed by drying, or rotary or vibratory bail mill followed by drying, or by air impact milling. Any other methods well known in the art can also be employed.

The powdered glass thus manufactured may be incorporated in a paint vehicle to form a corrosion inhibiting primer by two-stage ball milling, high speed dispersion or by other means well known in the art. We prefer to use an alkyd resin as the paint binder medium, but it will be appreciated by those skilled in the art that other conventional resins or binders can be employed, e.g. epoxy resins, acrylics or chlorinated rubbers.

Unlike prior corrosion resistant compositions, the glass compositions described herein are essentially colourless. Thus, they may be used in paint compositions as the sole pigment or together with the desired final colouring pigment. As such, a single coating of the present composition will in many instances be adequate for both corrosion resistance and final colour.

Thus, the present paint compositions may be the sole paint coating on a metal surface.

By way of example the glass compositions listed in Table I were individually prepared by blending together into a paste measured amounts of calcium carbonate, zinc orthophosphate and concentrated phosphoric acid. In each case the mix was fused for 1 hour at 1 100'C in a platinum/rhodium crucible and was then quenched to form a solid glass by pouring on to a cold steel plate. The glass was comminuted to a powder and the dissolution rate and 72 hour pH value. In each case the chemical composition of the glass was estimated from the batch weight making due allowance from the loss of phosphorus pentoxide by volatilisation. The results are summarised below.

Table 1 Composition Mole % Dissolution 72 hr Glass No. ZnO CaO P205 mg-cm-2 hr-1 pH 1 22.3 18.4 48.4 1.33 4.2 2 56.4 5.4 38.2 0.20 4.6 3 54.1 9.5 36.4 0.16 5.5 4 58.1 6.0 35.9 0.14 5.6 5 50.0 14.0 36.0 0.13 5.7 6 54.2 11.0 34.8 0.13 5.7 7 48.0 15.4 36.6 0.10 5.9 8 45.1 20.5 34.4 0.10 5.7 9 24.5 33.1 42.4 0.10 5.2 10 20.0 40.0 40.0 0.10 5.8 11 36.9 27.6 35.5 0.08 6.3 12 31.5 31.5 37.0 0.06 6.6 13 36.7 34.9 28.4 0.05 6.9 Two of the compositions for Table I (Nos 4 and 5) were evaluated by preparing test paints.

The glasses were first comminuted by successive crushing and milling to obtain a fine pigment grade powder. Particles greater than 10 microns in diameter were removed by air classification.

The pigments were each dispersed in a paint formulation having the following compositions: Ingredient Weight g SYNOLAC 9016X 167 Talc 108 TiO2 (R-CR2) 38 Yellow ion oxide 7.5 BENTONE 38 6.5 Active glass pigment 40 NUOSYN 18% Zirconium 4.0 NUOSYN 10% Cobalt 4.0 EXSKIN 2.0 Xylene 123 The paint was applied to clean mild steel coupons by air spraying and allowed to cure for several days. The resulting paint films, between 30 and 80 microns in thickness, were cross-cut with a sharp steel scriber. The coupons were then overcoated with a proprietory white alkyd gloss paint.The coated coupons were subjected to accelerated salt spray testing in accordance with British Standard 3900: Part F4: 1968, and SO2 fog in accordance with British Standard 1 391. For comparison coupons similarly treated with a proprietory zinc phosphate paint. After 500 hours the appearance of the coating, judged by lack of rusting at the cross cut, was excellent. In comparison the zinc phosphate painted coupons showed reverse rusting under the test conditons. The results are summarised in Table II.

These tests demonstrate the facility of formulating anti-corrosion paints for the glasses described herein. The glasses may be employed in a variety of applications, including their use in paints. In particular, where a vehicle paint is to be formulated from a relatively finely divided glass, we have found that the glass dissolution is advantageously less than 0.2 mg/cm2/hr.

Table II Results of accelerated testing of mild steel coupons coated with test paints.

Proportion of active in total pigment 500 hrs in 250 hrs in Active pigment (% w/w) salt spray 250 hrs in SO2 fog high humidity Pigment 4 1 2 Very little No blistering or Slight rusting rusting, even rusting of primer film: in cross-cut. No blistering.

Blistering almost entirely absent.

Pigment 5 1 2 Very little No blistering or Slight rusting rusting, even rusting of primer film: in cross-cut. No blistering.

Blistering almost entirely absent.

Zinc Severe rusting Moderate rusting Considerable orthophosphate* 40 of primer film of primer film. rusting and and cross-cut. Considerable pustuling visible Considerable blistering. through the over pustuling even coated portion.

under gloss coat.

"Commercially available high build primer for comparison.

Claims (18)

1. A glass composition for inhibiting corrosion of a steel surface by releasing corrosion inhibiting ions when in contact with water, said composition comprising a glass including phosphorus pentoxide as a glass forming oxide, and a mixture of zinc and calcium oxides which together comprise the glass modifying oxide content of the glass.

2. A glass composition as claimed in claim 1, wherein said glass has a composition lying within the shaded region of the composition diagram of Fig. 1 of the accompanying drawings.

3. A glass composition as claimed in claim 1 or 2, and comprising 1 5 to 60 mole % zinc oxide and 5 to 40 mole % calcium oxide, the remainder comprising phosphorus pentoxide.

4. A glass composition as claimed in claim 3, and comprising 20 to 60 mole % zinc oxide, 5 to 40 mole % calcium oxide and 28 to 50 mole % phosphorus pentoxide.

5. A glass composition as claimed in any one of claims 1 to 4, wherein the phosphorus pentoxide content of the glass is less than 40 mole %.

6. A glass composition as claimed in any one of claims 1 to 5, wherein the dissolution rate of the glass as hereinbefore defined is less than or equal to 0.20 mg/cm2/hr.

7. A glass composition as claimed in any one of claims 1 to 6, and in the form of a powder the average particle size of which is below 10 microns.

8. A glass composition substantially as described herein with reference to the accompanying drawing.

9. A paint formulation adapted to inhibit corrosion of a metal surface to which it is applied, said formulation including a particulate water soluble glass composition dispersed in a resin binder, said glass composition including phosphorus pentoxide as the glass forming oxide and a mixture of zinc and calcium oxides which together comprise the glass modifying oxide content of the glass.

1 0. A paint formulation, including a resin binder and a corrosion inhibiting pigment comprising a glass as claimed in any one of claims 1 to 8.

11. A paint formulation as claimed in claim 9 or 10 and including a further colouring pigment.

1 2. A paint formulation as claimed in claim 9, 10 or 11, wherein the resin binder is an alkyd resin, a chlorinated rubber, an epoxy resin or an acrylic resin.

1 3. A paint formulation substantially as described herein.

14. A process for inhibiting the corrosion of a metal surface, including applying to the surface a paint formulation as claimed in any one of claims 9 to 1 3.

1 5. A process as claimed in claim 14, wherein said paint formulation is the sole paint coating on the surface.

1 6. A process for inhibiting corrosion of a metal surface substantially as described herein.

1 7. A steel structure provided with corrosion protection by a process as claimed in claim 1 5 or 16.

18. A steel structure as claimed in claim 17 and comprising a vehicle body.