GB2112375A - Corrosion inhibiting water- soluble glass composition - Google Patents

Abstract

The composition contains P2O5, V2O5 and one or more Group IIA or IIB metal oxides. It is used in a paint formulation which includes the particulate water-soluble glass dispersed in a resin binder, preferably an alkyd acrylic, epoxy, chlorinated rubber or vinyl copolymer resin.

Description

SPECIFICATION Improvements in corrosion inhibition This invention relates to coating compositions for inhibiting corrosion of a metal surface to which they may be applied 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. 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. Furthermore 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 U.K. Patent 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 published specifications Nos. 23790/77 (C. F. Drake 58), No. 2062612 (C. F. Drake - A.

Maries - P. F. Bateson 73-2-1) and No. 2067129 ( C. F. Drake - A. Maries - P. F. Bateson 74-3-2) describe the use, as anti-corrosion materials, of various zinc/phosphorous 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.

Phosphorus pentoxide based glass pigments are restricted in their use in paint compositions by two constraints. The first is the extent of the composition region within which glasses may be formed.

Outside this region instability and identification may occur. The second constraint is the acid nature of the glass. Those glasses that have a relatively high phosphorus pentoxide content may provide an aqueous solution that is sufficiently acid to attack an adjacent metal surface thus nullifying any corrosion inhibition effect.

The object of the present invention is to minimise or to overcome these disadvantages.

According to one aspect of the invention there is provided a glass composition for inhibiting corrosion of a ferrous metal surface by releasing corrosion inhibiting ions when in contact with water, said composition comprising phosphorus pentoxide and vanadium pentoxide as glass forming oxides and one or more Group IIA or IIB metal oxides which comprise the glass modifying content of the glass.

According to another aspect of the invention there is provided 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 and vanadium pentoxide as the principal glass formers.

We have found that a proportion of the phosphorus pentoxide content of a phosphate glass may be replaced or substituted by vanadium pentoxide. This substitution extends the glass forming region of a glass composition and also provides a significant reduction in glass acidity. Thus the incorporation of vanadium pentoxide considerably expands the range of glasses that may be utilised.

In the following description the glass compositions are expressed, as is conventional, in terms of their constituent oxides. It will however be appreciated that glasses have complex structures and that the various constituents are not necessarily present in discrete oxide form. All compositions will be expressed in the form of molar proportions of oxide precursors, or their equivalents, due allowance being made for loss by volatilisation of some phosphorus pentoxide during the glass melting process.

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 corrosion inhibiting 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 total glass forming oxide (M2O5) 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 (Al2O3). 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. In particular we have found that the incorporation of vanadium pentoxide in a glass allows the dissolution rate to be raised to a relatively high value without the attendant disadvantage of an increase in acidity.

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

The accompanying drawing is a composition diagram illustrating the glass forming region of a typical phosphate glass system wherein a proportion of the phosphorus pentoxide glass former content is replaced by vanadium pentoxide. Typically such glasses may contain from 1% to 18% of vanadium pentoxide, the upper limit for most applications being equivalent to about 35% of the total M205 content. The glass modifiers of these glasses may comprise one or more Group IIA or IIB metal oxides.

In general we have found that the Group II glass modifier content of the glass should not exceed 72% as above this value the glasses become unstable and are subject to devitrification and/or phase separation. Similarly we have found that the phosphorus pentoxide control should not be less than 22%.

It will be understood that in any glass systems the boundaries of the glass forming region are not well defined. Thus, under certain conditions, it may be possible to make glasses outside the boundary.

The composite diagram should therefore be taken as a guide rather than as an absolute definition of the glass-forming region.

Advantageously the glasses may incorporate minor proportions (up to 5% in total) of Group III metal sesquioxides or Group IV metal oxides to provide dissolution rate adjustment.

The dissolution rate of the glass is a function of its composition, in general decreasing with increasing content of MO, e.g. calcium oxide, and/or alumina in the glass. We have also found that an increase in the vanadium pentoxide content leads to an increase in dissolution rate.

The soluble glass pigments may be present in a paint composition either as the entirety of the active pigment volume, or as a repiacement for certain conventional pigments when they may exhibit a synergistic effect on the inhibition of corrosion. In other applications glass pigments of different solubilities 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 glasses 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 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 be substituted by the metal carbonate, acetate, citrate or mixtures thereof. The phosphorus 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 slags, is then comminuted to a fine powder by one of 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 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, chlorinated rubbers or vinyl copolymers. In some applications the paint may include colouring pigments.

It may then be used as the sole 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 concentrated phosphoric acid, and vanadium pentoxide. In each case the mix was fused for 1 hour at 11 000C 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 24 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 ZnO CaO V2Os P205 M205 A 41.8 20.9 12.1 25.5 37.2 B 41.8 20.9 7.3 29.9 37.2 C 43.8 21.8 3.4 31.1 34.5 D 29.9 29.9 13.3 27.0 40.3 E 29.8 29.8 8.0 32.5 40.5 F 29.8 29.8 3.9 36.5 40.4 G 22.7 43.6 11.1 22.7 33.8 H 22.9 42.2 8.3 26.6 34.9 22.3 44.8 9.2 23.7 32.9 J 45.2 22.7 4.0 28.2 32.2 Samples of these glasses were tested for their corrosion inhibition effect by grinding each glass to a particle size of less than 38 microns and dispersing the powder in deionised water at a rate of 100 g/L. Mild steel rods were immersed in each sample and were periodically removed, washed and weighed. In each case a small initial weight gain was observed after which the weight remained substantially constant. This indicates the formation of a protective film on the steel surface following immersion in the solution.

Two of the compositions from Table 1 , A and B, 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 proprietary white alkyd gloss paint. The coated coupons were subjected to accelerated salt spray testing in accordance with British Standard 3900: Part F4: 1968, and SO2fog in accordance with British Standard 1391. For comparison coupons similarly treated with a proprietary zinc phosphate coupons showed reverse rusting under the test conditions.

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 they may be employed in the corrosion protection of vehicle bodies, freight containers and structural steel.

TABLE II Results of accelerated salt spray testing of mild steel coupons coated with high-build, short-oil alkyl test paints Proportion of active in total pigment Active pigment sample (% w/w) 250 hrs. in salt spray Pigment sample A 20 Very little cross-cut rusting and no staining. No blistering.

Pigment sample B 20 Some cross-cut rusting with slight staining. Slight blistering adjacent to cross-cut and on primer surface.

Zincorthophosphate* 40 Severe rusting of primer film and at cross-cut. Considerable pustuling over all primer surface.

* Commercially available high build primer for comparison.

Claims (14)

1. A glass composition for inhibiting corrosion of a ferrous metal surface by releasing corrosion inhibiting ions when in contact with water, said composition comprising phosphorus pentoxide and vanadium pentoxide as glass forming oxides and one or more Group IIA or IIB metal oxides which comprise the glass modifying content of the glass.

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

3. A glass as claimed in claim 1 or 2, and which comprises between 1 and 18 mole % vanadium pentoxide.

4. A glass as claimed in any one of claims 1 to 3, wherein the total M205 content comprises 35 mole %.

5. A glass as claimed in any one of claims 1 to 4, and which further includes a minor proportion of one or more Group III metal sesquioxides or Group IV metal oxides or mixtures thereof.

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

7. A glass as claimed in any one of claims 1 to 6 and in the form of a powder.

8. 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 and vanadium pentoxide as the principal glass formers.

9. A paint formulation, including a resin binder and a corrosion inhibiting pigment comprising a glass as claimed in any one of claims 1 to 7.

10. A paint formulation as claimed in claim 8 or 9 and including a further colouring pigment.

11. A paint formulation as claimed in claim 8, 9 or 1 0, wherein the resin binder is an alkyd resin, a chlorinated rubber, an epoxy resin, an acrylic resin, or a vinyl copolymer.

12. A paint formulation substantially as described herein.

13. 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 8 to 12.

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

1 5. A steel structure provided with a corrosion inhibiting paint as claimed in any one of claims 8to 12.